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Technical Manual for Design and Construction of Road Tunnels - Civil Elements

Chapter 9 - Sequential Excavation Method (SEM)

9.5 Ground Support Elements

This section addresses special ground support and material considerations that have evolved with the application of shotcrete supported SEM excavations. Chapter 6 also provides detailed discussions about rock reinforcement elements.

9.5.1 Shotcrete

The original name for shotcrete was "Gunite" when it was used for the purpose of taxidermy by spraying mortar on wire frames in the US in the early 1900's. In its early applications, sprayed dry mix material has also been used for the improvement of the fire resistance of timber supports in mines. During the course of the early 1930's, the term "Shotcrete" was introduced and has been widely used since. Development of equipment technology for the application of shotcrete progressed rapidly and the use of shotcrete for ground support purposes spread worldwide. In particular, the use of NATM / SEM, and the associated extensive use of shotcrete contributed to development of shotcrete which nowadays can be viewed as sprayed concrete, the major distinction between concrete and shotcrete being merely the method of placement (Vandewalle, 2005).

9.5.1.1 Effect of Shotcrete

When concrete is sprayed on a rough ground surface, it fills small openings, cracks and fissures and as initial support provides immediate support after excavation. It reduces the potential for relative movement of rock bodies or soil particles and, therefore, limits loosening of the exposed ground surrounding the tunnel. The adhesion depends on the condition of the ground surface, the dampness and presence of water and the composition of the shotcrete. Generally, the rougher the ground surface the better the adhesion. Dry rock surfaces have to be sufficiently dampened prior to application of shotcrete. Dusty or flaky surfaces, water inflow or a water film on the rock surface or other contaminant reduce the adhesion of shotcrete.

Modern admixtures improve the "stickiness" of shotcrete significantly such that rebound is reduced considerably. Fibers increase the adhesion and cohesion of the freshly applied ("green") shotcrete and therefore improve the build-up quality of the shotcrete. In turn, excessive stickiness of the shotcrete mix (as frequently observed when sodium silicate accelerators are used) can have an adverse effect. Too sticky shotcrete tends to accumulate around reinforcement bars, resulting in insufficiently compacted, low quality concrete or even voids or "shadows" behind the reinforcement bars.

In order to stabilize small wedges and slabs, shotcrete is applied locally. This application type does not form a continuous layer of shotcrete over an extended area to form a supporting member in the sense of a lining or structural shell. Rather, edges and corners generated by the intersection of discontinuities are filled with shotcrete bonding the bodies together thus forming local support.

Flashcrete: also referred to as sealing shotcrete, is applied immediately after excavation by spraying a thin layer of shotcrete if required to seal off the exposed ground surface. Flashcrete is often used in poor rock or soft ground (soil) in combination with (steel) fibers for reinforcement. This application limits desiccation, effects of humidity on sensitive ground material, softening due to contact with water, and loosening of the ground due to differential movement of ground particles. Flashcrete may be applied locally (and in areas where required) or over the entire exposed ground surface after excavation. Flashcrete is not considered to be an active support and, therefore is normally followed by a systematically applied initial shotcrete lining.

Shotcrete Face Support: In poor ground conditions a temporary face support may be required to restrict the ground from moving into the excavation. Dependent on the length of period through which the support is required and the ground conditions, the thickness and reinforcement of the face support varies. For tunnel stubs, permanent head walls are constructed with shotcrete. A domed face shape is of great importance in poor ground for successful face stabilization.

Experience gained from tunnel projects in soft ground demonstrates that ground deflections and hence surface settlements continue until a final, fully domed head wall with sufficient connection to the tunnel shotcrete initial lining is established.

Temporary Shotcrete Support: In poor ground conditions or where large tunnel cross sections are constructed, the excavation area must often be split into several drifts. To provide immediate support and, if required, ring closure for each sub-drift, temporary shotcrete support shells or linings are used. The thickness of the temporary lining is designed based on the cross sectional area of the drift to be supported and the period for which the support is required. The temporary shell is removed during subsequent construction steps that complete the excavation to the full tunnel opening. Figure 9-14 shows a typical SEM tunnel excavation with a temporary middle wall.

Typical Tunnel Excavation with Temporary Middle Wall

Figure 9-14 Typical Tunnel Excavation with Temporary Middle Wall (Beacon Hill Station, Washington)

Initial Shotcrete Lining: Initial shotcrete lining typically consists of 4 to 16 inch (100 to 400 mm) thick shotcrete layer mainly depending on the ground conditions and size of the tunnel opening, and provides support pressure to the ground. It is also referred to as shotcrete lining. A shotcrete ring can carry significant ground loads although the shotcrete lining forms a rather flexible support system. This is the case where the shotcrete lining is expected to undergo high deformations and hence ductility post cracking is of importance. By deforming, it enables the inherent strength and self-supporting properties of the ground to be mobilized as well to share and re-distribute stresses between the lining and ground. During deformation, stresses acting within the shotcrete lining are transferred into the surrounding ground. This process generates subgrade reaction of the ground that provides support for the lining. From the ground support point of view the design of the shotcrete lining is governed by the support requirements, i.e., the amount of ground deformations allowed and ground loads expected as well as economical aspects. The earlier the sprayed concrete gains strength the more the support restrains ground deformation. However, by increasing stiffness the support system increasingly attracts loads. It depends on the ground conditions and local requirements how stiff or flexible the support system has to be and thus what early strength requirements, thickness and reinforcement should be specified.

In shallow tunnel applications and beneath surface structures that are sensitive to deformations such as buildings, ground deformations and consequently surface settlements have to be kept within acceptable limits. The advantage of the mobilization of the self-supporting capacity of the ground can therefore be only taken into account to a very limited extent. Here, early strength of the shotcrete is required to gain early stiffness of the support to limit ground deflections. Under these conditions the shotcrete lining takes on significant ground loads at an early stage however in a generally low stress environment due to the shallow overburden. Early strength can be achieved with admixtures and modern cement types.

In contrast for tunnels under high overburden the prevention of ground deformation and surface settlement plays a secondary role. Excessive ground loads in squeezing ground and active tectonic pressures applied on the tunnel perimeter may be the design criteria for deep tunnels. By allowing the ground to deflect (without over-straining it) the ground's self-supporting capability, mainly shear strength, is mobilized. Consequently, the ground loads acting upon the shotcrete lining can be limited significantly because the ground assumes a part of the support function and a portion of the ground loads is dissipated before the initial support is loaded. For rock tunnels under high cover, early strength is not a necessity but final strength of the entire system is of importance. In special cases it may be required to construct "deformation joints" by implementing special yield elements to allow substantial deformation of the ground without generating uncontrolled cracks in the shotcrete lining while maintaining a defined support pressure during the process of deformation. Yield elements are designed such as to allow pre-scribed maximum deformation under defined lining loads (Section 8.3.3).

Rock reinforcement installed in rock tunnels augments the strength of the surrounding ground, controls deformation and limits the ground loads acting upon the shotcrete initial lining. Shotcrete support and rock reinforcement are designed to form an integrated support system in view of the excavation and support sequence. The design engineer must define the requirements for the support system based on thorough review of the ground response anticipated.

The effect of the shotcrete is heavily dependent on the radial and tangential subgrade reaction generated by the surrounding ground. Therefore, shape, shotcrete thickness and installation time have to be designed in accordance with the ground conditions and the capacities of the surrounding ground and the support system. Site personnel should assess the support requirements and, if necessary, adjust the designed support system based on observations in the field. Notwithstanding the need for reaction to site conditions, the designer should always be party to the decision making process prior to changing any support means on site. The design intent and philosophy must be taken into consideration when adjustments of the support system are made.

Friction between the ground and the sprayed concrete lining (tangential subgrade reaction) is paramount for the support system. This friction reduces differential movement of ground particles at the ground surface and contributes to the ground-structure interaction. Even the shotcrete arch not forming a closed ring provides substantial support to the ground, given that tight contact between the sprayed concrete and the ground is maintained.

The requirement for a ring closure, be it temporary or permanent, is governed by the size of the underground opening and the prevailing ground conditions. In a good quality rock mass, no ring closure is required. In low quality ground (weak rock and soil), it has been proven in numerous case histories that the time of support application after excavation, length of excavation round and time lag between the excavation of the top heading and the invert closure rules the ground and lining deflection. To reduce ground deflection and the potential for ground/lining failure, the excavation and support sequence must be designed such that an early ring closure of the shotcrete support in soft ground is achieved. Also the timely (immediately after excavation) installation of the shotcrete support members is of utmost importance. To achieve an early, temporary ring closure and to reduce excavation face size, partial drifts such as sidewall drifts, middle drifts and top heading, bench, and invert drifts can be used. These partial drifts are supported by temporary shotcrete support, such as temporary middle walls, invert supports, etc.

An important aspect of shotcrete linings is the design and construction of construction joints. These joints are located at the contact between shotcrete applications in longitudinal and circumferential directions between the initial lining shells of the individual excavation rounds and drifts. An appropriate location and shape as well as connection of the reinforcement through the longitudinal joints is of utmost importance to the integrity and capacity of the support system. Longitudinal joints have to be oriented radially, whereas circumferential joints should be kept as rough as possible. Splice bars/clips and sufficient lapping of reinforcement welded wire fabric maintain the continuity of the reinforcement across the joints. Rebound, excess water, dust or other foreign material must be removed from any shotcrete surface against which fresh concrete will be sprayed. The number of construction joints should be kept to a minimum.

In case of ground water ingress, the ground water has to be collected and drained away. Any build-up of groundwater pressure behind the shotcrete lining should be avoided for the following reasons: Increased ground water pressure in joints and pores reduces the shear strength in the ground, undue loads may be shed onto the shotcrete lining (unless it is designed for that, which is unusual for initial shotcrete linings); softening of the ground behind the lining; increased leaching of shotcrete; shotcrete shell will be detached from the ground.

9.5.2 Rock Reinforcement

As discussed in Chapter 6, rock reinforcement and rock mass act as a complex interactive system, where the individual elements always have to be seen in view of their interaction and interdependence. The overall strength of a reinforced rock mass with a joint system is governed by the characteristics of the joints (roughness, fill, rock material, orientation) and the contribution provided by the reinforcement elements. For the design of rock mass reinforcement systems, sufficient appreciation of the expected ground conditions and experience are of fundamental importance. Readers are referred to Section 6.5.2 for more detailed discussion for each type of rock reinforcement. The following focuses on the SEM applications and issues.

9.5.2.1 Types of Rock Reinforcement

Rock Dowels: (Figure 6-17), are passive reinforcement elements that require some ground displacement to be activated. In deep tunnels or under tunneling conditions where ground deflection is permitted or even desired, passive rock reinforcement is frequently installed. This applies for example to tunnel construction sequences where the excavation and support installation is carried out in sequences (e.g. top heading, bench, invert). In order to best use the support effect of the rock dowels, an early installation is required. The majority of ground deflections develop during excavation and closely behind the progressing tunnel face. In sequential rock tunneling using multiple drifts, ground deflection typically ceases after top heading excavation and support but commences again after a period of relative stability during excavation for bench and invert construction. Therefore, rock dowels should be installed right after excavation or close to the progressing excavation face.

Rock Bolts: (Figure 6-18) actively introduce a compressive force into the surrounding ground. This axial force acts upon the rock mass discontinuities thus increasing their shear capacity and is generated by pre-tensioning of the bolt. The system requires a 'bond length' to enable the bolt to be tensioned. Rock bolts frequently are fully bonded to the surrounding ground after tensioning, for long-term load transfer considerations.

Rock bolts are not only installed during construction. Rock bolts may also be used for existing under-ground openings, where further deformation of the ground and/or the support is to be inhibited or for additional support of existing structures that will undergo subsequent enlargement or be influenced by adjacent tunnel construction.

For tunnels constructed in an environment where ground deflection and surface settlement hast to be limited (e.g. shallow tunnels in urban areas), rock bolt aid in limiting the ground displacement caused by the SEM tunneling. Furthermore, during construction of large openings ground deflection limitation may be desired to avoid loosening (and hence weakening) of the rock mass. In high stress environments, special compressible elements have been developed, that are installed between the ground/support surface and the face plate of the bolt allowing a certain amount of displacement while the tension force at the bolt is kept constant.

Rock Anchors: Rock anchors are used under conditions where high anchor forces have to be accommodated often significantly higher than for example rock bolt forces. For instance in very large span tunnels, where high support forces have to be generated to stabilize the ground, anchors are frequently used.

Generally, it can be stated that pre-tensioning of bolts establishes a stiffer system of the reinforced rock mass after installation and minimizes the magnitude of shear displacement. The design and application of a pre-tensioned rock reinforcement system requires excellent knowledge of the ground conditions and ground behavior to avoid over-tensioning during ground displacement. In comparison, an initially untensioned rock dowel reinforcement may ultimately lead to the same strength and reinforced rock mass capacity, however, only along with larger deformations.

Table 9-5 summarizes commonly used rock reinforcement elements and application considerations for the installation as part of initial support in SEM tunneling in rock.

9.5.2.2 Practical Aspects

Several practical aspects related to rock dowel/bolt installation in the field have been summarized below based on experience in SEM tunneling. Each individual project has its own particularities and, therefore, this list is not exhaustive.

Layout of Rock Mass Reinforcement Pattern: While it also has to observe theoretical considerations, the design must take practical issues of installation into account. As a consequence of a design lacking practical considerations, rock mass reinforcement systems are frequently 'adjusted' on site to suit practical aspects without considering the ground conditions and the design intent. Such installed rock reinforcement systems may be of limited benefit or even have an adverse effect.

Grouting: Rock dowel/bolt grouting systems aim for the full embedment of a rock dowel/bolt in grout. Full embedment not only ensures bond over the entire length of the dowel/bolt but also provides corrosion protection. Regardless of the method used, the appropriate consistency of the grout material is the most important factor in achieving the required bond between the ground and the reinforcement element. This particularly applies for cementitious grout materials. While the available diameter of the grouting hose dictates the consistency of the grout material to some extent, too high or too low viscosity can lead to insufficient bond. It can frequently be observed that installation crews adjust grout mixing plants and pumps and do not visually check the consistency of the grout mix produced. Even with the use of the most sophisticated mixing and pumping devices, it is required to visually check the grout mix produced before commencing each installation operation. All foreign material must be removed prior to installation to ensure proper bond.

Table 9-5 Commonly used Rock Reinforcement Elements and Application Considerations for SEM Tunneling in Rock
No.NameMaterial*AnchorageTensionedInstallationGround **AdvantagesLimitations
1Steel rebar dowelDeformed (solid) steel rebarFully bonded using cement grout or resinNoRebar inserted into pre-drilled and grout filled hole;
Rebar inserted in pre-drilled hole together with grouting hose and grouted subsequently
Massive to highly jointed rock massLow cost; Availability; If properly installed, high performance and heavy duty supportRequires skilled and experienced installation personnel; collapsing boreholes hamper installation
2Glass Fiber DowelDeformed fiber glass barFully bonded using cement grout, more frequently with resinNoRebar inserted into pre-drilled and grout filled hole;
Rebar inserted in pre-drilled hole together with grouting hose and grouted subsequently
Massive to highly jointed rock mass; frequently used in areas to be excavated subsequently (e.g. face bolting, break-out areas)High performance heavy duty support; can be easily removed during subsequent excavations within reinforced rock massRequires skilled and experienced installation personnel; limited shear resistance; collapsing boreholes hamper installation
3Split SetLongitudinally split steel pipeFriction over entire length generated by spring action of pipeNoForced into pre-drilled borehole of slightly smaller diameter than outer diameter of split setMassive to jointed rock massImmediate support action; simple installation; no grouting requiredVery limited shear resistance; Light support only; very corrosion sensitive; cannot be used in collapsing borehole
4SwellexFolded, inflatable steel pipeFriction over entire length generated by inflation of tubeNoInserted into pre-drilled borehole and inflated with highly pressurized waterMassive to jointed rock massImmediate support action; Can achieve significant support capacityLimited shear resistance and durability; cannot be re-tightened; requires special equipment for inflation; higher material cost; collapsing boreholes hamper installation
5Grouted PipesPerforated steel pipeFully bonded with cement or resin groutNoInserted into pre-drilled borehole (or rammed into soft ground with thick walled pipes) and grouted through pipe and perforation holesJointed to heavily fractured ground (soil like)Simple installation; Availability; More controllable embedment resultsLimited shear resistance (depending on wall thickness); collapsing boreholes hamper installation
6Self-drilling dowelsThick walled steel pipes with disposable drill bitFully bonded with cement. or resin groutNoReinforcement element functions as drill rod, drill bit and dowel remains in ground after drilling and is grouted through flushing openingsJointed to heavily fractured rock massInstallation steps limited to two steps (fast installation); High performance heavy duty support;More expensive than bar reinforcement; May become trapped in collapsing boreholes as it does not have reverse cutting tools;
7Rammed DowelsSteel rebar or thick walled steel tubeShear resistance generated between ground and element (friction, adhesion)NoRammed into groundDecomposed rock, soilLeast ground disturbance during installation; Immediate support actionRelies on shear resistance generated between ground and element; requires ramming equipment; limited to soft ground conditions
8Steel rebar boltDeformed steel rebara. End anchored: cement grout or resin;

b. Fully bonded: two phase resin
Yesa. Grouting behind grout seal through grouting hose (aeration hose);

b. resin grout with two different setting times
Massive to highly jointed rock massLow cost; Availability; if properly installed, high performance heavy duty supportRequires skilled and experienced installation personnel; collapsing boreholes hamper installation

a. Requires grout seal;

b. Resin is more expensive than grout; Requires different types of resin
9Glass fiber boltDeformed glass fiber bara. End anchored: cement grout, resin;

b. Fully bonded: two phase resin
Yesa. Grouting behind grout seal through grouting hose (aeration hose);

b. resin grout with two different setting times
Massive to highly jointed rock massHigh performance heavy duty support; can be easily removed due to limited shear resistanceRequires skilled and experienced installation personnel; collapsing boreholes hamper installation

a. Requires grout seal;

b. Resin is more expensive than grout; Requires different types of resin
10Expansion Shell BoltSteel rebarMechanically end anchoredYesInserted in pre-drilled borehole, shell at end expanded by tightening the boltMassive to jointed rock mass; requires competent rock materialImmediate support effect; can provide high support capacity;Relatively expensive; Slip or rock crushing may occur; tends to lose tension due to vibration (blasting) and ground deformation

* Reinforcement material

** Ground conditions described are typical application examples; reinforcement elements may also be used in other ground conditions.

Contact: Frequently rock dowels and face plates as well as nuts are installed in time, but the nuts are not tightened or are tightened only after a long period of time and far behind the progressing excavation face. While tensioning of a fully bonded rock dowel does not have any effect on the strength of the integrated rock - reinforcement system (rock mass and reinforcement), it is important to tighten the nuts to ensure a tight fit of the face plate and, if used in combination with a shotcrete support, to aid an appropriate contact between the ground surface and the shotcrete support lining/face plate. If used without a shotcrete lining, tightening of nuts assists in limiting early deformation and loosening of the rock mass close to the opening.

Testing and monitoring: Pull-out-tests are an important tool to ensure adequate anchorage of rock bolts. While useful to check the bond strength and therefore, the support capacity of a tendon with a defined bonded anchorage section and a free section, pull-out tests are irrelevant when used for testing fully bonded rock dowels, because they do not provide any information on the overall performance of a fully bonded rock reinforcement. The conventional pull-out test, when used for fully bonded reinforcement, provides information on the shear capacity between the bolt and grout and the ground adjacent to the head of the tested element, but it does not yield any information of the overall bond along the reinforcement element or whether the element is fully embedded in grout.

Similar to above, monitoring the anchor forces between the ground surface and the face plate of a fully bonded rock dowel/bolt does not provide any information on the forces acting within the fully bonded reinforcement element over its length. Therefore, only monitoring devices (e.g. strain gages) mounted directly onto the shank along the reinforcement element can supply information on the performance and stresses acting within the reinforcement during ground deformation.

9.5.3 Lattice Girders and Rolled Steel Sets

As discussed in Chapter 6, lattice girders (Figure 6-20) are lightweight, three-dimensional steel frames typically fabricated of three primary bars connected by stiffening elements. Lattice girders are used in conjunction with shotcrete and once installed locally act as shotcrete lining reinforcement. The girder design is defined in the contract documents by specifying the girder section and size and moment properties of the primary bars. To address stiffness of the overall girder arrangement the stiffening elements must provide a minimum of five percent of the total moments of inertia. This percentage is calculated as an average value along repeatable lengths of the lattice girder. The arrangement of primary bars and stiffening elements is such as to facilitate shotcrete penetration into and behind the girder, thereby minimizing shadows. Lattice girders are installed to provide:

  • Immediate support of the ground (in a limited manner due to the low girder capacity)
  • Control of tunnel geometry (template function)
  • Support of welded wire fabric (as applicable)
  • Support for fore poling pre-support measures

In particular cases where, for example, immediate support is necessary for placing heavy spilling for pre-support, the use of rolled steel sets may be appropriate. In such instances steel sets are used for implementation of contingency measures. Steel sets of bell shaped profile (Heintzmann profile) are also used as structural members in temporary shotcrete sidewalls in multiple drift tunneling. Their primary purpose apart from increased capacity over lattice girders is their ease of removal when demolishing temporary shotcrete walls in multiple drift tunneling applications.

9.5.4 Pre-support Measures and Ground Improvement

When tunneling in competent ground, the ground surrounding the tunnel opening provides sufficient strength to ensure stand-up time needed for the installation of the initial SEM support elements without any pre-support or improvement of ground strength prior to tunneling.

With the significantly increased use of the SEM in particular in soft ground and urban areas over the past decades, traditional measures to increase stand-up time were adopted and further developed to cope with poor ground conditions and to allow an efficient initial support installation and safe excavation.

These measures are installed ahead of the tunnel face. They include ground modification measures to improve the strength characteristics of the ground matrix including various forms of grouting, soil mixing and ground freezing, the latter for more adverse conditions. Most commonly they include mechanical pre-support measures consisting of spiling methods installed ahead of the tunnel face often with distances of up to 60 to 100 feet (18 to 30 m) referred to as pipe arch canopies or at shorter distances, as short as 12 ft (3.6 m) utilizing traditional spiling measures such as grouted solid bars or grouted, perforated steel pipes. Ground improvement and pre-support measures can be used in a systematic manner over long tunnel stretches or only locally as required by ground conditions.

9.5.4.1 Pre-support Measures

Pre-support measures involve spiling or grouted pipe arch canopies that bridge over the unsupported excavation round. These longitudinal ground reinforcement elements are supported by the previously installed initial shotcrete lining behind the active tunnel face and the unexcavated ground ahead of the face. These mechanical pre-support measures are generally used to:

  • Increase stand-up time by preventing ground material from raveling into the tunnel opening causing potentially major over-break or tunnel instabilities
  • Limit over-break
  • Reduce the ground loads acting on the immediate tunnel face
  • Reduce ground deflection and, consequently surface settlements.

Mechanical pre-support measures are generally less intrusive than systematic ground modifications. They rely on the ground reinforcing action of passive reinforcement elements such as steel or fiberglass pipes/bars. Similar to passive concrete reinforcement the elements must directly interact with the surrounding ground to be efficient as reinforcement. This interaction can only be established by a tight contact between the reinforcement element and the ground. This interaction can be achieved by either fully grouting the pre-support elements to lock the reinforcement in with the ground or by ramming the reinforcement elements into ground if susceptible to this action in soft ground conditions. Loosely installed elements installed in soft rock or soil do not achieve their intended function and such installations must be avoided. In fractured, but competent rock, steel rebars loosely installed in boreholes may be acceptable but merely to limit over-break. Figure 9-15 displays closely spaced No. 8 rebar spiles bridging across an excavation round and keeping soft, cohesive fine soil materials in place. Spiles rest on the initial shotcrete lining (front) and on the unexcavated ground beyond the tunnel face. The narrow spacing allows even very soft and soils with little cohesion to bridge between individual spiles.

Spiling Pre-support by No. 8 Solid Rebars

Figure 9-15 Spiling Pre-support by No. 8 Solid Rebars (Berry Street Tunnel, Pennsylvania)

The effect of mechanical pre-support has frequently been misjudged. On one hand, the stiffness of the steel elements used for pre-support is often taken as basis for assessing an increase of the overall stiffness of the ground surrounding the pre-support. This can easily lead to an over-estimation of the pre-support function, as the longitudinal stiffness of the entire system must be taken into account in those considerations. On the other hand, the radial action of a systematic pre-support arch is often underestimated or not considered at all.

The longitudinal effect of a pre-support element is less governed by the stiffness of the reinforcement element than by the improvement of the tensile and shear capacity of its surrounding ground.

When grout is used to establish the bond between the reinforcing element and the ground, grouting pressure used for installation, type of grout and grouted length have a paramount influence on the effect and efficiency of the pre-support in particular in soft ground conditions.

Though it has been proven in countless applications that mechanical pre-support has the effects mentioned above, quantification of the effect by numerical analyses methods proved to be difficult involving efforts that go beyond the usual design efforts. Hence, the effect of pre-support is often assessed using simple approaches that result in very conservative assessments, thus underestimating the actual effect of pre-support. In many cases, the effect of pre-support is even ignored in a design and pre-support is viewed merely as an increase of the safety margin rather than a settlement-limiting element of the tunnel support.

Pre-Support in Rock Tunneling: Pre-support installation in fractured, yet competent rock mass types is typically aimed at limiting the over-break during and after excavation. Pre-spiling with steel rebars is a frequent method to keep rock fragments in place (Section 6.5.6). Dependent on the degree of fracturing, the rebars are installed in empty boreholes arranged around the perimeter of the roof, or the boreholes are filled with cement grout prior to insertion of the rebars. Alternatively, perforated steel pipes are used that are inserted into the boreholes and subsequently grouted. In a severely fractured rock mass where boreholes tend to collapse, self-drilling rock reinforcement pipes are used. With the grouted applications, grout may intrude into cracks and fractures introducing a limited cementing effect of the surrounding material.

In soft rock mass types, where fracturing and limited material strength result in conditions with low overall strength, grouted pipe spiling or grouted pipe arches are used for pre-support. If required, these pre-support measures are combined with groundwater drawdown measures to reduce the joint water pressure and to increase the frictional capacity along the joints.

Permeation grouting of the discontinuities is used to reduce the mass permeability and to increase the overall shear strength by cementing the rock fragments together.

Pre-Support in Soft Ground (Soil) Tunneling: Similar to soft rock, grouted mechanical pre-support measures are used to pre-stabilize soil or soil like ground. Dependent on the susceptibility of the soil to grout, these mechanical pre-support methods are combined with grouting systems that allow penetration of grout into the ground leading to cementation of the ground surrounding the pre-support. Penetrability of the ground and the intended purpose of the pre-support govern the selection of the grouting materials. While grout with standard cements has a limited capability for penetrating ground containing sand or smaller fractions, penetration results can be improved by the use of micro or ultra fine cement products or chemical grouting (resin grouting). The current market offers resin grouting materials with viscosity values close to water.

In many cases, particularly under shallow cover with the groundwater table in the lower part or below the tunnel invert, mechanical pre-support measures are sufficient as long as the support elements are sufficiently locked into the ground over their entire length by an appropriate grouting material. Any additional effect by grout material penetrating voids in vicinity of the installed pre-support is considered an additional benefit.

In very loose, generally non-cohesive ground, ground improvement measures may be required to cement the ground and to decrease the permeability of the soil.

Pre-Support Elements: Most commonly used mechanical pre-support elements include grouted pipe spiling of typically 2-inch (50 mm) diameter perforated steel pipes and rebar spiling using solid No. 8 (25 mm diameter) steel rebars as shown in Figure 9-15 . These are primarily installed in the area of the tunnel roof and shoulders, but may also be installed in the sidewall and invert if suitable and required. Grouting of these spiling elements establishes a tight contact between the reinforcement element and the surrounding ground. So-called self-drilling and grouted rebars (type IBO, ISCHEBECK or similar) provide for a very efficient installation of grouted, solid steel bars.

Grouted Pipe Arch Canopy: Pipe arch canopy methods involve a systematic installation of grouted pipes at a spacing of typically 12 inch (300 mm) around the tunnel crown. This installation typically involves one single row of pipes but under critical ground conditions and / or when surface settlements must be restricted may involve a double row of pipes. The pipes are installed at lengths typically not to exceed 15 to 24 meters (50 to 80 feet) using conventional drilling techniques at a shallow lookout angle from the tunnel and ahead of the tunnel excavation. Specialized drill bit and casing systems are utilized that aim at limiting and strictly controlling the over cut, i.e., annular void space between inserted pipe and the surrounding ground. They also provide for direction control and high installation accuracy. Drilling techniques include ODEX®, CENTREX®, ALWAG and similar methods.

The steel pipes are typically perforated and have a diameter of between 4.5 inch and 6 inch (114 mm to 150 mm). The steel pipes are grouted to facilitate contact between steel pipe and the surrounding ground and to create the desired arching effect around the tunnel opening during excavation. Depending on purpose and susceptibility of the ground to grouting, the perforated steel pipes may be grouted either from the single entry point at pipe end within the tunnel or using packers or double packers. Grouting with double packers will allow for targeted grouting with respect to location, grout mix, injected volumes, and pressures. These pipe arch systems have furthered the use of SEM applications in particular in urban settings under shallow overburdens and also in difficult ground conditions.

Figure 9-16 displays the installation of a steel pipe for an arch application for a 3-lane road tunnel in soft ground. The figure displays the steel pipe on a drill jumbo boom and a 4.5 inch (114 mm) steel pipe being drilled near the circumference of the shotcrete initial lining. Figure 9-17 displays previously installed pipe arch steel pipes exposed in the ground when opening a new excavation round.

Steel Pipe Installation for Pipe Arch Canopy

Figure 9-16 Steel Pipe Installation for Pipe Arch Canopy (Fort Canning Tunnel, Singapore)

Pre-support by Pipe Arch Canopy, Exposed Steel Pipes Upon Excavation of a New Round

Figure 9-17 Pre-support by Pipe Arch Canopy, Exposed Steel Pipes Upon Excavation of a New Round (Fort Caning Tunnel, Singapore)

Face Doweling: Face doweling forms a specific form of pre-support. Other than the mechanical pre-support installed in the tunnel roof and shoulder area, the face pre-support is installed within the excavation face to stabilize squeezing or raveling ground at the face prior to excavation. Passive elements are installed in the ground and usually grouted in place to increase the tensile and shear strength of the ground material. Since the reinforcement elements have to be excavated during subsequent excavation rounds, fiberglass reinforced resin dowels or pipes are frequently used. Steel elements for face doweling hamper the excavation progress and during excavation their removal transfers tension forces into the ground, promoting ground disturbance ahead of the progressing tunnel face. Face doweling can be combined with application of grouting methods to locally improve the overall strength of the ground within the tunnel cross-section and act with the face dowels.

Face support dowels are usually made of GFRP (glass fiber reinforced polyester resin) and provide significant tensile strength while allowing for easy removal during excavation due to the material composition and low shear resistance.

9.5.4.2 Ground Improvement

Ground improvement measures are primarily aimed at modifying the ground matrix to increase its shear (cohesion) and compressive strengths. An increase of the stiffness (deformation modulus) is coincidental to this improvement. These measures are frequently installed from the surface and well in advance of the tunnel excavation or are applied from within the tunnel ahead of the face. Ground improvement measures range from lowering of the groundwater table or reduction of the pore/joint water pressure to intrusive changes of the ground composition such as jet grouting, soil mixing or ground freezing.

Groundwater Draw Down: Draw down of the groundwater table reduces or eliminates the groundwater inflow into tunnels during construction and increases the effective shear strength of the ground. Groundwater flowing into the tunnel opening during construction not only causes unsafe conditions and increases equipment wear and tear; it also can promote ground instabilities. The reduction of the hydrostatic head reduces the water pressure acting within discontinuities and soil pores. Groundwater draw down can be carried out from the surface or from within the tunnel.

In fine-grained soils (fine sands, silts, clays) the reduction of the pore pressure results in a significant increase of the overall strength of the ground. Where gravity drainage is insufficient, vacuum wells or other means such as drainage by osmosis can be applied.

Permeation Grouting: Permeation grouting is frequently used to cement the ground matrix if it is sufficiently coarse and uniform to achieve reliable grout penetration. Microfine cement or chemical (resin) grouts are used for finer grained soils.

Where soils are not sufficiently uniform or groutable, other measures such as jet grouting or soil mixing are used. These methods actively modify the ground's fabric by mixing the ground with a cementing agent such as cement grout or lime. Jet grouting uses a high-pressure water-grout mix jet to cut the ground and mix it with the stabilizing agent generating improved soil columns of significant diameter. Readers are referred to Ground Improvement Methods Reference Manual (FHWA 2004) for more details.

Ground Freezing: Ground freezing is often considered as 'last resource' due to its high cost when compared to other ground improvement measures. However, ground freezing achieves a high degree of reliability of ground modification. This particularly applies for non-uniform soils. The frozen ground provides groundwater cut-off while its mechanical properties are sufficiently increased to allow an efficient and safe tunnel excavation and support installation under the protection of the frozen soil body. Ground freezing has provided solutions for tunneling under very complex conditions in urban settings.

Readers are also referred to Chapter 7 for discussions about the above ground improvement techniques. Chapter 15 presents a ground freezing application for jacked box tunnels.

9.5.5 Portals

9.5.5.1 General

This section describes the layout of temporary tunnel portal structures for highway tunnels that are frequently built with SEM tunneling. These structures provide a protection against rock fall, and stabilize the portal face from which SEM tunneling commences thus provide start-up condition for safe tunnel excavation.

Shotcrete canopies are also frequently used as an extension of the tunnel and are integrated into the final tunnel portal architecture. The tunnel final lining is cast against these shotcrete canopies and therefore the tunnel internal geometry is uniform from the cut-and-cover (shotcrete canopy) section into the mined tunnel. The shotcrete canopies are backfilled for the final condition.

9.5.5.2 Pre-Support and Portal Collar

The level of weathering and loosening of rock close to the surface must be addressed when starting tunnel construction. Even in generally good rock mass, surface near weathering and loosening requires pre-support at the portal.

After clearing the surface and installing required rock support at the portal face, a row of horizontal pre-spiling or grouted steel pipes should be installed to provide pre-support for the initial excavation rounds for the tunnel construction. Dependent on the degree and depth of weathering, this pre-support is typically 10 ft (3 m) to 60 ft (18 m) long and the reinforcement elements are grouted in place. The pre-support elements are typically spaced at 12-inch (0.30 m) centers around the future tunnel opening. Such tunnel pre-support at the portal is shown in Figure 9-18.

Pre-Support at Portal Wall and Application of Shotcrete for Portal Face Protection (Devil's Slide Tunnels, California)

Figure 9-18 Pre-Support at Portal Wall and Application of Shotcrete for Portal Face Protection (Devil's Slide Tunnels, California)

Following the pre-support installation, a reinforced shotcrete collar should be installed that is tied in with the protruding pre-support elements. The collar shall follow the tunnel perimeter extending from one sidewall to the other. In soft ground, the collar may extend over the entire tunnel perimeter. The collar provides stability to the ground in the immediate vicinity of the future tunnel opening and is structurally connected to the initial shotcrete lining for the first round of tunnel excavation.

9.5.5.3 Shotcrete Canopy

The shotcrete canopy comprises reinforced shotcrete and lattice girders. The canopy is founded on a strip foundation that extends over the entire length of the canopy. The length of the canopy is dependent on the rock fall protection required and on local conditions such as wind loads, temporary ventilation requirements, and needs of the final tunnel structure.

Portal canopies have to be designed for rock fall and snow loads, construction loads, dead loads, and any wind loads, as dictated by local site conditions. The canopy also serves as a counter form for final lining installation in the portal area. Figure 9-19 displays construction of a shotcrete canopy whereas the first three lattice girders and reinforcement have been placed and shotcrete is being sprayed against an expanded metal sheet placed on the outside of the lattice girders.

Shotcrete Canopy Construction after Completion of Portal Collar and Pre-support

Figure 9-19 Shotcrete Canopy Construction after Completion of Portal Collar and Pre-support (Schürzeberg Tunnel, Germany)

9.6 Structural Design Issues

9.6.1 Ground-Structure Interaction

The SEM realizes excavation and support in distinct stages with limitations imposed on size of excavation and length of round followed by the application of initial support measures. In particular the shotcrete lining has an interlocking function and provides an early, smooth support. To adequately address this sequenced excavation and support approach the structural design shall be based on the use of numerical, i.e. finite element, finite difference, or distinct element methods (see also Chapter 6). These numerical methods are capable of accounting for ground structure interaction. They allow for representation of the ground, the structural elements used for initial and final ground support, and enable an approximation of the construction sequence.

Embedded frame analyses have limitations in adequately describing the ground structure interaction. Due to this and the fact that these methods can not simulate excavation sequencing their use shall be limited to applications where the ground structure interaction phenomenon, in particular development of a ground-supporting arch, is of secondary importance. This is for example the case for shotcrete canopies that are frequently erected at tunnel portals as freestanding or backfilled reinforced shell structures and tunnel final linings.

9.6.2 Numerical Modeling

9.6.2.1 Two (2)-Dimensional and Three (3)-Dimensional Calculations

In general, use of two-dimensional models is sufficient for line structures. Where three-dimensional stress regimes are expected, such as at intersections between main tunnel and cross passages, or where detailed investigations at the tunnel face are undertaken such as for the behavior of pipe arch pre-supports, three-dimensional models should be used.

9.6.2.2 Material Models

In representing the ground, the structural models shall account for the characteristics of the tunneling medium. Material models used to describe the behavior of the ground shall apply suitable constitutive laws to account for the elastic, as well as inelastic ranges of the respective materials. For example when tunneling in rock, intact rock as well as the rock structure, i.e., the presence of discontinuities shall be taken into account. It is customary to apply Mohr-Coulomb or Drucker-Prager failure criteria for the representation of both rock and soil materials. Finite element programs that were developed initially for the simulation of underground excavations in rock such as Phase 2 by Rockscience, Inc. also allow use of rock mass material behavior using Hoek and Brown rock mass parameters (Hoek and Brown, 1980, 2002).

9.6.2.3 Ground Loads - Representation of the SEM Construction Sequence

Tunnel excavation causes a disturbance of the initial state of stress in the ground and creates a three-dimensional stress regime in the form of a bulb around the advancing tunnel face. Such a stress regime is indicatively displayed in Figure 9-20.

Stress Flow Around Tunnel Opening

Figure 9-20 Stress Flow Around Tunnel Opening (after Wittke, 1984 and Kuhlmann)

Far ahead of the advancing tunnel face the initial state of stress is represented by vertical and horizontal stress trajectories denoting major and minor principal stresses respectively (assuming that vertical stresses are higher than horizontal stresses in a geostatic stress field). At the tunnel face the stresses flow around the tunnel opening arching ahead of the tunnel excavation and behind it onto the newly constructed initial lining in longitudinal direction and to the sides of the opening perpendicular to the tunneling direction. At a distance where the tunnel is no longer affected by the three-dimensional stress conditions around the active tunnel face two-dimensional arching conditions are established.

The extent of the stress disturbance around an active heading depends mainly on ground conditions, size of the excavation and length of round. This disturbance begins up to two excavation diameters ahead of the active tunnel face as shown indicatively in Figure 9-21 . The SEM design dictates limits on excavation size and length of round and prescribes installation of initial support elements often following each individual excavation round directly or shortly thereafter. These requirements are portrayed in the Excavation and Support Class (ESC, see Chapter 9.5.3). Initial support elements are therefore installed within the shelter of a load-carrying arch around the newly created opening in an area where some pre-deformation has occurred. As the excavation of the tunnel advances the shotcrete hardens from an initially "green" shotcrete and becomes fully loaded at a distance of about one to two tunnel diameters from the face. Such sequencing combined with early support installation contributes to the development of the self-supporting capability of the ground. It further aids in minimizing deformations and ground loosening.

SEM Tunneling and Ground Disturbance

Figure 9-21 SEM Tunneling and Ground Disturbance (after OGG, 2007)

It is therefore important to portray this excavation and support sequencing closely in the numerical analyses. For shotcrete lining structural assessments it is important to distinguish between a "green" shotcrete when installed and when it has hardened to its 28-day design strength. Green shotcrete is typically simulated using a lower modulus of elasticity in the computations. A value of approximately 1/3 of the elastic modulus of cured shotcrete is commonly used to approximate green shotcrete in 2-D applications. In 3-D simulations the shotcrete may be modeled with moduli of elasticity in accordance with the anticipated strength gain in the respective round where it is installed.

Excavation and support installation sequencing can be readily realized in three-dimensional models. In two-dimensional modeling, however, auxiliary techniques must be utilized. A frequently utilized approach relies on the use of ground modulus reduction within the excavation perimeter prior to the insertion of the initial lining elements into the model. Other techniques involve the use of supporting forces applied to the circumference of the tunnel opening. Because of its frequent use the ground modulus reduction approach is used in describing a typical two-dimensional modeling sequence of SEM tunnel excavation and support of a line structure below. A calculation example is provided in Paragraph 9.7.3.7.

  • Represent the in-situ stresses including the geostatic stress field and surface loads as applicable.
  • Represent the excavation of the respective round by reducing the elastic modulus of the ground located within the geometric boundaries of the round to about 40% - 60% of its original value. The purpose of the modulus reduction is to achieve a pre-deformation of the ground prior to installation of the initial support measures. The extent of modulus reduction is only within the region where excavation takes places, i.e. a drift (top heading, bench, invert). It is an arbitrary measure applied to simulate an otherwise three-dimensional stress distribution at the face (see Figure 9-20) in two-dimensional computations. The value of 40-60% is a frequently used reduction amount and represents a typical range (Mohr and Pierau, 2004 and Coulter and Martin, 2004). A higher reduction will yield larger, a lower reduction will yield smaller deformations of the surrounding ground. A sensitivity analysis related to the actual reduction value is typically part of the computations.
  • Activate the initial support elements per design assumptions to represent the installation of initial support. Because the shotcrete will not have developed its design strength at this stage reduced shotcrete elastic properties (modulus) are initially taken into account and amount to about 1/3 of the hardened shotcrete. During subsequent simulation stages the shotcrete modulus is then increased to its 28-day design strength to represent a fully hardened shotcrete lining.
  • Remove the ground elements within the respective drift thereby completing excavation and support within the round.
  • Repeat this sequence until all drifts of the final tunnel cross section geometry have been excavated and supported.

Once accomplished, this completes modeling of the tunnel excavation and installation of initial support. The installation of the final tunnel lining generally occurs once all deformations of the tunnel opening have ceased. To account for this fact the calculations perform installation of the final lining into a stress-free state. The final lining becomes loaded only in the long-term resulting from a (partial) deterioration of the initial support (shotcrete initial lining and rock bolts if any), rheological long-term effects and ground water if applicable. Although modeling of the final lining is often undertaken by embedded frame analyses (see Chapter 10) its analysis within a ground-structure interaction numerical model will be most appropriate and can follow directly after the initial support is installed as follows:

  • Activate the structural elements representing the final tunnel lining.
  • If the modeling was carried out with temporary rock reinforcing elements without corrosion protection then all such supporting elements are deactivated.
  • If the ground water is generally aggressive and it may be assumed that the shotcrete initial lining will deteriorate long-term then it is assumed that no contributing support function may be derived from it for long-term considerations. This has been traditionally assumed on projects such as the Lehigh Tunnel, Cumberland Gap Tunnels and on NATM tunnels of the Washington, DC Metro. Washington Metropolitan Area Transit Authority (WMATA) has substantial experience with the design and construction of NATM tunnels in both soft ground and rock (Rudolf et al., 2007). To date it is customary on WMTA projects to assume that the shotcrete initial lining will deteriorate over time. Such computational approach will yield a conservative final lining design. Due to the nowadays high quality shotcrete fabrication however, and in particular in non-aggressive ground and ground water conditions it is admissible to assume that when the shotcrete initial lining is more than approximately 6-inch (150 mm) thick then 50% of its structural capacity may be taken into account in the final lining computations. The combined removal of initial support elements (rock reinforcement and shotcrete initial lining) will result in ground loads imposed onto the final lining in the long-term.
  • In addition to the ground loads, the concrete lining will be loaded with hydrostatic loads in un-drained or partially drained waterproofing systems. This load case generally occurs well before the final lining is loaded with any ground loads and shall be considered separately in the calculations.
  • Final lining calculations consider the existence of the waterproofing system, which is embedded between the initial shotcrete lining and the final lining. A plastic membrane will act as a de-bonding layer in terms of the transfer of shear stresses. Therefore simulation techniques should be used to simulate this "slip" layer. This is accomplished by only allowing the transfer of radial forces from the initial lining onto the final lining.

9.6.2.4 Ground Stresses and Deformations

Each step involving the simulation of excavation and installation of initial support allows for analysis of the ground response expressed in deformations, strains, and stresses. Both, elastic and, if yielded, inelastic portions of strains can be obtained and used to evaluate the state of stress in the ground and its capacity reserves. Stresses, strains and section forces are available in ground support elements such as dowels and rock bolts. The computational programs (for a selected list see Chapter 6) often provide such information in a user friendly display using numeric and graphic formats.

9.6.2.5 Lining Forces

Section forces and stresses are available for beam (2-D) or shell (3-D) elements. Section force and moment combinations are used to evaluate the capacity of the initial shotcrete and final concrete linings using ACI 318 or other accepted concrete design codes. Acceptance of codes is generally an owner driven process. For example, Washington Metropolitan Area Transit Authority (WMATA) allowed the use of the German Industry Standard DIN 1045 for the design of plain (unreinforced) cast-in-place concrete final linings (Rudolf et al., 2007 and Gnilsen, 1986).

Based on this evaluation the adequacy of lining thickness and its reinforcement (if any) is assessed. If the selected dimensions are found not to be adequate then the model must be re-run with increased dimensions and/or reinforcement. The process is an iterative approach until the design codes are satisfied.

These calculations do not distinguish between the type of lining application and therefore shotcrete and cast-in-place final linings are treated in the same manner within the program using the material properties and characteristics of concrete.

9.6.2.6 Ground Reinforcing Elements

Ground reinforcing elements are rock bolts and dowels. These are activated in the computations in accordance with the design of the SEM excavation and support installation. Once implemented and loaded during the simulation of excavation and support, section forces and stresses are available to evaluate their adequacy. Stresses and forces are compared with the capacity of the individual dowel or bolt.

9.6.2.7 Calculation Example

A calculation example (Appendix F) demonstrates the SEM tunneling analysis and lining design of a typical two-lane highway tunnel using the finite element code Phase2 by Rocscience, Inc. The calculation is carried out in stages and follows the approach laid out in 9.6.2.3 above and evaluates ground reaction as indicated in 9.6.2.4 and evaluates support elements as described in 9.6.2.5 and 9.6.2.6.

9.6.3 Considerations for Future Loads

Mainly due to its flexibility and ability to minimize surface settlements often in combination with ground improvement methods the SEM is frequently utilized for the construction of roadway tunnels in urban settings. In particular under such circumstances it is important to consider any future loads that may be imposed onto the tunnel for which the final linings must be designed. Such loads include among others buildings, foundations, and miscellaneous underground structures fulfilling future infrastructure needs. These can be readily implemented in the computation approach presented above in the form of external or internal modeling loads.

9.7 Instrumentation and Monitoring

9.7.1 General

An integral part of SEM tunneling is the verification by means of in-situ monitoring of design assumptions made regarding the interaction between the ground and initial support as a response to the excavation process.

For this purpose, a specific instrumentation and monitoring program is laid out in addition to general instrumentation programs connected with the overall tunneling work, i.e. surface and subsurface instrumentation. The SEM tunnel instrumentation aims at a detailed and systematic measurement of deflection of the initial lining. While monitoring of deformation is the main focus of instrumentation historically stresses in the initial shotcrete lining and stresses between the shotcrete lining and the ground were monitored to capture the stress regime within the lining and between lining and ground. Reliability of stress cells, installation complexity and difficulty in obtaining accurate readings have nowadays led to the reliance on deformation monitoring only in standard tunneling applications. Use of stress cells is typically reserved for applications where knowledge of the stress conditions is important, for example where high and unusual in-situ ground stresses exist or high surface loads are present in urban settings.

Monitoring data are collected, processed and interpreted to provide early evaluations of:

  • Adequate selection of the type of initial support and the timing of support installation in conjunction with the prescribed excavation sequence
  • Stabilization of the surrounding ground by means of the self-supporting ground arch phenomenon,
  • Performance of the work in excavation technique and support installation
  • Safety measures for the workforce and the public
  • Long-term stress/settlement behavior for final safety assessment
  • Assumed design parameters, such as strength properties of the ground and in-situ stresses used in the structural design computations (see Chapter 9.7).

Based on this information, immediate decisions can be made in the field concerning proper excavation sequences and initial support in the range of the given ground response classes (GRC) and with respect to the designed excavation and support classes (ESC).

9.7.2 Surface and Subsurface Instrumentation

The general instrumentation should include surface settlement markers, cased deep benchmarks, sub-surface shallow and deep settlement indicators, inclinometers, multiple point borehole extensometers, and piezometers (see Chapter 15).

The locations, types and number of these instruments should be determined by consultations between the civil, structural, geotechnical and SEM design teams to provide information on surface and subsurface structure settlements and to complement the SEM tunnel instrumentation readings.

9.7.3 Tunnel Instrumentation

Deformation Measurements Instruments are installed in the tunnel roof and at selected points along the tunnel walls to monitor vertical, horizontal, and longitudinal (in tunnel direction) deformation components. The number of points and their detailed location depends on the size of the tunnel and the excavation sequencing in multiple drift applications. As a minimum, the wall of each drift (including temporary) should be equipped with a device capable of measuring deformations. It is customary to install optical targets for this purpose. Figure 9-22 shows a series of deformation monitoring cross sections using optical targets in a SEM tunnel. Optical targets are the white reflecting points arranged in the tunnel roof and tunnel sidewalls.

Deformation Monitoring Cross Section Points

Figure 9-22 Deformation Monitoring Cross Section Points (Light Rail Bochum, Germany)

Stress Measurements If stress information is sought then measurements should be taken with a direct measuring tool that does not rely on any further conversions from say strains to stresses. For example, instruments based on strain gage principles require the knowledge of the elastic modulus of the material to covert strains to stresses. This introduces an additional parameter that must be estimated thus introducing a secondary uncertainty.

Stress measurements within shotcrete linings are frequently carried out using hydraulic pressure cells filled with mercury whereas ground stress measurements are carried out with cells filled with oil. If stress measurements are to be monitored then ground load cells and concrete pressure cells should be grouped in pairs.

9.7.4 SEM Monitoring Cross Sections

Monitoring devices are grouped into monitoring cross sections (MCS). These MCS are depicted with their respective instrument layout indicating location and number of instruments within that MCS. Typical MCSs are shown on the design drawings for each individual tunnel cross-section geometry and excavation sequence. Locations of the respective monitoring cross-sections are shown on dedicated instrumentation drawings by station references. An example deformation MCS is shown in Figure 9-23.

Typical Tunnel Monitoring Cross Section displaying Extensometers and Optical Targets
(a)
Detail A, view of Optical target displaying axes of measurement: Y=Vertical Displacement, X=Lateral Displacement, Z=Longitudinal DisplacementImage of Optical Target in place
(b)(c)

Figure 9-23 Typical SEM Deformation Monitoring Cross Section - a) Typical Tunnel Monitoring Cross Section displaying Extensometers and Optical Targets, b) Detail A, view of Optical target displaying axes of measurement: Y=Vertical Displacement, X=Lateral Displacement, Z=Longitudinal Displacement, c) Image of Optical Target in place.

During execution the installation of all MCSs is documented by a detailed description of the geological and tunneling conditions in the field using sketches showing the exact location of the instruments and the actual thickness of the shotcrete lining.

9.7.5 Interpretation of Monitoring Results

All readings must be thoroughly and systematically collected and recorded. An experienced SEM tunnel engineer, often the SEM tunnel designer, must evaluate the data, occasionally complemented by visual observations of the initial shotcrete lining for any distress, for example as indicated by cracking. To establish a direct relationship between the behavior of the tunnel and the ground as these react to tunnel excavation it is recommended to portray the development of monitoring values as a function of the tunnel progress. This involves a combined graph showing the monitoring value (i.e. deformation, stress or other) vs. time and the tunnel progress vs. time. An example is shown in Figure 9-24. In this example a prototypical deformation of a surface settlement point located above the tunnel centerline has been graphed on the ordinate (left vertical axis) vs. time on the horizontal axis. The same time horizontal axis is used to portray the tunnel excavation progress by station location on the right vertical axis. As can be seen from this graph the surface settlement increases as the top heading and later bench/invert faces move towards and then directly under that point and gradually decrease as both faces again move away from the station location of the surface settlement point. The settlement curve shows an asymptotic behavior and becomes near horizontal as the faces are sufficiently far away from the monitoring point indicating that no further deformations associated with tunnel excavation and support occur in the ground indicating equilibrium and therefore ground stability.

The evaluation of monitoring results along with the knowledge of local ground conditions portrayed on systematic face mapping sheets forms the basis for the verification of the selected excavation and support class (ESC) or the need to make any adjustments to it.

Prototypical Monitoring of a Surface Settlement Point Located Above the Tunnel Centerline in a Deformation vs. Time and Tunnel Advance vs. Time Combined Graph

Figure 9-24 Prototypical Monitoring of a Surface Settlement Point Located Above the Tunnel Centerline in a Deformation vs. Time and Tunnel Advance vs. Time Combined Graph

9.8 Contractual Aspects

SEM construction requires solid past experience and personnel skill. This skill relates to the use of construction equipment and handling of materials for installation of the initial support including shotcrete, lattice girders, pre-support measures, and rock reinforcing elements and even more importantly observation and evaluation of the ground as it responds to tunneling. It is therefore important to invoke a bidding process that addresses this need formally by addressing contractor qualifications and skills and payment on a unit price basis described below. For general contractual aspects refer to Chapter 14.

9.8.1 Contractor Pre-Qualification

It is recommended that the bidding contractors be pre-qualified to assure a skilled SEM tunnel execution. This pre-qualification can occur very early on during the design development but at a minimum should be performed as a separate step prior to soliciting tunnel bids. On critical SEM projects such as the NATM tunneling at Russia Wharf in Boston in the late 90's the project owner solicited qualifications from contractors at the preliminary design stage. This pre-qualification resulted in a set of pre-qualified contractors that were invited to comment on the design at the preliminary and intermediate design stages. This early process ensured that contractors were aware of the upcoming work and could plan ahead in assembling a qualified work force. Pre-qualification documents shall identify the scope of work and call for a similar experience gained on past projects by the tunneling company and key tunneling staff including project manager, tunnel engineers, and tunnel superintendents. As a minimum the documents lay out description of ground conditions, tunnel size and length, excavation and support cycles, and any special methods intended for ground improvement.

9.8.2 Unit Prices

It is recommended that SEM tunneling be procured within a unit price based contract. Unit prices suit the observational character of SEM tunneling and the need to install initial support in accordance with a classification system and amount of any additional initial or local support as required by field conditions actually encountered. The following shall be bid on a unit price basis:

  • Excavation and Support on a linear foot basis for all excavation and installation of initial support per Excavation and Support Class (ESC). This shall include any auxiliary measures needed for dewatering and ground water control at the face.
  • Local support measures including:
    • Shotcrete per cubic yard installed.
    • Pre-support measures such as spiling, canopy pipes and any other support means such as rock bolts and dowels, lattice girders, and face dowels shall be paid per each (EA) installed.
    • Instrumentation and monitoring shall be paid for either typical instrument section (including all instruments) or per each instrument installed. Payment will be inclusive of submitted monitoring results and their interpretation.
    • Ground improvement measures per unit implemented, for example amount of grout injected including all labor and equipment utilized.

Waterproofing and final lining installed to complete the typical SEM dual lining structure may be procured on either lump sum basis or on a per tunnel foot basis.

The quantity of local support (additional initial support) measures shall be part of the contract to establish a basis for bid.

9.9 Experienced Personnel in Design, Construction, and Construction Management

Because the SEM relies on tunneling experience it is imperative that experienced personnel be assigned from the start of the project, i.e., in its planning and design phase. The SEM design must be executed by an experienced designer. At this level of project development it is incumbent upon the owner to select a team that includes a tunnel designer with previous, proven, and relevant SEM tunneling design and construction experience.

The SEM tunnel contract documents have to identify minimum contractor qualifications. In this case it is secondary whether the project is executed in a design-bid-build, design-build or any other contractual framework. For example, if the project uses the design-build framework then it is imperative that the builder take on an experienced SEM tunnel designer.

The construction contract documents must spell out minimum qualifications for the contractor's personnel that will initially prepare and then execute the SEM tunnel work. This is the case for field engineering, field supervisory roles and the labor force who must be skilled. SEM contract documents call for a minimum experience of key tunneling staff by number of years spent in the field on SEM projects of similar type. Experienced personnel will include Senior SEM Tunnel Engineers, Tunnel Superintendents and Tunnel Foremen. All of such personnel should have a minimum of ten (10) years SEM tunneling experience. These personnel are charged with guiding excavation and support installation meeting the key requirements of SEM tunneling:

  • Observation of the ground
  • Evaluation of ground behavior as it responds to the excavation process
  • Implementation of the "right" initial support.

Knowledgeable face mapping, execution of the instrumentation and monitoring program, and interpretation of the monitoring results aid in the correct application of excavation sequencing and support installation. Figure 9-24 displays a typical face mapping form sheet that is used to document geologic conditions encountered in the field. While this form sheet portrays mapping for rock tunneling, mapping of soft ground conditions is similar and lays out the characteristics of anticipated soil conditions. Face mapping should occur for every excavation round and be formally documented and signed off by both the contractor and the owner's representative.

The Senior SEM Tunnel Engineer is generally the contractor's highest SEM authority and supervises the excavation and installation of the initial support, installation of any local or additional initial support measures and pre-support measures in line with the contract requirements and as adjusted to the ground conditions encountered in the field. As a result the ground encountered is categorized in accordance with the contract documents into ground response classes (GRCs) and the appropriate excavation and support classes (ESC) per contract baseline. Any need for additional initial support and/or pre-support measures is assessed and implemented. This task is carried out on a daily basis directly at the active tunnel face and is discussed with the Owner's representative for each round. The outcome of this process is subsequently documented on form sheets that are then signed by the Contractor's and Owner's representatives for concurrence.

This frequent assessment of ground conditions provides for a continuous awareness of tunneling conditions, for an early evaluation of adequacy of support measures and as needed for implementation of contingency measures that may involve more than additional initial support means. Such contingency measures may include heavy pre-support and face stabilization measures or even systematic ground improvement measures.

To be able to support this on-going evaluation process on the owner's behalf the construction management (CM) and inspection team must also include SEM experience. These CM supervisory personnel are independent of the executing party and it is recommended that it include a designer's representative. Represented in the field the designer is able to verify design assumptions and will aid in the implementation of the design intent.

Engineering Geological Tunnel Face Mapping
Click image to enlarge

Figure 9-25 Engineering Geological Tunnel Face Mapping

However, it is often the case that the CM role is filled by a construction management entity that has been assigned an overall role for a project of which the tunneling may only be a subset of the work. If this is the case it is important that the CM be thoroughly familiar with the SEM tunnel design and its design basis. For this purpose it is recommended that the CM participate in the design review process during design development from an early stage through the bidding of the tunnel work. If it is not possible to integrate the tunnel designer within the CM staff then the CM should be augmented by third party SEM experienced personnel who then oversee the tunnel execution in the field.

The key of safe and successful SEM tunneling is a solid knowledge of SEM principles and thorough experience with its execution.

References

Rudolf, J. and Gall, V, 2007: "The Dulles Corridor Metrorail Project - Extension to Dulles International Airport and its Tunneling Aspects", 2007 RETC Conference Proceeding, June 10-13.

Rabcewicz, L. v.: "Patentschrift, Oesterreichisches Patent Nr. 165573", 1948. (Patent Entry, Austrian Patent Nr. 165573, 1948).

Kuhlmann, D.: "Tunnelbau". In: Wayss & Freitag am Donnerstag, Gespraeche mit Wissenschaft und Praxis, Frankfurt, a.M., Germany.

Gnilsen, R., 1986: "Unreinforced Concrete Tunnel Lining - Design Concepts". Technical Bulletin, Volume 1.

Vandewalle, M., 2005: "Tunneling is an Art". NV Bekaert SA.

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Updated: 06/19/2013
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