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

Chapter 9 - Sequential Excavation Method (SEM)

9.1 Introduction

The Sequential Excavation Method (SEM), also commonly referred to as the New Austrian Tunneling Method (NATM), is a concept that is based on the understanding of the behavior of the ground as it reacts to the creation of an underground opening. In its classic form the SEM/NATM attempts to mobilize the self-supporting capability of the ground to an optimum thus achieving economy in ground support. Building on this idea practical risk management and safety requirements add to and dictate the required tunnel support. Initially formulated for application in rock tunneling in the early 1960's, NATM has found application in soft ground in urban tunneling in the late 60's and has since then enjoyed a broad, international utilization in both rural and urban settings.

A large number of tunnels have been built around the world using a construction approach which was loosely termed NATM. During the years of discussions and the application of NATM a variety of terms have been used for the same construction approach. These terms were primarily aimed at describing the construction approach rather than the region of its reported origin. While in the 70's and early 80's the term "Shotcrete Method" was frequently used in Germany and Switzerland, besides NATM, developments in the UK in the late 90's led to the use of the term "Sprayed Concrete Lining" or SCL. Alternatively, "Conventional Tunneling Method" was used in Austria and Germany . As the NATM is largely based on an observational approach, the term "Observational Method" was introduced and used in many countries. The term "Conventional" as opposed to TBM driven tunnels has recently found its way into publications by the International Tunneling Association's (ITA) Working Group 19. In the German speaking countries in Europe namely in Austria and Germany very recent standards and codes use the term "Cyclic Tunneling Method."

In the US, where NATM was systematically applied for the first time in the late 70's and early 80's for the construction of the Mount Lebanon tunnel in Pittsburgh and the Redline tunnels and Wheaton Station of the Washington, DC metro the term adopted was NATM. Gradually, however, the term has been and is being abandoned in the US and replaced by Sequential Excavation Method or SEM. Today, SEM is becoming increasingly popular in the US for the construction of tunnels, cross passages, stations, shafts and other underground structures (Gildner et. al., 2004).

The SEM offers flexibility in geometry such that it can accommodate almost any size of opening. The regular cross section involves generally an ovoid shape to promote smooth stress redistribution in the ground around the newly created opening. By adjusting the construction sequence expressed mainly in round length, timing of support installation and type of support it allows for tunneling through rock (Chapter 6), soft ground (Chapter 7) and a variety of difficult ground conditions (Chapter 8). Depending on the size of the opening and quality of the ground a tunnel cross section may be subdivided into multiple drifts.

Application of the SEM involves practical experience, earth and engineering sciences and skilled execution. The SEM tunneling process addresses:

  • Ground and excavation and support classification based on a thorough ground investigation
  • Definition of excavation and support classes by:
    • Round length (maximum unsupported excavation length)
    • Support measures (shotcrete lining and its reinforcement, ground reinforcement by bolts or dowels in rock)
    • Subdivision of the tunnel cross section into multiple drifts or headings as needed (top heading, bench, invert, side wall drifts)
    • Ring closure requirements
    • Timing of support installation (typically every round)
    • Pre-support by spiling, fore poling, and pipe arch canopy
    • Local, additional initial support by dowels, bolts, spiles, face support wedge, and shotcrete
  • Instrumentation and monitoring
  • Ground improvement measures prior to tunneling
  • A key support element is shotcrete mainly due to its capability to provide an interlocking, continuous support to the ground. Implementation of ground improvement measures in the form of dewatering, grouting, ground freezing and others and of pre-support measures in the various forms of spiling have further widened the range of SEM applications mainly in urban tunneling. These are specified to enhance the quality of the ground prior to and during the tunneling process. The SEM features typically a dual lining cross section by which a waterproofing membrane is inserted between an initial shotcrete and a final, typically cast-in-place concrete lining as addressed in Chapter 10 Tunnel Lining. An instrumental element of SEM tunneling is the monitoring of deformations of tunnel and surrounding ground (Chapter 15). Evaluation of monitoring allows for the verification of design assumptions or adjustment of the tunneling process.
  • Lastly, because SEM tunneling allows for an adjustment to ground conditions as encountered in the field it benefits from a unit-price contract form. Geotechnical baseline reports (GBR) as discussed in Chapter 4 and Geotechnical Design Summary Reports (GDSR) facilitate the adjustment process and aid in risk sharing between owner and contractor. Geotechnical investigations are discussed in Chapter 3.

9.2 background and Concepts

The origins of the NATM lie in the alpine tunnel engineering in the early 1960s. In 1948, Ladislaus von Rabcewicz applied for a patent for the use of a dual lining system with the initial lining being allowed to deform. The NATM is based on the philosophy that the ground surrounding the tunnel is used as an integrated part of the tunnel support system. The deformable shotcrete initial lining allows a controlled ground deflection to mobilize the inherent shear strength in the ground and to initiate load redistribution. The key for the successful use of a relatively thin lining layer applied to the excavation surface lies in the smooth tunnel shape to avoid stress concentrations and the tight contact between the shotcrete lining and the surrounding ground to provide an intense interaction between the support and the ground. In order to augment the support provided by the initial lining, rock reinforcement is used in response to the rock mass conditions. The rock reinforcement avoids the development of wedge failure (keystone), and it generates a rock mass ring with significantly improved strength characteristics around the opening.

Smooth, concavely rounded excavation surfaces initiate confinement forces and limit bending and tension forces in the lining and the ground in the vicinity of the tunnel opening. This is of particular importance for tunneling in ground with limited stand-up time, where fracturing and weathering reduce the ground's natural shear strength.

NATM was the first concept, where the ground and its strength were used as a building material and became an integrated part of the tunnel support system. Rather than implementing stiff support members that attract high loads to fight the ground deformation, the flexible, yet strong shotcrete lining shares with and re-distributes loads into the ground by its deflection.

Rabcewicz summarizes the philosophy of NATM in his patent of 1948 (Rabcewicz, 1948) as follows: "NATM is based on the principle that it is desirable to take utmost advantage of the capacity of the rock to support itself, by carefully and deliberately controlling the forces in the readjustment process which takes place in the surrounding rock after a cavity has been made, and to adapt the chosen support accordingly." By briefly reviewing the stress conditions around a newly created cavity and the interaction between ground and its support needs the following lays out the principle approach taken in NATM tunnel design (Rabcewicz et al., 1973).

The stress conditions around a cavity after Kastner are schematically provided in Figure 9-1 . The primary stress σ0 in the surrounding ground before any cavity is created depends mainly on the overburden, the unit weight and any tectonic stresses σs. Following tunnel excavation the tangential stresses will increase next to the tunnel circumference (solid line σt0). If the induced tangential and radial stresses (σt and σr) around the tunnel opening exceed the strength of the surrounding ground yielding will occur. Such yielding will create a plastified zone that reaches to a certain distance R into the ground beyond the tunnel circumference (dashed line R).

Schematic Representation of Stresses Around Tunnel Opening

Figure 9-1 Schematic Representation of Stresses Around Tunnel Opening (Rabcewicz et al., 1973)

A schematic illustration of the relationships between the radial stresses σr, deformation of the tunnel opening Δr, the required outer and inner supports pi a and pi I respectively, and the time of support application T is provided in Figure 9-2. According to Rabcewicz the outer support or outer arch (pi a) involves the ground itself, its reinforcement by rock bolts and any support applied to the opening itself ranging from sealing shotcrete (flashcrete) to a structural initial lining involving reinforced shotcrete or concrete and steel ribs. The inner support involves a secondary lining that is applied after the tunnel opening with the help of the outer arch has reached equilibrium. The σr /Δr curve, often referred to as ground reaction curve schematically describes the relationship between deformation of the tunnel opening and tunnel support provided by the outer arch. At any intersection point between the support pi and the σr curve equilibrium is reached for the respective support. It is characteristic for the NATM that the intersection between the support and the σr curve takes place at the descending side of the curve. Undesirable loosening of the ground starts at point B of the σr curve if the minimum support pi min is not provided. Within the ascending side of the σr curve the ground has lost strength and consequently its supporting capacity and thus requires enhanced tunnel support to passively support the overburden.

Examination of curves Figure 9-1 and Figure 9-2 exemplifies the relationship between timing of support installation, yielding of the ground and the amount of support needed. The minimum support is required at point B to prevent loosening and loss of strength in the surrounding ground. It will result in the largest deformations but the most economical tunnel support. Curve 1 which intersect the σr curve in point A will require enhanced support pia but result in less deformation Δr and a higher factor of safety. Selection of a stiffer outer arch in curve 2 will result in more support loads because the ground has not been allowed to deform and mobilize its strength and consequently led to a decrease of the safety factor.

The capacity of the inner arch is chosen to satisfy a desired safety factor s. This will depend on specific needs and assuming that the initial tunnel support (outer arch) will deteriorate over time then pi a may be used as guidance to arrive at a desired safety factor. C and C' denote a loaded and unloaded condition of the inner arch respectively.

The σr /Δr curve may be approximated by means of numerical modeling using the deformation and strength characteristics of the ground along with the specific geometry of the opening and the envisioned excavation sequencing (Rabcewicz, 1973).

Schematic Representation of Relationships Between Radial Stress, Deformation of the Tunnel Opening, Supports, and Time of Support Installation

Figure 9-2 Schematic Representation of Relationships Between Radial Stress σr, Deformation of the Tunnel Opening Δr, Supports pi, and Time of Support Installation T (Rabcewicz et al., 1973)

While the NATM had its origins in alpine tunneling in fractured or squeezing rock, its field of application expanded dramatically in the 70's and the following decades. The superb flexibility of the construction concept to adapt to a wide range of ground conditions and tunnel shapes in combination with significant developments in construction materials, installation techniques as well as ground treatment methods formed the basis for a radical expansion from the alpine rock tunneling into soft ground tunneling. The use of NATM thus expanded from rural tunneling in rock into urban tunneling in predominantly soft ground and highly built-up environments with sensitive structures above the tunnel alignment.

The major focus in rock tunneling in rural settings is to find equilibrium in the surrounding ground with the best possible economy in the amount of initial support installed. In urban settings however, in particular when tunneling at shallow overburden depths in soft ground the main goal is to minimize the impact on the surface and adjacent structures thus to minimize settlements. As shown in Figure 9-2 less and delayed support installation will be associated with larger deformations of the tunnel Δr and consequently with larger surface settlements when tunneling at shallow depth. Curve No. 1 describes a relatively "soft" support that is applied later than the support represented by curve No. 2, which is applied earlier and is "stiffer." The curves point out that in order to reduce settlements generally an early and stiffer support should be used. Reduction of the round length and subdivision of the tunnel cross section will aid in applying support to the ground early thus reducing deformations. The stiffness of the support can be increased by increasing the initial shotcrete lining thickness and using shotcrete with early and high strength development.

Today's tunnel construction economies require tunneling approaches that are competitive to fully mechanized tunneling methods by TBMs with their high initial capital cost while being adjustable to project specific space demands. The main field of SEM application is, apart from rural railway and highway tunnels, in the construction of tunnel schemes with complex geometries, short tunnels, large size tunnels and caverns in urban areas at shallow depths. Shallow tunnel depths frequently involve the challenge of soft ground tunneling. With the help of modern equipment for rapid excavation, modern high quality construction materials (mainly shotcrete), and modern ground support installation techniques as well as the overarching SEM concept, complex and challenging underground structures can be built in practically all types of ground. A major advantage of the SEM is its flexibility.

9.3 SEM Regular Cross Section

9.3.1 Geometry

The shape of the tunnel cross section is designed to comply with SEM principles, which are to (as effectively as possible) activate the self-supporting arch in the surrounding ground. To accommodate this principle cross section geometries shall be curvilinear, consisting of compound curves in both arch and invert (if constructed in soft ground like conditions). Any straight walls and sharp edges in the excavation cross section shall be avoided. Thus the geometry of the excavation cross section will enable a smooth flow of stresses in the ground around the opening, minimizing loads acting on the tunnel linings. While adhering to these principles the excavation cross section shall be optimized in size to achieve economy. The layout of the invert will depend on the ground conditions in which the tunnel is constructed. In competent rock formations the tunnel invert will be flat, whereas in weak rock and soft ground tunnels the invert will be rounded to facilitate ring closure and stability.

9.3.2 Dual Lining

The SEM regular cross section is of dual lining character and consists of an initial shotcrete lining and a final, cast-in-place concrete or shotcrete lining. A waterproofing system is sandwiched between the initial and final linings. The waterproofing system consists of a flexible, continuous membrane (typically PVC). A regular cross section is developed for each tunnel geometry: the main tunnel, widenings, niches, cross passages, and other miscellaneous structures. A typical regular SEM cross-section for a two-lane highway tunnel is shown in Figure 9-3 distinguishing between a rounded (right side) and flat (left side) invert. A rounded invert is typically associated with tunneling in soft ground whereas a flat invert is used in competent ground conditions, typically rock. As discussed in Chapter 2, the tunnel cross section is designed around the project clearance envelope including tolerances. Figure 9-4 displays a completed SEM tunnel section for a three-lane road tunnel showing rounded cast-in-place concrete tunnel walls. The alignment of the tunnel is curved to accommodate alignment needs of an urban environment. In the front the SEM tunnel abuts a straight tunnel wall of an adjoining cut-and-cover box tunnel. Figure 9-4 also displays tunnel installations including lighting and jet fans for tunnel ventilation.

Regular SEM Cross Section

Figure 9-3 Regular SEM Cross Section

Three-Lane SEM Road Tunnel Interior Configuration

Figure 9-4 Three-Lane SEM Road Tunnel Interior Configuration (Fort Canning Tunnel, Singapore)

9.3.3 Initial Shotcrete Lining

The initial shotcrete lining is the layer of shotcrete applied to support the ground following excavation. It has a thickness ranging generally from 4 to 16 inches (100 to 400 mm) mainly depending on the ground conditions and size of the tunnel opening. It is reinforced by either welded wire fabric or steel fibers; the latter have generally replaced the traditional welded wire fabric over the last ten to fifteen years.

Occasionally structural plastic fibers are used in lieu of steel fibers. This is the case where the shotcrete lining is expected to undergo high deformations and ductility post cracking is of importance. Where the shotcrete lining is greater than about 6 inches (150 mm) it further includes lattice girders. Depending on loading conditions and purpose rolled steel sets may replace lattice girders or act in combination.

9.3.4 Waterproofing

The SEM uses flexible, continuous membranes for tunnel waterproofing. Most frequently PVC membranes are used at thicknesses of 80 to 120 mil (2.0 to 3.0 mm) depending on the size of the tunnel. Only in special circumstances, for example when contaminated ground water is present, special membranes are applied using hydrocarbon resistant polyolefin or very light density polyethylene (VLDPE) membranes.

The impermeable membrane is backed by a geotextile that also acts as a protection layer, and in drained systems as a drainage layer behind the membrane. This waterproofing system is placed against the initial lining and prior to installation of the final lining. Prior to waterproofing system installation all tunnel deformations must have ceased.

In drained system applications water is collected behind the membrane and conducted to perforated sidewall drainage pipes located at tunnel invert elevation on each side of the tunnel. From there collected water is conveyed via transverse, non-perforated pipes to the tunnel's main roadway drain. In undrained systems the membrane and geotextile wrap around the entire tunnel envelope and prevent water seepage into the tunnel thereby subjecting it to hydrostatic pressures. If this is the case the tunnel invert geometry and structural design must be adapted to accommodate for the hydrostatic head. Utilization of drained vs. undrained systems is discussed in Chapter 1.

Over the past decades a so called "compartmentalization system" has been developed and nowadays supplements the installation of flexible membrane based waterproofing systems. The purpose of this compartmentalization is to provide repair capability in case of leakage. In particular, when the tunnel is not drained and the waterproofing has to withstand long-term hydrostatic pressures, installation of such systems provides a cost effective back up and assures a dry tunnel interior. Compartmentalization refers to the concept of subdividing the waterproofing membrane into individual areas of self-contained grids (compartments) by means of base seal water barriers. These water barriers are specifically formulated for the purpose of creating these compartments. They feature ribs of 1.3-inch (30 mm) minimum height to properly key into the final lining, which is cast (or sprayed) against the waterproofing. In case of water leakage the water infiltration is limited to the individual compartment thus preventing uncontrolled water migration over long distances behind the final lining. Within each compartment control and grouting pipes are installed. These pipes penetrate through the final lining and are in contact with the membrane. Figure 9-5 displays an installed PVC waterproofing system with compartments, control and grouting pipes, and hoses prior to final lining installation. Control and grouting pipes serve a twofold purpose; should leakage occur then water would find its path to these pipes and exit there thus signaling a breach within the compartment. Once detected, the same pipes may be used for injection of low viscosity, typically hydro-active grouts into the compartments. The injection of grout is limited to leaking compartment(s) and once cured provides a secondary waterproofing layer in the form of a membrane that acts as a remedial waterproofing layer.

Waterproofing System and Compartmentalization

Figure 9-5 Waterproofing System and Compartmentalization (Automated People Mover System at Dulles International Airport, Virginia)

9.3.4.1 Smoothness Criteria

To provide a suitable surface for the installation of the waterproofing system, all shotcrete surfaces to which the membrane is to be applied must meet certain smoothness criteria. These are expressed in the waviness of the shotcrete surface to which the waterproofing system will be applied. The waviness is measured with a straight edge laid on the surface in the longitudinal direction. The maximum depth to wavelength ratio should be generally 1:5 or smoother. The surface has to be inspected prior to installation of the waterproofing system and all projections should be removed or covered by an additional plain shotcrete layer, which meets the smoothness criteria. The SEM design documents will address required smoothness criteria and set those in relation to the waterproofing system to be used.

9.3.5 Final Tunnel Lining

The final permanent lining for a SEM tunnel may consist of cast-in-place concrete or shotcrete. Cast-in-place concrete can be un-reinforced or reinforced. Shotcrete is generally fiber reinforced. Chapter 10 provides general discussions about permanent tunnel lining. The following addresses design and construction considerations specifically for SEM application.

9.3.5.1 Cast-in-Place Concrete Final Lining

The traditional final lining consists of cast-in-place concrete at a thickness of generally 12 inches for two-lane road tunnels. While the lining may generally remain unreinforced, structural design considerations and project design criteria will dictate the need for and amount of reinforcement. The Lehigh Tunnel ( Pennsylvania ) and Cumberland Gap Tunnels ( Kentucky / Tennessee ) are the first road tunnels built in the US in the late 80's and early 90's using SEM construction methods. Both feature unreinforced, 12-inch thick cast-in-place concrete final linings. The flexible membrane based waterproofing is in particular beneficial in unreinforced cast-in-place concrete lining applications in that it acts as a de-bonding layer between the initial and final linings and therefore reduces shrinkage cracking in the final lining.

To ensure a contact between the initial and final linings, contact grouting is performed as early as the final lining has achieved its 28-day design strength. With this grouting the contact is established between the initial lining and final tunnel support. Any deterioration or weakening of the initial support will lead to an increased loading of the final support by the increment not being supported by the initial lining. The loads can be directly transferred radially due to the direct contact between initial and final linings.

Cast-in-place final concrete linings (concrete arch placed on sidewall footings) are frequently installed in pour lengths not exceeding 30 feet (10 meters). This restriction is important to limit surface cracking in general and becomes mandatory if unreinforced concrete linings are used. A 30 feet (10 meter) long section in a typical two-lane highway tunnel is also practical in terms of formwork installation and sequencing and duration of concrete placement.

Adjacent concrete pours feature construction joints that are true lining separators designed as contraction joints. The inside face at joint location shall be laid out with a trapezoidally shaped joint. A continuous reinforcement is not desired in construction joints to allow their relative movement in particular for thermal deformation effects.

9.3.5.2 Water Impermeable Concrete Final Lining

Use of water impermeable cast-in-place concrete linings as an alternative to membranes is generally not considered due to the high demands on construction quality and exposure to freeze thaw conditions in cold climates. Elaborate measures are needed to prevent cracking. Detailed arrangement of construction joints is needed as well as complex concrete mix designs to suppress excessive hydration heat. The curing requires elaborate procedures. These aspects generally do not render water impermeable concrete practical in road tunnels. If selected these construction aspects have to be addressed in detail in specifications and working procedures and they have to be rigidly enforced.

9.3.5.3 Shotcrete Final Lining

Shotcrete represents a structurally and qualitatively equal alternative to cast-in-place concrete linings. When shotcrete is utilized as a final lining in dual lining applications it will be applied against a waterproofing membrane. The lining thickness will be generally 12 inches (300 mm) or more and its application must be carried out in layers with a time lag between layer applications to allow for shotcrete setting and hardening. Its surface appearance can be tailored to the desired project goals. It may remain of a rough, sprayer type shotcrete finish, but may have a quality comparable to cast concrete when trowel finish is specified. Shotcrete as a final lining is typically utilized when the following conditions are encountered:

  • The tunnels are relatively short in length and the cross section is relatively large and therefore investment in formwork is not warranted, i.e. tunnels of less than 100-250 m (300-800 feet) in length and larger than about 8-12 m (25-40 feet) in springline diameter.
  • The access is difficult and staging of formwork installation and concrete delivery is problematic.
  • The tunnel geometry is complex and customized formwork would be required. Tunnel intersections, as well as bifurcations qualify in this area. Bifurcations are associated with tunnel widenings and would otherwise be constructed in the form of a stepped lining configuration and increase cost of excavated material.

Figure 9-6 displays a typical shotcrete final lining section with waterproofing system, welded wire fabric (WWF), lattice girder, grouting hoses for contact grouting and a final shotcrete layer with PP fiber addition.

Typical Shotcrete Final Lining Detail

Figure 9-6 Typical Shotcrete Final Lining Detail

Readers are referred to Chapter 10 for detailed discussion about utilizing shotcrete as a final lining.

9.3.5.4 Single Pass Linings

Under special circumstances the initial shotcrete lining alone or with the addition of an additional shotcrete layer designed to withstand long-term loads may be used as a single support lining for the long term. Although labeled "single pass" this final shotcrete lining may be applied in multiple shotcrete application cycles. Use of a single pass lining will generally be limited to conditions where the ground water inflow is not of concern and deterioration of the shotcrete product over the life time of the tunnel lining can be excluded or partially tolerated. In multiple layer applications the shotcrete surface to which additional layers will be applied must be sufficiently clean and free of any layer that may cause de-bonding over the long term (Kupfer, et al., 1990 and Hahn, 1999). Specially detailed construction joints and high quality shotcrete must be required to assure water tightness and long-term integrity.

9.4 Ground Classification and SEM Excavation and Support Classes

9.4.1 Rock Mass Classification Systems

A series of qualitative and quantitative rock mass classification systems have been developed over the years and are implemented on tunneling projects worldwide. Section 6.3 provides an overview of the most commonly used rock mass classification systems including Terzaghi's qualitative classification (Table 6-1), and quantitative systems such as the Q system and the Rock Mass Rating (RMR) system.

Rock mass classification systems aid in the assessment of the ground behavior and ultimately lead to the definition of the support required to stabilize the tunnel opening. While the above quantitative classification systems lead to a numerical rating system that results in suggestions for tunnel support requirements (Section 6.5), these systems cannot replace a thorough design of the excavation and support system by experienced tunnel engineers.

9.4.2 Ground Support Systems

In the early years of the use of NATM (SEM) in Austria, Switzerland and Germany, standards and codes used descriptive (qualitative) categories to define ground support classes. Recent standards, codes and guidelines implemented in Austria and Germany utilize a process-oriented approach (OGG, 2007). This approach defines the process of using relevant parameters from ground investigation to derive a ground response classification and subsequently assess tunnel support needs. This forms a more objective basis for all parties involved and promotes the understanding of the rationale in retrospect by persons that have not been involved in the design process. It also provides a common platform for contractors, owners and engineers to negotiate the project specific challenges in the field during actual construction.

All classification systems have in common that they should be based on thorough ground investigation and observation. The process from the ground investigation to the final definition of the ground support system can be summarized in three models:

  • Geological Model
  • Geotechnical Model
  • Tunnel Support Model

9.4.2.1 Geological Model

A desk study of the geological information available for a project area forms the starting point of the ground investigation program. Literature, maps and reports (e.g. from the US Geological Survey) form the basis for a desk study. Subsequently and in coordination with initial field mapping results, a ground investigation program is developed and carried out. The geological information from the ground investigation, field mapping, and the desk study are compiled in the geological model.

9.4.2.2 Geotechnical Model

With the data from the geological model in combination with the test results from the ground investigation program and laboratory testing the ground response to tunneling is assessed. This assessment takes into account the method of excavation, tunnel size and shape as well as other parameters such as overburden height, environmental issues and groundwater conditions. The geotechnical model assists in deriving zones of similar ground response to tunneling along the alignment and Ground Response Classes (GRC) are defined. These GRCs form the baseline for the anticipated ground conditions. Typically, the ground response to an unsupported tunnel excavation is analyzed in order to assess the support requirements for the stabilization of the opening (OGG, 2007).

9.4.2.3 Tunnel Support Model

After assessing the ground support needs, excavation and support sequences, subdivision into multiple drifts, as well as the support measures are defined. These are combined in Excavation and Support Classes (ESCs) that form the basis for the Contractor to develop a financial and schedule bid as well as to execute SEM tunnel work.

9.4.3 Excavation and Support Classes (ESC) and Initial Support

Excavation and Support Classes (ESCs) contain clear specifications for excavation round length, subdivision into multiple drifts, initial support and pre-support measures to be installed and the sequence of excavation and support installation. They also define means of additional initial support or local support or pre-support measures that augment the ESC to deal with local ground conditions that may require additional support.

In SEM tunneling initial support is provided early on. In soft ground and weak rock it directly follows the excavation of a round length and is installed prior to proceeding to the excavation of the next round in sequence. In hard rock tunneling initial support is installed close to the face. The intent is to provide structural support to the newly created opening and ensure safe tunneling conditions. Initial support layout is dictated by engineering principles, economic considerations, and risk management needs.

The amount and design of the initial support was historically motivated mainly by the desire to mobilize a high degree of ground self support and therefore economy. This was possible at the outset of SEM applications in "green field" conditions where deformation control was of a secondary importance and tolerable as long as equilibrium was reached. Nowadays, however, safety considerations, risk management, conservatism and design life, and the need for minimizing settlements in urban settings add construction realities that ultimately decide on the layout of the initial support.

Initial support is provided by application of a layer of shotcrete to achieve an interlocking support with the ground. Shotcrete is typically reinforced by steel fibers or welded wire fabric. Plastic fibers are used for reinforcement only occasionally. With higher support demands of the ground and with shotcrete thicknesses of generally 6 inches or greater lattice girders are embedded within the shotcrete. Occasionally and if needed by special support needs rolled steel sets are used in lieu of, or in combination with lattice girders. Initial support also includes all measures of rock reinforcement in rock tunneling. Types of rock reinforcement are provided in Section 9.7.1.

Figure 9-7 and Figure 9-8 show a prototypical ESC cross section and longitudinal section respectively. Figure 9-7 displays a cross section without a closed invert on the left side and ring closure on its right side. Invert closure is typically required in weak rock conditions and squeezing ground. Figure 9-7 includes elements of typical initial support including rock bolts/dowels, initial shotcrete lining and tunnel pre-support. The arrangement of rock bolts/dowels is typical and varies depending on the excavation and support. The table in Figure 9-8 provides details of initial support measures for a prototypical ESC Class IV. In that sense, the SEM is a prescriptive method which defines clearly and in detail tunnel excavation and initial support means.

Prototypical Excavation Support Class (ESC) Cross Section

Figure 9-7 Prototypical Excavation Support Class (ESC) Cross Section

Prototypical Longitudinal Excavation and Support Class (ESC)

Figure 9-8 Prototypical Longitudinal Excavation and Support Class (ESC)

9.4.4 Longitudinal Tunnel Profile and Distribution of Excavation and Support Classes (ESCs)

SEM contract documents contain all Excavation and Support Classes (ESCs) assigned along the tunnel alignment in accordance with the Ground Response Classes (GRCs) and serve as a basis to estimate quantities. A summary longitudinal section along the tunnel alignment shows the anticipated geological conditions, the GRCs with the relevant description of the anticipated ground response, hydrological conditions and the distribution of the ESCs. Figure 9-9 displays a prototypical longitudinal profile with an overlay of GRCs and corresponding ESCs, which form a baseline for the contract documents.

Prototypical Longitudinal Profile

Figure 9-9 Prototypical Longitudinal Profile

Geological data, Ground Response Classes, Excavation and Support Classes, the Longitudinal Tunnel Profile as well as design assumptions and methods shall be described and displayed in reports that become part of the contract documents. When defining the reaches and respective lengths of GRCs and corresponding ESCs it is understood that these are a prognosis and may be different in the field. Therefore contract documents establish the reaches as a basis and call for observation of the ground response in the field and the need for their adjustment as required by actual conditions encountered. Actual conditions must be accurately mapped in the field to allow for a comparison with the baseline assumptions portrayed in the GRCs. For that purpose standard form sheets are developed as portrayed for a typical SEM rock tunnel mapping in Section 9.9.

9.4.5 Tunnel Excavation, Support, and Pre-Support Measures

Table 9-1 and Table 9-2 exemplify the use of most common initial support measures, along with excavation and support installation sequencing frequently associated with SEM road tunnels depending on the basic types of ground encountered, i.e. rock and soft ground respectively. These tables indicate basic concepts to derive Excavation and Support Classes (ESCs) for typical ground conditions portrayed. The support and pre-support means addressed in the tables are further detailed in Section 9.7 Ground Support Elements.

Table 9-1 builds on the use of Terzaghi's Rock Mass Classification. According to this Classification, it can be distinguished between the following rock mass qualities:

  • Intact Rock
  • Stratified Rock
  • Moderately Jointed Rock
  • Blocky and Seamy Rock
  • Crushed, but Chemically Intact Rock
  • Squeezing Rock
  • Swelling Rock

The column labeled "Excavation Sequence" in Table 9-1 lists typical heading sequences used for road tunnels in ground conditions portrayed. Further subdivison of the headings into multiple drifts either for the purpose of construction logistics or to handle extraordinary ground conditions is not addressed. Table 9-2 characterizes the typical soils characteristics in column 1 directly.

Table 9-1 Elements of Commonly Used Excavation and Support Classes (ESC) in Rock
Ground Mass Quality - RockExcavation SequenceRock Reinforce-
ment
Initial Shotcrete LiningInstallation LocationPre-SupportSupport Installation Influences ProgressRemarks
Intact RockFull face or large top heading & benchSpot bolting (fully grouted dowels, Swellex ®)Patches to seal surface in localized fractured areasTypically Several rounds behind face or directly near face to secure isolated blocks/slabs/ wedgesNoneNo 
Stratified RockTop heading & benchSystematic doweling or bolting in crown considering strata orientation (fully grouted dowels, Swellex®, rock bolts)Thin shell (fiber reinforced) typically 4 in (100 mm) to bridge between rock reinforcement in top heading; alternatively chain link mesh; installed with the rock reinforcement.Two to three rounds behind faceNoneNo or eventually 
Moderately Jointed RockTop heading & benchSystematic doweling or bolting in top heading considering joint spacing (fully grouted dowels, Swellex®, rock bolts)Systematic shell with reinforcement (welded wire fabric or fibers) in top heading and potentially bench; dependent on tunnel size thickness of 6 in (150 mm) to 8 in (200mm); installed with the rock reinforcement.One to two rounds behind faceLocally to limit over breakYes 
Blocky and Seamy RockTop heading &benchSystematic doweling or bolting in top heading & bench considering joint spacingSystematic shell with reinforcement (welded wire fabric or fibers) in top heading & bench; depending on tunnel size thickness 8 in (200 mm) to 12 in (300 mm)At the face or maximum one round behind faceSystematic spiling in tunnel roof or parts of itYes 
Crushed, but Chemically Intact RockTop heading, bench, invertN/ASystematic shell with reinforcement (welded wire fabric or fibers) and ring closure in invert; dependent on tunnel size thickness 12 in (300 mm) and more; for initial stabilization and to prevent desiccation, a layer of flashcrete may be requiredAfter each roundSystematic grouted pipe spiling or pipe arch canopySupport installation dictates progressIf water is present, groundwater draw down or ground improvement is required
Squeezing RockTop heading, bench, invertSystematic doweling or bolting in top heading & bench considering joint spacing; extended lengthSystematic shell with reinforcement (welded wire fabric or fibers) and ring closure in invert; dependent on tunnel size thickness 12 in (300 mm) and more; potential use for yield elements; for initial stabilization and to prevent desiccation, a layer of flashcrete may be requiredAfter each roundSystematic grouted pipe spiling or pipe arch canopySupport installation dictates progress 
Swelling RockTop heading, bench, invertSystematic doweling or bolting in top heading & bench considering joint spacing; extended lengthSystematic shell with reinforcement (welded wire fabric or fibers) and ring closure in invert; dependent on tunnel size thickness 12 in (300 mm) and more; potential use for yield elementsAfter each roundSystematic grouted pipe spiling or pipe arch canopy may be required depending on degree of fracturingSupport installation dictates progressDeepened invert for additional curvature
Table 9-2 Elements of Commonly Used Soft Ground Excavation and Support Classes (ESC) in Soft Ground
Ground Mass Quality - SoilExcavation SequenceInitial Shotcrete LiningInstallation LocationPre-SupportSupport InstallationRemarks
Stiff/hard cohesive soil - above groundwater tableTop heading, bench & invert; dependent on tunnel size, further sub-divisions into drifts may be requiredSystematic reinforced (welded wire fabric or fibers) shell with full ring closure in invert; dependent on tunnel size 6 in (150 mm) to 16 in (400 mm) typical; for initial stabilization and to prevent desiccation, a layer of flashcrete may be requiredInstallation of shotcrete support immediately after excavation in each round. Early support ring closure required. Either temporary ring closure (e.g. temporary top heading invert) or final ring closure to be installed within one tunnel diameter behind excavation face.Typically none; local spiling to limit over-breakSupport installation dictates progressOverall sufficient stand-up time to install support without pre-support or ground modification
Stiff/hard cohesive soil - below groundwater tableTop heading, bench and invert; dependent on ground strength, smaller drifts required than aboveSystematic reinforced (welded wire fabric or fibers) shell with full ring closure in invert; dependent on tunnel size 6 in (150 mm) to 16 in (400 mm) typical; for initial stabilization and to prevent desiccation, a layer of flashcrete may be required; frequently more invert curvature than aboveInstallation of shotcrete support immediately after excavation in each round. Early support ring closure required. Either temporary ring closure (e.g. temporary top heading invert) or final ring closure to be installed within less than one tunnel diameter behind excavation face; typically earlier ring closure required than aboveTypically none; locally pre-spiling to limit over-breakSupport installation dictates progressSufficient stand-up time to install support without pre-support or ground improvement; dependent on water saturation, swelling or squeezing can occur
Well consolidated non-cohesive soil - above groundwater tableTop heading, bench & invert; dependent on tunnel size, further sub-divisions into drifts may be requiredSystematic reinforced (welded wire fabric or fibers) shell with full ring closure in invert; dependent on tunnel size 6 in (150 mm) to 16 in (400 mm) typical; for initial stabilization and to prevent desiccation, a layer of flashcrete is requiredInstallation of shotcrete support immediately after excavation in each round. Early support ring closure required. Either temporary ring closure (e.g. temporary top heading invert) or final ring closure to be installed within less than one tunnel diameter behind excavation faceFrequently systematic pre-support required by grouted pipe spiling or grouted pipe arch canopy; alternatively ground improvementSupport installation dictates progressStand-up time insufficient to safely install support without pre-support or ground improvement
Well consolidated non-cohesive soil - below groundwater tableTop heading, bench & invert; dependent on tunnel size, further sub-divisions into drifts may be required; Pocket excavation and/or face stabilization wedge may be requiredSystematic reinforced (welded wire fabric or fibers) shell with full ring closure in invert; dependent on tunnel size 6 in (150 mm) to 16 in (400 mm) typical for initial stabilization and to prevent desiccation, a layer of flashcrete is requiredInstallation of shotcrete support immediately after excavation in each round. Early support ring closure required. Either temporary ring closure (e.g. temporary top heading invert) or final ring closure to be installed within less than one tunnel diameter behind excavation faceFrequently systematic pre-support required by grouted pipe spiling or grouted pipe arch canopy; groundwater draw down or ground improvementSupport installation dictates progressStand-up time insufficient to safely install support without pre-support or ground improvement; Running ground conditions or boiling may occur
Loose non-cohesive soil - above groundwater tableTop heading, bench & invert; dependent on tunnel size, further sub-divisions into drifts may be required; Pocket excavation and/or face stabilization wedge may be requiredSystematic reinforced (welded wire fabric or fibers) shell with full ring closure in invert; dependent on tunnel size thickness 6 in (150 mm) to 16 in (400 mm) typical for initial stabilization and to prevent desiccation, a layer of flashcrete is requiredInstallation of shotcrete support immediately after excavation in each round. Early support ring closure required. Either temporary ring closure (e.g. temporary top heading invert) or final ring closure to be installed within less than one tunnel diameter behind excavation faceSystematic pre-support required by grouted pipe arch canopy; alternatively ground improvementSupport installation dictates progressStand-up time insufficient to safely install support without pre-support and/or ground improvement
Loose non-cohesive soil - below groundwater tableTop heading, bench & invert; dependent on tunnel size, further sub-divisions into drifts may be required; Pocket excavation and/or face stabilization wedge may be requiredSystematic reinforced (welded wire fabric or fibers) shell with full ring closure in invert; dependent on tunnel size thickness 6 in (150 mm) to 16 in (400 mm) typical for initial stabilization and to prevent desiccation, a layer of flashcrete is requiredInstallation of shotcrete support immediately after excavation in each round. Early support ring closure required. Either temporary ring closure (e.g. temporary top heading invert) or final ring closure to be installed within less than one tunnel diameter behind excavation faceSystematic pre-support required by grouted pipe arch canopy frequently in combination with ground improvementSupport installation dictates progressStand-up time insufficient to safely install support without pre-support or ground improvement; Running ground conditions or boiling may occur
9.4.6 Example SEM Excavation Sequence and Support Classes

While Section 9.5.3. introduced excavation and support classes in a prototypical context the following tables show examples on how, based on a ground classification, excavation and support classes were realized on selected projects. Grouped into two main types of ground, rock and soft ground, the examples are shown in tables Table 9-3 and Table 9-4 for rock and soft ground respectively.

The three examples in Table 9-3 outline tunnel constructions in three different characteristic rock mass types ranging from intact to fractured rock. The examples have rock mass reinforcement as a common element of initial support while systematic shotcrete support is used in stratified and fractured rock. Tunnel cross sections typically have horse-shoe-like shapes and no structural tunnel invert closure.

For the tunnel construction in intact rock, drill-and-blast excavation with round lengths of up to 12 feet (3.7 m) was utilized at the Bergen Tunnel in New Jersey . The initial tunnel support consisted of spot bolting to support loose rock blocks and slabs. Shotcrete was not systematically used as initial shotcrete lining but for local sealing of the rock face and for smoothening of the rock surface prior to waterproofing installation. Support was generally installed as required by field conditions.

The construction for the Zederhaus tunnel in Austria in stratified rock required systematic rock doweling and initial shotcrete lining installation. Excavation was carried out using drill-and-blast techniques with round lengths of typically 6 feet and 6 inches (2 m). The initial shotcrete lining was installed after each excavation round, whereas the installation of the rock dowels lagged 1 to 2 rounds behind the excavation. The bench excavation followed in a distance to the top heading excavation to suit the tunnel construction logistics.

A dense, systematic rock doweling pattern and an initial shotcrete lining were installed after each excavation round when tunneling through fractured rock at the Devil's Slide tunnel project in California . Drill-and-blast techniques and road headers were employed for excavation depending on ground quality. The maximum length of round in the top heading was limited to 7 feet and 2 inches (2.2 m), while the bench excavation was limited to twice that length. There was no restriction on the distance between the top heading and bench construction.

The three examples in Table 9-4 are taken from typical soft ground tunneling projects where different sizes of tunnels were constructed at different overburden depths.

The three examples show the typical, rounded tunnel geometry with a systematic initial shotcrete lining that is closed in the curved invert. The support is installed after each excavation round prior to commencement of the next round in sequence.

The shallow cover of maximum about 16 feet (5 m) combined with soft ground conditions required the systematic installation of a grouted steel pipe arch pre-support canopy over the entire tunnel length at the Fort Canning Tunnel in Singapore. The tunnel cross section was split into top heading, bench and invert excavation with a shotcrete invert closure. To enable longer advances of the top heading ahead of the final invert closure, a temporary shotcrete invert was provided in the top heading. Excavators were used for the excavation of residual soils with round lengths limited to 3 feet and 4 inches (1 m) in the top heading and 6 feet and 8 inches (2 m) in the bench and invert.

Table 9-3 Example SEM Excavation and Support Classes in Rock
DescriptionCross SectionLongitudinal SectionPhoto
Intact Rock:
  • Spot bolting
  • Occasional sealing shotcrete
  • Full face or top heading/bench excavation
  • Round Length
    • Top Heading: 8'-12" (2.5-3.7 m)
    • Bench: Up to 16'-0" (4. 9 m)
  • Dimensions
    • Height: 20'-0" (6 m)
    • Width: 29'-0" (8.8 m)
Example: Bergen Tunnels, NJ
Intact rock cross section Intact rock longitudinal section Intact rock photograph
Stratified Rock:
  • Systematic rock doweling
  • Systematic shotcrete initial lining
  • Top heading excavation
  • Bench excavation follows distant
  • Round Length
    • Top Heading: 6'-6" (2 m)
    • Bench: 6'-6" (2 m)
  • Dimensions
    • Height: 29'-6" (9 m)
    • Width: 36'-0" (11 m)
Example: Zederhaus, Austria
Stratified rock cross section Stratified rock longitudinal section Stratified rock photograph
Fractured Rock:
  • Systematic rock doweling
  • Systematic shotcrete initial lining
  • Top heading excavation
  • Bench excavation follows any time
  • Round Length
    • Top Heading: 7'-2" (2.2 m)
    • Bench: 13'-0" (4.0 m)
  • Dimensions
    • Height: 28'-0" (8.5 m)
    • Width: 36'-5" (11.1 m)
Example: Devil's Slide Tunnels, CA
Fractured rock cross section Fractured rock longitudinal section Fractured rock photograph
Table 9-4 Example SEM Excavation and Support Classes in Soft Ground
DescriptionCross SectionLongitudinal SectionPhoto
Soft Ground - shallow cover:
  • Systematic pre-support
  • Systematic shotcrete initial lining support with early ring closure
  • Top heading excavation (with temporary invert), bench and invert excavation
  • Round Length
    • Top Heading: I - 3'-3" (1 m)
    • Top Heading: II - 6'-6" (2 m)
    • Bench III/Invert IV - 6'-6" (2 m)
  • Dimensions
    • Height: 38'-0" (11.6 m)
    • Width: 48'-0" (14.7 m)
Example: Fort Canning Tunnel, Singapore
Soft Ground - shallow cover, cross section Soft Ground - shallow cover, longitudinal section Soft Ground - shallow cover, photograph
Soft Ground - deep level:
  • Systematic shotcrete support with early ring closure
  • Top heading excavation closely followed by bench/invert excavation
  • Round Length
    • Top Heading: 3'-3" (1 m)
    • Bench: 6'-6" (2 m)
  • Dimensions
    • Height: 20'-3" (6.3 m)
    • Width: 20'-3" (6.3 m)
Example: London Bridge Station, London, UK
Soft Ground - deep level, cross section Soft Ground - deep level, longitudinal section Soft Ground - deep level, photograph
Soft Ground - deep level:
  • Systematic shotcrete support with early ring closure
  • Sub-division into sidewall drifts
  • Top heading excavation closely followed by bench and invert excavation
  • Round Length
    • Top Heading: 3'-3" (1 m)
    • Bench: 6'-6" (2 m)
    • Invert: 6'-6" (2 m)
  • Dimensions
    • Height: 30'-2" (9.2 m)
    • Width: 37'-0" (11.3 m)
Example: London Bridge Station, London, UK
Soft Ground - deep level, cross section Soft Ground - deep level, longitudinal section Soft Ground - deep level, photograph

The tunnels built for London Bridge subway station in London, UK, located at approximately 80 feet (25 m) depth below ground surface, were excavated in over-consolidated clays using excavators and road headers with maximum round lengths of 3 feet and 4 inches (1 m) and 6 feet and 8 inches (2 m) in the top heading and bench/invert respectively. While the smaller running tunnels were excavated and supported in a staggered full face sequence in a top heading and bench/invert arrangement, the 37 feet (11.3 m) wide turn-out was constructed using a single-sidewall drift with a top heading, bench and invert excavation in each partial drift. The temporary middle wall provided temporary sidewall support for the first tunnel half during construction. During the enlargement to the full tunnel size the temporary middle wall was removed.

9.4.7 Excavation Methods

During the history of application of the SEM/NATM, tunneling methods for a wide variety of ground conditions have been developed. With the further development and refinement of support means, the application field of the SEM has ever been expanded. From its original implementation in alpine, "green field" rock and soft rock tunnels the focus moved into urban areas and soft ground tunneling. SEM tunneling is typically accomplished in hard rock using drill-and-blast excavation techniques (Section 6.4.1), medium hard and soft rock using a road header (6.4.3) and in soft ground using backhoe excavation.

Figure 9-10 through Figure 9-13 display such SEM excavations from hard rock through soft ground. Figure 9-10 displays drilling of a face in a rock tunnel for a drill-and-blast excavation. A close up of the drilling at the face is shown in Figure 9-11 that also displays the shotcrete initial lining installed close to the face. The rock face has been sealed by a layer of flashcrete. Figure 9-12 shows a close up of a road header boom excavating a medium hard, jointed rock mass. Figure 9-13 displays tunnel construction of a soft ground tunnel in a top-heading, bench, and invert excavation using backhoes. The backhoe is in the background at the tunnel face.

Face Drilling for Drill-and-Blast SEM Excavation

Figure 9-10 Face Drilling for Drill-and-Blast SEM Excavation (Andrea Tunnel, Austria)

Shotcrete Lining Installed at the Face in a SEM Tunnel Excavated by Drill-and-Blast

Figure 9-11 Shotcrete Lining Installed at the Face in a SEM Tunnel Excavated by Drill-and-Blast (Andrea Tunnel, Austria)

Road Header SEM Excavation in Medium Hard, Jointed Rock

Figure 9-12 Road Header SEM Excavation in Medium Hard, Jointed Rock (Devil's Slide Tunnels, California)

Soft Ground SEM Excavation Tunnel Using Backhoes

Figure 9-13 Soft Ground SEM Excavation Tunnel Using Backhoes (Fort Canning Tunnel, Singapore)

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Updated: 06/19/2013
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