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Technical Manual for Design and Construction of Road Tunnels - Civil Elements
Chapter 11 - Immersed Tunnels
For the assessment of loads, the density of materials should be based on actual measurements made on samples from the same source as will be used for construction. For the design of individual sections, the least favorable loading should be used. The design should take into account the fact that the specific gravity of water may vary according to depth, prevailing weather conditions and season. The effect of suspended material should be taken into account in determining the specific gravity of water. The maximum hydrostatic load should be used for structural calculations. To ensure flotation (during launching or floating of the elements), the minimum relevant specific gravity of water should be used, and to prevent flotation (after placement of the element) the maximum should be used. The maximum and minimum values for each material used must be specified. The design must take into account any particular current regimes expected. This must include consideration of current speed, depth, direction, any interface between contra-flowing currents and the turbulence engendered thereby.
The loads to be considered in the design of structures along with how to combine the loads are given in Section 3 of the AASHTO LRFD specifications. It divides loads into two categories: Permanent Loads and Transient Loads. Paragraph 3.3.2 "Load and Load Designation" of the AASHTO LRFD specifications defines following permanent loads that are applicable to the design of immersed tunnels:
DC = Dead Load: This load comprises the self weight of the structural components as well as the loads associated with nonstructural attachments. Nonstructural attachments can be signs, lighting fixtures, signals, architectural finishes, waterproofing, etc. Typical unit weights for common building materials are given in Table 3.5.1-1 of the AASHTO LRFD specifications. Actual weights for other items should be calculated based on their composition and configuration. [These items have essentially well defined weights.]
DW = Dead Load: This load comprises the self weight of wearing surfaces and utilities. Utilities in tunnels can include power lines, drainage pipes, communication lines, water supply lines, etc. Wearing surfaces can be asphalt or concrete. Dead loads of wearing surfaces and utilities should be calculated based on the actual size and configuration of these items. [The weights of these items are generally less well defined, may be removed or replaced, and have different load factors.]
EH = Horizontal Earth Pressure Load. This load is generated by the backfill material and any armoring located above the backfill. The properties of the backfill material should be well defined. The value of the horizontal earth pressure should be calculated based on the properties of the specified backfill material. At-rest pressures should be used in the design of immersed tunnels.
EL = Accumulated locked-in force effects resulting from the construction process including secondary forces from post tensioning.
ES = Earth surcharge load. This is the vertical earth load due to fill over the structure that was placed above the original ground line. This load may be generated by armoring that is placed over the backfill.
EV = Vertical pressure from the dead load of the earth fill. This is the vertical earth load due to fill over the structure up to the original ground line. The properties of the backfill material should be well defined. The value of the vertical earth pressure should be calculated based on the properties of the specified backfill material.
Paragraph 3.3.2 "Load and Load Designation" of the LRFD specifications defines following transient loads that are applicable to the design of immersed tunnels:
CL = Construction Load: These loads are not explicitly defined in the AASHTO LRFD specifications, but must be considered when designing immersed tunnels. They include loads imposed when the tunnel section is launched, transporting loads such as loads imposed when towing the sections, wave action on the floating section, current loads when the section is being outfitted or placed, the loads imposed when the section is floating and concrete is placed in or on the section, wind on the section when it is being towed or when it is moored and being outfitted.
CR = Creep: Creep can be a factor in the design of concrete immersed tunnels and should be considered accordingly.
CT = Vehicular Collision Force: Inside the tunnel, this load would be applied to individual components of the tunnel structure that could be damaged by vehicular collision. Typically, tunnel walls are very massive or are protected by redirecting barriers so that this load need be considered only under very unusual circumstances. It is preferable to detail tunnel structural components so that they are not subject to damage from vehicular impact.
CV = Vessel Collision Force: This load could is generated by a sinking ship coming to rest over the tunnel. The magnitude of this load is a function of the type and size of vessels using the waterway over the tunnel. A study of the vessel traffic should be performed and the load determined based on the results of that study. Another category of this load is anchor impact. Should a ship drop its anchor in the vicinity of the tunnel, this will impart a significant load on the tunnel. This load should not be applied in combination with the vessel collision force. The following section provides guidelines for computing these loads.
EQ = Earthquake. This is a load that should be considered in areas where seismic activity is expected. This is discussed in Chapter 13.
IM = Vehicle dynamic load allowance: This load can apply to the roadway slabs of tunnels. An equation for the calculation of this load is given in paragraph 126.96.36.199 of the AASHTO LRFD specifications.
LL = Vehicular Live Load: This load can apply to the roadway slabs of tunnels and should be applied in accordance with the provisions of paragraph 188.8.131.52 of the AASHTO LRFD specifications.
PL = Pedestrian Live load. Pedestrian are typically not permitted in highway tunnels, however, there are areas where maintenance and inspection personnel will need access, areas such as ventilation ducts when transverse ventilation is used, plenums above false ceilings, and safety walks. These loads are transmitted to the lining through the supporting members for the described features.
SE = Settlement: Allowance should be taken of immediate settlements during the first week or so after placement of the element due to compaction of the foundation material (this could easily be 1 inch or 25 mm), expected long term movements due to placement of backfill and subsequent movements of the underlying materials, and movements resulting from the placement and backfilling of adjacent tunnel elements. Lateral movements can occur in soils that are non-uniform laterally and where the soil surface is sloping. Proper preparation of the foundation and placement of the backfill can minimize these effects. For the typical highway tunnel, the overall weight of the structure is less than the soil it is replacing. As such, unless backfill in excess of the original ground elevation is placed over the tunnel, settlement will not be an issue for immersed tunnels. If settlement is anticipated due to poor subsurface conditions or due to the addition of load onto the structure or changing ground conditions along the length of the tunnel, it is recommended that a pile foundation be used.
SH = Shrinkage: Shrinkage usually results in cracking. In the case of concrete immersed tunnels, detailing and construction methods should be employed to minimize shrinkage in order to minimize the resulting cracking. Shrinkage can also occur in the concrete placed as part of steel shell tunnel sections. The effect of this force should be accounted for in the design or else the structure detailed to minimize the effect of shrinkage.
SL = Support Loss: This loading is not defined in AASHTO since it is unique to immersed tunnels. It should include loss of support (subsidence) below the tunnel or to one side, and storms and extreme water levels with a probability of being exceeded not more than once during the design life (considering appropriate static and dynamic effects for each). A loss of support of not less than 10% of the length of an immersed tunnel element and uneven support from the foundation over the full width of the tunnel element should be considered.
TG = Temperature Gradient. Concrete immersed tunnel elements are typically massive members that have a large thermal lag. Combined with being surrounded by an insulating soil backfill that maintains a relatively constant temperature, the temperature gradient across the thickness of the members can be measurable. This load should be examined on a case by case basis depending on the local climate and seasonal variations in average temperatures. Steel tunnel sections may be thinner and would have a smaller thermal lag, which would help reduce this effect. However, it is recommended that this load be studied for all tunnel types. Paragraph 4.6.6 of the AASHTO LRFD specifications provides guidance on calculating this load. Note that paragraph C3.12.3 allows the use of engineering judgment to determine if this load need be considered in the design of the structure.
TU = Uniform Temperature. This load is used primarily to size expansion joints in the structure. If movement is permitted at the expansion joints, no additional loading need be applied to the structure. Since the structure is very stiff in the primary direction of thermal movement, the effects of the friction force resulting from thermal movement can be neglected in the design.
WA = Water load. This load represents the hydrostatic pressure expected outside the tunnel structure. Immersed tunnel structures are typically detailed to be watertight. Hydrostatic pressure acts normal to the surface of the tunnel. The design should take into account the specific gravity of the water which can be saline. Both maximum and minimum hydrostatic loads should be used for structural calculations as appropriate to the member being designed. The designer should take into account the fact that the specific gravity of water may vary according to depth, prevailing weather conditions and season. The effect of suspended material should be taken into account in determining the specific gravity of water. The maximum hydrostatic load should be used for structural calculations. To ensure flotation (during launching or floating of the elements), the minimum relevant specific gravity of water should be used, and to prevent flotation (after placement of the element) the maximum should be used. Two water levels should be considered: normal (observed maximum water level) and extreme, 3 ft (1 m) above the 200-year flood level. The buoyancy force should be carefully evaluated to ensure that the applied dead load effect is larger than the applied buoyancy effect. Frequently, structural member sizes will have to be increased to ensure that the buoyancy is completely resisted by the dead load or ballast added to the tunnel to counteract the buoyancy effect. The net effect of water pressure on the tunnel, i.e., the buoyancy, is the difference between hydrostatic loads on upward and downward facing surfaces. The total uplift force is equal to the weight of water displaced. Friction effects (the theoretical force required to dislodge the wedge of material over the tunnel) of backfill should not be taken into account, but the weight of material vertically above the structure may be taken into account.
Some of the loads shown in paragraph 3.3.2 of the LRFD specifications are not shown above because they are not applicable to the design of highway immersed tunnels as described below.
BR = Vehicular Breaking Force: This load would be applied only under special conditions where the detailing of the structure requires consideration of this load. Under typical designs, this force is resisted by the mass of the roadway slab and need not be considered in design.
CE = Vehicular centrifugal force: This load would be applied only under special conditions where the detailing of the structure requires consideration of this load. Under typical designs, this force is resisted by the mass of the roadway slab and need not be considered in design.
DD = Downdrag: This load comprises the vertical force applied to the exterior of the tunnel that can result from the subsidence of the surrounding soil due to the subsidence of the in-situ soil below the bottom of the tunnel. This load would not apply to immersed tunnels since it requires subsidence or settlement of the material below the bottom of the structure to engage the downdrag force of the tunnel. For a typical immersed tunnel, the overall weight of the structure is usually less than the soil it is replacing. As such, unless backfill significantly in excess of the original ground elevation is placed over the tunnel, settlement will not be an issue.
FR = Friction. The structure is very stiff in the direction of thermal movement. Thermal movement is the source of the friction force. In a typical tunnel, the effects of friction can be neglected.
IC = Ice load. Since the tunnel is not subjected to stream flow and unlikely to be exposed to the weather in a manner that could result in an accumulation of ice or icebergs, this load does not apply to immersed tunnel design.
LS = Live Load Surcharge: This load would be generated by vehicles traveling over or adjacent to the tunnel. Since immersed tunnels are constructed under water, this load does not apply.
WL = Wind on live load. The tunnel structure is not exposed to the environment, so it will not be subjected to wind loads.
WS = Wind load on structure. The tunnel structure is not exposed to the environment, so it will not be subjected to wind loads when in service, however, when the tunnel section is being towed to the tunnel site, this is a potential loading. See construction loads (CL) listed above.
Section 3 of the LRFD specifications provides guidance on the methods to be used in the computations of these loads. Loadings unique to immersed tunnels such as anchor and ship impact are calculated as follows.
11.3.3 Ship Anchors
The effect of an anchor impacting the underwater tunnel structure directly or being dragged across the line of the tunnel structure should be considered. Either the tunnel structure should be designed to resist the full loading imposed by the design anchor system, or the backfill / armor system should be designed to mitigate the loading, in which case the tunnel structure should be designed for the demonstrable reduced load. Rupture of the waterproofing membrane should not occur. The design anchor should be selected as appropriate to shipping using or expected to use the waterway, based on the relevant section of Lloyd's Rules.
The penetration depth of a falling anchor through tunnel roof protection material should be estimated. The formulae given in CEB Bulletin d'Information No 187, August 1988, reproduced for reference below provide a good design method to calculate the anchor penetration depth in granular material:
Penetration Depth of a Falling Anchor through Granular Material:
|x||penetration depth (m)|
|de||equivalent diameter of striking area of anchor (m)|
|mw||mass of anchor reduced by the mass of the displaced water (kg)|
|ma||mass of anchor in air (kg)|
|Er||modulus of elasticity in the longitudinal direction of the layer (N/m2)|
|vi||impact velocity of anchor (m/s)|
|A||cross-sectional striking area of anchor (m2)|
The calculated maximum penetration depth should not exceed 90% of the total thickness of the protection layer covering the tunnel using the 5% fractile value for Er. The dynamic load factor (DLF) ratio of the static equivalent load on the tunnel roof to the triangular dynamic load pulse F = mwvi/Td may be obtained from Figure 11-16 below using the minimum duration of impact Td = x/vi (where x is calculated with the 95% fractal value for Er), and the natural period T0 of the affected element.
11.3.4 Ship Sinking
The primary sunken ship design case should be assumed to consist of a ship of the size approximating those using or expected to use the waterway. The imposed loading of a ship on the tunnel should be taken as an appropriate uniform loading over an area not exceeding the full width of the tunnel times a length as measured on the longitudinal axis of the tunnel of 100 feet (30 m). Collision impact loading should not be considered.
If appropriate, a secondary sunken ship design case should be assumed to consist of a smaller vessel, such as a ferry or barge, sinking and impacting the tunnel structure with the stem or sternpost in a manner similar to that of a dropped anchor. A static equivalent concentrated load of 225 kips (1,000 kN) working on an area of 3.3 x 6.6 ft2 (1x2 m2) directly on the tunnel roof should be considered.
Figure 11-16 Graph of Dynamic Load Factor (DLF) Against Td/T
The intensity of uniformly distributed loading from a sunken ship should be determined by methods such as that outlined in Chapter 6 of the State-of-the-Art Report, 2nd Edition, International Tunnelling Association Immersed and Floating Tunnels Working Group, Pergamon, 1997. In the absence of data to the contrary, it may be assumed that the ship will exert a pressure of 1 ksf (50 kN/m2).
11.3.5 Load Combinations
The loads described above should be factored and combined in accordance with the LRFD specification and applied to the structure. Paragraph 12.5.1 gives the limit states and load combinations that are applicable for buried structures as Service Limit State Load Combination I and Strength Limit State Load Combinations I and II. These load combinations are given in Table 3.4.1-1. In some cases, the absence of live load can create a governing case. For example, live load can reduce the effects of buoyancy. Therefore, in addition to the load cases specified in Section 12 of the AASHTO LRFD specifications, the strength and service load cases that do not include live load should be used, specifically Strength III and IV and Service IV. In addition, vessel collision forces and earthquake forces must be considered in the design of immersed tunnels. These loads are contained in the Extreme Event I and II load combinations. Combining the requirements of Section 12 and Section 3 as described above results in the following possible load combinations shown in Table 11-1 for use in the design of immersed tunnels:
When developing the loads to be applied to the structure, each possible combination of load factors should be developed. Assessment can then be used to eliminate the combinations that obviously will not govern.
|Load Comb. Limit State||DC||DW||EH* |
|ES||EL||LL, IM||WA||TU, CR, SH, CL||TG||EQ**|
|Extreme Event I||1.25||0.90||1.50||0.65||1.35||0.90||1.50||0.75||1.00||gEQ||1.00||0.00||0.00||0.00||0.00||gSE|
|Extreme Event II||1.25||0.90||1.50||0.65||1.35||0.90||1.50||0.75||1.00||0.5||1.00||0.00||0.00||0.00||0.00||gSE|
* The load factors shown are for at-rest earth pressure. At-rest earth pressure should be used for all conditions of design of immersed tunnel structures.
# The load factors shown are for rigid frames. All immersed tunnel structures are considered rigid frames.
** EQ is used only in Extreme Event I, CT and CV are used, one at a time in Extreme Event II.
*** gSE is computed is considered on project specific basis. It should be determined based on the certainty that anticipated settlements can be accurately predicted.
11.3.6 Loads during Fabrication, Transportation and Placement
During fabrication, load effects caused by placement of concrete while the element is afloat or by settlements of the foundation (in case of concrete elements), and other items should be evaluated. Some of these loads may cause locked-in stresses that must be considered together with stresses due to external loads.
Particular care must be taken during the placement of concrete while an element is afloat to ensure not only that stresses stay within limits, but also that the deflected shape due to the weight of the new concrete is within acceptable limits. At all times when the element is afloat, stresses due to waves should be checked to ensure that all limit states are satisfied; the wave height and length used in design must be specified for each stage of construction and for towing so that measures can be taken to move the element to a place of safety when forecasts predict conditions that exceed allowable limits. If the freeboard is such that waves could run over the top of an element, this loading should also be taken into consideration.
During transportation and while moored at the outfitting pier or elsewhere and even while in the fabrication yard, a tunnel element can be subject to wind loads that should be considered.
The tunnel element may be suspended from lifting hooks during immersion and may be placed on temporary supports in the final location pending completion of the foundation. All limits states must be satisfied. Temporary supports if used should be released before backfill is placed. When adjacent tunnel elements are connected by shear keys, the effects due to relative differential settlements of each tunnel element during progressive backfilling operations must be taken into account.
Paragraph 3.4.2 of the AASHTO LRFD specifications provides guidance for minimum load factors to be used when investigating loads that occur during construction. The following Table 11-2 reflects the load combinations and load factors to be used when evaluating immersed tunnel sections for construction loads.
11.4 Structural Design
Historically there have been three basic methods used in the design of immersed tunnels:
- Service load or allowable stress design which treats each load on the structure equally in terms of its probability of occurrence at the stated value. The factor of safety for this method is built into the material's ability to withstand the loading.
- Load factor design accounts for the potential variability of loads by applying varying load factors to each load type. The resistance of the maximum capacity of the structural member is reduced by a strength reduction factor and the calculated resistance of the structural member must exceed the applied load.
§ Load and resistance factor design takes into account the statistical variation of both the strength of the structural member and of the magnitude of the applied loads.
The fundamental LRFD equation can be found in paragraph 184.108.40.206 of the AASHTO specification. This equation is:
|ΣηiγiQi ≤ fRn = Rr||11-2|
(AASHTO Equation 220.127.116.11-1)
In this equation, η is a load modifier relating to the ductility, redundancy and operation importance of the feature being designed. The load modifier η is comprised of three components:
ηD = a factor relating to ductility = 1.0 for immersed tunnels constructed with conventional details and designed in accordance with the AASHTO LRFD specification.
ηR = a factor relating to redundancy = 1.0 for immersed tunnel design. Typical cast in place and prestressed concrete structures are sufficiently redundant to use a value of 1.0 for this factor. Typical detailing using structural steel also provides a high level of redundancy.
ηI = a factor relating to the importance of the structure = 1.05 for immersed tunnel design. Tunnels usually are important major links in regional transportation systems. The loss of a tunnel will usually cause major disruption to the flow of traffic, hence the higher importance factor.
γ is a load factor applied to the force effects (Q) acting on the member being designed. Values for γ can be found in Table 11-1 above.
f is a resistance factor applied to the nominal resistance of the member (R) being designed. The resistance factors are given in the AASHTO LRFD specifications for each material in the section that covers the specific material. Specifically, Section 5 covers Concrete Structures and in general, the resistance factors to be used in concrete design can be found in Section 5. However, Section 12 of the AASHTO LRFD specifications gives the following values to be used for f in Table 12.5.5-1:
For Reinforced Concrete Box Structures:
f = 0.90 for flexure
f = 0.85 for shear
Since the walls, floors and roofs of immersed tunnel elements will experience axial loads, the resistance factor for compression must be defined. The value of f for compression can be found in Section 18.104.22.168.1 of the AASHTO LRFD specification given as:
f = 0.75 for compression
Values for f for precast construction are also given in Table 12.5.5-1. However, only rarely under unusual circumstances will a casting yard be set up to create the same controlled conditions that exist in a precast plant. Therefore, it is recommended that the f values given for cast-in-place concrete be used for the design of immersed tunnels.
Structural steel is also used in immersed tunnel construction. Structural steel is covered in Section 6 of the AASHTO LRFD specification. Paragraph 22.214.171.124 gives the following values for steel resistance factors:
For Structural Steel Members:
ff = 1.00 for flexure
fv = 1.00 for shear
fc = 0.90 for axial compression for plain steel and composite members
Rr is the calculated factored resistance of the member or connection.
11.4.2 Structural Analysis
Structural analysis is covered in Section 4 of the AASHTO LRFD specifications. Section 4 describes a number of analysis methods that are permitted. It is recommended that classical force and displacement methods be used in the structural analysis of concrete immersed tunnel elements. Other methods (as described below) may be used, but will rarely yield results that vary significantly from those obtained with the classical methods. The modeling should be based on elastic behavior of the structure as per AASHTO paragraph 126.96.36.199. Steel immersed tunnels can also be analyzed using the same structural model except that the efficiency of any curvature of the steel members will not be fully utilized. Most general purpose structural analysis programs have routines based on these principles for dimensional models.
Since all members of a concrete immersed tunnel element are subjected to bending and axial load, the secondary effects of deflections on the load affects to the structural members should be accounted for in the analysis. AASHTO LRFD specifications refer to this type of analysis as "large deflection theory" in paragraph 188.8.131.52. Most general purpose structural analysis software have provisions for including this behavior in the analysis. If this behavior is accounted for in the analysis, no further moment magnification is required. Alternatively finite element models can be used. These models can identify load sharing, account for secondary effects and identify load paths
Steel immersed tunnel elements, are complex assemblies of plates that might be curved, stiffeners and diaphragms. Simplifying these systems to the point where classical methods of analysis can be used often undermines the efficient use of materials that can result from complex load paths. Steel structures lend themselves well to sophisticated computer modeling such as finite element models. These models can identify load sharing, account for secondary effects and identify load paths. It is recommended that these models be used in the analysis of steel immersed tunnel sections.
Paragraph 4.5.1 of the AASHTO LRFD specifications states that the mathematical model used to analyze the structure should include "...where appropriate, response characteristics of the foundation". The response foundation for an immersed tunnel element can be modeled through the use of a series of non-linear springs placed along the length of the bottom of the section. These springs are considered non-linear because they should be specified to act in only one direction, the downward vertical direction. This model will provide the proper distribution of loads to the bottom of the model and give the designer an indication if buoyancy is a problem. This indication is seen in observing the calculated displacements of the structure. A net upward displacement of the entire structure indicates that there is insufficient resistance to buoyancy.
Structural models for computer analysis are developed using the centroids of the structural members. Due to the thickness of the walls and the slabs of an immersed tunnel, it is important when calculating the applied loads, that the loads are calculated at the outside surface of the members. The load is then adjusted according to the actual length of the member as input. For example, if the out to out bottom width of a tunnel structure is 90 feet and the bottom of the bottom slab is located 15 feet below the water table, the buoyancy force on the bottom slab is calculated as:
62.4pcf x 15ft x 1ft (along length of tunnel) = 936plf for a total load on the bottom of the tunnel of:
936plf x 90ft = 84,240lbs
If the outside walls of the tunnel are 4ft thick, then the length of the structural model is 90ft - 4ft = 86ft
Using 86ft, the applied buoyancy force is 936plf x 86ft = 80,496lbs. This computation underestimates the buoyancy force by 5 percent. Given that the load factor for the buoyancy force is 1.00, this could result in a buoyancy problem with the tunnel. The solution would be to apply the actual calculated load as follows: 84,240lbs / 86ft = 980plf. This results in a slightly conservative estimate of the load for bending and shear, but an accurate estimate of the buoyancy effect including the axial load in the side walls.
This problem is not as prevalent in a finite element model. However, the designer should be careful that sufficient load is being applied to the model to be sure that the actual conditions are being modeled as closely as possible.
11.5 Watertightness and Joints Between Elements
11.5.1 External Waterproofing of Tunnels
External waterproofing for tunnel elements should be considered for both steel tunnels and concrete tunnels. The waterproofing should envelop every part of the element exposed to soil or water with materials impervious to the surrounding waters. For steel tunnels the outer steel membrane would act as waterproofing membrane, while for concrete elements either steel or synthetic membrane should be used. For steel waterproofing membranes used on either concrete or steel elements, an appropriate corrosion protection and monitoring system should be used to ensure that the minimum design thickness is maintained during the life of the facility or an added sacrificial thickness should be provided. Non-structural steel membranes should be no less than 1/4 in (6 mm) thick.
The membrane should be watertight. Typical materials used for concrete elements include two coats of a spray-applied elasticized epoxy material; steel plates; and flexible PVC waterproofing sheet. Minimum thickness should be no less than 0.06 inch (1.5 mm), and anchored to the concrete using T-shaped ribs. The materials of the waterproofing system should have a proven resistance to the specific corrosive qualities of the surrounding waters and soils. The materials of the system should be flexible and strong enough to span any cracks that may develop during the life of the structure. Bituminous membranes are not recommended. The waterproofing system should preferably adhere at every point to the surfaces to which it is applied so that, if perforated at any one location, water may not travel under it to another. The areas of free water flow between the membrane and the underlying concrete in case of leakage should be limited to no more than 100sf (10m2). For a steel tunnel, the membrane could be the external steel shell, provided that an adequate corrosion protection is provided either by cathodic protection or additional sacrificial thickness. Steel plates should be joined using continuous butt welds. All welds should be inspected and tested for soundness and tested for watertightness. Notwithstanding the provision of a membrane, the underlying structural concrete should be designed to be watertight.
Depending upon the type of waterproofing used, it may require protection on the sides and top of the tunnel elements to ensure that it remains undamaged during all operations up to final placement and during subsequent backfilling operations.
Joints between immersed tunnels elements can be classified as described below.
Immersion Joint (or Typical Joint) The immersion joint is the joint formed when a tunnel section is joined to a section that is already in place on the seabed. After placing the new element, and joining it with the previously placed element, the space between the bulkheads (dam plates) of the two adjoining elements is then dewatered. In order to dewater this space, a watertight seal must be made. A temporary gasket with a soft nose such as the Gina gasket (Figure 11-17 ) is most often used. In addition an omega seal is also provided after dewatering the joint from inside the joint.
Figure 11-17 Gina-Type Seal
For immersion joints, the primary compression or immersion seal is usually made of natural or neoprene rubber compounds. The most common cross-section used today is the "Gina" type. This consists of a main body with designed load/compression characteristics and an integral nose and seating ridge. The materials used should have a proven resistance to the specific corrosive qualities of the water and soils and an expected life no shorter than the design life of the tunnel unless the gasket is considered temporary. For flexible joints, a secondary seal is usually required in case of failure of the primary seal. It is usually manufactured from chloroprene rubber to an overall cross-section corresponding to that known as an "Omega" type (Figure 11-18 ), the materials having proven resistance against the specific corrosive qualities of the water and soils, oil, fungi and micro-organisms, oxygen, ozone and heat.
Figure 11-18 Omega Type Seal
Figure 11-19 shows a typical immersion joint.It is essential that immediately after dewatering of the chamber between the two bulkheads, an inspection of the primary seal is made so that any lack of watertightness can be remedied. Similarly, the secondary seal of a flexible joint should be pressure tested up to the expected maximum service pressure via a test pipe and valve to ensure that it too can function as required; after a successful testing, the chamber between the seals should be de-watered.
Closure or Final Joint: Where the last element has to be inserted between previously placed elements rather than appended to the end of the previous element, a marginal gap will exist at the secondary end. This short length of tunnel sometimes is completed as cast-in-place and is known as the closure or final joint.
The form of the closure or end joint is dependent on the sequence and method of construction. Closure joints may also be immersion joints, although details may need to be different. Potential options for the closure joints include:
Figure 11-19 Gina-type Immersion Gasket at Fort Point Channel, Boston, MA.
- Place the last element between two previously placed elements and dewater one joint between the newly placed element and the one of the previously placed elements. Then insert under water closure form plates and place tremie concrete around the closure joint to seal it. The joint can then be dewatered and interior concrete can be completed from within the joint. Other methods such as telescopic extension joints and wedge joints have been developed to make the closure joint similar to the immersion joint.
- Construct both end (terminal) joints first, lay the tunnel elements outwards from these and complete the immersed tunnel with a special closure (final) joint.
- Construct one terminal joint first and lay all the immersed tunnel elements outwards from that side and backfill over the top of the final element, using a soil-cement mixture (or other reasonably watertight material) in the vicinity of the second terminal joint. Construct the structures abutting the second terminal joint after the immersed tunnel is complete.
- Lay and complete the immersed tunnel with or without a special closure joint and backfill at the terminal elements using a soil-cement mixture (or other reasonably watertight material) in the vicinity of both terminal joints. Construct the structures abutting the both terminal joints after the immersed tunnel is complete.
Earthquake Joint This may be an immersion joint of special design to accommodate large differential movements in any direction due to a seismic event. It also applies to a semi-rigid or flexible joint strengthened to carry seismic loads and across which stressed or unstressed prestressing components may be installed.
Segment or Dilatation Joint Moveable segment joints must be able to transmit shear across the joint and well as allowing dilatation and rotation. The joints contain an injectable rubber-metal waterstop as well as neoprene and hydrophilic seals.
11.5.3 Design of Joints between Elements
All immersed tunnel joints must be watertight throughout the design life, and must accommodate expected movements caused by differences in temperature, creep, settlement, earthquake motions, method of construction, etc. Displacements in any direction should be limited so that the waterproof limits of a joint are not exceeded. Joint shear capability should take into account the influence of normal forces and bending moments on the shear capacity of the section; the design should take account of shear forces generated where the faces of the joints are not normal to the tunnel axis. Joints must be ductile in addition to accommodating longitudinal movements. Tension ties may be used to limit movement so that joints do not leak or break open, especially during a seismic event.
The axial compression of tunnel elements and bulkheads due to depth of immersion should be taken into account in determining joint dimensions at installation.
The design of primary flexible seals at tunnel joints must be designed to take into account the maximum deviations of the supporting frames relative to their theoretical location, the maximum deviation of the planes of the frames, and any relaxation of the seal. The seal is required to have a minimum compression of 3/8 inch (10 mm) greater than the compression required to maintain a seal. Just in case an initial seal is not obtained after immersion and joining, it may be advisable in some cases for the immersion joint to be designed so that a backup method of obtaining an initial seal is available.
For flexible joints, a secondary seal (omega) capable of carrying the full water pressure should be fitted across the inside of the joint and should be capable of being inspected, maintained and replaced. The seal should be capable of absorbing the long-term movements of the joint. The secondary seals should be provided with a protective barrier against damage from within the tunnel. All joints in the tunnel should be finished to present a smooth surface.
The metal hardware in joints should have a design life adequate to fulfill its purpose throughout the design life of the joint. Nuts and bolts for primary and secondary seals should be stainless steel. Plate connections between elements should be corrosion-protected to ensure that the design life is obtained.
The mounting procedure or the mounting surface for the primary seal of immersion joints must allow for fine adjusting and trimming of the seal alignment in order to compensate for construction tolerances. It is recommended that the gasket be protected from accidental damage until the time of immersion. All embedded parts, fixings, including the bolts and their corrosion protection system, mating faces, clamping bars and other fixings, must have a design life at least equal to that of the tunnel structure. Where clamping bars and other fixings are used for the secondary seal, these need to have a design life at least equal to that of the secondary seal. The gasket assembly should have provision for injection in case of leakage.
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