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Technical Manual for Design and Construction of Road Tunnels - Civil Elements
Chapter 11 - Immersed Tunnels
This chapter describes the structural design of immersed tunnels in accordance with the AASHTO LRFD Bridge Design Specifications (AASHTO). The intent of this chapter is to provide guidance in the interpretation of the AASHTO specifications in order to have a more uniform application of the code and to provide guidance in the design of items not specifically addressed in AASHTO. The chapter begins with a basic description of immersed tunnel construction methodology.
Immersed tunnels consist of very large pre-cast concrete or concrete-filled steel tunnel elements fabricated in the dry and installed under water. More than a hundred immersed tunnels have been built to provide road or rail connections. They are fabricated in convenient lengths on shipways, in dry docks, or in improvised floodable basins, sealed with bulkheads at each end, and then floated out. Tunnel elements can and have been towed successfully over great distances. They may require outfitting at a pier close to their final destination. They are then towed to their final location, immersed, lowered into a prepared trench, and joined to previously placed tunnel elements. After additional foundation works have been completed, the trench around the immersed tunnel is backfilled and the water bed reinstated. The top of the tunnel should preferably be at least 5 ft (1.5 m) below the original bottom to allow for sufficient protective backfill. However, in a few cases where the hydraulic regime allowed, the tunnel has been placed higher than the original water bed within an underwater protective embankment.
Immersed tunnel elements are usually floated to the site using their buoyant state. However, sometimes additional external buoyancy tanks attached to the elements would be used if necessary. The ends of the tunnel elements are equipped with bulkheads (dam plates) across the ends to keep the inside dry, located to allow only about 6 to 8 ft feet (2 m to 2.5 m) between the bulkheads of adjacent elements at an immersion joint; this space is emptied once an initial seal is obtained during the joining process. The joints are usually equipped with gaskets to create the seal with the adjacent element. They are also equipped with adjustment devices to allow placement of the elements on line and grade. The tunnel elements will be lowered into their location after adding either temporary water ballast or tremie concrete. Figure 11-1 shows an illustration of the placement of an immersed tunnel.
Figure 11-1 Immersed Tunnel Illustration
11.1.1 Typical Applications
Immersed tunnels may have special advantages over bored tunnels for water crossings at some locations since they lie only a short distance below water bed level. Approaches can therefore be relatively short. Compared with high level bridges or bored tunnels, the overall length of crossing will be shorter. Tunnels can be made to suit horizontal and vertical alignments. They can be constructed in soils that would be a real challenge to a long-span bridge structure and under such conditions may be very cost competitive. However, immersed tunnels have potential disadvantages in term of environmental disturbance to the water body bed. They may have impact on fish habitats, ecology, current, and turbidity of the water. Furthermore, impacts on navigation in all navigable waterways should be considered and often extensive permitting would be required. In addition, many of the water bodies such as harbors or causeways have contaminated sediments requiring special handling. The use of immersed tunnel techniques might encounter such contaminated ground and would require its regulated disposal. For very long crossings where navigation is important, bridge-tunnel combinations can provide a most economical solution; long trestle bridges extend out from the shores through relatively shallow water to man-made islands at which the transition between bridge and tunnel is made, with the tunnel extending across the usually deeper navigation channels. The Chesapeake Bay Bridge-Tunnel in Norfolk, Virginia, was completed in 1964, is over 17 miles long and has immersed tunnels at each of the two main shipping channels, one of which is shown in Figure 11-2.
Figure 11-2 Chesapeake Bay Bridge-Tunnel
11.1.2 Types of Immersed Tunnel
Two main types of immersed tunnel have emerged, known as steel and concrete tunnels, terminology that relates to the method of fabrication. Both types perform the same function after installation. Steel tunnels use structural steel, usually in the form of stiffened plate, working compositely with the interior concrete as the structural system. Concrete tunnels rely on steel reinforcing bars or prestressing cables. The steel immersed tunnel elements are usually fabricated in ship yards or dry docks similar to ships, launched into water and then outfitted with concrete while afloat. Concrete immersed elements are usually cast in dry docks, or specially built basins, then the basin is flooded and the elements are floated out. Steel tunnels can have an initial draft of as little as about 8 feet (about 2.5 m), whereas concrete tunnels have a draft of almost the full depth. Tunnel cross-sections may have flat sides or curved sides.
Historically, concrete tunnels have predominantly been rectangular, which is particularly attractive for wide highways and combined road/rail tunnels. In Europe, Southeast Asia and Australia, virtually all immersed tunnels are concrete. In Japan, steel and concrete tunnels are in approximately equal numbers. Although most tunnels in North America are steel tunnels, there are also concrete immersed tunnels.
Steel tunnels have been circular, curved with a flat bottom, and rectangular (particularly in Japan), but the predominant shape in the US has been the double-shell tunnel, which is a circular shell within an octagonal shape. Most or all of the concrete in steel tunnels is placed while the steel shell is afloat, in direct contrast to concrete tunnels that are virtually complete before being floated out. The order in which concrete is placed for a steel tunnel is tightly controlled to minimize deformations and the resulting stresses. Steel immersed tunnels can be categorized into three sub-types: Single shell, double shell and sandwich.
11.1.3 Single Shell Steel Tunnel
In this type, the external structural shell plate works compositely with the interior reinforced concrete and no external concrete is provided. The shell plate requires corrosion protection, usually in the form of cathodic protection. The Hong Kong Cross-Harbour tunnel (Figure 11-3 and Figure 11-12 ) and San Francisco BART trans-bay tunnel are typical of this type.
Figure 11-3 Cross Harbour Tunnel Hong Kong
Early examples of the single shell type are The Detroit River tunnel (1910), and the Harlem River tunnel (1914); both are rail tunnels and being the first two immersed transportation tunnels ever built, have similarities to single-shell tunnels. Of the eight existing single-shell immersed tunnels in the world, three are for rail in Tokyo, Japan, and three are for rail in the US . Two road tunnels have been constructed using the single shell method: The Baytown Tunnel in Texas (since removed) and the Cross Harbor Tunnel (Figure 11-12 ) in Hong Kong . Figure 11-4 shows the BART tunnel in San Francisco, a transit tunnel built in 1969. It is 5800m long and consists of 57 elements, all end launched.
Figure 11-4 BART Tunnel, San Francisco
The initial draft of a single shell tunnel is less than that for other immersed tunnel types because of the elimination of the outer shell. However, leaks in the steel shell may be difficult to identify and seal; subdividing the surface into smaller panels by using ribs will improve the chances of sealing a leak. Great care and considerable testing is required to ensure that the welds are defect free. The risks of permanent leakage can be higher in single shell immersed tunnels than in other types. To avoid this, the external structural steel shell often requires a positive form of corrosion protection.
11.1.4 Double Shell
A double shell tunnel element is comprised of an internal structural shell that acts compositely with concrete placed within the steel shell. The top and invert concrete outside the structural shell plate is also structural. A second steel shell is constructed outside the structural steel shell to act as formwork for ballast concrete at the sides placed by tremie. In this configuration the interior structural shell plate works compositely with internal reinforced concrete while it is protected by external concrete placed within non-structural steel form plates. Figure 11-5 shows the cross section of the Second Hampton Roads Tunnel in Virginia . The steel portion of the double shell tunnel element is often fabricated at a shipyard. Prior to launching, the invert concrete may be placed to make the element more stable during towing and outfitting and to internally brace the steel elements. Due to the double shell configuration, this element is stiffer than the single shell section. However, due to the potential for rough conditions during towing and in particular during launching if not constructed in a dry dock, internal bracing may be required until the tunnel element is in its final position.
Multiple bores are created by linking sections with diaphragms. The diaphragms also serve to stiffen the steel shell. Diaphragms are spaced along the length of the tunnel element. Longitudinal stiffeners in the form of plates or T-sections are used in the longitudinal direction of the element between diaphragms to stiffen the shell. Figure 11-6 is a photograph of double shell tunnel elements constructed for the Fort McHenry Tunnel in Baltimore, Maryland.
Figure 11-5 Double Shell - Second Hampton Road Tunnel, Virginia
11.1.5 Sandwich Construction
This construction type consists of a structural concrete layer sandwiched between two steel shells. Both the inner and outer shells are load carrying and both act compositely with the inner concrete layer. The concrete is un-reinforced and is formulated to be non-shrink and self consolidating. The inner surfaces of the steel shells are stiffened with plates and L-shaped ribs that also provide the connection required for composite action with the internal concrete. The internal concrete, once cured, carries compression loads and also serves to stiffen the steel shells. The steel shells carry the tension loads. Figure 11-7 shows a schematic of this type of construction.
Figure 11-6 Fort McHenry Tunnel, Baltimore
Figure 11-7 Schematic of Sandwich Construction
As with the other types, the steel shells are fabricated at a shipyard, launched and towed to the tunnel site. Internal diaphragms between the two shells stiffen the section sufficiently to resist the loads imposed during transport and outfitting. Once at the outfitting pier, the internal concrete is placed and the element draft increases. The element is towed to its location along the tunnel alignment and the final ballast and structural concrete is placed so that it can be lowered into place.
The steel sandwich construction provides a double layer of protection against leaks. However it is a very complex arrangement that requires carefully defined and executed procedures for fabrication and concreting. Distortion of the section during welding and poor quality welds can be costly mistakes for this type of construction. A recent example of a tunnel using this methodology is the Bosphorus crossing in Istanbul, Turkey where the end sections of each element are made in this way. Figure 11-8 shows two elements afloat while being outfitted. Several tunnels of this type exist in Japan.
Figure 11-8 Bosphorus Tunnel, Istanbul, Turkey
11.1.6 Concrete Immersed Tunnels
Cast-in-place concrete is a versatile and durable material. It is easily formed into any shape or configuration to meet the needs of a specific project. Due to the fact that concrete is heavy, immersed tunnel elements constructed from concrete will float usually with very large drafts. In fact the freeboard for concrete elements is often less than a foot resulting in almost the entire element being underwater when being towed into position. This requires careful planning when using a concrete element. The path from the fabrication site to the tunnel alignment must contain water deep enough for the element to pass. Therefore, concrete elements are usually cast in a basin constructed close to the project site. A dredged channel may be required from the basin to the tunnel alignment. Once the concrete elements have been fabricated, the basin is flooded. The elements are towed out of the basin and to the tunnel alignment. Figure 11-9 shows construction of a concrete immersed tunnel crossing the Fort Point Channel in Boston .
Figure 11-9 Fort Point Channel Tunnel, Boston
Considerable development in the design and construction of concrete immersed tunnels has occurred over recent years, particularly in the use of materials and construction methods that reduce the number of construction joints. Water-cement ratio has been substantially reduced, and there have been efforts to reduce the heat of hydration, both of which result in fewer through-cracks during the curing of the concrete. Reducing the through cracks is key to making the sections waterproof. Figure 11-10 shows an above-ground fabrication facility and a transfer basin for the Øresund crossing in Denmark .
The length of concrete cast in a single operation for a full-width segment (bay) of a tunnel element has increased in length from some 30 ft (10 m) to about 60 ft (20 m) over the years, despite the very large volumes of concrete to place, and the expansion and contraction that occur during the first few days due to the heat of hydration.
To prevent cracking due to heat of hydration, mitigation measures have been used including concrete cooling using refrigerated pipes cast into the concrete, mix design, low heat cement such as ground granulated blast furnace cement, shielding from the elements and proper curing. Each of these measures has advantages and disadvantages. All aspects of these measures must be understood in order for them to be implemented. For example, high percentages of blast furnace slag will slow down the set of the concrete; pressure due to any additional height of liquid concrete needs to be considered in the formwork design.
Figure 11-10 Fabrication Facility and Transfer Basin, Øresund Tunnel, Denmark
Typically, the floor slab is cast first, followed by walls and then roof. Techniques have evolved permitting the outer walls and even the base slab to be cast with the roof slab, thereby reducing the number of construction joints in the exterior. Since construction joints are particularly susceptible to leakage, most often due to thermal restraint, it is most desirable to minimize their numbers.
Prestressing has been used in certain cases to resist bending moments and to reduce cracking. Some tunnels are prestressed transversely, and some have a nominal longitudinal prestressing applied. Careful detailing and good workmanship should be able to eliminate virtually all deleterious cracking in concrete.
The construction of an immersed tunnel consists of excavating an open trench in the bed of the body of water being crossed. Tunnel elements are fabricated off site, usually at a shipyard or in dry docks. Elements constructed on launching ways are launched similar to ships by sliding them into the water. Elements constructed in dry docks, are floated by flooding the dry dock. The ends of each element are closed by bulkheads to make the element watertight. The bulkheads are set back a nominal distance from the end of the element, resulting in a small space at the ends of the adjoining sections that is filled with water and will require dewatering after the connections with the previous element is made. After fabrication and launching, the elements are towed into position over the excavated trench, once positioned and attached to a lowering device (lay barge, pontoons, crane, etc), ballast is placed in or on the element so that it can be lowered to its final position. Sometimes ballasting of the element is achieved by water ballast in temporary internal tanks or by adding concrete. After placing the element in its position, connection is made between the newly placed element and the end face of the previously placed element or structure to which it is to be joined. Once the element is in its final position butted up against the adjacent element, the water within the joint between two elements is pumped out. After any remaining foundation work has been completed and locking fill is in place, the joint can completed and the area made watertight. Once locking fill is in position, another element can be placed. The bulkheads can then be removed, making the tunnel opening continuous. For safety reasons, the bulkheads at the joint to the most recently placed tunnel element are left in position. The tunnel is then backfilled and a protective layer of stone is placed over the top of the tunnel if required.
Variations in the construction method deal primarily with materials and location of the fabrication site at which the sections are constructed.
11.2.2 Trench Excavation
The most common method of excavation for immersed tunnels is the use of a clamshell dredger (Figure 11-11 ). Sealed buckets should be used for contaminated materials and/or to reduce turbidity in environmentally sensitive areas. Cutter suction dredgers have also been used and are able to remove most materials other than hard rock. Blasting may be required in certain areas, though it is highly environmentally undesirable.
Figure 11-11 Sealed Clam Shell Dredge
The tunnel trench should be dredged to longitudinal profiles and bottom widths taking potential sloughing of the sides and accuracy of dredging into account so that the necessary bottom width and profile can be maintained during lowering of the elements and placing of the foundation materials. Over-dredged areas should be refilled with materials conforming to design requirements for foundation materials. Dredging should be carried out in at least two stages: removal of bulk material; and trimming. The trimming should involve removal of at least the last 3 feet (1.0 m) above final dredge level. All silt or other material that may accumulate on the bottom of the trench should be cleared immediately before placing the element. Dredging methods and equipment should be designed to limit the dispersal of fine materials in the water. Turbidity or silt curtains or other measures should be used where appropriate. Methods, materials, and mitigation measures should be used to avoid or reduce to acceptable levels the impacts of excavation, filling and other operations on the marine environment.
Trench excavation in any waterway is an environmentally sensitive issue. Once the environmental conditions have been set by the planning and permitting process, extreme care should be taken to meet these conditions. Trench excavation underwater is a difficult and complex process that can be complicated by contaminated materials, tides, storms and construction restrictions in waterways due to environmental concerns associated with fish migration and mating patterns and with ecology and marine life. Scheduling of construction activities, environmentally friendly construction techniques and equipment and innovative methods of dealing with contaminants must be considered in the design of the excavation and backfill.
Locations, elevations and dimensions of all underwater utility lines and marine structures should be determined in the area of the dredging and protection should be provided if required. Excavations should be evaluated for stability using appropriate limit state methods of analysis. Temporary slopes offshore should be designed for a minimum factor of safety of 1.3. Side slopes of the trench should not be steeper than 2 horizontal to 1 vertical in soil, nor steeper than 1 horizontal to 4 vertical in rock provided the minimum specified factor of safety is achieved. The design should ensure that the bottom of any excavation is stable. The design should take into account excavation base stability against heave in any cohesive soils. Remedial measures such as ground improvement may be required to provide stability of the excavation base against heave.
Special requirements to handle the disposal of dredged materials are usually specified. Contaminated materials must be disposed of in special spoil containment facilities, while uncontaminated materials, if suitable, can be reused for backfill. Materials for reuse must be stored in areas where excess water can drain away. For most immersed tunnel projects where spoil containment facilities are required, the quality and quantity of the wet material are such that existing facilities are too small or unsuitable. A dramatic increase in dredging and disposal costs over the past three decades due primarily to continually tightening environmental restrictions present significant challenges to the disposal of unwanted material. Unique solutions were developed for various projects including: the use of the dredged materials to construct a manmade island such as for the Second Hampton Road Tunnel in Virginia or to reclaim a capped confined disposal facility (CDF) as a modern container terminal such as the case of the Fort McHenry Tunnel in Baltimore.
11.2.3 Foundation Preparation
Once the trench excavation is complete, installation of the foundation should begin. Two types of foundations are used in immersed tunnel construction, continuous bedding (screeded foundation or pumped sand) or individual supports.
Continuous Bedding Continuous bedding should consist of clean, sound, hard durable material with a grading compatible with the job conditions. These include applied bearing pressure, the method with which the bedding is placed and the material onto which the bedding is placed. The foundation thickness should not be less than 20 inches (500) mm and preferably less than 4.5 feet (1.4 m). The gap between the underside of the tunnel and the trench bottom should be filled with suitable foundation material. The foundation can be prepared prior to lowering the elements (screeded), or it can be completed after placing the elements on temporary supports in the trench (pumped sand); foundations formed after placement have included sand jetting, sand flow and grout. For a screeded foundation, the bedding is fine graded with a screed to the line and grade required for section placement, or a stone bed may be placed with a computer-controlled tremie pipe ("scrading"). Settlement analyses for the immersed tunnel should be performed and should consider compression of the foundation course placed beneath the tunnel elements. Analyses should also be performed to estimate the longitudinal and transverse differential settlement within each tunnel element, between adjoining tunnel elements, and at the transitions at the ends of the immersed tunnel. Measures should be taken to prevent sharp transitions from soil to rock foundations. Varying the thickness of the continuous bedding can accomplish this. Alternately the tunnel structure should be designed to resist the load effects from the potential differential settlement of the sub-foundation material.
Individual Supports Individual supports usually consist of driven piles. Pile foundations should be designed in accordance with generally recognized procedures and methods of analysis. The piles should be designed to fully support all applied compression, uplift and lateral loads, and any possible down-drag (negative friction) loads from compressible soil strata. The load-bearing capacity, foundation settlement and lateral displacement should be evaluated for individual piles and for pile groups, as appropriate. The load capacity for bearing piles should be confirmed by static and/or dynamic pile load testing in accordance with recognized standards. The piles and tunnel sections are usually detailed to be adjustable in order to fine tune the horizontal and vertical placement of the tunnel. Once the tunnel sections are in their final potions, the adjustment is locked off and a permanent connection between the tunnel and pile may be made. The space between the bottom of the tunnel section and the bottom of the trench below the tunnel section is then filled with granular material. This process must be carefully controlled so that the bottom of the trench is not disturbed and that the void is completely filled. Since in most cases, the weight of the tunnel section being placed is less than the weight of the soil it is replacing, pile foundations are rarely used.
11.2.4 Tunnel Element Fabrication
For steel tunnels, fabrication is usually done by modules, each module being in the range of 15ft (5m) long, spanning between diaphragms. The modules are then connected and welded together to form the completed shell of the tunnel element. Electro-slag and electro-gas welding are not permitted, and all groove and butt welds are full-penetration welds. Measures need to be taken to eliminate warping and buckling of steel plates resulting from their local overheating during welding. Welds must be tested by non-destructive methods; it is recommended that ultrasonic testing be supplemented by X-ray spot-check testing. In some cases, stress relieving may be necessary. The placing of keel concrete should be done in such a way that it avoids any overstressing or excessive deflections in the bottom shell and its stiffeners. All length and angular measurements for tolerances need to be made while the structure is shielded from direct sunlight to eliminate errors due to warping from differential temperatures. Figure 11-12 shows the completed fabrication of a tunnel element for the Hong Kong Cross Harbor Tunnel, almost ready to be side launched.
Figure 11-12 Hong Kong Cross Harbour Tunnel is Nearly Ready for Side Launching.
Concrete tunnel elements are usually constructed in a number of full-width segments to reduce the effects of shrinkage. The segment joints may be construction joints with reinforcement running through them, or they may be movement joints. All joints must be watertight. Tight controls on casting and curing must be maintained to minimize cracking. Differential heat of hydration can be controlled by the use of high percentages of blast furnace slag to replace Portland cement or by using internal cooling system. Where concrete segments are cast with movement joints, they are joined together using temporary or permanent post-tensioning to form complete elements at least during transportation and installation. Care must be taken to ensure that long-term movements of short segments free to move are acceptable.
Tunnel elements are generally fabricated to be approximately 300 to 400 feet in length each. The actual length is a function of the capacity of the fabrication facility, restrictions along the waterway used to float the elements to the construction site, restrictions at the tunnel including accommodation of marine traffic during construction, currents, element shape and the availability of space for an outfitting pier, and the capacity of the equipment used to lower the elements into place.
All construction hatches, openings, etc., need to be sealed, by welding or other secure means, upon completion of concreting or other works for which they were required. Before the launching or floating of elements, bulkheads, manholes and doors, etc. should be inspected to ensure that they are secure and watertight. When no longer needed, any temporary access manholes through the permanent structure should be closed and a permanent seal made.
As tunnel elements are installed, the actual installed length of tunnel and position should be monitored so that any changes to the overall length of future tunnel elements and the orientation of the end faces can be adjusted as required to ensure fit with the actual surveyed positions of installed tunnel elements. This is especially important prior to fabrication and placement of the closure (last) element.
11.2.5 Transportation and Handling of Tunnel Elements
The stability of tunnel elements must be ensured at every stage of construction, especially when afloat. In checking tunnel elements for stability while floating, due attention must be paid to effects of variations in structural dimensions, including results of thermal and hydrostatic effects. Items to consider include:
When a storm warning is issued, or forecast wave heights are expected to exceed operational limits, all marine operations should be ceased temporarily; marine plant and floating tunnel elements should be sent to their designated storm moorings or shelters. It is recommended that an emergency berth be identified for tunnel elements, preferably within or close to the placement site. Special measures may be required to control tunnel elements in areas with currents or navigation channels. Figure 11-13 shows the transportation of a tunnel element to its final position.
11.2.6 Lowering and Placing
After outfitting at their final destination, immersed tunnel elements are prepared for immersion and lowering onto prepared foundations in a trench in the bed. The equipment used may typically be provided on a purpose-built catamaran straddling the element (Figure 11-14). Other methods include the placement of pontoons on top of elements (Figure 11-13), or cranes have sometimes been used. In Boston, for the Fort Point Channel tunnel, vertical buoyancy tubes were attached to the top of the elements and immersion by progressively adding water ballast was done.
Figure 11-13 Osaka Port Sakishima Tunnel Element Transported to Site with Two Pontoon Lay Barges.
Figure 11-14 Catamaran Lay Barge.
To lower an element to its final position, it is usual for either a temporary ballasting system to be used or for the element weight to be such that the element will itself have sufficient negative buoyancy. The method of immersion must:
The calculation of the factor of safety may include items such as external ballast, for example concrete blocks or internal ballast water tanks.
Lowering equipment should be designed to enable the lowering operation to be effectively controlled from a central control point and to make available at the central control accurate information on the position of the element and the loads on the lowering and the holding lines.
Elements are lowered and butted up to preceding elements. Thereafter, the joint between them is dewatered. A typical joint between elements includes watertight bulkheads (dam plates); watertight access bulkhead doors; joint seal and gaskets, dewatering equipment including any pumps and piping; location devices to guide the element horizontally and vertically into place relative to the preceding element, provision for shear keys (horizontal and vertical) and vertical and horizontal adjustment devices such as wedges, jacks and shims.
Tunnel elements should be installed at an elevation that considers an allowance for settlement such that after completion of the foundation works and all backfilling, they will be expected to be located within a tolerance of 2 inches (50 mm) laterally and vertically from their theoretical location, or any such lower figure on which the design methods are based. The allowance for settlement included in the determination of the installation level should be determined before installation. Notwithstanding the above, the relative location laterally and vertically should not be more than 1 inch (25 mm) across any joint. The relative location vertically across the terminal joints to other structures should not exceed 2 inches (50 mm).
Where foundation pads are used for temporarily supporting tunnel elements, any requirements for preloading and all subsequent behavior of the pads should be determined. The effect of potential hard spots beneath the tunnel element created by the foundation pads should be evaluated. Settlement of the foundation pads should be measured from the time of installation through any period of preloading until the tunnel element no longer requires support by the pads.
Permanent survey markers are needed within and on top of each element so that at any time its position relative to its position at time of casting is known. Survey towers or other markers or systems are needed so that the position of the element during lowering and placing is accurately known.
11.2.7 Element Placement
Element placement is the most delicate of all operations involving immersed tunnel elements. The needed duration of weather windows must be defined as well as "go/no-go" hold points. Some recent tunnels where prevailing currents could affect placing operations have used a weather-forecasting modeling system to forecast the required window; this may require monitoring of the hydrological and meteorological conditions concurrently to develop a forecasting model. Such a model should provide an understanding of the relationship between observed flow and meteorological and hydrological conditions. The last "go/no-go" decision should be based upon the current waves, and other physical conditions staying below the designed upper limits with a statistical probability of more than 90%. In all cases, the actual current at the element position should be checked immediately before lowering and continuously observed during the lowering and placing operation.
The element should have sufficient negative buoyancy to maintain stability and control of the tunnel element during immersion, so that the element can be lowered safely to its final position. The design should enable the negative buoyancy to be increased, if required, to give the minimum factors of safety given in Clause 11.2.6. Figure 11-15 shows the placement of a tunnel element using a catamaran lay barge.
Figure 11-15 A Tunnel Element is Being Placed.
Valves for dewatering of immersed joints should be operated from inside the previously placed tunnel element. No watertight doors or hatches should be opened until it can be confirmed that there is no water on the other side. Access must be maintained to the inside of the first element that is placed from the time when the element is placed until completion of permanent access through one of the terminal joints. Where hydrostatic pressure exists on a temporary bulkhead, the next two bulkheads should remain in place (one at the remote end of the same element, and the immediately adjacent one in the next tunnel element). Watertight doors in these bulkheads should remain closed at all times when the last tunnel element is unoccupied by personnel. Watertight doors should not be opened until the absence of water on the far side has been confirmed. The stability of the installed immersed tunnel elements during removal of temporary ballast and joint dewatering must be controlled to ensure that necessary factors of safety are maintained for the element as a whole, not only for the ends and for the sides, and so that the bearing pressure on the foundation remains approximately uniform.
After lowering and initial joining of each immersed tunnel element, its position should be precisely surveyed before the next element is placed. Settlement monitoring of tunnel elements should be carried out using the survey markers installed inside the elements. Levels should be recorded weekly until completion of backfilling of the subsequent element to ensure no remedial action is required and monthly thereafter until settlement becomes negligible.
The design should take into account the suitability of excavated material for use as backfill. The design should ensure that backfill placed next to the immersed tunnel is placed uniformly on both sides of the structure to avoid imbalanced lateral loads on the structure. The maximum difference in backfill level outside such structures above the locking fill should be 3 ft (1 m) until the lower side has been filled to its final level. Elements with more than 3 ft (1 m) difference in backfill level should be designed to accommodate the resulting transverse loads.
All fill materials subject to waves and currents should be designed to prevent scour and erosion. All underwater filling and rock protection material should be placed in a way that avoids damage to the waterproofing membranes (if present) or to the structure from impact or abrasion. The material should be placed in even layers on either side of the tunnel to avoid unequal horizontal pressures on the structures, and should be placed by means of buckets or tremie.
Prior to and during the placing of fill, the trench should be checked for sediment. Sediment that is detrimental to the performance of the material being placed should be removed.
Backfill should be provided around the tunnel. In seismic areas where there is a risk of liquefaction, the foundation and backfill should be designed as free-draining to prevent the development of excess pore-water pressure during and following a seismic event. Armor protection, if needed, should be provided to prevent long-term loss of backfill at the sides and on top of the tunnel.
The backfill usually consist of the following:
11.2.9 Locking Fill
Selected locking fill is placed in the trench to a minimum level of half the height of each element after the joint to the adjacent tunnel has been dewatered. Locking fill should extend at least 6 ft (2 m) horizontally from the tunnel element before being allowed to slope down not steeper than 1:2. Locking backfill is placed in layers of uniform thickness not exceeding 2 ft (600mm), such that lateral and vertical forces on the tunnel element are minimized and no displacement of the element occurs. Placement of locking backfill proceeds from the inboard (jointed) end of tunnel elements and progresses towards the outboard end of tunnel elements in a manner that produces a uniformly dense backfill bearing tightly against the tunnel periphery.
The locking fill must be a granular, clean, sound, hard, durable material that will compact naturally and that will remain stable under both non-seismic and seismic conditions (where required). It may include crushed sound rock or gravel. Well graded sub-angular sand may be included. Sand fill, if used, must be free-draining.
11.2.10 General Backfill
General backfill should be used to fill the remainder of the trench above the selected locking fill up to the underside of any protection layer, or to the pre-existing seabed level if no protection layer is used. General backfill should be placed by a method that avoids segregation or misplacement of the fill.
The properties of general fill must suit the proposed design and method of placing. General fill may comprise soft cohesionless material that will remain stable. General fill must be free from clay balls and be chemically inert. Often the dredged materials for the trench are suitable as general backfill.
11.2.11 Protection Blanket
The elevation of the top of the protection layer should approximate pre-existing seabed levels unless instructed otherwise. However in certain situations, the top of the tunnel can extend above the original seabed in an underwater embankment if permitted. In this situation, the protective blanket shall be provided above the embankment backfill.
Rock protection blanket material should consist of hard inert material, usually sound, dense, newly quarried rock in clean angular pieces, well graded between 1 inch to 10 inches (25 mm and 250 mm). The material should be durable for at least the design life of the tunnel. The method of placing this material must ensure that the large-size stones do not penetrate the general backfill and must cause no damage to waterproofing of the tunnel (if used). The protection layer should not be placed by bottom dumping.
11.2.12 Anchor Release Protection
In navigable waters, anchor release protection should be provided, if required, and if the tunnel cover extends above the bed. Rock armor for anchor release bands should be of sound, dense, newly quarried rock in clean angular pieces and well graded. The intent of the anchor release protection is to bring the anchor to the surface and choke the gape (the space between the hook and the shank). The size of anchor should be for vessels plying those waters. The material needs to be durable for at least the design life of the tunnel.