13 Analysis of Earthquake Effects

All immersed tunnels should be designed for seismic events appropriate to their location [1, Chapter 8]. Seismic events during the construction phase should also be considered. Liquefaction of soils around an immersed tunnel should be avoided, perhaps requiring special measures to be taken. An appropriate level of risk should be agreed on with the client because cost implications may be significant. It may be appropriate to consider three magnitudes of earthquake loading:

• The functional evaluation earthquake (FEE), also known as the design basis earthquake (DBE), should be used first to design the structure for either limited or full performance. A magnitude corresponding to a return period of one to three times the design life is appropriate.

• The SEE should be checked to ensure compliance with minimum performance for life safety and survivability of a design made to FEE. The SEE is the most severe seismic event considered at the location. A performance level agreed on with the client may or may not assume progressive collapse under SEE, but ductility of the structure must be ensured to prevent sudden fracture. A return period of 1000 years or more should be used. As an alternative, however, selecting the maximum credible earthquake (MCE) (i.e., the maximum foreseeable earthquake) or that of large earthquakes that occur at a lesser frequency may sometimes be appropriate.

• A smaller serviceability limit state earthquake, corresponding to a 5- to 10-year return period, may also be included as an ordinary static live load to be combined with other live loads.

For each of these magnitudes, an acceptable structural response to or performance with these loads, including the extent of cracking, movement, damage, formation of plastic hinges, etc., needs to be defined and agreed on with the owner so that the corresponding allowable design stresses and displacements can be determined. (Structural response resulting in collapse or catastrophic inundation is not acceptable.) Typical acceptable performance criteria are

• Minimum performance level

• Significant damage, repairable or perhaps not.

• May require full closure or replacement of tunnel.

• Emergency vehicles and slow-moving traffic can still pass any flooding on roadway.

• Limited lighting and ventilation.

• Limited performance level

• Intermediate damage, repairable over 12 months.

• Limited emergency and public traffic possible within hours.

• Limited leakage.

• Full lighting and ventilation.

• Full performance level

• Light damage requiring minor repairs.

• Public traffic immediately.

• No significant loss of service.

Not all combinations of earthquake and performance level may be useful to consider. Those that should be used for design would depend on the strategic importance of the tunnel route, the availability of alternative routes, the risks that the owner is prepared to carry, and the cost. Dynamic structural response analysis and assessment of displacements using soil-structure interaction may be necessary and may need to include the effective mass of water. An effective soil drainage system can reduce soil pore pressures.

28.4.6 Methods of Constructing Elements: Concrete and Steel Concrete

An element is a length of tunnel that is floated and immersed as a single rigid unit. The rigidity may be temporary and later released. In other words, elements either may be monolithic or may consist of a number of discrete segments stressed temporarily together longitudinally for ease of transportation and placing. After placing, the stressing may be removed so that each segment may act as a minielement that is free to move at each segment joint. Some Dutch tunnels and the 0resund tunnel between Denmark and Sweden consist of such released segments. The ability to use discrete segments can depend on subsurface conditions, acceptable displacements, and sufficient capacity to resist seismic effects. Monolithic Elements

Monolithic elements are cast in bays (equivalent to segments), typically with the floor slab cast first, and followed by the walls and roof in either one or two operations (Figure 28.12). Special efforts to reduce cracking adjacent to previously cast concrete could include low-heat concrete mixes and cooling pipes embedded in the concrete. Reinforcement is continuous across construction joints. Discrete Segments

Discrete segments lend themselves to being cast in a single continuous operation, either horizontally (0resund Tunnel) or vertically and then rotated (Tuas Bay Tunnel). Either way has the advantage of eliminating horizontal construction joints in the walls and the associated thermal cracking. If assembly can be achieved reasonably quickly, a basin or dock sized for a single element can be used. Operations can be tailored to obviate the need to store completed elements before immersion.

External walls and slabs are usually 1 m or more in thickness. These must therefore be considered thick concrete, so special precautions must be taken to avoid cracking during casting. It is particularly important to avoid cracking caused by heat of hydration because such cracks will leak. Steel

A steel tunnel is usually designed to be able to float initially with little or none of the internal concrete having been placed. The bulk or all of the concrete is then placed after launching, either close to the fabrication site or more usually at an outfitting site close to the immersion site. If a graving dock is used (Figure 28.12), a compromise must be reached between the size, the number of reuses, and the schedule.

Testing and repair of any cracks and leaks is needed before submerging the elements. Figure 28.11 shows the Cross Harbour Tunnel on the quayside in Hong Kong almost ready for side launching.

Ease of construction can be achieved by a high degree of mechanization and line fabrication. Prefabricated forms, work shelters (heated in winter), shop welding of any steel plate, and prefabrication of reinforcement cages and prestressing cables can all be used to advantage. Waterproofing

The need for waterproofing concrete tunnels is still a hotly debated issue. A barrier between concrete and salt water, warmer waters, or corrosive waters would appear particularly beneficial. Waterproofing will also reduce the amount of water penetrating the remaining cracks, particularly if the waterproofing adheres to the concrete. In case of fire, concrete that is not saturated with water is less likely to spall since the formation of steam is less likely.

Steel tunnels do not need waterproofing since the structural steel shell serves this purpose well, yielding and not cracking when overloaded, although measures to inhibit corrosion of the steel may be required.

28.4.7 Tunnel Joints Final Joint

Some tunnels are constructed progressively from one end to the other, after which the landside structures are completed. Others may require the last element to be inserted rather than appended to the end of the previous element. To achieve this, a small final gap will have to remain. This closure or final joint corresponds to a short length of tunnel that will need to be cast in place, inserted like a wedge, or jacked out like a piston. Methods used include tremie concrete to seal the joint and dewatering to complete the joint in the dry from the inside. Construction Joints

These are horizontal or vertical connections between monolithic parts of a structure. Usually, a waterstop is placed in such a joint. Typically, this would be one of the traditional types of waterstops, although hydrophilic and groutable waterstops are sometimes used. Special moveable watertight joints are required between discrete segments of a tunnel element, where used, designed as expansion joints and perhaps also with shear capacity. Immersion Joints

These joints are the ones that are dewatered between tunnel elements subsequent to the immersing and joining operations at the seabed. The joints may remain flexible, or they can be made rigid, as has been common with many steel tunnels. Flexible joints are generally sealed with a temporary immersion gasket or soft-nosed gasket in compression. The use of a secondary independent flexible seal, capable of being replaced from within the tunnel, is common practice (often an omega seal). Each seal should be capable of resisting the external hydrostatic pressure and should allow for expected future movements. Protection should also be provided against damage to seals from within the tunnel, such as impact damage or airborne contaminant damage. Seismic Joints

A seismic joint, which can be an immersion joint of special design, may be required to accommodate large differential movements in any direction due to a seismic event. Such a joint would most likely be located at significant changes in cross-section. Figure 28.13 shows a typical seismic joint of BART in San Francisco. Semirigid or flexible joints between elements may also need to be strengthened to carry seismic loads and to prevent catastrophic inundation, typically by using stressed or unstressed pre-stressing components across the joints or by using bearings and shear keys. Terminal Joints (Land Connections)

The terminal joints between the shore ends of an immersed tunnel and the land portions may also be immersion joints. Direct joint connections may be made to land-based structures such as cut-and-cover

FIGURE 28.13 Seismic joint of BART, San Francisco, CA (© Parsons Brinckerhoff, Inc. 1967, with permission).

tunnels or ventilation buildings. These structures may be constructed either before or after placing the immersed tunnel, depending on schedule constraints and local conditions. For a bored or mined tunnel connection, the backfill around the end of the immersed tunnel would first need to be made relatively impermeable, such as by grouting, to allow boring to continue into the end of the immersed tunnel.

Steel immersed tunnels, because of their shallow draft capabilities, may eliminate the need for cut-and-cover tunnel construction in poor ground. After backfilling, the end of the tunnel can be exposed and open depressed highway sections constructed against it. The end of the immersed tunnel was exposed at the Second Downtown Elizabeth River Tunnel in Virginia (Figure 28.14).

Wherever the cross-section changes significantly, seismic actions may generate significant differential movements and the design must accommodate these. Drains, sumps, and pumping stations are required at both portals to remove rainwater that falls within the open sections, and at the tunnel low points to remove wash and leakage water.

Ventilation buildings, if needed, should preferably be sited on land, where they cannot be hit by shipping. The end faces of the buildings may also make suitable interfaces between immersed tunnel elements and on-shore techniques.

28.4.8 Construction Aspects

Dimensional and density checking of the concrete is necessary at all stages of construction to ensure that the design weight is not exceeded, since the tunnel might then not float: it is easier to add more weight than to remove it, and weight can always be added externally.

Temporary bulkheads are needed at the ends of each element, just visible in Figure 28.9 and clearly visible in Figure 28.14. They need to be watertight and yet reasonably simple to remove later. Bulkheads, if made of steel, are often designed for reuse.

Typical foundation layers are 600 mm to 1 m thick above the bottom of the predredged trench. They are mostly formed either by a screeded mattress of stone placed before the tunnel element is immersed or by jetted sand after the element is set on temporary supports, conditions to be allowed for in design. Such supports at the free end of the element are made adjustable, while the other end is first guided and then held by the previously laid element. Other types of design including injection of concrete are sometimes used. Before immersing (Figure 28.1), elements are usually held by temporary lifting hooks while ballast is

FIGURE 28.14 Second Downtown Elizabeth River Tunnel, Virginia (© Parsons Brinckerhoff, Inc. 1983, with permission).

added to provide the necessary negative buoyancy. Survey towers or similar devices are attached to the elements to enable monitoring of position after lowering starts, while bollards and other towing equipment are removed. Calculations to ensure stability at all stages of placement are necessary. Backfilling must be carried out in such a manner that unbalanced lateral forces do not move the element.

28.4.9 Protection against Ship Traffic and Currents

Immersed tunnels should be protected against falling anchors by a layer of either graded material or sacrificial concrete (see Chapter 6 on ''Hazard Analysis'' in Ref. [1]). The same reference also provides guidance on expected loads from sinking ships, although this is very much dependent on the cargo that the ships would be carrying. Typical results for actual ships passing a particular location, assuming the worst cargo, can be plotted as shown in Figure 28.15. These results may vary further, depending on

• Partially sunken vessels, which may increase these values, particularly if the vessel has a rigid stern post that can produce point loading.

• Critical water depths over which the vessel must pass.

• The water depth at the tunnel location.

If storms cause vessels to drag their anchors, it may be desirable to protect the tunnel further with rock berms over which anchors would be forced. Once clear of the tunnel, the anchors could re-engage.

Fully sunken ship loadings on tunnel

Fully sunken ship loadings on tunnel

Vessel design draft, ft

FIGURE 28.15 Ground pressures from fully sunken ships (courtesy of Christian Ingerslev).

Vessel design draft, ft

FIGURE 28.15 Ground pressures from fully sunken ships (courtesy of Christian Ingerslev).

At the proposed site of the tunnel, the depth to the structure below the clearance envelope required for the shipping channel will vary according to the amount of overdredge expected during maintenance and the amount of protective backfill required over the tunnel. Protection may be required to the surface of the bed near the tunnel against scour, not only from currents but also from propellers. At some vessel terminals where propellers are kept turning to maintain position, significant local scour can occur. Overdredge of 1 m is not unusual, depending on the method used for dredging. Anchor loads are usually small compared with structural capacity but may cause local surface damage if insufficient backfill is provided [1]. The type and grading of backfill layers are selected so that they do not damage the tunnel structure and waterproofing, if used, and so that material does not get washed away under anticipated currents.

It may not be necessary to completely bury a tunnel and restore the original bed levels (a floating tunnel is an extreme case). In such cases, the effects of hydraulic intrusion of a tunnel into an existing waterway regime may require study since water flow is obstructed. For a floating tunnel, tidal and current effects may also cause dynamic and fatigue effects. In some instances all of these measures have been implemented on a single project.

28.5 Seismic Analysis and Design 28.5.1 Introduction

Tunnels, in general, have performed better during earthquakes than have above ground structures such as bridges and buildings. Tunnel structures are constrained by the surrounding ground and, in general, cannot be excited independent of the ground or be subjected to strong vibratory amplification, such as the inertial response of a bridge structure during earthquakes. Adequate design and construction of seismic resistant tunnel structures, however, should never be overlooked. Their seismic performance could be vital, particularly when they comprise important components ofa critical transportation system (e.g., a transit system) in which little redundancy exists.

The general procedure for seismic design and analysis of tunnel structures should be based primarily on the ground deformation approach, that is, the structures should be designed to accommodate the deformations imposed by the ground. The analysis of the structure response can be conducted first by ignoring the stiffness of the structure, leading to a conservative estimate of the ground deformations. This simplified procedure is generally applicable for structures embedded in rock or stiff or dense soil.

In cases where the structure is stiff relative to the surrounding soil, the effects of soil-structure interaction must be taken into consideration.

28.5.2 Performance Record during Earthquakes

Seismic performance data of underground structures, including tunnels, are relatively scarce. Data reported in several studies [2,9-12] show that

• The greatest risk to all types of tunnels (including bored and mined, cut-and-cover, and immersed) occurs when the following conditions exist.

• An active fault intersects the tunnel.

• A landslide intersects the tunnel.

• Soils surrounding the tunnels liquefy.

• Damage potential to tunnels reduces with increasing overburden, due partly to the attenuation of ground-shaking intensity with depth.

• Damage is greater in soils than in competent rock.

• Seismic performance of shallow cut-and-cover rectangular tunnels has been relatively poor in comparison to that of bored tunnels, as evidenced during the 1995 Kobe, Japan, earthquake [13,14]. These rectangular box-type structures are particularly vulnerable at the joints connecting the slabs with the walls or columns when subjected to cyclic racking deformations imposed by the ground.

• Immersed tunnels are susceptible to permanent ground deformations resulting from liquefaction induced settlements, lateral spread, and uplift (flotation), and slope instability (landslides) in soft cohesive soils. Joints connecting tube segments are particularly vulnerable to the relative movements between two adjacent segments during shaking, noting that water tightness is one of the critical performance requirements of immersed tunnels.

• Damage potential to bored tunnels due to ground-shaking effects (excluding permanent displacements due to faulting, landslides, and liquefaction) increases with ground-shaking intensity and decreases with better tunnel lining and support system. Figure 28.16 presents performance data of bored tunnels under the effects of seismic ground shaking alone [10]. The figure suggests that

• Ground shaking caused little damage in tunnels for peak ground acceleration (PGA) less than about 0.2g, where g is the gravity of acceleration.

• Tunnels with ductile lining (reinforced concrete or steel) tend to have performed better, with damage observed only when the PGA exceeds 0.5g.

28.5.3 Design and Analysis Approach for Ground-Shaking Effects General

Underground tunnel structures undergo three primary modes of deformation during seismic shaking: ovaling or racking, axial, and curvature deformations (see Figure 28.17 and Figure 28.18) [2,12]. The ovaling or racking deformation is caused primarily by seismic waves propagating perpendicular to the tunnel longitudinal axis, causing deformations in the plane of the tunnel cross-section. Vertically propagating shear waves are generally considered the most critical type of waves for this mode of deformation. The axial and curvature deformations are induced by components of seismic waves that propagate along the longitudinal axis. Evaluation of Axial and Curvature Deformations Free-Field Deformation Procedure

This procedure assumes that the tunnel lining conforms to the axial and curvature deformations of the ground in the free field (i.e., without the presence of the tunnel). While conservative, this assumption provides a reasonable evaluation because, in most cases, the tunnel lining stiffness is considered relatively

Damage state:

FIGURE 28.16 Rock surface peak ground acceleration for various tunnels (courtesy of M.S. Power, Summary and Evaluation of Seismic Design of Tunnels, Draft report submitted to Multidisciplinary Center for Earthquake Engineering Research (MCEER) 1998).

Damage state:

Unlined Timber or Concrete Reinforced concrete masonry or steel pipe

FIGURE 28.16 Rock surface peak ground acceleration for various tunnels (courtesy of M.S. Power, Summary and Evaluation of Seismic Design of Tunnels, Draft report submitted to Multidisciplinary Center for Earthquake Engineering Research (MCEER) 1998).

Tunnel before wave motion

Tunnel during wave motion

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