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Sheet piles are used mainly for temporary ground support to deep excavations and basements where side slopes would be impracticable or uneconomic, and as in-ground cut-off barriers.

Suitable for excavation support as embedded retaining walls and for steel sheet piled cofferdams (a closed cell to exclude water for foundation construction). Depending on soil conditions, exposed heights as retaining walls may be up to 12m, suitably propped by

Problems to be avoided

Section of guide walls

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struts or tied back by ground anchors as excavation proceeds. Maximum driven depth for retaining walls and in-ground cut-offs is 25m in ‘soft’ soils.

For groundwater control, piles should toe in to an impermeable stratum, but where this is not possible the precautions for partially penetrating cut-offs must be observed (3.4).

Generally, temporary in-ground barriers using sheet piles are not particularly cost-effective compared with slurry trenches as excavation is likely to be easier than driving piles. It can be difficult to achieve relatively impermeable joints—at least 1,000 times less permeable than the surrounding soil is desirable.

Not suitable where noise and vibration restrictions apply or where large cobbles and boulders will damage piles and ‘clutches’ (the interlocking joints to the sheet piles) during driving or prevent driving to required depth.

Where de-clutching of the piles has occurred due to driving through cobbles, injection grouting or jet grouting behind the piles can considerably improve the water-tightness as a cofferdam and cut-off.

Sheet pile walls may be left in place as permanent back-shuttering to the basement or shaft.

Ground investigation should be as for diaphragm walls, with the additional assessment of the driving resistance of the soil based on comprehensive penetration testing of granular soils.

Earth pressures for design will tend to be based on the lower values for the geotechnical parameters provided by the ground investigation, whereas drivability will be limited by the higher values.

Investigation for cofferdams must include determination of groundwater and tidal movements. Also wave, scour and impact forces may be significant for marine sheet piling.

Over-consolidated clay soils may be difficult to penetrate with sheet piles, and as porewater pressures change with time, effective shear strength changes may affect lateral pressures.

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Design for cofferdams and embedded retaining walls generally is based on determining resistance to bending and shearing forces imposed by lateral pressure on the wall.

As for other retaining walls (5.4, 5.6), total and effective stress conditions in the soil may need to be considered at different times during construction and service life.

Methods of deciding design values of soil parameters based on peak, critical and residual test results for different limit states are given in BS 8002:1994. Also, empirical rules are set out to allow for:

• softening of clay layers due to water entering tension cracks by applying the ‘minimum equivalent fluid pressure’ on the side of the wall

• a minimum ‘surcharge’ load of 10kN/m² resulting from construction plant and materials at the top of walls.

Determining the size of structural members, props and tie-back anchors requires specialist knowledge; worked examples of cofferdam design are given by Williams and Waite (1993), and Gaba et al. (2003) describe current best practice for embedded walls generally.

Groundwater lowering outside the retaining wall using wells or wellpoints will reduce the lateral pressure and, provided that there is a fail-safe system to ensure that the drawdown is maintained, effective stress parameters may be used.

Once water is removed from a cofferdam, pumping, from either sumps in the formation or wellpoints, will be required to keep the formation dry for construction. Particular attention has to be paid to avoid piping (3.2) and loss of fines at this stage causing instability and danger to personnel in a partially penetrating cofferdam.

Design for in-ground cut-offs assumes that sheet piles can withstand high hydraulic Driving sheet piles using vibrators: Indicative depths in granular soils

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gradients and have low overall permeability—de-clutching or damage during driving will severely compromise effectiveness. Ideally piles should toe in to an aquiclude.

Construction is carried out using a variety of sheet pile sections, piling hammers and methods depending on application, plant availability and environmental factors. Main contractors generally carry out the work in accordance with the ICE Specification (1996a).

Piles are principally ‘U’-section, known as the ‘Larssen’ pile, or ‘Z’-section, known as the ‘Frodingham’ pile. Straight web and ‘H’-section piles are also in general use, and many combinations can be made by welding different sections together to improve the bending moment resistance.

Z-section clutches are generally considered more water-tight and are preferred for embedded cantilever walls. U-sections are used in pairs to give the specified bending resistance; but with a thicker section than the equivalent Z-section, they are useful in dense and difficult ground. Pile manufacturers and steel stockholders provide information on the structural properties of the various types currently available.

Butt welding on site to extend pile length must ensure that piles are straight and that the joints are staggered when driven and not at the point of maximum bending moment in the wall.

Hammers for driving piles may be simple drop-hammer types especially in heavy clays, but generally double-acting diesel or hydraulic impact hammers are used for most types of ground.

Medium-frequency vibrating hammers are used where noise restrictions apply (Tomlinson, 1994) and will effectively drive piles in sands, gravels and soft clays.

Hydraulic thrusters (‘silent’ drivers) are useful in clays where noise and vibration would be a problem.

Methods of driving all require a form of guide frame, whether for use as a perimeter wall to an excavation, as a cofferdam or as an in-ground cut-off in order to ensure that the sheets are driven vertically and the clutch connections are as tight as possible. Overhead working space is essential.

The simplest method for short piles in sandy soil is to pitch and drive each single pile to required depth along-side the previous pile; care is needed to prevent twisting and leaning out of vertical. This is generally favoured for high-speed vibrating hammers driving medium-section piles.

The ‘panel’ method (BS 8004:1986), controls the verticality and alignment by pitching and interlocking five piles in the guide frame and then driving each to, say, half the required penetration. The next panel is pitched to lock into the last upstanding pair and then driven. The hammer then completes the driving of the first panel and the pitching/driving procedure is repeated for the complete wall. This technique requires a long-jib crane to pitch the piles and care is needed to ensure that the clutches engage smoothly otherwise high friction hinders driving.

The order of driving piles must be carefully planned for cofferdams so that watertight closure at a corner is achieved. Other types of cofferdam and construction methods are detailed in Tomlinson (2001).

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Maintenance of cofferdams is generally concerned with eliminating leaks at clutches which appear at and above formation level after excavation.

In marine conditions leaks can be sealed by dumping PFA in the water outside the sheet piles, which is then drawn into the clutches by gravity flow.

Leaks below cofferdam formation level must not be allowed to cause reduction in the passive resistance of the soil or wash-out of fines. Sealing by injection or jet grouting can be effective in coarse soils. Where leaks are to be controlled by pumping, care must be taken not to remove fines from the soil; internal sumps must have filters, but external dewatering may be indicated as above.

Leaks in an in-ground cut-off wall can be considerably reduced by grouting.

Corrosion of steel is generally not a problem for temporary sheet piles in undisturbed soil, but for long-term use, coating with epoxide paint prior to driving is appropriate.

However, where corrosion is likely to cause a severe reduction in the steel sections (marine conditions; hot, humid atmosphere), impressed current or cathodic protection using sacrificial anodes may be needed.

Caution: safety procedures must be established and enforced for all forms of pile driving. Safety cages are required for personnel when pitching piles and guiding into the clutch. The pile gates on frames when used as access for personnel must have walkways with safety rails. Shackles for rigging must be inspected regularly.

Driving sheet piles using double-acting hammer Indicative depths in granular soils

Depths depend on type of strara encountered and construction methods used.

It may be necessary to move up a size to achive the required penetration or greater depth at lower N-value. Note that nomenclature may be different for different manufactures.

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Safety procedures for working in cofferdams must include:

• provision of safe working areas, access and alternative exit routes

• life jackets and means of rescue when working adjacent to water

• audible and visual warnings of failure of any part of the system.

Monitoring of the cofferdam must be carried out regularly by experienced personnel and recorded on Form 91 (Part 1 Section B) in accordance with the Factories Act (HMSO, 1988 and TSO, 1996a) to check:

• safe access

• stability of internal struts and frames

• ingress of fines and movement of the sheet piles

• ground movement in and around the cofferdam

• the dewatering system and associated piezometers.

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5.6 Bored Pile Walls