The NBS has just been expanded to include piling and will shortly include a section on embedded retaining walls Piles and retaining walls
The 1996 Specification for Piling and Embedded Retaining Walls1 has now been adopted as the base specification for nbs. It comprises:
specification - the clauses, the only part that should be referred to from a contract
contract documentation and measurement - these include procuring a piling contractor, responsibility for design and construction of piles and measurement of piling work and guidance on pile testing
guidance notes - on what should be specified and why.
Making the choice
The choice of pile and retaining walls will be influenced by:
ground conditions - weak soils overlying competent strata, stiff clays, presence of archaeological artefacts, general ground variability, negative skin friction (downdrag), presence of obstructions, aggressive ground conditions (especially the presence of elevated sulphates)
construction and environmental constraints - noise, vibrations near existing sensitive structures
construction considerations - location, access, work area
structural constraints - magnitude of loads, tensile and compressive axial loads, lateral loads, sensitivity of structure to total and differential settlement, loads due to general ground deformation, construction sequence
For characteristics of retaining wall types see box. Site investigation is essential - see article on previous page.
Pile load testing
The information obtained from pile load tests may be used to justify the use of better soil parameters for design or a lower factor of safety. It may be used to prove the efficacy of a particular pile type in difficult ground conditions. It can be used to justify a pile whose construction was considered suspect.
Preliminary pile load test (also known as a trial pile test) - this test is carried out in advance of the piling contract to prove the use of the pile in the particular ground conditions found at a particular location, and to verify or improve the soil parameters proposed in the design. Tests should be considered for each distinct pile type and set of soil conditions.
Contract pile load test (also known as a working pile test) - this load test is carried out on piles which are to be incorporated into the works to prove the load settlement performance of the piles at loads that may be encountered in service. Loading is typically only to 1.5 times the pile's rated capacity.
Pile dynamic and integrity testing
Dynamic testing involves driving the pile with a hammer and measuring its response using accurate instrumentation. It is sometimes used to extrapolate the result of pile load testing and may be used to reduce the number of load tests that might otherwise be required.
Integrity testing is used to prove that piles have not become damaged, either during installation or subsequently. It involves the use of non- destructive testing methods. See Integrity Testing in Piling Practice2.
Retaining wall deflections
An embedded wall is often chosen instead of a battered slope because of a need to control ground movements behind it. This may be because of structures or buried services in the retained ground which must not be damaged. This retained ground will settle and deflect horizontally in response to the deflection of the retaining wall. Some deflection of the retaining wall inevitably results from the excavation of the soil in front of it. Hence control of movements, for instance by propping or ground anchors, and prediction of the wall and ground movements, may form part of the design.
Retention of groundwater
Control of groundwater is required temporarily, to provide a clean dry space for construction. No embedded retaining wall is completely watertight in the long term. Hard-hard secant pile walls or diaphragm walls (see box) may provide an acceptable level of water-retention if a low grade of basement space is all that is required.
For higher grades of space, structural facing walls should be designed to bs8110 (in front of contiguous or hard-soft secant pile retaining walls for Grade 1 space) or the more onerous bs8007 (for Grade 2 space), or drained cavities (for Grades 3 and 4 space). See also in the specification1, guidance note 11.9, plus bs8102: 1990 and ciria Report 1393.
1 Specification for Piling and Embedded Retaining Walls, Thomas Telford.
2 Integrity Testing in Piling Practice, MJ Turner, ciria Report 144 (ciria, tel 0171 222 8891), 1977.
3 Water-resisting basements, ciria Report 139 (ciria, tel 0171 222 8891), 1995.
Tim Chapman is an associate with Arup Geotechnics. John Elkington and Colin McGregor are with National Building Specification.
Diaphragm: adjacent reinforced-concrete panels. High cost
Hard-hard secant pile: overlapping adjacent reinforced-concrete piles. High cost
Hard-soft secant pile: every second pile is formed from a weaker cement- bentonite mix to ease installation. Only temporary water retention. Medium cost
Contiguous pile: piles normally spaced with a gap between. No water retention. Lower cost
King-post: steel members embedded in piles, which can be widely spaced. Waling planks are inserted between the steel members' flanges to form the face of the wall. Limited water retention. Lower cost
Steel sheet piles: interlocking steel lengths which can be driven to form a retaining wall. Some water retention. Lower cost