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Secant and Contiguous Piled Walls
Arguably the most common specialist method of soil retention
is the construction of bored pile retaining walls. These are formed by
the construction of either continuous flight auger or rotary auger bored
piles to a predetermined layout and sequence and permit a vertical faced
excavation. They are particularly appropriate where site boundary restrictions
prohibit alternative forms of soil retention eg battered excavations such
as inner city developments where adjacent structures are often positioned
immediately adjoining the party wall boundary. May Gurney Geotechnical
operate a modern fleet of rigs, some of which have been specially designed
and manufactured to allow piles to be installed in close proximity to these
adjacent properties.
Piled retaining walls are typically designed to be free standing (cantilever), or propped in their temporary i.e. during construction or permanent conditions. Cantilever walls are normally more heavily reinforced and of a larger section size than propped walls of similar retained height in order to provide adequate stiffness to control deflections at the head of the wall. Propping to walls may be provide in a variety of ways eg ground anchors, raking shores, temporary soil bunds, basement/ground floor construction, bracing props and the like. May Gurney Geotechnical possess a complete in-house design and construct capability.
Contiguous bored pile walls consist of a line of individual piles positioned at close centres and are formed by conventional continuous flight auger or rotary auger bored pile techniques. Because the piles are discreet the retaining function of the wall relies upon the soil between the piles arching. Consequently they are therefore not normally appropriate for use in granular soils with a high ground water table as in these conditions loss of fines and settlement of the ground at the rear of the wall may be experienced. Temporary guidewalls are not normally necessary with this form of wall unless very onerous positional tolerances are required.
Secant bored pile walls consist of overlapping primary (often referred to as female) piles and secondary (male) piles. The primary piles are usually, though not always, un-reinforced. They are typically formed using a cement-bentonite concrete mix for hard/soft walls, or a low strength concrete for hard/semi-hard walls. A structural concrete mix in both primary and secondary piles is used for hard/hard walls. Although not watertight structures these types of walls are ideally suited for use in granular or water bearing strata to control water flows and allow basement excavation to proceed with only limited pumping. As with a contiguous bored pile wall a cavity drain or similar is recommended. The construction of temporary guidewalls in advance of the piling operations is an essential component of these walls and is required to ensure the overlap between adjacent piles is maintained over the depth of the excavation.
All bored pile retaining walls should be tied at the head with a capping beam in order to maximise the efficiency and performance of the wall.
Whilst some walls are left exposed in areas such as plant rooms the vast majority are faced using a variety of finishes such as sprayed concrete, brickwork/block work cavity walls and marble cladding.
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