LDA (Large Diameter Auger) piles, more commonly known as bored piles, are used mainly as part of the foundation works for major structures or retaining structures.
Piles may be used in isolation or in groups. They can also form a mixed, continuous, secant or tangent piled curtain retaining wall or a composite curtain wall like a Berlin-type wall or similar. Piles are also used as plunge column (or stanchions) for structural incorporation into a building with basement levels.
Columns of this type are installed before any earthworks begin and can take part of the load imposed by the superstructure when it is constructed in parallel with the basement levels. They may be battered where specified in the installation standards.
Bored piles and barrettes differ in terms of their right cross-sectional shape:
Large-diameter bored (LDA, Kelly) piles are the most traditional and common form of piling worldwide. They are capable of supporting very high loads and coping with complex soil conditions. They are used chiefly in the foundations of major structures, but may also be used to provide support within secant pile walls.
Cost-efficient piles with a higher load-bearing capacity ratio
Piles can be constructed on land or from floating platforms
A range of interchangeable tools is available to cope with different types of terrain
The LDA technique requires the use of powerful hydraulic piling rigs which can use interchangeable attachments such as augers, buckets or core barrels.
These tools can cope with all types of granular or cohesive soils, including rock up to a compressive strength of 100MPa:
LDA piles use a temporary steel casing to support unstable strata and ensure safe working conditions at pile boring platform level. The casing is installed by the piling rig using a range of techniques developed for different soil types: vibration, oscillation or rotation. After concreting, the casing is removed immediately for re-use, although in some applications it may be left in place to provide protection or reinforcement.
As with diaphragm wall, carrier fluids such as bentonite or polymer may be a more effective way of stabilising a deep pile bore than using a longer casing.
The general principle here is to maintain a positive hydrostatic fluid load in the bore above groundwater levels. The bore excavation phase is followed by substitution or removal of the drilling mud, installation of steel reinforcement, and concreting of the pile. Concreting begins at the bottom of the bore using tremie pipes.
Bored piles generally take the form of cast-in-situ concrete reinforced with a steel rebar cage.
However, reinforcement can also be provided by prefabricated concrete and/or steel elements in preparation for the installation of retaining walls (e.g. Berlin-type walls) or plunge columns.
Soletanche Bachy uses a range of complementary techniques to improve the performance of standard LDA piles.
Under-ream, or “bell”, piles are perfectly suited to providing exceedingly high-bearing capacities in stable cohesive strata. Under-reaming of piles to produce enlarged bases has traditionally been used to enhance end-bearing capacity and provide economical support for structural mega-columns. The Group manufactures its own top-hinged tools that are operated mechanically during excavation. They can produce piles with shafts up to 2.4m and enlarged bases up to 7.2m (3 times the shaft diameter).