Micropiles, also known as minipiles, are a deep foundation element constructed using high-strength, small-diameter steel casing and/or threaded bars or simply constructed as small diameter concrete bored pile with normal steel bars. Micro piles can be installed by various means depending on ground conditions and load requirements. Most commonly, micro piles are designed so that their load bearing capacity is essentially derived from the cross section of the high yield steel reinforcement or tube. The grout or concrete cover provides the protective cover but may also be designed to contribute to the external capacity of the micro pile by virtue of skin friction.

Bored piles, also known as replacement piles, are a commonly-used form of building foundation that provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics. Bored piles are piles where the removal of spoil forms a hole for a reinforced concrete pile which is poured in situ. The spoil is replaced by the pile, hence ‘replacement’ piles as opposed to displacement piles where soil is forced away by driving or screwing the pile. Bored piles are used primarily in cohesive subsoils for the formation of friction piles and when forming pile foundations close to existing buildings. They are popular in urban areas as there is minimal vibration, where headroom is limited, where there is no risk of heave, and if there is a need to vary the length of the piles.

Bored piles are drilled using buckets and/or augers driven by percussion boring (vibratory hammers) or through rotary boring (twisting in place). In unstable soil strata, the use of bentonite fluid assists in stabilising the bore especially in large diameter deeper piles and allows the insertion of heavily reinforcing steel cages. This is known as flush boring (see more below). If the boring and pouring takes place simultaneously, piles are known as continuous flight auger (CFA) piles. Piles are known as large diameter piles if they are 600 mm or more in diameter. Small diameter piles of less than 600 mm are sometimes placed in groups under a common pile cap to receive heavy loads.

A secant pile wall consists of overlapping (secant) piles to form structural or cutoff walls and achieve the required water tightness. The design can incorporate steel bar or beams for reinforcement and anchors can provide additional lateral support, if needed.

Contiguous pile walls are installed using rotary bored or CFA techniques. This type of retaining wall is constructed with spaces between adjacent reinforced concrete piles and is used on a range of engineering or building projects including but limited to road or rail underpasses, underground shafts/tanks, cut and cover tunnels/portals or basements. Contiguous piled walls are used in dry and cohesive soils and are often the quickest and more economic method of constructing retaining walls. In certain ground conditions it may be necessary for the design to incorporate temporary props or ground anchors to provide additional lateral support to the wall. Contiguous bored pile walls are formed by constructing piles using the CFA, LDA or CSP technique. Occasionally, a guide wall is constructed at platform level to aid positional and verticality tolerances. Piles are constructed in a sequence such that no one pile’s integrity is affected by constructing another immediately adjacent pile next. Piles are constructed so that they virtually touch each other with typical gaps of between 50mm-150mm. The soil in the gaps is exposed during excavation and can if necessary be grouted or sealed to form a watertight wall. Normally a secondary lining wall is added to provide a permanent works finish to the wall.