Civil Engineering Reference
In-Depth Information
Figure 3.39
Photograph of Horsfall tunnel showing ground support with micro-piles. (Photograph
courtesy of Skanska UK Plc.)
augers can drill unsupported holes whilst in cohesionless soils a steel temporary
casing fitted with a cutting shoe at its leading edge is drilled-in using a drill fluid.
The fluid is usually water, but can consist of a drillingmud. In a typical system, such
as that operated by Fondedile Foundations Limited, the fluid is transmitted down
the inside of the drill casing to return cuttings to the surface via an annulus that is
formed between the casing and the soil. Casing is added as the drilling advances,
often using short lengths as headroom is frequently limited in the applications
where micro-piles are employed.
At the required depth, the borehole is flushed clean and grouted with a sand
cement mix and the tremie method is used to introduce the grout at the base
of the borehole. Reinforcement is added and the casing withdrawn with further
grout being tremied into the bore to make up for the casing volume. In self-
supporting cohesive soils, it is not necessary to employ a temporary casing and
drilling can be carried out using an auger or drag bit. Micro-piles can also be
installed to bear into weak rock by using rock roller bits or down-hole hammers
to advance the bore. The steps involved in this method of micro-pile formation
are illustrated in Figure 3.40. When installing micro-piles through existing foun-
dations, it may be necessary to step-down the temporary casing size as drilling
