Pile Design and Construction Practice, Fifth edition

66 Types of pile
Where timber or steel piles are pitched and driven at the bottom of drilled-in tubes, the
operation of removing the soil and obtaining a clean interior in which to place concrete is
tedious and is liable to provoke argument as to the standard of cleanliness required.
2.6 Minipiles and micropiles
Minipiles are defined as piles having a diameter of less than 300 mm. Generally, they range
in shaft diameter from 50 to 300 mm, with working loads in the range of 50 to 500 kN. The
term ‘micropile’ is given to those in the lower range of diameter. Neither micropiles nor
minipiles are specifically mentioned in EC7, but if load bearing, it should be assumed that
the EC7 design rules apply. BS EN 14199: 2005 Execution of special geotechnical works –
Micropiles covers requirements for using steel bars or tubes for reinforcement, concrete and
grout materials, and the use of additives. They can be installed by a variety of methods.
Some of these are as follows:
(1) Driving small-diameter steel tubes followed by injection of grout with or without withdrawal
of the tubes
(2) Driving thin wall shells in steel or reinforced concrete which are filled with concrete
and left in place
(3) Drilling holes by rotary auger, continuous flight auger, or percussion equipment
followed by placing a reinforcing cage and in-situ concrete in a manner similar to
conventional bored pile construction (Section 2.4.2) and
(4) Jacking-down steel tubes, steel box-sections, or precast concrete sections. The sections
may be jointed by sleeving or dowelling.
One of the principal uses of minipiles is for installation in conditions of low headroom,
such as underpinning work (Section 9.2.2), or for replacement of floors of buildings damaged
by subsidence. Where minipiles are used for underpinning in clays susceptible to heave
and shrinkage, it is advisable to insert a sleeve into a pre-bored hole over the top 2 to 3 m of
the shaft. In this case the pile must be considered as a column over the sleeved length and
designed accordingly.
Pali radice or ‘root piles’ (Section 9.2.2) are a form of grouted minipile used mainly for
underpinning where the piles are installed at appropriate angles through the structure or
foundations to transfer load to competent strata.
2.7 Factors governing choice of type of pile
The advantages and disadvantages of the various forms of pile described in Sections 2.2 to
2.5 affect the choice of pile for any particular foundation project and these are summarized
in the following subsections:
2.7.1 Driven displacement piles
Advantages
(1) Material forming pile can be inspected for quality and soundness before driving
(2) Not liable to ‘squeezing’ or ‘necking’