9.7 Piled foundations in karst
The design and construction of piles for structures on land underlain by limestone formations
which exhibit karst conditions such as wide fissures and solution cavities present several
unique challenges. Because variations in rockhead and cavitation can occur over short
distances it is difficult to produce an overall geological model of the site to determine if
shallow foundations can be used or whether piles can be founded on ‘competent rock’. The
first requirements are therefore to assess the depth and strength of the overburden, the extent
of cavities and the degree of infilling under each foundation by drilling a series of closely
spaced probe holes using a combination of rotary percussive rigs capable of installing casing
and rotary coring drills. Waltham and Fookes(9.38)give an engineering classification of karst as
a means of identifying foundation difficulties but they point out that there is no simple answer
to the number of probes which may be required. The probes are usually taken to a depth of at
least 3 m below rockhead and any void encountered, or to a similar depth below the anticipated
depth of rock socket of each pile. Because of the possibility of vertical faces in the rockhead
and cavities it is advisable to include a percentage of raked probe holes in the investigation.
The selection of the pile installation method is critical, as it may be necessary to overcome
random boulders in the overburden, remove and replace weak material in cavities through
which the pile has to pass, and finally found on competent rock or form a socket in rock,
ensuring that sound rock also exists within the bearing zone. Large driven piles are not
usually feasible and the most effective method is the drilled and cast-in-place pile, usually
with permanent steel casing sealed into the rock at the top of the socket.
The removal of cavity and fissure infill debris and replacement with cement grout to
allow uncased holes to be drilled for piles is expensive and rarely achieves the desired
results. Flushing/grout holes are required at less than 1 m centres under and around the pile
group and flushing water is necessary in quantities greater than 150 l per min and pressure
greater than 10 bar – potentially causing pollution of surrounding water courses. If sufficient
grout can then be injected it may be possible to place concrete in the open pile hole, or as
temporary casing is withdrawn, without loss of fluid concrete. However, for pile diameters up
to 1 200 mm rotary-percussive rigs which can simultaneously install permanent casing
(duplex drilling) are generally considered the most cost-effective installation method. For
larger-diameter piles, the use of a powerful casing oscillator and a drilling method to clean
out the pile and form the rock socket are recommended (see Section 3.3.2); above this
diameter, shaft or caisson construction techniques may be necessary. Whichever method is
used it is essential to probe below the base of the pile to check for cavities. Pile drilling may
have to be continued until a sound bearing for the rock socket is located. Jet grouting could be
used to consolidate any cavity infill within the bearing zone below the sound rock socket –
again high grout pressure and volume (450 bar and 350 l per min) will be required with
adequate venting to the surface and pollution control.
Grouting to form a mattress of sound bearing material at rockhead for driven H-piles can
be effective in karstic limestone(9.39). In order to limit the flow of grout away from closely
centred grout holes it was necessary in this case to vary the slump depending on injection
rate and pressure and to use secondary and tertiary injection holes where grout loss
occurred. Verifying the treatment is difficult and reliance has to be placed on detailed grouting
records. Specially serrated driving shoes were needed to minimize the slipping of the pile
tip along uneven bedrock surfaces to achieve full bearing capacity.
‘Micropiles’, with working capacity of 890 and 1 160 kN, were used for bridge piers
by inserting 245 mm diameter thick wall steel tubes into grout-filled pre-drilled holes as
472 Miscellaneous piling problems