11.1.3 Drilling in rock
Weak rocks can be drilled by percussion equipment, but this technique is useful only to
determine the level of the interface between the rock formation and the soil overburden.
Little useful information is given on the characteristics and structure of the rock layers
because they are reduced to a gritty slurry by the drilling tools and drilling should be stopped
as soon as it is evident that a rock formation has been reached. Some indication of the strength
of weak rocks can be obtained from standard penetration tests (see Section 11.1.4). Percussion
boring can provide reliable information from rocks which have been weathered to a stiff or
hard clayey consistency such as weathered chalk, marl or shale. Open drive tubes, or preferably
the thin-wall pushed-in tubes, can be used in these weathered rocks to obtain undisturbed
samples for laboratory testing. The improved core drilling equipment described by Binns(11.1)
is now the preferred method of sampling weak and weathered rock. Hammering sample
tubes into shattered rock will not produce useable samples and frequently leads to confusion
and error in determination of rockhead.
The most reliable information on the strength and compressibility of rocks is obtained by
rotary core drilling, supplemented as necessary by in-situ tests. The core diameter must be
large enough to ensure complete or virtually complete recovery of weak or heavily jointed
rocks to allow reliable assessment to be made of bearing capacity. The percentage core recov-
ery achieved and the Rock Quality Designation (RQD) should be recorded. All cores should
be stored in secure, correctly sized core boxes and selected cores should be coated in wax or
wrapped in aluminium foil to preserve in-situ moisture content. Generally, the larger the core
size the better will be the core recovery. Drilling to recover large diameter cores, say up to the
ZF size (165 mm core diameter), can be expensive, but the costs are amply repaid if claims
by contractors for the extra costs of installing piles in ‘unforeseen’rock conditions can be
avoided. Also, by a careful inspection and testing of the cores to assess the effects of the joint
pattern on deformability, and to observe the thickness of any pockets or layers of weathered
material, the required depth of the rock socket (see Section 4.7.3) can be reliably determined.
It must be remembered that drilling for piles in rock by chiselling and baling or by the
operation of a rotary rock bucket (Figure 3.28) will form a weak slurry at the base of the
pile borehole which may make it impossible to ascertain the depth to a sound stratum for
end-bearing piles. Whereas if there has been full recovery of the cores from an adequate
number of boreholes together with sufficient testing of core specimens the required base level
of the piles can be determined in advance of the piling operations.
Investigation of chalk for piled foundations requires attention to defining the ‘marker beds’
(marl and flints), variability of the chalk with depth, possible fissures and dissolution cavities,
leading to determination of the ‘grades’as given in the revised engineering classification of
chalk(4.43)(also see Appendix). Exploration should continue for at least 5 m below the tip of
the longest pile anticipated. Percussion boring can cause disturbance, and is best used in low
and medium-density chalks. Rotary drilling in most grades will produce cores, but even with
high-quality large-diameter cores identification of the fracture size is difficult.
11.1.4 In-situ and laboratory testing in soils and rocks
Vane teststo determine the undrained shearing strength of soft silts and clays have little
application to piling operations. Shaft friction in these soils contributes only a small
proportion of the total pile resistance and it is of no great significance if laboratory tests
502 Ground investigations, contracts and pile testing