Civil Engineering Reference
In-Depth Information
Weathered sedimentary rocks frequently exhibit erratic strength profiles and weak
zones below less weathered beds must be detected if they are present.
Percussive drilling combined with penetration resistance measurement may give
some guide to the driving resistance of driven piles, but significant zones may be missed
and there is generally little substitute for adequately detailed logs prepared from drill
cores in order to establish a rock grade. In variably weak rock core, laboratory tests
such as the point load test or unconfined compression tests tend to favour the stronger
zones as the weakest material is often lost and therefore the results need to be care-
fully interpreted to avoid drawing the wrong (and unsafe) conclusions about the rock
strength.
The results of tests carried out on rock cores in the field (e.g. point load tests) should
be interpreted in a similar way. Useful information on the in-situ rock mass may
however be obtained from permeability tests (packer tests for instance) which reflect
the fracturing, fissuring and jointing of the rock structure. Where detailed information
of in-situ rock condition or rock grades is required, especially in formations such as
chalk or weak Triassic strata, a purpose-excavated shaft is recommended. Additional
information can be obtained by carrying out plate-bearing tests at selected horizons
as the shaft is excavated.
Settlement of piled foundations on rock may be estimated from in-situ modulus
measurements using the pressuremeter or by plate-bearing tests carried out over a
range of depths, though with adequate geological information, settlement may be
shown to be too small to justify any extensive investigation. Pressuremeter testing is
comparatively cheap, but suffers the disadvantages of possible disturbance of the rock
when drilling the pocket for the probe, and 'sample' size and orientation effects. The
plate-bearing test is costly, but is likely to give superior information.
It is however of critical necessity to recognize the presence of clay layers in rocks that
are vertically or near vertically fractured or jointed (i.e. 'blocky'). Piles end bearing
in such rocks may give a satisfactory blow count during driving, or may, if drilled
or augered, appear to be within apparently satisfactory, massive strata. When loaded,
poor load-settlement behaviour is obtained as a result of the toe load being transferred
via a column of blocks to the clay layers which compress, or may undergo lateral yield.
It is essential that the local geology is understood at the time of the investigation if
this condition is to be identified correctly and emphasis needs to be placed on good
drilling to achieve high core recovery, combined with accurate and suitably detailed
core logging.
Adequate information relating to the groundwater conditions is particularly impor-
tant for bored or drilled piles, and detailed notes should be made of water encountered
during drilling operations. Some qualitative information may be obtained from the
quantity of water/mud returns if a fluid flushing medium is being used, and water
levels in drilled holes should be recorded if the borehole is left standing for any length
of time. In continuous aquifers, simple standpipes may be adequate to establish the
hydrostatic head, but in more complex ground, piezometers will be required to isolate
water pressures associated with a particular structure.
Limestone is a special case as it may contain large cavities that can transfer consider-
able quantities of water with strong cross flows. Pile installation can be compromised
in such strata and grouting may not be a successful precursor to the piling. As with
the problem posed by clay layers in blocky rock, an understanding of the geology is a
