20.3 Gravel and sand

COMMENTARY ON 20.3

Gravel and sand (coarse soils) are generally not self-supporting; boreholes usually need to be lined with temporary casings and excavations tend to collapse unless trench sheets or other support systems are used.

In boreholes below the water table, some sands tend to "blow" up the hole, loosening the ground below. The tendency to "blow" is usually reduced but might not be completely eliminated by keeping the borehole full of water.

When planning an intrusive investigation, where gravel and sand is likely to be encountered, the following should be taken into account.

  • a) Excavations (see 24.2 and 24.3) can be of limited applicability because vertical faces in coarse soils are inherently unstable. In very favourable conditions such as in sands above the water table it might be possible to obtain from observation pits block samples cut by hand (see 25.10.1) which are quality Class 2 or even Class 1. However, under typical circumstances, trial pits allow the ground to be inspected without personnel entry and Class 4 samples, suitable for particle size distribution, to be obtained from the excavating equipment. Excavations extending below groundwater level experience water inflow due to the permeable nature of coarse soils; this exacerbates instability and hampers reliable description and sampling of the ground.
  • b) Borehole considerations vary to some extent depending on the drilling method.
    • 1) Dynamic sampling (see 24.6) is hindered by borehole wall instability except when using equipment with a simultaneous casing capability, and the addition of water to the borehole to counteract blowing is not readily accommodated.
    • 2) Cable percussion boring (see 24.8) can readily accommodate the use of casings to support the borehole walls. However, the method can militate against the recovery of samples whose grading is representative of the soil in situ. Samples taken with the shell are disturbed and are likely to be Class 5 because they are deficient in fines; the shell arisings should be tipped into a bucket or tank and allowed to settle to mitigate this problem. Below the water table, in addition to keeping the borehole full of water to mitigate against blowing, the shell should be withdrawn slowly and if necessary an undersized shell used.
    • 3) Resonance drilling (see 24.7) provides support to the walls of the borehole as an integral part of the method. However, the addition of water to the borehole to counteract blowing is not readily accommodated.
    • 4) Rotary drilling (see 24.10) is generally only viable with simultaneous casing as would be the case with wireline coring and some proprietary open hole systems.
  • c) It is generally not possible to recover undisturbed samples with a tube sampler. The action of forcing a sampler into sand tends to cause a change in volume, even if the area ratio is small (see 25.4.1.3b)), and hence the density of the sample might not be representative of the stratum although the recovered sample might be representative of the particle sizes. In some sands, a piston sampler is effective (see 25.5); this has minimal effect on the density but the water content of the samples might still be unrepresentative of the stratum so it is questionable whether Class 3 can be achieved. In sand and fine gravel samples suitable for particle size distribution, Class 4 samples are usually obtained by using the split-barrel standard penetration test sampler (see 25.4.5). Larger Class 4 samples can sometimes be obtained by using the 100 mm open-tube sampling equipment with a core catcher fitted above the cutting shoe (see 25.4.4). The quality class of samples recovered by dynamic sampling and resonance drilling is discussed in 25.8 and 25.9 respectively. In very favourable circumstances and with a combination of sophisticated equipment and flush it might be possible to recover relatively undisturbed samples, say Class 2, by rotary core drilling (see 25.7).

Given that it is very difficult to obtain representative samples, field testing is likely to be required to obtain an indication of relative density, compressibility and permeability; there are various methods of field testing which should be evaluated for use to obtain an indication of the properties of the ground (see Section 7).

Investigation of such soils requires a range of tests to characterize the in-situ condition; these should include a selection of the following:

  • i) Within boreholes the standard penetration test gives some indication of the relative density. However, the results might not be representative of the ground if sand is loosened, as happens when there is blowing below the water table which leads to an underestimate of relative density, or in coarse gravels which can lead to an overestimate.
  • ii) The static cone penetration test is an alternative method by which the relative density can be assessed and is not prone to the loosening due to blowing which leads to an underestimate.
  • iii) Dynamic probing could give useful results provided the density is not too high. However, the dynamic probe generally only provides quantitative data on the relative density by correlation with the standard penetration test or static cone penetration test and some of the published correlations might not be reliable.
  • iv) Approximate values of the strength and compressibility parameters can be estimated empirically from the results of the standard penetration test or, preferably, from the results of the static cone penetration test. Pressuremeter tests might also be used.
  • v) Plate tests provide more direct determination of strength and compressibility either in a dry excavation or within a large diameter borehole.
  • vi) In-situ permeability may be assessed from borehole permeability tests or by pumping tests.

20.4 Silt

COMMENTARY ON 20.4

Silt is a difficult material to sample and test and, in considering methods, it is sometimes necessary to distinguish between finer grained, cohesive silts and coarser grained silts, which approach fine sand in behaviour. Sample tubes can either loosen or densify a silt depending upon grading, water content and tube geometry.

When planning an intrusive investigation where silt is likely to be encountered, the following should be taken into account.

  • a) Excavation face stability is influenced by the grading, the behaviour of finer grained silt is similar to that of clay (see 20.5) whereas that of coarser grained silt is more like a sand (see 20.3)
  • b) Borehole considerations are similarly influenced by grading.
  • c) Depending on the clay and/or sand content, silt might be sufficiently cohesive to allow the recovery of samples using an open-tube sampler. Because of the relatively low permeability, these samples might permit a reliable determination of water content, even when water has been added to the borehole. Silts are often sensitive to disturbance during sampling, and hence samples taken with the 100 mm open-tube thick walled sampler are usually only Class 3 at best; there can be a tendency to liquefy with dynamic driving of tube samplers. In low or medium strength silt. Class 2 samples can be obtained using a thin-wall sampler, e.g. piston or statistically pushed UT100.

Field testing (see Section 7) might be used in addition laboratory testing on samples and the various methods should be evaluated.

Investigation of such soils requires a range of tests to characterize the in-situ condition; these should include a selection of the following:

  • 1) Depending on the clay and/or sand content, which determines material behaviour, the standard penetration test might be used to obtain an indication of the relative density. However, blowing and disturbance caused by the borehole tools can lead to the standard penetration test giving erroneously low results.
  • 2) As with sand and gravel (see 20.3), a more reliable indication of relative density or, where the silt behaves as a fine-grained material, undrained shear strength might be obtained using the static cone penetration test.

BS 5930:2015 Code of practice for ground investigations