20 The effect of ground conditions on the selection of methods of intrusive investigation

20.1 General


The various methods of forming exploratory holes described in Clause 25 are not all equally suited to each type of soil, or to rock. Similarly, the sampling techniques described in Clause 26 do not work in all ground irrespective of its nature. In fine soils undisturbed sampling is often practicable thereby allowing determinations of engineering parameters (most commonly strength and stiffness) by laboratory test, although field testing might also be used. In coarse soils undisturbed sampling is extremely difficult so generally field testing is used to obtain the engineering parameters. Field testing is described in Section 7; some types of test are carried out within boreholes, e.g. the standard penetration test, whereas others are investigation points in their own right, e.g. the cone penetration test.

The character of the ground, as anticipated from the desk study, should be taken into account in the planning of an investigation. The factors involved in the choice of the most suitable procedures for intrusive investigations in the range of soil types and in rock are covered in 20.2 to 20.11 where frequent reference is made to sample quality classes and these are defined in 26.2. The aspects covered in 20.2 to 20.11 are excavations (trial/observation pits and trenches) and boreholes (by the most relevant drilling methods) together with sampling and field testing.

NOTE For further information, see BS EN 1997-2:2007, Section 3 and BS EN ISO 22475-1:2006, Section 5. An overview of methods of forming exploratory holes and sampling from them, along with other aspects of investigation, is given by Clayton et al., 1995 [22].

Appropriate sample quality should form the basis of the selection of the methods for intrusive fieldwork for all investigation phases. The quality class of sample needed to conform to testing requirements in the laboratory should dictate the category of sampler to be used and thereby influence the selection of the appropriate method(s) of investigation. Where the nature of the ground precludes the recovery of samples of an adequate quality class, the use of field testing for the assessment of engineering parameters should be evaluated.

When working with interlayered ground (e.g. soil/rock or clay/gravel), the drilling method should be selected with the aim of ensuring recovery of appropriate quality samples of both the stronger and weaker, or finer and coarser, materials.

Groundwater conditions should be determined (see Clause 26) using appropriate methods of exploration.

20.2 Boulders and cobbles


The presence of boulders and cobbles (particles of dimensions greater than 63 mm), whether as the principal or secondary fractions, presents significant difficulties to drilling and sampling. The largest particles obstruct the drilling process for most type of boreholes, often being the obstacle to any further progress. Where they do not cause early termination, these particles often have to be broken into smaller fragments by the drilling tools to allow them to pass up the hole. This results in samples which are unrepresentative both because they do not contain the largest particles and because the sample recovered is not of sufficient size to be representative of in-situ conditions (see BS 1377-2:1990, Figure 10).

When planning an intrusive investigation, which is likely to encounter soils containing very coarse particles (and a matrix of finer soil), the following, which relate to the boulder and cobble fraction, should be taken into account and prioritized over the considerations for gravel and sand fractions (see 20.3) and silt and clay fractions (see 20.4 to 20.6).

  • a) Dry excavations (see 24.2 and 24.3) are the best method for examining this type of ground, although there are depth limitations. They enable the structure of the ground to be inspected, disturbed samples to be obtained and field tests for the determination of the in-situ density, undrained strength and deformation characteristics to be carried out on the matrix. However, it is not normally economic to extend excavations below the groundwater table where the ground is sufficiently permeable to allow inflows.
  • b) Boreholes can often be difficult to advance through the cobbles and more particularly the boulders.
    • 1) Dynamic sampling (see 24.6) is unlikely to get past very coarse particles.
    • 2) Cable percussion boring (see 24.8) using a shell and, where necessary, supporting the sides of the borehole with casing is more suitable than dynamic sampling because a chisel can be used to attempt to break up or push to one side particles that are too large to enter the shell. Where the matrix is fine soil it is often necessary to resort to alternate use of the chisel and claycutter. However, progress is slow and might not be successful in getting past the boulders. Disturbed samples recovered by the shell are Class 5 and give a very poor guide to the character of the ground, because the coarse particles are broken up by the chisel and much of the matrix is washed out.
    • 3) Resonance drilling (see 24.7) and rotary drilling (see 24.10) are usually faster and less likely to be terminated due to effective refusal.
    • 4) Rotary open hole drilling (see 24.11.2) would be expected to penetrate the very coarse particles. Simultaneous casing systems allow tube samples and standard penetration tests to be attempted as the hole is advanced.
    • 5) Rotary core drilling (see 24.11.3) can be successful when the clay matrix is stiff or very stiff. The core recovery and quality tends to increase with core size. In general, the core size should not be less than about 90 mm diameter (PWF size) and better results might be obtained with cores of 100 mm diameter or larger. Considerable care and expertise is required in the drilling process. The core samples give an indication of the structure of the ground, but the water content and the strength might be altered depending on the drill flush and drilling method.
  • c) Samples from boreholes are unlikely to be representative of the full range of particle sizes. The sampling methods described in 25.4 to 25.6 and 25.8 are generally not suitable for this type of ground. However, it might be possible to recover samples of the matrix where this is a fine soil using the 100 mm open-tube sampler, but the sampler frequently cannot be driven past the coarser particles. The quantity and grain size of the coarse and very coarse particles determines what class of sample can be recovered. Clays containing a small proportion of fine and medium gravel might be sampled with thin wall samplers and might yield Class 1 samples. Conventional 100 mm thick-walled open-tube samplers yield Class 2 samples at best. With increasing percentages of the coarse fraction, primarily gravel, the quality of recovered sample is likely to decrease markedly. In resonance drilling, the coarsest particles are likely to be missing from samples taken with the sonic barrel. However, both resonance and rotary drilling might give an indication of the prevalence and nature of very coarse particles and their dimension along the axis of the borehole.

As it is virtually impossible to recover undisturbed samples, field testing is likely to be required to obtain an indication of density, compressibility and permeability; there are various methods which should be evaluated for use (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 where the matrix is a coarse soil, or undrained shear strength (derived by correlation) where the matrix is a fine soil. However, the test gives unrealistically high results where the tool bears directly on a boulder or cobble. It is common practice to use the 60° cone in place of the cutting shoe because it is less susceptible to damage by the coarser particles.
  • ii) The static cone penetration test and dynamic probing can give useful results provided the density/strength of the matrix is not too high. However, both are likely to be terminated due to effective refusal on encountering a boulder or cobble.
  • iii) While such ground is not generally suitable for self-boring pressuremeters, test pockets can be formed such that down hole pressuremeter testing might be undertaken.
  • iv) Plate load tests might be used as a means of obtaining more accurate strength and stiffness parameters. Such tests can be performed in boreholes but more commonly in shallow excavations. A plate of sufficient diameter in relation to the largest particle size should be used. If boulders occur, the site beneath the test should be excavated to ensure that the results are not affected by very large particles.
  • v) The borehole permeability test might give a reasonable indication of permeability and the results can also be used to give a guide to the proportion of fine particles in the soil. A more reliable assessment of permeability is obtained from a pumping test.

NOTE The use of alternate tube samples and standard penetration tests is a frequently adopted expedient where the matrix is a fine soil.