20.5 Soft and firm clay

COMMENTARY ON 20.5

Soft and firm clays are amenable to investigation by excavation and by borehole. Undisturbed tube samples can be recovered which allow laboratory testing for strength, compressibility and permeability.

When planning an intrusive investigation where soft and firm clays are likely to be encountered, the following should be taken into account.

  • a) Excavations (see 24.2 and 24.3) might require support in lower strength clays. Samples obtained from excavating equipment or by hand are likely to be Class 3 or, if water is present, Class 4. Better quality samples might be obtained pushing sample tubes.
  • b) Boreholes of most types can be used although considerations vary to some extent depending on the drilling method.
    • 1) Dynamic sampling (see 24.6) generally has to resort to successive reduction in diameter because the borehole walls squeeze in the lower strength clays.
    • 2) Cable percussion boreholes (see 24.8) generally need to be cased. Samples obtained from the tools used to advance the borehole are likely to be Class 3 or, if water is present, Class 4.
    • 3) Resonance drilling (24.7) is not subject to borehole squeezing as casing is an integral part of the method.
    • 4) Rotary drilling (24.10) is viable but less commonly used than cable percussion boring and dynamic sampling. Considerable care should be taken to avoid disturbance by the drilling fluid. Samples obtained by using a thin-walled piston or open-tube sampler (such as the UT100) can be Class 1. Samples taken with a thick wall open-tube (OS-TK/W) sampler (such as the U100) suffer disturbance, the degree of disturbance increasing with the sensitivity of the clay to remoulding, and with the depth at which the sample is taken; such samples could be Class 2 or 3. The quality classes obtained by dynamic sampling, resonance drilling and rotary core drilling are discussed in 25.8, 25.9 and 25.7 respectively.
  • c) Clays that exhibit permeable fabric, e.g. sand partings or laminae, might require a balancing water head in the borehole as for sands.

NOTE There might be a significant amount of disturbance in laminated clays due to the drilling process. In addition, samples of laminated clays soften through migration of porewater from the more permeable laminations into the finer laminations. Piston samplers, including samplers up to 250 mm diameter, can be used to obtain in these clays provided the consistency is not too high.

Field testing (see Section 7) might be used in addition to 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:

  • i) The static cone penetration test might be used to provide continuous in-situ shear strength profiles.
  • ii) The in-situ vane test might be used to measure the undrained shear strength.
  • iii) Other types of testing, e.g. pressuremeter and plate tests, might also be used to measure strength and stiffness.

20.6 Stiff and very stiff clays

COMMENTARY ON 20.6

Stiff and very stiff clays are amenable to investigation by excavation and by borehole in the same way as soft and firm clays. Undisturbed tube samples can be recovered which allow laboratory testing for strength, compressibility and permeability.

When planning an intrusive investigation where stiff and very stiff clays are likely to be encountered, the following should be taken into account.

  • a) Excavations (see 24.2 and 24.3) are generally expected to stand unsupported in the short term, although this does not obviate the need for precautions if there is to be person entry. Samples obtained from excavating equipment or by hand are likely to be Class 3 or, if water is present, Class 4.
  • b) Boreholes of most types are suitable:
    • 1) Dynamic sampling (see 24.6) and cable percussion method (see 24.8) require less diameter reduction or casing than in clays of soft and firm consistencies (see 20.5).
    • 2) Resonance drilling (see 24.7) can be used.
    • 3) Rotary core drilling (see 24.11.3) has been used in these clays with considerable success although there is the potential for causing a change of water content.
    • 4) Mechanical augering (see 24.9) using continuous flight augers, with solid or hollow stems would be viable, although this can lead to difficulties if granular horizons are also encountered.
  • c) Samples obtained by using a thin-wall sampler (OS-T/W) can produce class 1 samples in clay of up to very stiff consistency. Ideally the sampler should be pushed statically into the ground, although this might limit the length that can be recovered. Samples of clays in the stiff consistency range taken with a 100 mm thick-walled open-tube sampler (OS-TK/W) usually suffer significant material disturbance and do not produce samples better than Class 2. With very stiff clays there is even greater disturbance and the samples can be no better than Class 3. The quality classes obtained by dynamic sampling and resonance drilling are discussed in 25.8 and 25.9 respectively. Rotary core drilling can produce Class 1 samples although sometimes only a lesser quality is achieved (see 25.7).

Field testing (see Section 7) might be used in addition laboratory testing on samples and the various methods should be evaluated, particularly where design considerations warrant greater accuracy.

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

  • i) Plate tests in large diameter boreholes can be used in the determination of undrained strength and deformation characteristics, although they can give erroneous results below the water table.
  • ii) Self-boring Pressuremeters or downhole pressuremeters can also be used to obtain in-situ values for shear strength and deformation parameters.
  • iii) Standard penetration tests give a useful indication of shear strength but this method does not provide a direct measurement; published correlations can be used to derive shear strength values and other ground parameters.

20.7 Peat

COMMENTARY ON 20.7

Peat is a material of low strength and very high compressibility. In many investigations establishing the thickness of the peat is all that is required because the engineering solution is to excavate and replace or to found below it.

When planning an intrusive investigation which is likely to encounter peat, the following should be taken into account.

  • a) Excavations (see 24.2 and 24.3) can be problematic, both because vertical faces can be unstable and because where the peat occurs as the surficial layer it might not have sufficient strength to support typical excavating plant
  • b) Boreholes are likely to require the use of plant which exerts very low bearing pressures, methods which use lighter weight equipment such as dynamic sampling (see 24.6) are advantageous in this regard.
  • c) It is very difficult to recover undisturbed samples; piston samples (see 25.5) and block samples taken with a large samplers (see 25.10.2) are most likely to achieve quality Class 1.

Field testing (see Section 7) might be used and the various methods should be evaluated for inclusion in the intrusive investigation as follows:

  • 1) Dynamic probing, either hand-operated or mechanical, might be used to determine the thickness of peat, although these techniques generally do not provide any quantitative information on the shear strength.
  • 2) The static cone penetration test might be used to provide continuous in-situ shear strength profiles.

NOTE Various alternative to the "standard" cone such as the T-bar and piezoball have been developed specifically for cone penetrometer testers (CPT) in peat.

20.8 Anthropogenic ground

The presence and approximate extent of fill on a site from previous human activity such as engineering is normally identifiable from the desk study because it results from some previous engineering works. It is also usually possible to obtain an indication of the nature of the selected materials used; the relevant considerations for the type of material (see 20.2 to 20.7) should be taken into account when planning the investigation.

Made ground is placed with little or no control; the ground investigation should assess the variation in character and quality across the site, which is often random. When planning an intrusive investigation likely to encounter made ground, the following should be taken into account.

  • a) Pits and trenches are particularly useful for investigating the nature and variability of made ground (see 25.2).
  • b) Conventional methods of boring, sampling and in-situ testing, as appropriate to the character of the ground, should be used to obtain information on the thickness and properties and, to some extent, types of made ground at the particular locations of the boreholes or in-situ test. Boreholes should always be fully cased through uncontrolled made ground to avoid contamination of the natural groundwater and underlying strata.
  • c) In combustible fills, temperature measurements should be carried out as a routine part of the investigation.

NOTE 1 Certain types of made ground do not lend themselves to any type of conventional sampling or in-situ testing. The investigation might have to rely on visual detailed descriptions of the material as the main way of assessing physical properties.

NOTE 2 On waste tips, burning materials below ground might give rise to toxic or flammable gas from the borehole. Tip fires can also create voids, which might collapse under the weight of an investigation rig. Lagoons within waste tips might be areas of very soft ground.

BS 5930:2015 Code of practice for ground investigations