Section 7: Field tests

37 General


See Section 3 for detailed recommendations on the planning of ground investigations, where it is recommended that all ground investigations consist of both field and laboratory investigations.

Utilities should be identified before any intrusive work commences (see 19.2.4).

Appropriate field tests should be selected, taking into account the information and parameters that are required for the geotechnical design. This section covers both in-situ field tests used as part of the ground investigation process and also large scale field tests or trials targeted at specific specialist requirements. For the former application, Table 33 (based on Lunne et al., 1997 [74]) and BS EN 1997-2:2007, Table 2.1 should be used along with any other available tables to aid the selection of tests based on their applicability, given the ground conditions likely to be encountered and the parameters to be established.

Field tests should be used at all stages of ground investigations (for example, to assist in the siting of boreholes, trial pits, etc.).

The larger scale field tests should be used where the mass characteristics of the ground might differ appreciably from the material characteristics determined by laboratory testing (but they can also be used to validate design procedures).

NOTE 1 These differences normally arise from several factors, the most important of which are the extent to which the laboratory samples are representative of the mass and the quality of sample that can be obtained for laboratory testing. Factors affecting sample quality are considered in Clause 25 and attention is drawn to factors affecting the representative nature of a laboratory sample. These factors are partly related to the in-situ conditions of stress, pore pressure and degree of saturation, and can be altered from an unknown in-situ state by the sampling processes. Consequently, their influence cannot be accounted for in laboratory testing.

The material tested in situ by a field test is analogous to a laboratory sample, and should be deemed to be an "in-situ sample".

NOTE 2 The in-situ conditions of the sample can be affected by the process of gaining access to the position, e.g. drilling a borehole, digging a trial pit or pushing a probe, but for large scale tests the effect is usually very much less than for a laboratory sample and even for smaller scale tests, e.g. probing tests, any disturbance is likely to be localized and repeatable.

The controlling effects of the nature, orientation, persistence and spacing of discontinuities (see Anon, 1972 [75]), the nature of any filling and the size of sample required for it to be representative should all be taken into account. The selection and preparation of samples in the field should be subjected to the same requirements as for laboratory samples to ensure that they are representative. Considerable attention should be given in the field to these aspects, because normally fewer large scale field tests can be carried out than laboratory tests and it might not be possible to visually examine the in-situ sample.

NOTE 3 The scale of sample to be tested in a field test depends on the nature of the ground and type of test, and might vary from a fraction of a metre, such as in-situ triaxial state or stress measurements, to several metres for field trials, to one or two kilometres in a pumping test.

Table 33 The applicability and usefulness of in-situ tests
Group Device Soil parameters Ground type
Soil type Pro- file u φ’A) su ID Mv cv k Go σh OCR σ–ε Hard rock Soft rock Gravel Sand Silt Clay Peat
Pen- etrom- eters Dynamic probing C B C C C C C C B A B B B
Mechanical (CPTM) B A/B C C B C C C C C C A A A A
Electric (CPT) B A C B A/B C B B/C B C C A A A A
Piezocone (CPTU) A A A B B B B A/B B B B/C B C C C A A A A
Seismic (SCPT/ SCPTU) A A A B A/B A/B B A/B B A B B B C C A A A A
Flat dilato- meter (DMT) B A C B B C B B B B C C C A A A A
Standard pene- tration test (SPT) A B C C B C C C B A B A B
Resistivity probe B B B C A C C A A A A
Pres- surem- eters Pre-bored (PBP) B B C B C B C B C C C A A B B B A B
Self boring (SBP) B B AB) B B B B AB) B AC) A/B B A/ BC) B B B A B
Full displace- ment (FDP) B B C B C C C AC) C C C C B B A A
Others Vane B C A B/C B A B
Plate load C C B B B C C A C B B B A B B A A A
Screw plate C C C B B B C C A C B C A A A A
Borehole permeability C A B A A A A A A A B
Hydraulic fracture B C C B B B C A C
Crosshole/ downhole/ surface seismic C C A B A A A A A A A
Applicability: A = high / B = moderate / C = low / —  = none
Soil parameter definitions: u = in situ static pore pressure / φ' = effective internal friction angle / su = undrained shear strength / ID = density index / Mv = constrained modulus / cv = coefficient of consolidation / k = coefficient of permeability / Go = shear modulus at small strains / σh = horizontal stress / OCR = overconsolidation ratio / σ-ε = stress-strain relationship
A) Depends on soil type. / B) Only when pore pressure sensor fitted. / C) Only when displacement sensor fitted.

Field/in-situ tests should be used as part of all investigations but are particularly valuable where the preparation of representative laboratory samples is complicated by one or more of the following conditions.

  • a) The spacing of the bedding, fabric and discontinuity features in the mass is such that a sample representing the mass would be too large for laboratory test equipment. The discontinuities are assumed to govern the geomechanical response of the material on the scale of the structure concerned.
  • b) It is difficult to obtain samples of adequate quality because of the lack of cohesion or irreversible changes in mechanical properties; these result from changes in the pore pressure, the degree of saturation and stress environments during sampling, and from physical disturbance resulting from the sampling procedure.
  • c) It is difficult to determine the in-situ conditions, such as pore water pressure, degree of saturation and stress environments for reproduction in the laboratory testing.
  • d) Sample disturbance due to delays and transportation from remote sites is excessive.
  • e) The zone of interest might be inaccessible to sampling equipment.

The larger scale field tests are expensive and should not be undertaken before obtaining a comprehensive understanding of the geology and nature of the ground.

NOTE 4 Standards are now generally available for the more common in-situ ground investigation tests but the more specialist and large scale tests are generally designed to take account of the nature of the works and the character of the ground based on the findings of the initial ground investigation. Careful observation is required in all field tests. Continuous recording equipment can be used to improve the precision, e.g. to note small changes during the test, and can increase the quality of the data obtained.

BS 5930:2015 Code of practice for ground investigations