33.3 Mass characteristics of soils
33.3.1 Range of application
The mass characteristics of soils should also be given. Reference should be made to Table 7.
33.3.2 Scale of consistency, strength and relative density
The consistency of clays and silts should be assessed in the field, as given in Table 7 and as follows in Table 8.
| Field description term | Consistency description definition [after BS EN ISO 14688-1:2002, 5.14] |
| Very soft | Finger easily pushed in up to 25 mm |
| Soft | Exudes between fingers Finger pushed in up to 10 mm |
| Firm | Moulded by light finger pressure Thumb makes impression easily |
| Stiff | Cannot be moulded by fingers, rolls in the hand to a 3 mm thick thread without breaking or crumbling Can be indented slightly by thumb |
| Very stiff | Crumbles in rolling a 3 mm thick thread, but can then be remoulded into a lump Can be indented by thumb nail Cannot be moulded but crumbles under pressure |
NOTE 1 These subdivisions can be approximate, particularly in material of low plasticity. NOTE 2 The description of materials of higher consistency than very stiff should use rock strengths and material names. NOTE 3 Soils varying around the boundary between soil (very stiff consistency) and rock (extremely weak strength) can be described as, for example, "very stiff CLAY/extremely weak MUDSTONE". A boundary on the log at rockhead if it is a simple transition is preferable. |
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The description of consistency in silt should be made of a sample that is representative of the in-situ condition. The consistency terms given in Table 7 and Table 8 should be used.
NOTE 1 Silts are conventionally described as fine soil, but depending on their grading, can behave as coarse soil. In these circumstances use of relative density terms rather than consistency might be appropriate.
Where the sample is disturbed but some guidance as to the in-situ consistency can be sensibly given, the consistency term should be given in brackets, e.g. (Probably firm) brown CLAY. No consistency term should be given when this would be unreliable.
NOTE 2 Direct measurements of the undrained shear strength (e.g. field or hand vane, laboratory unconsolidated undrained triaxial test or laboratory vane) provide a classification of the undrained strength using the scale given in Table 9. This may be used in discussion of the material strength in the report text but does not form part of the soil description required for exploratory hole or exposure logs.
| Term based on measurement | Undrained strength classification definition, su, in kPa [from BS EN ISO 14688-2:2004, 5.3, Table 5] |
| Extremely low strength Very low strength Low strength Medium strength High strength Very high strength |
< 10 10 – 20 20 – 40 40 – 75 75 – 150 150 – 300 |
Strength determination for a stratum should be given using a number of individual determinations by specified means, i.e. triaxial, laboratory vane and hand vane tests (see BS EN 1997-2:2007, Annex M, P and I).
NOTE 3 The soil/rock boundary is set at an undrained shear strength of 300 kPa (an unconfined compressive strength of 0,6 MPa) (see BS EN ISO 14688-2 and BS EN ISO 14689-1). The field assessment of this boundary is not easy, being beyond the range of hand vanes, but such fine soils, in their saturated condition, break in a brittle manner and can only be scratched with a thumbnail.
If the soil includes fissures or other discontinuities the field consistency description should apply to the intact material between discontinuities. If a (lower) mass consistency which includes the effect of discontinuities can also be assessed in the field, this should be separately and clearly reported. Further information on the discontinuities should be given at the end of the main description, after a full stop for clarity.
NOTE 4 The consistency and discontinuity terms are linked in the word order, giving, for example, "stiff fissured", "firm sheared".
If a mineral cement appears to be present the nature and degree of cementing should be noted, e.g. "slightly iron oxide cemented sand" or preferably using rock strength terms, e.g. "very weak carbonate cemented SANDSTONE".
NOTE 5 It is also useful to note whether slaking occurs on immersing the air dry material in water.
Laboratory or field test results should also be used to provide a strength classification; however, the strength measured might not be representative of the stratum as a whole due to the effects of sampling disturbance, in-situ test disturbance and scale effects; these can lead to significant underestimation of field strength. Discrepancies between results of various tests used in strength measurements should be reported bearing in mind that the different tests involve different sample sizes, stress paths, strain rates and stress orientations. Strength classification terms should not be given on borehole logs.
NOTE 6 Laboratory tests on fissured materials usually give lower results than the intact material strength.
NOTE 7 The use of empirical relationships between standard penetration test "N" values and strength of fine soils might also be useful (see articles on the standard penetration test by Stroud and Butler, 1975 [43] and Stroud, 1989 [44]).
The relative density of sands and gravels should be determined in boreholes by N-values obtained from the standard penetration test and a classification in terms of N-values as shown in Table 10.
| Term | Classification based on uncorrected SPT N-values for use of borehole logs |
| Very loose | 0 — 4 |
| Loose | 4 — 10 |
| Medium dense | 10 — 30 |
| Dense | 30 — 50 |
| Very dense | > 50 |
The numerical SPT N-values on the log should be uncorrected (see CIRIA R143 [45]) and the relative density classification term should be applied in accordance with Table 10. Particular care should be taken in applying these classification terms in coarse gravels; they should not be used for very coarse soils.
The relative density of coarse soils in observation pits and trenches may be assessed using the field tests given in Table 11. If the density is assessed in this manner, the results should be included together with the test method on the field log. The application of a relative density term should be provided as extra information after the main description, not as the first sentence of the description. For example, the material observed in an exposure could be described as:
- Grey silty SAND. Assessed as loose as fairly easy to excavate with a spade; or
- Brown very sandy GRAVEL. Assessed as medium dense using a geological pick.
The descriptive terms obtained in this way should not be confused with those obtained from the standard penetration test (SPT) in boreholes, but the source of the descriptor is obvious in practice. These field tests should not be used on borehole samples.
| Density term | Excavation by spade or pick A) | Penetration of light horizontal blows of geological pick | Penetration of geological pick by pushing |
| Very loose | Very easy to excavate with a spade | 100 mm (full depth) | 75 mm — 100 mm (full length) |
| Loose | Fairly easy to excavate with a spade or to penetrate with a crowbar | 50 mm — 100 mm | 25 mm — 75 mm |
| Medium dense | Difficult to excavate with a spade or to penetrate with a crowbar | 25 mm — 50 mm | 10 mm — 25 mm |
| Dense | Very difficult to penetrate with a crowbar. Requires a pick for excavation | 5 mm — 25 mm | 2 mm — 10 mm |
| Very dense | Difficult to excavate with a pick | <5 mm | <2 mm |
| A) See BRE,1993. | |||
33.3.3 Discontinuities
Discontinuities should be described in accordance with BS EN ISO 14688-1 and BS EN ISO 14689-1.
NOTE Discontinuity types include fissures and shear planes.
The spacing of discontinuities should be described using the spacing scale given in Table 7. Their surface texture, e.g. rough, smooth, polished, striated, should be described (see Table 7 and BS EN ISO 14689-1) as should any colour changes or staining on discontinuities and any infilling; infilling can be present, e.g. sand in vertical dessication cracks within clay. Where possible in exposure, the orientation or trend of discontinuities should be given by stating direction of dip and angle of dip (e.g. 180º / 40°); their persistence and openness should also be stated.
33.3.4 Bedding
Bedding should be described in accordance with BS EN ISO 14688-1. Layer boundaries are not always horizontal and can include irregular features that should be recorded, e.g. perigalcial structures such as ice wedges.
The thickness of bedding units should be described using the terms in Table 7; in a homogeneous soil this is marked by bedding planes or, possibly, colour changes, and not necessarily discontinuities.
NOTE Interstratified deposits are those in which there are layers of different types of material, which might be of constant thickness, or might thin out locally or occur as lenses.
If beds of alternating or different soil types are too thin to be described as individual strata, the soil should be described as interbedded or interlaminated, using the terms in Table 7, as appropriate. Where the soil types are approximately equal, "thinly interlaminated SAND and CLAY" would, for example, be appropriate. Where one material is dominant, the subordinate material should be described with a bed thickness and a bed spacing (using the bedding and discontinuity spacing terms respectively), e.g. "SAND with closely spaced thick laminae of clay". Where two or more soils types are present in a deposit, arranged in an irregular manner, the soils should be described as mixed, e.g. "SAND with gravel size pockets (20 mm — 35 mm) of CLAY".
The spacing of sedimentary features, such as shell bands, and of minor structures, such as root holes in soils, should also be reported as measurements or using the spacing terms for discontinuities. There are descriptive terms that have no size connotation (e.g. pocket, lens, inclusion); where such terms are used their size, spacing and frequency should be defined and reported.
Any special bedding characteristics, e.g. cross-bedding, graded bedding, should be described besides disturbed bedding structures, including slump bedding or convoluted bedding.
