34 Field procedures for description of principal inorganic soil type


Underlining is recommended in BS EN ISO 14688-1 to identify interlayered soils. This is not recommended for UK use but does not conflict with UK practice.

34.1 Choice of procedure

The simple tests described below should be used for the field assessment and naming of soils, in conjunction with Table 7. The procedures below should not replace laboratory testing, which is necessary to determine relevant properties as and when required; indeed, the results of the testing should be reported together with the original description. Operators should also check their results against laboratory tests to ensure that their judgement is sound.

34.2 Field assessment of grading

The distinction between coarse and fine soils should be made on the basis of whether they stick together when wet and remould; the water content might need to be adjusted to correctly assess this.

NOTE 1 The size of a particle is that of the square sieve aperture through which it would pass.

NOTE 2 The coarse silt/fine sand boundary (0,063 mm) can also be assessed by eye, the coarse silt particles only being visible with the aid of a hand lens, and by feel as the silt particles are not gritty.

NOTE 3 The boundary between gravels and sands is at 2 mm and so readily visible; the distinction between gravelly and sandy fine soils is also readily made. Particles of 2 mm size are approximately the largest that cling together when moist.

NOTE 4 The boundary between silts and clays is made using simple hand tests, see 34.3.

NOTE 5 In visually assessing the particle size distribution, an additional judgement is required in order to report the relative proportions by weight rather than by volume; a ratio of 10:7 might be appropriate.

34.3 Field assessment of fine soil

34.3.1 General


The principal constituents of fine soils are CLAY and SILT. To complete a correct field description of a fine soil it is necessary to distinguish between these two soil types. This can be achieved through a series of hand tests described below.

To begin the field assessment, a representative sample of the material for examination should be selected and all particles larger than medium sand removed. This soil should be moulded into a ball about 25 mm diameter until it is soft but not sticky, like putty; water should be added or the ball allowed to dry out as necessary in order to achieve the correct consistency.

NOTE Water might have to be added to the soil whilst carrying out the following hand tests. Generally, silt soils require more frequent addition of water than clay soils as silts tend to dry faster than clays.

34.3.2 Dilatancy test

  • Smooth the soil ball into a pat in the palm of one hand with the thumb, the blade of a knife or a small spatula.
  • Shake horizontally, striking the side of the hand vigorously against the other hand several times. Alternatively the pat can be manipulated between the fingers of both hands.
  • Squeeze the pat between thumb and fingers.
  • Record the speed with which water appears while shaking, and disappears while squeezing as none, slow or rapid.

34.3.3 Toughness test

  • Shape the test pat into an elongated pat and roll by hand on a smooth surface or between the palms into a thread about 3 mm in diameter. If the sample is too wet to roll easily, it should be allowed to dry. Fold the sample threads and reroll repeatedly until the thread crumbles at a diameter of about 3 mm when the soil is near the plastic limit. Record the pressure required to roll the thread. Also, record the strength of the thread. After the thread crumbles, the pieces ought to be lumped together and kneaded until the lump crumbles. Record the toughness of the material during this kneading.
  • Describe the toughness of the thread and lump as low, medium or high.

34.3.4 Plasticity test

On the basis of observations made during the toughness test, describe the plasticity of the material in accordance with the criteria in Table 23.

Table 23 Terms for description of plasticity
Description Criteria
Non plastic A 3 mm thread cannot be rolled at any water content.
Low The thread can barely be rolled and the lump cannot be formed when drier than the plastic limit.
Medium The thread is easy to roll and not much time is required to reach the plastic limit. The thread cannot be rerolled after reaching the plastic limit. The lump crumbles when drier than the plastic limit.
High It takes considerable time rolling and kneading to reach the plastic limit. The thread can be rerolled several times after reaching the plastic limit. The lump can be formed without crumbling when drier than the plastic limit.

34.3.5 Dry strength test

  • Remould the soil into several balls with a diameter of about 12 mm and allow them to dry naturally.
  • Test the strength of the dry balls or lumps by crushing between the fingers. Note the strength as none, low, medium, high or very high.
  • The presence of high-strength water-soluble cementing materials, such as calcium carbonate, might cause high dry strengths. The presence of calcium carbonate can be detected from the intensity of the reaction with dilute hydrochloric acid.

34.3.6 Feel test

  • Smear the soil between the fingers or with a blade and note the feel as smooth or silky and whether the surface polishes.

NOTE The fingertips are remarkably sensitive for this test.

34.3.7 Test for behaviour in air and water

  • Prepare a ball of soil and place it in a bucket or tub of clean water and note the speed with which the ball disintegrates.
  • Smear moist soil over a smooth surface (e.g. glass or plastic) or the back of the hand and note the rate of drying and whether the dry soil can be brushed off.

34.3.8 Cohesion test

  • Compress it the ball or pat between the fingers and note whether it ruptures or deforms.

34.3.9 Selection of principal soil type

The naming of the soil as CLAY or as SILT should be based on the results of these hand tests as in Table 24.

When the various hand tests give conflicting results, one of the hybrid terms "silty CLAY" or "clayey SILT" should be used.

Table 24 Decision on fine soil type from results of hand tests
Soil description CLAY Silty CLAY Clayey SILT SILT
Dilatancy None None to slow Slow Slow to rapid
Toughness High Medium Low to medium Low or thread cannot be formed
Plasticity High Medium Low Non-plastic
Dry strength High to very high Medium to high Low to medium None to low
Feel Smooth, sticky (when wet) Smooth Silky Silky, gritty
Behaviour in water Disintegrates slowly if at all Disintegrates slowly Disintegrates Disintegrates rapidly
Behaviour in air Dries slowly with shrinkage Dries slowly with shrinkage Dries quickly, brushes off Dries quickly, brushes off
Cohesion Deforms without rupture. Maintains shape and moisture during handling. Deforms without rupture. Maintains shape and moisture during handling. Moisture drains Slumps, moisture drains

35 Classification of soils


A full soil description (in accordance with Clause 33) gives detailed information on the colour, nature (plasticity and particle characteristics) of a soil, as well as on the state (consistency, strength, relative density) and structure (bedding, discontinuities) in which it occurs in a sample, borehole or exposure. Few, if any, soils have identical descriptions. For the purposes of engineering interpretation on a particular project it is often useful to classify the soils on the basis of geological origin of the strata, on some engineering property or properties of the strata, or on any of a large number of combinations of geological and engineering parameters. Such classifications are usually adopted to provide a framework for description and assessment of the ground conditions at a site or series of sites for the particular engineering problem in hand.

Probably the most common type of classification, however, places a soil in a limited number of groups with shorthand identifiers on the basis of grading and plasticity of disturbed samples. These characteristics are independent of the particular condition in which a soil occurs, and disregard the influence of the structure, including fabric, of the soil. For this and other reasons, such a classification might appear to differ from the field description of a soil determined in accordance with Clause 33, although it can be useful for those soils to be used as construction materials and is widely used in Europe (see DIN 18196). The more general classification scheme for this purpose, which was devised by the Department of Transport (see Manual of contract documents for highway works [14]), is widely used in the UK. It is recommended that the field descriptions stand as a record of the undisturbed character of the soil. The classification can provide additional useful information as to how the disturbed soil behaves when used as a construction material under various conditions of water content.

A soil should be classified on the basis of the geological origin or engineering characteristics. This classification is usually based on the results of testing, and so does not appear on borehole or field logs but can be added as an appropriate coding for data transfer.

NOTE The classification presented in BS EN ISO 14688-2, Annex B is not preferred in UK practice as it takes no account of plasticity or water content.