Soil sampling - Foundations.

There are two main types of soil sample which can be recovered from boreholes or trial pits

(a) Disturbed samples, as their name implies, are samples taken from the boring tools examples are auger parings, the contents of the split-spoon sampler in the standard penetration test, sludges from the shell or wash-water return, or hand samples dug from trial pits

(b) Undisturbed samples, obtained by pushing or driving a thin-walled tube into the soil, represent as closely as is practicable the in-situ structure and water content of the soil. It is important not to over-drive the sampler as this compresses the contents

It should be recognized that no sample taken by driving a tube into the soil can be truly undisturbed

Disturbance and the consequent changes in soil properties can be minimized by careful attention to maintaining a water balance in the borehole That is, the head of water in the borehole must be maintained, while sampling, at a level corresponding to the piezometric pressure of the pore water in the soil at the level of sampling This may involve extending the borehole casing above ground level or using bentonite slurry instead of water to balance high piezometric pressures

The care in sampling procedure and the elaborateness of the equipment depends on the class of work which is being undertaken, and the importance of accurate results on the design of the works.

BS 5930 recommends five quality classes for soil sampling following a system developed in Germany

The classification system, the soil properties which can be determined reliably from each class, and the appropriate sampling methods are shown in Table 11

In cohesive soils sensitive to disturbance, quality classes 1 and 2 require a good design of sampler such as a piston or thin-walled sampler which is jacked or pulled down into the soil and not driven down by blows of a hammer Class 1 and 2 sampling in soils insensitive to disturbance employs open-drive tube samplers which are hammered into the soils by blows of a sliding hammer or careful hand-cut samples taken from trial pits There is a great difference in cost between piston and open sampling, but the engineer should recognize the value of good quality if this can result in economies in design, for example, good-quality sampling means higher indicated shear strengths, with higher bearing pressures and consequently reduced foundation costs

In certain projects good sampling may mean the difference between a certain construction operation being judged possible or impossible, for example the placing of an embankment on very soft soil for a bridge approach If shear strength as indicated by poor-quality sampling is low, then the engineer may decide it is impossible to use an embanked approach and will have to employ an expensive piled viaduct On the other hand in 'insensitive' clays such as stiff glacial till the sampling procedure has not much effect on shear strength and thick-wall open samplers may give quite adequate information Also, elaborate samplers such as the fixed piston types may be incapable of operation in clays containing appreciable amounts of large gravel

Table 1.1 Quality classification for soil sampling

The presence of discontinuities in the form of pockets or layers of sand and silt, laminations, fissures, and root holes in cohesive soils is of significance to their permeability, which in turn affects their rate of consolidation under foundation loading, and the stability of slopes of foundation excavations The use of large-diameter sample tubes may be justified to assess the significance of such discontinuities or 'fabnc' to the particular foundation problem

The engineer should study the foundation problem and decide what degree of elaborateness in sampling is economically justifiable, and he should keep in mind that in-situ tests such as the vane or cone tests may give more reliable information than laboratory tests on undisturbed samples If in-situ tests are adopted, elaborateness an undisturbed sampling is unnecessary and the 'simple' class is sufficient to give a check on identification of soil types

A good practice, recommended by Rowei2 is to adopt continuous sampling in the first boreholes drilled on a site An open-drive sampler with an internal split sleeve is used to enable the samples to be split longitudinally for examination of the soil fabric The critical soil layers can be identified and the appropriate class of sampling or zn-situ testing adopted

BS 5930 gives details and dimensions of five types of soil samplers for use in boreholes

These are Thin-walled samplers

General-purpose 100mm diameter open-tube sampler

Split-barrel standard penetration test sampler

Thin-walled stationary piston sampler

Continuous sampler

Thin-walled samplers which are pushed rather than hammered into the soil cause the minimum of moisture content changes and disturbance to the fabric of the soil Sample diameters are generally 75—100 mm, but tubes up to 250 mm can be provided for special purposes The thin-wall sampler is suitable for use in very soft to soft clays and silts

One type of thin-walled sampler, not described in BS 5930, is the Laval sampler developed in Canada for sampling soft clays ' It has been shown to provide samples of a quality equal to those obtained by conventional hand-cut block sampling The tube is hydraulic- ally pushed into a mud-supported borehole to recover samples 200 mm in diameter and 300 mm long The tube is over-cored before withdrawal

The general-purpose 100 mm diameter open-tube sampler was developed in the UK as a suitable device for sampling the very stiff to hard clays, gravelly glacial till, and weak weathered rocks such as chalk and marl

In this respect the detachable cutting shoe is advantageous It can be discarded or reconditioned enabling many reuses of the equipment However, the relatively thick-walled tube and cutting shoe do cause some disturbance of the fabric of the soil and moisture content changes within the sample The equipment is suitable for geotechmcal category 2 investigations

The split-barrel standard penetration test sampler is used to make the in-situ soil test The tube has an internal diameter of 35 mm and recovers a disturbed sample suitable for classes 3 and 4 in Table 11 Some indication of layering or laminations can be seen when the sampler is taken apart

Thin-walled stationary piston samplers are suitable for quality class 1 m Table 11 and for geotechnical category 3 investigations Diameters range from 75 to 100 mm with special types up to 250 mm They recover good samples of very soft to soft clays and silts, and sandy soils can sometimes be recovered Special thin- wall piston samples are used in stiff clays

The Delft continuous sampler is an example of this type It is made in 29 and 66 mm diameters with a penetration generally up to 18 m, but samples up to 30 m can be recovered in favourable soil conditions

It is designed to be pushed into the ground using the 200 kN thrust of the standard cone penetration test sounding machine (see below)

The samples from the 66 mm tubes are retained in plastic liners which can be split longitudinally to examine the stratification and fabric of the soil

Figure 1.2 Lack of information on shear strength at foundation level due to adoption of uniform sampling depths in all boreholes