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Clay cutter Fig. 14.2 Clay cutter and sample tube.

100 mm sample tube of dropping the cutter from some 1.5 to 3 m above the soil and is largely carried out by hand, although this practice is going out of fashion and when site conditions are suitable the operation is often powered. In compact sands and gravels water is generally added if the deposit is not already wet. The material is removed by means of a shell which is dropped in a similar manner to the clay cutter: it is fitted with a clack (a hinged lid) that closes as the shell is withdrawn and retains the loose particles. In extremely hard granular deposits a chisel is sometimes necessary to achieve break up of the soil.

Boreholes in sands and gravels, and most deep boreholes in clay, must be lined with steel tubes to prevent collapse of the sides, the casings (of slightly larger diameter than the cutter) being hammered or surged downwards as boring proceeds.

In order to prove bedrock a minimum penetration of 1.5 to 3 m is generally required. Penetration into soft rocks is sometimes possible with the chisel but for hard rocks a diamond drill becomes necessary, particularly if rock cores are to be obtained.

Setting out of trial pits and I or boreholes

An accurate setting out by theodolite is not necessary, since lining in with structures marked on an Ordnance Survey map or using a compass survey will give all the accuracy needed. For heavy structures, boreholes require to be some 15 to 30 m apart and should be taken down to about 1.5 times the width of the structure unless rock is encountered at lower depths. For roads the boreholes need not be closer than 300 m centres unless vegetation changes indicate variations in soil conditions, and need not go beyond 3 m below formation level.

Guidance on site investigation in the form of a handbook for engineers, is given by Clayton et al. (1995).

14.3.2 Sampling

Two types of soil sample can be obtained: disturbed sample and undisturbed sample.

Disturbed samples

The auger parings or the contents of the shell can be collected as disturbed soil samples. Such soil has been remoulded and is of no use for shear strength tests but is useful for identification tests wl and wp, particle size distribution, etc.).

Disturbed samples are usually collected in airtight tins or jars or in plastic sampling bags, and are labelled to give the borehole number, the depth, and a description.

Undisturbed samples (cohesive soil)

In a trial pit samples can be cut out by hand if care is taken. Such a sample must be placed in an air tight container and as a further precaution should first be given at least two coats of paraffin wax.

The hand auger can be used to obtain useful samples for unconfined compression tests and employs 38 mm sampling tubes with a length of 200 mm (Fig. 14.1). The auger is first removed from the rods and the tube fitted in its place, after which the tube is driven into the soil at the bottom of the borehole, given a half turn, and withdrawn. Finally, the ends of the tube are sealed with paraffin wax.

With the boring rig, 100 mm diameter undisturbed samples are collected, the sampling tube being 105 mm diameter and usually 381mm long (Fig. 14.2) but dimensions can vary, e.g. tubes 106 mm diameter and 457 mm long are also used. The tube is first fitted with a special cutting shoe and then driven into the ground by a falling weight in a similar manner to the standard penetration test; during driving any entrapped water, air or slush can escape through a non-return valve fitted in the driving head at the top of the tube. After collection the sample is sealed at both ends with paraffin wax and, as a further precaution, sealing caps are screwed on to the tube.

For soils such as soft clays and silts that are sensitive to disturbance a thin-walled sample tube can be used. Because of the softness of the soil to be collected the tube is simply machined at its end to form a cutting edge and does not have a separate cutting shoe. The thin-walled sampler is similar in appearance to the sample tube shown in Fig. 14.1 but can have an internal diameter of up to about 200 mm.

These sampling techniques involve the removal of the boring rods from the hole, the replacement of the cutting edge with the sampler, the reinsertion of the rods, the collection of the sample, the removal of the rods, the replacement of the sampler with the cutting edge and, finally, the reinsertion of the rods so that boring may proceed. This is a most time-consuming operation and for deep bores, such as occur in site investigations for off-shore oil rigs, techniques have been developed to enable samplers to be inserted down through the drill rods so that soil samples can be collected much more quickly.

Degree of sample disturbance

No matter how careful the technique employed there will inevitably be some disturbance of the soil during its collection as an 'undisturbed' sample, the least disturbance occurring in samples cut from the floor or sides of a trial pit. With sample tubes, jacking is preferable to hammering although if the blows are applied in a regular pattern there is little difference between the two.

The degree of disturbance has been related (Hvorslev, 1949) to the area ratio of the sample tube:

where De and D; are the external and internal diameters of the tube respectively. It is generally agreed that, for good undisturbed 100 mm diameter samples, the area ratio should not exceed 25 per cent, but in fact most cutting heads have area ratios = 28 per cent. For 38 mm samples the area ratio should not exceed 20 per cent. Thin-walled sample tubes, of any diameter, have an area ratio of about 10 per cent.

Undisturbed samples (sands)

If care is taken it may be possible to extract a sand sample by cutting from the bottom or sides of a trial pit. In a borehole, above ground water level, sand is damp and there is enough temporary cohesion to allow samples to be collected in sampling tubes, but below ground water level tube sampling is not possible. Various techniques employing chemicals or temporarily freezing the ground water have been tried, but they are expensive and not very satisfactory; the use of compressed air in conjunction with the sampler evolved by Bishop (1948), however, enables a reasonably undisturbed sample to be obtained.

Owing to the fact that sand is easily disturbed during transportation any tests on the soil in the undisturbed state should be carried out on the site, the usual practice being to use the results of penetration tests instead of sampling.

Frequency of sampling

Samples, both disturbed and undisturbed, should be taken at every change of stratum and at least at every 1.5 m in apparently homogeneous material.

Continuous sampling

In some cases, particularly where the soil consists of layers of clay, separated by thin bands of sand and silt and even peat, it may be necessary to obtain a continuous core of the soil deposits for closer examination in the laboratory. Such sampling techniques are highly specialised and require the elimination of friction between the soil sample and the walls of the sampler. A sampler which reduces side friction by the use of thin strips of metal foil placed between the soil and the tube was developed by Kjellman et al. (1950) and is capable of collecting a core 68 mm in diameter and up to 25 m in length.

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