25.8 Dynamic samplers

25.8.1 Window sampling

COMMENTARY ON 25.8.1

The procedures for window sampling are described in 24.7. BS EN ISO 22475-1 includes window sampling in the sampling by using sampler category whereas Baldwin and Gosling, 2009 [34] contend that the method falls into the sampling by drilling category. The distinction might be academic but the quality class of the sample is dependent on the downhole equipment, i.e. the hollow steel tube which both advances the hole and recovers the sample.

The sampling tubes are available in lengths of generally 1 m, 2 m or 3 m and a series of some five or six nominal diameters, these being between about 45 mm and 95 mm. The wall thickness of the cutting shoe is typically about 5 mm giving area ratios of about 25% for the largest diameter increasing to about 65% for the smallest.

The soil recovered in the window sampler tube(s) can only be partly viewed through the "window" itself. In order to properly describe the recovered material, the contents of the tube should be placed into a length of plastic channel. This allows both description to be made and, where required, sampling into small disturbed or bulk disturbed containers. However, a full soil profile can only be recovered under favourable conditions and, in practice, zones of material loss occur. In all cases, the material recovered is fully disturbed and this should be taken into account when considering laboratory testing.

NOTE BS EN ISO 22475-1 assigns quality class 5 to window sampling; Baldwin and Gosling, 2009 [34] contend that that this is unduly pessimistic and that the quality class is normally 4 or, in favourable circumstances, 3. This is significant in allowing laboratory specimens to be recovered from the samples, provided these are taken immediately after recovery.

25.8.2 Windowless sampling

COMMENTARY ON 25.8.2

The procedures for windowless sampling are described in 24.7. Dynamic sampling with the windowless equipment does not appear explicitly in BS EN ISO 22475-1:2006 unless the term "window" in Table 3 of that standard is taken as generic for dynamic sampling. If this is the case, then windowless sampling would be categorized as sampling by use of sampler as suggested by Baldwin and Gosling, 2009 [34]. In effect the sampling hole is advanced by taking a series of consecutive tube samples.

It is debateable whether the sampler tubes equate to the tube samplers discussed in 25.4. Windowless sampling tubes are generally 1 m long and are available in a series of five or six nominal diameters, these being between about 45 mm and 100 mm. Due to the plastic liner inserted in the steel sampler tube house, wall thickness of the cutting shoes are about 10 mm, giving area ratios of about 50% for the largest diameter increasing to about 115% for the smallest plastic liner.

The soil recovered should be retained in the sampler's liner tube(s) until it has been examined and described. At the point of examination, a full soil profile, i.e. approaching 100% recovery, might be available for viewing. Despite the full profile, the material in the tube should be deemed to be relatively disturbed due to the sampler geometry. The samples should be preserved for longer term temporary retention, either in the liners or as bag samples following examination, and a record made of the logger's description.

NOTE Irrespective of whether the sample is retained in the plastic liner or as disturbed samples in other containers, a quality class normally 4 or in favourable circumstances 3 has been suggested (see Baldwin and Gosling, 2009 [34]).

25.9 Resonance samplers

COMMENTARY ON 25.9

With resonance drilling as described in 24.8, samples are recovered with the sonic barrel and retained either in its plastic liner (where fitted) or in a polythene "sausage".

The soil or rock recovered should be retained in the sampler's liner tube(s) or polythene "sausage" until it has been examined and described. At the point of examination, a full soil profile, i.e. approaching 100% recovery, might be available for viewing. The samples should be preserved for longer term temporary retention, either in the liners or as bag samples.

NOTE BS EN ISO 22475-1 indicates that the quality classes are 4 and 5 for cohesive and granular soils respectively irrespective of whether water is present. However, arguably classes 3 and 4 are more realistic, the former being on the proviso that undue heat is not generated by the drilling equipment.

25.10 Block samples

25.10.1 Block samples cut by hand

COMMENTARY ON 25.10.1

Block samples cut by hand from materials exposed in excavations are normally taken in fine grained soils (but also in weaker rock) to provide samples for the laboratory determination of strength, deformation and modulus values. The procedure is often used for obtaining specially oriented samples and, in such cases, both the location and the orientation needs to be recorded before the sample is separated from the ground.

The cutting of a block sample often takes an appreciable time during which there is the potential for the water content to change; the following precautions should be taken.

a) No extraneous water should be allowed to come into contact with the sample.
b) The sample should be protected from the wind and the direct sunlight.
c) Immediately after the sample has been cut, the orientation should be marked and then it should be coated to preserve the water content and density such as muslin and paraffin wax and packed as described in 25.11.6.
d) Remoulded soil should be carefully removed from the sampling location.

In addition to the measures to preserve water content, the sample should be protected from disturbance during handling and transport to the laboratory, see 25.11.6. This can take the form of a bespoke container.

25.10.2 Block samples taken with a large sampler

COMMENTARY ON 25.10.2

There are two proprietary types of this generic sampler detailed in BS EN ISO 22475-1, namely the Sherbrooke sampler and the Laval sampler. The former appears to have been most widely used for sampling soft cohesive deposits or peat. BS EN ISO 22475-1 defines these samplers as Category A and capable of achieving Class 1 samples.

The equipment to be used and the procedure to be followed for recovering block samples with a large sampler should be in carried out in accordance with BS EN ISO 22475-1, Annex C, C.15.