25.5 Stationary piston sampler
The principles of design for open-tube samplers (see 25.4.1) should be applied to the tube of a piston sampler.
NOTE As with open-tubes, BS EN ISO 22475-1 identifies thin- and thick-walled types of piston sampler (PS-T/W and PS-TK/W respectively) although the latter are not common in the UK. Figure 4 shows the basic details of a piston sampler.
25.5.2 Thin-walled stationary piston sampler (type PS-T/W)
COMMENTARY ON 25.5.2
The thin-walled stationary piston sampler consists of a thin-walled sharpened sample tube containing a close-fitting sliding piston, which is slightly coned at its lower face. The sample tube is fitted to the drive head, which is connected to hollow drill rods. The piston is fixed to separate rods, which pass through a sliding joint in the drive head and up inside the hollow rods. Clamping devices, operated at ground level, enable the piston and sample tube to be locked together or the piston to be held stationary while the sample tube is driven down. Figure 4 shows the basic details of a stationary piston sampler.
An alternative to the double rod and mechanical locking mechanism is the hydraulically operated piston sampler.
The sampler is normally used in low strength fine soils and can give Class 1 samples in silt and clay, including sensitive clay. Its ability to take samples below the disturbed zone and to hold them during recovery gives an advantage over the thin-walled sampler described in 25.4.3. Although normally used in soft clays, special piston samplers have been designed for use in stiff clays (see Rowe, 1972 ).
The sample diameter is normally 75 mm or 100 mm, but samplers up to 250 mm diameter are used for special soil conditions. Typically, for the smaller diameter tubes, samples up to 1 m long can be taken.
Initially, the piston should be locked to the lower end of the sample tube to prevent water or slurry from entering the sampler. With the piston in this position, the sampler should be pushed below the bottom of the borehole. When the sample depth is reached, the piston should be held stationary and the sample tube pushed by a static thrust until the drive head encounters the upper face of the piston. An automatic clamp in the drive head prevents the piston from dropping down and extruding the sample while the sampler is withdrawn.
NOTE In clays, and possibly other soils, of firm or lower consistency, piston samples can be taken by progressive penetration from the ground surface (rather than from the base of a borehole); an hydraulic jack might be required to assist the penetration and/or extraction operation.
25.5.3 Other stationary piston samplers
COMMENTARY ON 25.5.3
Variations on the piston sampling method are also available, for example the Mostap sampler as used with cone penetrometer systems takes samples which are typically 1 m long and 35 mm to 65 mm in diameter. The Mostap geometry does not comply with the area ratio criterion for a thin-walled piston sampler. However, the sample is packed in a nylon stocking that is connected to the piston and this stocking reduces the friction between the sample and the sample tube so lessening disturbance.
The Mostap sampler should be pushed into the ground with a cone locked in front of the sample cutting tube. At the desired depth a fishing tool should be lowered down the tube string. By pulling the fishing tool the cone and piston are unlocked and held stationary. The sample cutting mouth with the rest of the assembly should then be pushed further into the soil and so the sample is taken.
|1 Piston rod||4 Thin-walled sampler tube|
|2 Hollow drill rod||5 Piston|
|3 Drive head containing device for clamping piston rod|
25.6 Continuous soil sampling
COMMENTARY ON 25.6
Continuous soil sampling can produce samples up to 30 m in length in softer soils such as fine alluvial deposits. This is of particular value for identifying the soil fabric (see Rowe, 1972 ) and gives results superior to those that can be obtained by consecutive drive sampling. The Swedish system (see Kjellman, Kallstenius and Wager, 1950 ) takes samples 68 mm in diameter using steel foils to eliminate inside friction between the sample and the tube wall. The Delft system (see Begeman, 1966 ) uses lighter equipment and offers two sizes of sample, 29 mm and 66 mm; this is a thick walled sampler but, akin to the Mostap sampler (see 25.5.3), it uses stockinette (either impregnated or surrounded by fluid and plastic lining tubes) to reduce friction, lessen disturbance and support the sample.
Alternative systems for taking semi-continuous samples have been developed to recover consecutive soil samples. An example of this is the Goudsche Machinefabriek (GMF) proprietary system (see French, Woolgar and Saynor, 2000 ) where a 66 mm diameter plastic liner is inserted into the sampling tube with cutting shoe. The assembly is hydraulically pushed into the ground for 750 mm under controlled conditions, for example with a cone penetrometer rig. The liner tube with sample can then be recovered using a wireline tool. The sample is labelled and stored. Another liner is then dropped inside the sampling tube and latched into the end section. The procedure is repeated until the required depth is achieved or the limiting capacity of the equipment is reached.
Continuous and semi-continuous samplers are not readily available in the UK but where the ground conditions are suitable their use should not be discounted. Specialist contractors familiar with the equipment should be engaged to advise on its use and/or operate the sampler.