6.4.2 Sampling using open-tube or piston samplers
6.4.2.1 General
For recovering samples from boreholes in cohesive, sandy and organic soils, open-tube or piston samplers canbe used. These samplers generally consist of a sampler tubewith or without a piston and a sampler head with connection to the extension rods. The open-tube sampler (thin-walled and thick-walled) can be used in boreholes. The piston sampler can be pushed directly into soft to medium stiff soil.
6.4.2.2 General geometry
6.4.2.2.1 Tube inner diameters between 50 mm and 120 mm are common, but diameters up to 250 mm are used for special soil conditions. The lower end of the tube shall be shaped to form a cutting edge.
6.4.2.2.2 The sampling tube length should preferably be not greater than20 times the sample diameter. An effective sampling length of 0,45 m to 1,00 m is considered sufficient for ordinary soil testing. Longer tubes can be used if friction reducing systems are applied.
6.4.2.3 Detailed geometry
6.4.2.3.1 The material of the sampling tube shall be rigid, resistant to corrosion and with a smooth surface. The thickness of the tube wall shall be chosen so that the tube resists distortion when pushed into the soil.
6.4.2.3.2 The thin-walled tube samplers used shall meet the following requirements, which apply by analogy to samplers with other internal diameters:
a) the edge taper angle should not exceed 5 °;
b) the area ratio, Ca (see 3.3.11), should be less than 15 %;
c) taper angles between 5° and 15° and area ratios up to 25 % may only be used if it is demonstrated that the quality class is not affected;
d) for tube samplers with Ca exceeding 15 %, the angle of the cutting edge shall decrease as the wall thickness increases;
e) the tolerances on the cutting edge and the sample tube should be chosen to give a minimum inside clearance ratio, Ci (see 3.3.12), less than 0,5 %. When assessing the inside clearance, the worst case of manufacturing tolerances shall be applied.
6.4.2.4 Preparation of tubes
6.4.2.4.1 Prior to sampling, the sampler and its component parts should be carefully inspected, especially the cutting edge. Defective or damaged components should be replaced. In order to keep the sample as undisturbed as possible during extraction, transport and handling in the laboratory, samplers with rigid, low-friction liners are recommended.
6.4.2.4.2 The inside of the sampling tube or liner shouldbe clean and smooth without any protruding edges or irregularities, which can cause disturbance of the sample. The tubes and liners shall have smooth walls to minimise friction in the soil. Tubes which are corroded on the inside, or have damaged cutting edge, shall not be used.
6.4.2.5 Field procedure
6.4.2.5.1 The sampler shall be pushed or driven into the soil (see column 4 of Table 3). If dynamic driving is used, the drop weight used shall impinge directly onto the sampler head, its mass being sufficient to effect the required penetration of the tube by a minimum number of blows from a small height.
6.4.2.5.2 Before sampling from the bottom of the borehole, any loose or disturbed material shall be removed. The sampler should be carefully lowered into a borehole as soon as practicable after the borehole bottom has been cleaned. The sampler tube shall be pushed down to at least 200 mm below any disturbed material at or below the base of the borehole. If a casing is used, samples shall be taken from the undisturbed soil below the casing.
6.4.2.5.3 The depth of the borehole and the position of the sampler shall be checked exactly when the sampler enters the borehole. The sampler shall not bear upon the soil at the bottom when the sampler reaches its full depth.
6.4.2.5.4 The sampler advance should be made in one continuous motion to the predetermined depth, and the length of advance should be measured. This length shall be assessed for each type of sampler. It is preferred to use not more than 90 % of the effective length. Advance in excess of the effective length is not allowed.
6.4.2.5.5 After driving, the sample shall be sheared off at the bottom edge of the sampler tube by rotating the rods or by slowly raisingthe sampler. The sampler should be carefully withdrawn without any vibrations or shocks in order to keep the sample undisturbed. It is often advisable to keep the sampler in position for a few minutes so that sufficient adhesion is developed between the sample and the sampling tube or liner.
6.4.2.5.6 After withdrawal the sampler should be disassembled and, if necessary, the samples carefully extracted without any bending or torsion of the sample. The sampling tube and the cutting edge should be checked for any deformations. Any such deformations should be noted in the sampling record. The occurrence of loosened soils or cuttings in the upper end shall also be checked and noted in the record.
6.4.2.5.7 The sampling process can disturb the soil underneath the sampler. This influence shall be considered.
