6.3 Sampling by drilling (continuous sampling)
6.3.1 General
6.3.1.1 This sampling method allows
- the identification and description of the soil at the site penetrated by the borehole;
- the differentiation of distinct soil layers and changes ofsoil material;
- the sampling aswell as the investigation and testing of samples of all strata and depths.
NOTE Continuous sampling, combined with a sampling method according to category A (see Table 2), normally gives the most valuable information on the ground conditions out of all the ground investigation methods by drilling. Sampling by drilling is therefore the preferred sampling method for heterogeneously-layered soils.
6.3.1.2 Drilling methods and equipment shall be selected as a function of the required sampling category (see Table 2 and Table 4), tests and/or groundwater measurements to be carried out in the borehole.
6.3.1.3 Boreholes shall be stabilised, usually by casing, as drilling proceeds to prevent collapse of the borehole and caving.
6.3.1.4 When drilling below groundwater surface, the diameters of borehole casings and tools and the water level in the casing pipe shall be selected as to preclude the inflow of soil into the pipe. To prevent the drilling and cleaning tools from creating hydraulic failure in the soil, they shall be selected with sufficient annular clearance and withdrawn slowly. An adequate water pressure shall be maintained in the borehole.
6.3.2 Sampling by rotary drilling
6.3.2.1 Sampling by rotary dry core drilling
6.3.2.1.1 In sampling by rotary dry core drilling, a tube system fitted with a bit at its lower end is rotated and fed into the soil by the drill rig via the drill string. This action produces a core sample within the tube system. The sampling tool can be single tube with a preferred borehole diameter of 100 mm to 200 mm or a hollow stem auger with a preferred borehole diameter of 100 mm to 300 mm. No flushing medium is used.
6.3.2.1.2 This technique is used for clay, silt and fine sand. If a hollow stem auger is used as a sampling tool, it will also be suitable for medium and coarse sand as well as organic soils. Sampling by rotary dry core drilling is generally unsuitable for sampling coarse gravel, cobbles and boulders.
6.3.2.2 Sampling by rotary core drilling
6.3.2.2.1 In sampling by rotary core drilling, a tube system fitted with a bit at itslower end is rotated and fed into the soil by the drill rig via the drill string. This action produces a core sample within the tube system. The sampling tool can be single tube, double tube or triple tube. The preferred borehole diameter is between 100 mm and 200 mm. Flushing medium is used.
6.3.2.2.2 The single-tube corebarrel consists of a core tube with a bit at its lower end and a corebarrel head that attaches to the drill rods at its upper end. A core lifter can be fitted between the bit and the core tube or directly within the bit. The flushing medium passes between the inside of the core tube and the recovered soil core, continuously washing the length of the recovered sample.
6.3.2.2.3 The double-tube corebarrel consists of two concentric tubes and a bearing arrangement in the corebarrel head which allows the inner tube to remain stationary whilst the outer tube is rotated by the drill string. A core lifter is generally fitted between the core bit and the inner tube. The flushing medium passes through the annulus between the inner and outer tubes thus protecting the recovered core sample from damage. The double-tube corebarrel can be fitted with an optional additional plasticlining tube within the inner tube. When such a liner is fitted, the standard core bit and core lifter shall be replaced by a core bit and core lifter with a reduced inner gauge. The fitting of such a plastic liner will assist in improving core recovery in certain soil types and contain and protect the sample during transport. The double-tube corebarrel can also be fitted with an extension to the inner tube that passes through and protrudes just ahead of the core bit for use in very soft soil types.
6.3.2.2.4 The triple-tube corebarrel is similar in construction to the double-tube design but is fitted with an additional third tube within the inner tube as standard. This third tube is generally a thin wall steel tube split in half longitudinally so that, when it is removed from the inner tube, the top half can be removed to view the core sample. In some cases, the split inner tube can be replaced by a plastic liner. The triple-tube corebarrel can also be fitted with an extension to the inner tube that passes through and protrudes just ahead of the core bit for use in very soft soil types.
6.3.2.2.5 Sampling by rotary core drilling is generally suitable for clay, clayey and cemented composite soils and boulders; it is unsuitable for all non-cohesive soils.
6.3.2.2.7 After recoveryof the corebarrel to the surface, the recovered core shall be handled in such a way that it as far as possible maintains its natural state. Extraction shall be made horizontally with a suitable extruder and in the same direction as it entered the barrel.
