C.15 Method of sampling using a large sampler

C.15.1 Method of sampling using a Sherbrooke block sampler

a) Preparation of the borehole

The preparation of the borehole for a Sherbrooke block sampler requires use of a solid auger with a diameter of 400 mm. The borehole can be supported by mud, or be cased down to the sampling level. Before lowering a large sampler into the borehole (see Figure C.34), any loose debris or disturbed material is removed from the bottom of the borehole using a flat bottom auger with a diameter of 400 mm.

b) Sampling procedure with sample recovery

The Sherbrooke block sampler carves a cylindrical soil block of 250 mm in diameter by three cutting knives. These tools have an annular motion that permits carving of a 5 cm wide slot around a clay cylinder. At each cutting tool, water or mud is fed from the surface to help evacuate the clay cuttings during sampling. The sampler is connected to an ordinary drill rod system, that provides rotation of the sampler at about 5 r/min during the carving. The rate of vertical progression can vary with the clay types, but generally extends to 25 min to 30 min. When carving of the about 350 mm high soil cylinder is completed, a horizontal diaphragm fixed at each cutting tool is activated from the surface and pushed into the lower end of the soil block. An additional 5 min are added to let these bottom diaphragm elements cut their way under the sample as the sampler continues to rotate. The closure of the diaphragms separates the sample from the surrounding soil, and provides support beneath the sample when this is lifted to the surface. The sample is separated very slowly the first few centimetres to permit good circulation ofwater under the sample to avoid suction.

Key

• 1 control of vertical progression (manually)
• 2 annulus slot
• 3 rotation (mechanic or electrical)
• 4 water orbentonite mud
• 5 borehole 400 mm in diameter
• 6 water circulating at each leg
• 7 sample being carved (bottom diaphragm opened)
• 8 cuttingtools at every 120°

C.15.2 Method of sampling using a Laval sampler

a) Preparation of the borehole

The preparation of the borehole for a Laval sampler is either prepared by the previous sampling or by means of a solid auger with a diameter of 400 mm. The borehole can be supported by mud, or be cased down to the sampling level. Before lowering a large sampler into the borehole (see Figure C.35), any loose debris or disturbed material is removed from the bottom of the borehole using a flat bottom auger with a diameter of 400 mm.

b) Sampling procedure with sampling recovery

The sampler assembly is lowered into the borehole with the sampler hooked up inside the coring tube and with the head valve open; the mud can then flow freely through the sampler. When the lower edge of the coring tube reaches the bottom of theborehole, the coring tube is held fixed from the surface and the sampler is unhooked by pulling up and turning the central rod slightly. As the tube sampler is pushed down into the soil by a continuous thrust, the mud flows out of the tube through the head valve of the sampler. To make sure that no pressure is applied on the soil sample, the movement of the sampler is stopped when the head of the sampler has reached an elevation of approximately 50 mm above the top of the sample. The head valve is then closed and the coring operation is carried out by rotating the coring tube, at the same time injecting under pressure the bentonite mud. This flows through the drill rod down between the sampling and coring tubes, around the lower remoulding ring, and up outside the coring tubeinto the borehole. The injection of mud aims at washing the remoulded clay out of the teeth and cutters of the remoulding ring. When the coring ring has reached a depth of approximately 20 mm below the edge of the sampler, the coring is stopped and the sampler rotated through 90°, pulled up gently, and hooked back on the collar of the coring tube ready to be retrieved from the borehole.

The soil samples are extruded immediately after sampling in the field. They are cut out with a wire in slices 130 mm or 200 mm high, depending on the type of tests to be carried out. The slices are put on waxed plywood board, wrapped in special paper,sandwiched between layers of a paraffin wax and vaseline mixture, and are then ready to be transported and stored.

Key

• 1 sample
• 2 borehole 300 mm in diameter
• 3 coring tube
• 4 sampling tube
• 5 cutting teeth
• ^a The tube sampler is pushed down.
• ^b The head valve is closed byscrewing the inner string ofrods.
• ^c The coring operation is carried out by rotating the coring tube.
• ^d The sampling tube is hooked back on the collar of the coring tube and the sampler is retrieved from the borehole.