SOIL SAMPLES AND SAMPLERS - FOUNDATIONS SITE EXPLORATION.

Soil samples can be of two types:

(i) Disturbed samples.
(ii) Undisturbed samples.

A disturbed sample is that in which the natural structure of soil gets partly or fully modified and destroyed although with suitable precautions the natural water content may be preserved. Such a soil sample should, however, be representative of the natural soil by maintainlng the original proportion of the various particles intact. An undisturbed sample is that in which the natural structure and properties remain preserved.

The sample disturbance depends upon the design of the samplers and the method of sampling. To take undisturbed samples from bore holes properly designed sampling tools are required. The sampling tube when forced into the ground should cause as little remoulding and disturbance as possible. The design features of the sampler, that govern the degree of disturbance are (i) cutting edge (ii) inside wall friction and (iii) non-return valve.

Fig. 2.18 shows a typical cutting edge of a sampler, with the lower end of the sampler, with the lower end of the sampler tube. The following terms are defined with respect to the diameters marked in Fig. 2.18.






FIG. 2.18 LOWER END OF A SAMPLER.




The area ratio should be as low as possibie. It should not be greater than 25 percent; for soft sensitive soil, it should preferably not exceed to porcent. The inside clearance should lie between 1 to 3 percent and the outside clearance should not be much greater than the inside clearance. The walls of the sampler should be smooth and should be kept properly oiled so that wall friction is minimum. Lower value of inside clearance allows the elastic expansion of soil and reduces the frictional drag. The non-retum valve, invariably provided in samplers, should permit easy and quick escape of water and air when driving the sampler.


Types of Samplers

The samplers are classified as thick wall or thin wall samplers depending upon the area ratio. Thick wall samplers are those having the area ratio greater than 10 percent. Depending upon the mode of operation, samplers may be classified in the following three common types : (i) open drive sampler (including split spoon samplers), (ii) stationary piston sampler and (iii) rotary sampler.

The open drive sampler is a tube open at its lower end. The sampler head is provided with vents (valve) to permit water and air to escape during driving. The check valve helps to retain sample when the sampler is lifted up. The tube may be seamless or it may be split in two parts; in the latter case it is known as split spoon sampler.

The stationary piston sampler consists of a Sample cylinder and the piston system. During lowering of the sampler through the hole, the lower end of the sampler is kept closed with the piston. When the desired sampling elevation is reached, ihe piston rod is clamped, thereby keeping the piston stationary, and the sampler tube is advanced down into the soil. The sampler is then Iifted up, with piston rod clamped in position. The sampler is more suitable for sampling soft soils saturated sands.

Rotatory samplers are the core barrel type having an outer tube provided with cutting teeth and a removable thin wall liner inside. It is used for firm to hard cohesive soils and cemented soils.

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