Sampling is conducted to characterize the soil material and carry out laboratory testing in order to derive its physical, chemical, mechanical or dynamic properties. A major assumption of this methodology is that the soil sample is representative of the ground from which it was retrieved. In general, samples are divided in 2 categories:
- Disturbed samples, usually obtained by drilling methods and
- Undisturbed samples that are taken by carefully cutting blocks of soil of by driving tubes into the soil mass.
Undisturbed soil sampling is a challenging process. The factors that affect the quality of an undisturbed soil sample are the following:
- Side friction: When using tube equipment, the friction between the soil and the tube tends to compress the sample during recovery process. To address this issue, the diameter of the cutting edge can be slightly reduced, decreasing the friction inside the tube.
- Volume displacement: In general, any type of sampling causes the displacement of a certain soil volume affecting its initial conditions. If the soil consists of gravel particles, the impact is greater.
- Lateral expansion: When a soil sample is retrieved, its confining pressure is removed causing its lateral expansion.
- Variation in water content: The recovery methods and the sample handling until laboratory testing is conducted cause a variation in the water content of the soil from its initial condition.
- Hydrostatic pressure: Hydrostatic pressure is lost when a sample is recovered and this may cause disturbance mostly by creating gas-bubble voids in the soil sample.
- Weather conditions: Weather may influence the quality of a sample after its recovery. During a cold day, the sample may freeze if it remains unprotected whereas in a hot environment, it may lose a large amount of its water content.
- Handling and Transportation: Every process that is conducted from the sample's recovery until testing (e.g. transportation, storage) may disturb the soil.
- Professionalism of the working group: The disturbance of a soil sample will remain as low as possible given that all members of the crew (drilling crew, engineers, laboratory technicians) follow the standard procedures.
Sample disturbance is inevitable and occurs in all sampling procedures. However, if sampling is conducted properly, the effects of disturbance can be significantly reduced and the sample can be considered representative of the ground from which is was taken.