What is Sample Disturbance in Soil Sampling?

During collection of soil samples from either shallow or deep location, there may have disturbance from small to large degree. When boring is of wash boring, a suspension of soil is collected; obviously this is a very disturbed sample. This method represents a soil that has almost lost all ambient properties (completely disturbed structure, altered natural moisture content etc); only provides information about changes in the layers.

With this complete disturbance, some methods provides less disturbed sample and there have some special method that provides least disturbed sample. There have some special method that provides nearly undisturbed sample. We have discussed about block sampling in our preceding post.

A soil sample may be disturbed in three consecutive stages either in any one stage or may be in combination of stages. These are:

a. Disturbance before sampling

b. Disturbance during sampling

c. Disturbance after sampling

Disturbance before sampling

During sampling generally we remove or cut out soils over the sample at desired depth. This method reduces vertical stress over the sample within a short duration reducing overburden pressure quickly. In some cases, soil layer beneath the sample may be deflected up for the same consequence of stress relief. In some sampling operation continuous collection of samples is applied depending on extent and requirement of soil investigation. In continuous sampling process, additional disturbance is observed due to shearing of previous sample by applying torsion to separate it from parent soil.

Disturbance During Sampling

During sampling the soil sample entered into the sampler by forcing them to take their position in sampler. We know there have some friction between sampler and soil and other mechanism that disturb the sample.

We can summarize this mechanism as below:

a. Disturbance by displacing soil by wall of sampler

b. Friction between sampler and sample

c. Adhesion between sampler and sample

Another factor that influences the disturbance is inside clearance between cutting shoe and sample.

Disturbance After Sampling

When a soil sample is collected carefully, least disturbing it which is practical during sampling and before sampling, may be disturbed by next steps of sampling process like

-transportation

-sealing

During transporting, in many cases, sample becomes disturbed. Sometimes in case of improper sealing of tube, moisture lose may be taken place.

In designing a soil sampler, the main criteria are that they don’t produce much disturbance. In determining disturbance of soil during sampling some parameters like ci, co, Ar, RQD and recovery ratio etc are used and specific limit of these parameters for different soil type are provided that will be discussed in upcoming posts.