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What is Recovery Ratio and RQD in Soil Sampling? What are the Limits of Sample Disturbance Parameters?

In our previous post we have discussed about soil sample disturbance parameters. We will learn another two parameters here then we will provide some limits for determining degree of sample disturbance.

Recovery ratio of soil sample

This is another parameter that also defines degree of sample disturbance. This is a ratio of length of recovered sample to inserted length in sampling tube and expressed as percentage. This is used for rock or cohesive core soil. An unit recovery ratio (length of sample recovered=sampler insertion into desired stratum), means sample was not compressed in the tube due to friction. When it becomes more than unity, a potential looseness of sample due to

• arrangement of
  • roots 
  • stones
• removal of preloads

Soil sample in core sampler
RQD or Rock quality designation

The sampling quality of rock sample can be defined by RQD. It is a ratio of recovered total length of piece of core to sampler penetration length into rock. The pieces of core usually have length 100 mm or greater. This is also expressed as percentage.


Now we will provide some limits of these parameters:

Ci=inside clearance ratio—----should lie between (1-3)%

Co=outside clearance ratio----—should be greater than Ci

Ar=area ratio----–this parameter should be as less as possible and should be consistent with requirement of strength of sampling tube.

For stiff soil, Ar should be ≤20%

For soft soil, Ar should be ≤10%

To have undisturbed sample recovery ratio should lie between (96-98)%

Rock classification based on rock quality designation as shown in following table 

Table:  Classification of Rock Quality

RQD(%)
Rock Quality
90 – 100
Excellent
75 – 90
Good
50 – 75
Fair
25 - 50
Poor
0 - 25
Very Poor

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