RQD = (Sum of core pieces >= 10 cm)/ total drill length *100
Following are the methods of obtaining RQD :
a. Direct Method
International Society for Rock Mechanics(ISRM) recommends a core size of at least NX (size 54.7 mm) drilled with double-tube core barrel using a diamond bit. All the artificial fractures should be ignored while calculating the core length for RQD.
The relationship between RQD and engineering quality of the rock mass as proposed by Deere in 1968 is given as below:
S.No. RQD (%) Rock Quality
1. <25 Very Poor
2. 25-50 Poor
3. 50-75 Fair
4. 75- 90 Good
5. 90- 100 Excellent
|RQD determination - Direct Method ( Photo credit- A Practical Approach to Civil Engineering by Singh and Goel)|
1. Seismic Method : The seismic survey method makes use of the elastic properties of strata that affect the velocity of the seismic waves travelling through them. This method is cheap, rapid and relatively easy to apply. We can find out the following information from this method:
a) Location and configuration of bed rock and geological structures in the subsurface.
b) The effect of discontinuities in rock mass may be estimated by comparing the in-situ compressional wave velocity with laboratory sonic wave velocity of intact drill core obtained from the same rock mass.
RQD(5) = (Vi/Vl)^2 * 100
Where, Vi is the in-situ compressional wave velocity and Vl is compressional wave velocity in the intact rock core.
2. Volumetric Joint Count Method:
The RQD can be determined by counting the number of joints(discontinuities) per unit volume Jv. A simple relationship(given by Palmstrom, 1982), RQD = 115 - 3.3*Jv , can be used to convert Jv into RQD for clay free rock masses.
Here Jv is the number of joints per cubic meter of the rock mass.
There are few other methods, which I shall cover up in the upcoming posts.