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RQD - Rock Quality Designation

posted Jul 20, 2019, 7:31 AM by jeffery jim

Many civil engineers are not good in interpreting the borehole log especially when it comes to rock. The most essential thing to see here is the RQD in order to determine the quality of the rock.

Some specialists or consultants allow pile socketing at the value of 0%. In actual fact, this is depending on the geological setting of the area as well as the allowance when assigning pile with bearing capacity. Apart from that, the method of fine tuning the socket (chiseling) as well as socket washing. Therefore, socketing is allowed at RQD 0% when the designer allows for skin friction rather than the bearing capacity. So, take note on this.

Apart from that, RQD value will guide specialist in determining the possible anchorage depth or length of a dowel or nail for both soil and rock. This is to ensure that anchors are behind the possible soil slip circle and/or behind the possible rock wedge failure. It also tell stories of possible fissures in geological sense.

I usually allow minimum 30% unless there are contractual hindrances. RQD value of 30% can be considered fairly poor where 76% to 90% is considered as good. In similar nexus, good rock is hard to find especially in Sabah where most rocks are sedimentary rocks except for rocks at Mount Kinabalu which are igneous rocks.

Usually, what we see as hard (sedimentary) rocks on rock surface are actually Class III or Class IV. You need to go very deep down to 20 to 30 meters to find Class II and Class I usually is below 60 meters. By rule of thumb, at 20-30 meter depth, the RQD may go up to 80% but at certain stratum and less likely as a whole depth. Slope of a hill usually have even lower value; 30% for certain continuous strata or 50% at certain stratum.

Recovery is derived by ratio of recovery core with fractures against the total core run. This is a way to determine the volume ratio compare to void or stratum/strata due to other formation, foreign to the rock itself.

RQD can be termed as total solid rock samples without fractures at the length of 100mm or more (and met the soundness requirement) against the total core run. This is the measurement of segments' fissures or fractures along the total core sample.

So i hope this article will guide you greenhorns when interpreting borehole logs for engineering application.

*The photo below indicates RQD of close to 0%

Image may contain: outdoor