RQD Index in Tunneling

 

RQD Index in Tunneling

As noted previously , the RQD concept was developed for tunneling, firstly as an aid in siting tunnels and shafts in the best ground conditions possible, and secondly as a guide in assessing tunneling conditions and selecting the initial supports. Papers published in the 1969- 1970 period , presented tables relating tunnel support and RQD based on the University of Illinois sponsored research efforts. Cecil  published in 1970 his work on correlation of RQD with rock bolt-shotcrete support as used in Scandinavia.
rqd rock

Merritt in 1972 made use of his recent experience and the works cited above to present an improved version based on 58 cases which included tunnel widths ranging from 2 m (6 ft) to 20 m (60 ft). He compared the support criteria as shown by his system as a function of tunnel width and RQD with those proposed by Peck  and Cecil . Table I is based on Merrill's Fig. 3 and has been selected for a 6-m (20-ft)-wide tunnel, a common tunnel size for pressure tunnels and a single-t rack rapid trans it tunnel.

Merritt  points out problems associated with any attempt to precisely correlate rock quality with the tunnel support actually used:

RQD Index in Tunneling

rqd rock

Unfortunately, the selection of tunnel support docs not always depend upon the actual rock conditions.

The method preferred by the contractor may be based on a favorable unit price for steel sets as opposed

to bolts, or the lack of adequate equipment for the rapid placement of either sets or bolts. The preference

for set in the first place may be based on an overcautious safety program . .. although no method for

predicting tunnel support criteria is foolproof, the writer believes this system can be of great value for

design and estimating support purposes. The RQD method of core analysis is simple, inexpensive, and

reproducible and it has an advantage over joint frequency, for example, in that joints can only be counted

in recovered core . rqd rock .. The RQD support criteria system has limitations in areas where the joints contain

thin clay fillings .. .

rqd rock