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

Rock Quality designation (RQD)

Definition

The Rock Quality designation (RQD) index has been used for over 20 years as an index of rock quality. It measures the percentage of "good" rock within a borehole. It was developed by the senior author originally as a means of qualitatively describing whether a rock mass provided favorable tunneling conditions. It is now used as a standard parameter in drill core logging and forms a basic element of several rock mass classification systems [1,2]. Perhaps its greatest value is its simplicity , which allows for the delineation of zones of poor quality rock that could adversely affect engineering structures.

This article presents the background for the development of the RQD, the recommended procedure
for measuring RQD, and examples of its use in practice.
rqd rock

Background

rqd rock
In 1963 a paper was published by Deere [3 ] entitled "Technical Description of Rock Cores for Engineering Purposes" in the first volume of Felsmeclwnik and lngenieurgeologie (Rock Mechanics and Engineering Geology). This would have been an excellent international forum for introducing the RQD concept but it was not included because it had not as yet been devised. It was in the following year that the senior author developed the RQD concept to assist in the siting and the decision of tunnels and large caverns in granite at the Nevada Test Site. In 1965 it was extended to the design of highway tunnels in massive quartzite, gneiss. and schist in North Carolina .
Because of its success in these early applications to tunnels as actually designed and built, the RQD concept appeared worthy of a continuing research effort. It was at the University of Illinois that the RQD concept was first applied to a wider range of rock engineering problems.
In 1967 Deere and his colleagues at the University of Illinois [4] presented for the first time in published form the RQD concept of rock quality logging together with some correlations with velocity indexes, fracture frequency, and in situ modulus values. The method or measuring RQD was given as well as a brief discussion of some of the difficulties involved in determining it.
The published work that introduced RQD to an international audience, and that no doubt was responsible for its rapid growth in use in many countries, was Rock Mechanics in Engineering Practice ( 1968) [5]. This contained chapters by Deere [6] and by Hendron [7] in which the RQD concept and applications were discussed.
Research continued at the University of Illinois on tunneling and the application of the RQD index under the sponsorship of the U.S . Air Force and the U.S. Department of Transportation. This research lead to several publications in the late 1960's and early 1970's [8-12]. During the 1970's the RQD index began to be used as a basic parameter in several classification systems for
rock masses (Bieniawski [I ,13], Barton ct al[2]).
rqd rock

References

[I] Bieniawski, 7 .. T ., " Engineering Classification of Jointed Rock Masses . '· Transactions of the South African Institute of Civil Engineers, Vol. 15, 1973, pp. 335- 344.
[2] Barton, N . . Lien, R., and Lunde, J., " Engineering Classification of Rock Masses for the Design of
Tunnel Support," Rock Mechanics. Vol. 6 , 1974 , pp. 1!!9-2.16.
[3] Deere, D. U., " Technical Description of Rock Cores for Engineering Purposes.' " Felsmachanik and
 lngenieurgeologie (Rock Mechanics and Engineering Geology). Vol. I, No. I, 1963. pp. 16- 22.
[4] Deere , D. U. , Hendro n, A. J ., Jr. , Patton, r. D. , and Cording . E. J .. " Design of Surface and Near Surface Construction in Rock,' ' in Failure: and llreokogc of flock. C. Pairhurst, Ed ., Society of Mining Engineers of AIMF. , New York , 1967. pp. 237- 302.