Development of Rock mass classification systems(Part-2)

2.2       Lauffer’s Classification System
            This classification system involving stand up time for an unsupported span of a tunnel (Fig. 2) is known as Lauffer’s classification. Lauffer (1958) proposed that the stand-up time for an unsupported span (already excavated tunnel length or distance between the face and the nearest support) of tunnel is related to the quality of the rock mass in which the span is excavated.
Figure 2 Span of Tunnel

Pacher and his group modified the Lauffer’s classification and presently Lauffer’s classification is a part of the general tunneling method. This classification system is also applicable for underground excavations and important because the same rock mass may predict different stand-up times for short and long spans.

2.3       Deere’s Rock Quality Designation index (RQD)
            Deere (1964) proposed a quantitative index obtained directly from measurements of rock pieces from core drilling. The percentage of intact core pieces longer than 100 mm (4 inches) in total length of core recovered are identified separately and summed up to obtain RQD. The core should be at least 54.7 mm in diameter. Deere also proposed a relationship between RQD and the engineering quality of the rockmass (Table 3).

Table 3          Rock mass Quality defined by RQD (Deere, 1964)

Rockmass Quality
< 25
Very Poor
25 to 50
50 to 75
75 to 90
90 to 100

The following relationship is used to calculate RQD.

The procedure for the determination of the length of core pieces and the calculation of RQD are given in Fig. 3. RQD index is an indicator of rock mass conditions and is an essential input parameter for the Geomechanics and Q Systems of classification. In case of unavailability of core data, RQD can be estimated from the joint data. Palmstrom (1982) suggested that number of discontinuities per unit volume helps to estimate value of RQD. The following relationship for RQD is suggested:

RQD = 115-3.3 JV

            Where Jv is the sum of the number of joints per unit length for all joint (discontinuity) sets known as the volumetric joint count. RQD is a directionally dependent parameter and its value may change significantly, depending upon the borehole orientation. The use of the volumetric joint count can be quite useful in reducing this directional dependence. RQD is intended to represent the rock mass quality in situ. When using diamond drill core, care must be taken to ensure that fractures, which have been caused by handling or the drilling process, are identified and ignored when determining the value of RQD. When using Palmström's relationship for exposure mapping, blast induced fractures should not be included when estimating Jv. Cording and Deere (1972), Merritt (1972) and Deere and Deere (1988) have attempted to relate RQD to Terzaghi's rock load factors and to rockbolt requirements in tunnels.

Figure 3 Procedures for Measurement and Calculation of RQD (Deere, 1989)

2.4       Rock Structure Rating (RSR)
            Wickham et al. (1972) described a quantitative method for describing the quality of a rock mass and for selecting appropriate support on the basis of their Rock Structure Rating (RSR) classification. Most of the case histories, used in the development of this system, were for relatively small tunnels supported by means of steel struts. This system was the first to make reference to shotcrete support. It is worth examining the RSR system in some detail since it demonstrates the logic involved in developing a quasi-quantitative rock mass classification system. The significance of the RSR system is that it introduced the concept of rating each of the components listed below to arrive at a numerical value of RSR = A + B + C.

2.4.1 Parameter A (Geology)
            General appraisal of geological structure on the basis of:
a. Rock type origin (igneous, metamorphic, and sedimentary).
b. Rock hardness (hard, medium, soft, and decomposed).
c. Geologic structure (massive, slightly faulted/ folded, moderately faulted/ folded, intensely faulted/folded).

2.4.2   Parameter B (Geometry)
            Effect of discontinuity pattern with respect to the direction of the tunnel drive on the basis of:
a. Joint spacing.
b. Joint orientation (strike and dip).
c. Direction of tunnel drive.

2.4.3   Parameter C (Joint & Groundwater)
            Effect of groundwater inflow and joint condition on the basis of:
a. Overall rock mass quality on the basis of A and B combined.
b. Joint condition (good, fair, poor).
c. Amount of water inflow (in gallons per minute per 1000 feet of tunnel).
Figure 4         Direction of Drive

            Three tables from Wickham et al. (1972) paper are reproduced in Tables 4, 5 and 6 which can be used to evaluate the rating of each of these parameters to arrive at the RSR value (maximum RSR = 100). For example, a hard metamorphic rock which is slightly folded or faulted has a rating of A = 22 (from Table 4). The rock mass is moderately jointed, with joints striking perpendicular to the tunnel axis which is being driven east-west, and dipping at between 20to 500. Table 5 gives a rating for B = 24 for driving with dip (defined in Fig. 4).  The value of A + B = 46. For joints of fair condition (slightly  weathered and altered) and a moderate water inflow of between 200 and 1,000 gallons per minute, Table 6 gives a rating for C = 16. Hence, the final value of the rock structure rating RSR = A + B + C = 62. A typical set of prediction curves for a 24 foot (7.2 m) diameter tunnel are given in Figure 5 which indicates that, for the RSR value of 62 derived above, the predicted support would be 2 inches (5cm) of shotcrete and 1 inch (25 mm) diameter rockbolts spaced at 5 foot (1.5 m) centres. As indicated in the figure, steel sets would be spaced at more than 7 feet (2.1 m) apart and would not be considered a practical solution for the support of this tunnel. For the same size tunnel in a rock mass with RSR = 30, the support could be provided by 8 WF 31 steel sets (8 inch (200 mm) deep wide flange I section weighing 31 lb per foot) spaced 3 feet (0.9 m) apart, or by 5 inches (125 mm) of shotcrete and 1 inch (25 mm) diameter rockbolts spaced at 2.5 feet (750 mm) centres. In this case it is probable that the steel set solution would be cheaper and more effective than the use of rockbolts and shotcrete. Although the RSR classification system is not widely used today, Wickham et al.’s work played a significant role in the development of the modern classification systems.

Figure 5         RSR support estimates for a 24 ft. (7.3 m) diameter circular tunnel. Note that rockbolts and shotcrete are generally used together. (After Wickham et al. 1972)

Table 4          Rock Structure Rating: Parameter A: General area geology
Table 5          Rock Structure Rating: Parameter B: Joint pattern & direction of drive
Table 6       Rock Structure Rating: Parameter C: Groundwater, joint condition
a) Dip: flat: 0-20; dipping: 20-50; and vertical: 50-90
b) Joint condition: good = tight or cemented; fair = slightly weathered or altered; poor = severely weathered, altered or open.


posted by Geology on 07:22