Development of Rock mass classification systems(Part-I)
1. Introduction
Strength of rock mass largely depends on the density, nature and extent of discontinuities within it. It also relates to the rock strength, weathering and water condition.
Many rock mass classifications developed by engineers with a sound background of geology, serve as the practical basis for the design of complex structures. The most used, earliest and well known classification systems are:
Ø Tarzaghi’s ‘Rock Load’ classification which was developed in 1946.
Ø Lauffer’s classification (1958) was a considerable step forward in the art of tunneling since it introduced the concept of the stand-up time of the active span in a tunnel, which is highly relevant in determining the type and amount of tunnel support.
Ø The classification of Deere et al. (1967) introduced Rock Quality Designation (RQD) Index, which is simple and practical method of describing the quality of rock core from boreholes.
Ø The concept of rock structure rating (RSR) developed by Wickham et al., (1972 & 1974) was the first system featuring classification rating for weighting the relative imported of rock mass parameters.
Ø The Geomechanical classification (RMR) system proposed by Bieniawski (1973) and tunneling quality (Q) system proposed by Barton et al. (1974) were developed independently and both provide quantitative data for the selection of modern tunnel reinforcement measures such as rock bolts and shotcrete.
Ø The Q system have been developed specifically for tunnel and chambers; whereas the Geomechanical classification, although initially developed for tunnels has been applied to rock slope engineering, foundations, ground rippability and mining problems.
A brief account of the six main classification schemes which have been widely used is described and discussed below:
2. Rock mass classification systems
2.1 Terzaghi's Rock mass Classification
The earliest reference to the use of rock mass classification for the design of tunnel support is in a paper by Terzaghi (1946) in which the rock loads, carried by steel sets, are estimated on the basis of a descriptive classification. While no useful purpose would be served by including details of Terzaghi's classification in this discussion on the design of support, it is interesting to examine the rock mass descriptions included in his original paper, because attention was drawn to those characteristics that dominate rock mass behaviour, particularly in situations where gravity is the dominant driving force. The clear and concise definitions and the practical comments included in these descriptions are good examples of the type of engineering geology information, which is most useful for engineering design. Table 1 gives the details the equations for calculating the rock load for different rock conditions. Figure 1 depicts the rock load on the tunnel support system.
Terzaghi's descriptions are:
A. Intact rock contains neither joints nor hair cracks. Hence, if it breaks, it breaks across sound rock. On account of the injury to the rock due to blasting, spalls may drop off the roof several hours or days after blasting. This is known as a spalling condition. Hard, intact rock may also be encountered in the popping condition involving the spontaneous and violent detachment of rock slabs from the sides or the roof.
B. Stratified rock consists of individual strata with little or no resistance against separation along the boundaries between the strata. The strata may or may not be weakened by transverse joints. Spalling condition is quite common in such rock.
C. Moderately jointed rock contains joints and hair cracks, but the blocks between joints are intimately interlocked that vertical walls do not require support. Both spalling and popping conditions may be encountered in rocks of this type.
D. Blocky and seamy rock consists of chemically intact or almost intact rock fragments which are entirely separated from each other and imperfectly interlocked. Vertical walls in such rock may require lateral support.
E. Crushed but chemically intact rock has the character of crusher run. If most or all of the fragments are as small as fine sand grains and no re-cementation has taken place, crushed rock below the water table exhibits the properties of water-bearing sand
F. Squeezing rock advances slowly into the tunnel without perceptible volume increase. A prerequisite for squeezing is a high percentage of microscopic and sub-microscopic particles of micaceous minerals or clay minerals with low swelling capacity.
G. Swelling rock advances into the tunnel chiefly on account of expansion. The capacity to swell seems to be limited to those rocks that contain clay minerals such as montmorillonite, with a high swelling capacity.
Table 1 Terzaghi’s Rock Load Classification
S.No | Rock Condition | Rock Load Hp (ft) | Remarks |
1 | Hard and intact | Zero | Light lining required only if spalling or popping occurs |
2 | Hard stratified or schistose | 0-0.5B | Light support, mainly for protection against spalls. Load may change erratically from point to point |
3 | Massive, moderately jointed | 0-0.25B | |
4 | Moderately blocky and seamy | 25B-0.35(B+H1) | No side pressure |
5 | Very blocky and seamy | (0.35-1.1)(B+H1) | Little or no side pressure |
6 | Completely crushed | 1.10(B+H1) | Considerable side pressure Softening effects of seepage toward bottom of tunnel require either continuous support for lower ends of ribs or circular ribs |
7 | Squeezing rock, moderate depth | (1.10-2.10)(B+Ht) | Heavy side pressure, invert struts requires circular ribs are recommended |
8 | Squeezing rock, great depth | (2.10-4.50)(B+Ht) | |
9 | Swelling rock | Up to 250 ft | Circular ribs are required in extreme cases, use yielding |
Rock load Hp in feet on tunnel roof with width B (ft) and Height (ft) at depth of more than 1.5 (B+Hp).
Table 2 Terzaghi’s Rock Load Classification (Modified by Deere et al., 1982)
Rock Condition | RQD | Rock Load Hp in m | Remarks |
Hard and intact | 95-100 | Zero | Same as Terzaghi (1946) |
Hard stratified or schistose | 90-99 | 0-0.5B | Same as Terzaghi (1946) |
Massive, moderately jointed | 85-95 | 0-0.25B | Same as Terzaghi (1946) |
Moderately blocky and seamy | 75-85 | 0.25B-0.2(B+H1) | Reduced by 50% from Terzaghi’s values because water table has little effect on Rock load |
Very blocky and seamy | 30-75 | (0.2B-0.0)(B+H1) | |
Completely crushed | 3-30 | 0.6-1.10(B+H1) | |
Squeezing rock, moderate depth | NA | (1.10-2.10)(B+Ht) | Same as Terzaghi (1946) |
Squeezing rock, great depth | NA | (2.10-4.50)(B+Ht) | Same as Terzaghi (1946) |
Swelling rock | NA | Up to 75 m | Same as Terzaghi (1946) |
Rock load Hp in m of rock roof of support in tunnel with width B (m) and Height (m) at depth of more than 1.5 (B+Hp).
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