Development of Rock mass classification systems(Part-IV)
2.6 Q-system
The Q-rating is developed by assigning values to six parameters. These are:
(a) RQD;
(b) Number of discontinuity sets;
(c) Roughness of the ’most unfavourable’ discontinuity;
(d) Degree of alteration or filling along the weakest discontinuity;
(e) Water inflow; and
(f) Stress condition.
Q = RQD/Jn * Jr/Ja * Jw/SRF
Where
RQD = rock quality designation,
Jn = joint set number (related to the number of discontinuity sets),
Jr = joint roughness number (related to the roughness of the discontinuity surfaces),
Ja = joint alteration number (related to the degree of alteration or weathering of the discontinuity surfaces),
Jw = joint water reduction number (relates to pressures and inflow rates of water within the discontinuities), and
SRF =stress reduction factor (related to the presence of shear zones, stress concentrations and squeezing and swelling rocks).
Table 8 Q-System parameters
Table 8 Q-System parameters
The motivation in presenting the Q-value in this form is to provide some method of interpretation for the three constituent quotients.
The first, RQD / Jn, is related to the rock mass geometry: Q increases with increasing RQD and decreasing number of discontinuity sets. RQD increases with decreasing number of discontinuity sets, so the numerator and denominator of the quotient mutually reinforce one another. Basically, the higher the value of this quotient means the better the ’geometrical quality’ of the rock mass.
The second quotient, Jr / Ja, relates to the ’inter-block shear strength’ with high values of this quotient representing better ‘mechanical quality’ of the rock mass: the quotient increases with increasing discontinuity roughness and decreasing discontinuity surface alteration. The different discontinuity sets in the rock mass may have different roughnesses and degrees of alteration, so the Q-system uses the worst case.
The third quotient, Jw / SRF, is an ’environmental factor’ incorporating water pressures and flows, the presence of shear zones, squeezing and swelling rocks and the in situ stress state. The quotient increases with decreasing water pressure or flow rate, and also with favourable rock mass strength to in situ stress ratios.
The Q-system is more complex to use than the RMR system. Table 8 shows the full range of classes for the six parameters involved in the system
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