Question

If the Q-system value of a rock mass is low, which specific numerical support requirement must be increased to compensate for poor rock quality?

Accepted Answer

When the Q-system value—a numerical index used to classify rock mass quality and determine tunnel support requirements—is low, the specific numerical requirement that must be increased is the Support Pressure (P_roof), which represents the amount of force the rock mass exerts on the tunnel ceiling. The Q-system calculates this value by assessing parameters like rock block size, inter-block shear strength, and active stress, resulting in a number where lower values indicate weaker, more fractured, or more hazardous rock. Because poor-quality rock has less structural integrity and is more prone to collapsing under its own weight or shifting into the tunnel void, you must install stronger support measures, such as denser steel rib spacing, thicker shotcrete (a mixture of cement, sand, and aggregate projected pneumatically at high velocity), or more robust rock bolts, to generate the higher support pressure required to stabilize the opening. Think of this like reinforcing a bridge: if the foundation soil is soft and shifting rather than solid rock, you must build a much thicker, stronger concrete base to carry the same weight without sinking.

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If the Q-system value of a rock mass is low, which specific numerical support requirement must be increased to compensate for poor rock quality?

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