The 1992 edition of IS 13365 Part 2 outlines a quantitative framework for classifying rock mass quality to predict support pressures in subterranean excavations like tunnels and mines. This standard is crucial for geotechnical and mining professionals to evaluate rock stability and design effective support systems using parameters such as RQD, joint attributes, water pressure, and stress factors.
Overview
The 1992 edition of IS 13365 Part 2 outlines a quantitative framework for classifying rock mass quality to predict support pressures in subterranean excavations like tunnels and mines. This standard is crucial for geotechnical and mining professionals to evaluate rock stability and design effective support systems using parameters such as RQD, joint attributes, water pressure, and stress factors.
Audience
Contents
Structure
This section covers the classification of rock mass quality, accounting for stress reduction and water pressure effects on excavation stability.
Stress Reduction Factor (SRF) classifications describe various weakness zones and competent rock stress categories, detailing associated risk and SRF ranges.
Joint Water Reduction Factor (Jw) categories are also outlined with corresponding water pressure ranges.
IS 13365 Part 2 directs users to IS 11315 Part 11 (1985) for methodologies related to Core Recovery and Rock Quality Designation (RQD).
Key formulas include the calculation of RQD from volumetric joint count and classification tables for RQD and Q-values used in rock mass evaluation.
Defines the formula for calculating rock mass quality Q as a product of RQD over joint set number, joint roughness over alteration, and water reduction over stress reduction factors.
Includes detailed explanations and tables for RQD calculation, joint set number (Jn), joint roughness (Jr), joint alteration (Ja), joint water reduction factor (Jw), and stress reduction factor (SRF).
Guidelines for field data collection and parameter assessment are included to ensure accurate Q estimation.
Presents empirical relationships to estimate ultimate and short-term support pressures on roof and walls based on rock mass quality indices Q and correction factors for overburden and tunnel closure.
Includes key formulae and notes on critical tunnel closure limits and support system recommendations.
Explains the calculation of equivalent unsupported dimension (span, diameter, or height) using rock mass quality Q.
Defines Excavation Support Ratio (ESR) values for different types of excavations and describes conditions for permanent unsupported openings along with correction factors related to tunnel closure.
Outlines procedures for collecting field data including core or excavation lengths depending on rock uniformity, evaluation of shear zones, and methods for obtaining separate Q values for roof, floor, and walls.
Includes instructions on adjusting joint water reduction factors for specific conditions and rounding numerical values per standards.
Frequently Asked
Rock mass quality (Q) is computed using the formula:
[ Q = \frac{RQD}{J_n} \times \frac{J_r}{J_a} \times \frac{J_w}{SRF} ]
where RQD is the Rock Quality Designation calculated from the volumetric joint count (Jv) by the relation:
[ RQD = 115 - 3.3 \times J_v ]
If RQD is less than 10%, it is taken as 10% for calculations. The parameters Jn (joint set number), Jr (joint roughness), Ja (joint alteration), Jw (joint water reduction factor), and SRF (stress reduction factor) are determined through field assessments. Rock mass quality is then classified using corresponding tables for RQD and Q values to assist support pressure predictions.
Support pressure prediction depends chiefly on the classification of rock mass quality, which incorporates geological and geomechanical attributes such as rock strength, the number and nature of joint sets, water pressure within joints, in-situ stress conditions, and the geometry of the opening. These parameters collectively determine the load-bearing capacity of rock and guide the design of suitable support systems to ensure stability and safety.
Joint water pressure reduces the effective normal stress acting on rock joints, thereby decreasing the shear strength of the rock mass. This effect is quantified by the Joint Water Reduction Factor (Jw), which lowers the overall rock mass quality (Q). A decreased Jw leads to a lower Q value, indicating poorer rock quality and consequently higher required support pressures. Accurate assessment of Jw is essential for designing reliable support structures, and drainage measures can improve Jw, reducing support demands.
IS 13365 Part 2 presents empirical correlations to estimate support pressures using rock mass quality indices and correction factors. Ultimate roof support pressure (Pru) is calculated by:
[ P_{ru} = 20 \times Q_{ru}^{-1/3} \times f \times f' ]
Ultimate wall support pressure (Pwu) uses:
[ P_{wu} = 120 \times Q_{wu}^{-1/8} \times f \times f' ]
Short-term roof and wall support pressures (Pri and Pwi) are similarly determined with adjusted Q values and the same correction factors for overburden (f) and tunnel closure (f'). These formulas help estimate the load requirements for various support systems based on rock mass behavior.
The allowable unsupported span dimension (De) is calculated from rock mass quality (Q) using the equation:
[ D_e = 2 \times Q^{0.4} ]
This formula provides the maximum span, diameter, or height for self-supporting underground openings without additional support. A higher Q value indicates better rock quality, permitting larger unsupported spans while maintaining stability. This calculation is fundamental in tunnel design to ensure safety during excavation.
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