Publication: Evaluation of rock overstressing in a deep underground excavation of pahang-selangor raw water transfer tunnel
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Date
2020-07-01
Authors
Azit, Romziah
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Abstract
Rock overstressing issues such as rock burst or spalling failure will significantly influence the overall performance of an underground excavation. This study focuses on the stress induced problems with special reference to prediction of spalling failures due to rock mass overstressing of a deep underground excavation. Stress distribution was analyzed to determine potential overstressing based on estimated rock physical and mechanical properties, and in-situ stress. The in-situ stress measurement was conducted to determine the coefficient of lateral stress (Ko) at high overstressing potential location. Rock mass strength parameters were estimated based on the Hoek-Brown failure criterion. A comparison between the Mohr-Coulomb elastic-perfectly plastic and Cohesion Softening Friction Hardening (CSFH) elastic-brittle plastic material model was performed to predict spalling failure. The parametric study for selected method was conducted to determine appropriate parameters for simulation of real spalling cases. These parameters are used as an input of numerical model to predict the depth of spalling. Thus, numerical simulation of Pahang-Selangor Raw Water Transfer tunnel excavation was conducted to assess the rock overstressing, depending on the in-situ stress conditions and rock compressive strength. Furthermore, back-analysis of rock mass classification was conducted to identify the appropriate rock strength estimation for numerical analysis. Distribution analysis results were initiate the potential of rock overstressing will occur at 600m depth and more. From the in-situ stress measurement the Ko value was 0.38, thus the spalling failure type was predicted. The CSFH model was found suitable for spalling failure prediction with some modification made at certain strength parameters. The parametric analysis was identified that the peak friction angle in the range of 10o and assumption of residual cohesion equal to rock mass cohesion is suitable for to determining the spalling depth. Validation of the numerical analysis indicated that the modified CSFH is suitable for spalling failure prediction with respect to location, depth and shape. Thus, from the field observation of spalling failure cases the predicted values indicate a satisfactory match between the numerical predicted and observed spalling depth. Furthermore, these parameters will serve as reference for spalling failure prediction of a Main Range Granite in Peninsular Malaysia.