TITLE: AN IMPROVED ROCK MASS BEHAVIOR NUMERICAL MODEL AND ITS APPLICATIONS TO LONGWALL COAL MINING The rock mass constitutive models should include elastic moduli, strength and stiffness of intact rock as well as those of joints and geometric properties of joints. The post-failure behavior of intact rock and joints must also be specified. A direct application of the above comments is in longwall coal mining where the coal as well as the immediate roof and floor strata may undergo controlled brittle failure and associated weakening in tension and shear based on post- failure characteristics of the rock mass. In addition to controlled failure and weakening of the rock mass ahead and over the longwall face, large scale caving and compaction of caved materials occur behind the longwall face. Itasca’s Cave-Hoek three dimensional constitutive model has the ability to model longwall mining process that involve the above mentioned mechanism of rock mass failure and compaction. However, its testing to date is limited. The overall goals of research are two-fold: 1) Develop numerical modeling approaches that consider the caving behavior of jointed rock masses in design and analysis, and 2) Apply these techniques in designing stable chain-pillars and set-up rooms for longwall coal mining. Specific objectives are to: 1) Develop an improved constitutive model for prediction of post-peak behavior of rock masses typical of longwall mining in Illinois, 2) Implement the improved model for predicting gob material behavior using FLAC3D numerical code (most commercial codes do not have a built in model for gob material) and its effects on load transfer into gate entries, 3) Identify mechanisms of instability in setup rooms, 4) Develop alternate 3- and 4-entry set-up room geometries using 3-D numerical analyses, 5) Implement and field demonstrate developed geometries, and 6) Monitor performance of implemented geometries through field monitoring. An alternative method to estimate the residual strength of a rock mass is developed. A yielded rock mass and a rock fill have several common characteristics including dilation behavior under low confinement and extensive crushing of contact points under high stress, which decrease dilation. The residual strength takes on an initial value in the immediate post-peak (corresponding to near-zero porosity) condition, then degrades to an ultimate residual strength that is lower as a result of bulking, a corresponding increase in porosity, and a drop in interlock under continued shear. The following comments summarize the key findings of this research: • The model for predicting rock fill material shear strength was used as a residual strength criterion. A relationship for estimating Hoek-Brown residual parameters as a function of equivalent roughness of rock fill particles and basic friction angle was used. • Macro-level measurements around setup rooms and gate entry development areas indicated that most of the observed ground control problems may be related to subsidence movements over the setup rooms area. • Mechanisms that may be responsible for poor ground conditions in setup rooms and adjoining gate entries were identified. Collected field data and numerical analyses results tend to support the identified mechanisms. • The integrated field monitoring and numerical modeling study here assisted the cooperating coal company to plan for additional supports in development entries impacted by the fault zone and in taking appropriate safety measures while the longwall face advanced toward the fault and crossed it.