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|Title:||Geomechanical model study of the behavior of underground openings in rock subjected to static loads. Report 2, Tests on unlined openings in intact rock|
|Authors:||University of Illinois at Urbana-Champaign. Department of Civil and Environmental Engineering.|
United States. Defense Atomic Support Agency.
Heuer, R. E.
Hendron, Alfred Joseph.
|Publisher:||Weapons Effects Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Contract report (U.S. Army Engineer Waterways Experiment Station) ; N-69-1 rept.2.|
Abstract: Model laws governing the design of geomechanical model studies of underground openings in rock subjected to static loads are developed using dimensional analysis and the theory of models. The significant variables influencing the prototype which are considered in this study are free-field stresses, intact rock properties, rock mass discontinuity properties, and the opening geometry. Body forces are considered insignificant as a first approximation. The prototype chosen for study is a short section of a long circular tunnel which is underground at a depth of more than 4 tunnel diameters. The development of a modeling material which successfully models the intact properties of rock is described. It was found that modeling materials described in the literature by previous investigators were not satisfactory because they lacked a sufficiently high angle of internal friction. Satisfactory frictional strength in the materials was achieved by developing sand/plaster of Paris mixtures with a dense packing of sand grains. The size of the model tested was 24" x 24" x 8". The design and development of a loading apparatus which allows independent control of the three principal stresses on the block is described. Uniformly distributed loads of 96 tons were applied to the 24" x 8" faces of the model. To maintain a plane strain condition in the model, loads of up to 144 tons were applied to the 24 11 x 24" faces to null strains parallel to the eight inch dimension. Friction along the loading faces was controlled with sheets of teflon. Friction losses were reduced to the order of ten percent or less of the applied loads. The development of an instrumentation system for the model is described. Of particular interest is the development of techniques for imbedding electrical resistance strain gages within the model sand/plaster material to measure radial and circumferential strains at points within the model around the tunnel. Model behavior observed was analyzed and compared to theoretical predictions and field observations. Fractures observed were shear fractures oriented at an angle of (45°- 0/2) to the direction of the maximum principal stress. Some tentative observations are made concerning the feasibility of utilizing relatively large scale models to study the behavior of underground openings, and suggestions are made for testing of more complex geomechanical models of underground openings.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Contract Report|
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|CR-N-69-1-Report-2.pdf||12.19 MB||Adobe PDF|