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Next: CONCLUDING REMARKS Up: 無題 Previous: FORMULATION OF FMBEM FOR

SIMULATION

In order to investigate the characteristics of the improved conical-end borehole strain measurements method (ImCESM) method, simulations of the overcoring process are made by the FMBEM. For simplicity, the rock mass is assumed as linear with unit Young's elastic modulus and Poisson's ratio $\nu=1/4$, and it is in the state of unit magnitude of stresses. As an initial state, a conical-end borehole attached at its apex with a small-diameter inspection borehole is formed already. And the strain-gages are placed at the prescribed points on the surface of the conical-end. The aim of this simulation is to monitor the developments of strains released as the progress of the ovecoring with the same diameter as that of the borehole.

The size of the model and the boundary element model at initial state are shown in Fig.2. The DOF of the initial state before overcoring are 13824. The boundary element meshes in detail around the conical-end (x1=500-536.5[mm]) are shown in Fig.4. Although the length of borehole is several-tens meters in the practical applications, only 500[mm] part of the length of the borehole is considered in this analysis because the released strains induced by overcoring at far distance from the conical-end are negligible. The length of ovecoring denoted by L are chosen successively as $0.0,7.3,14.6,\cdots,73$[mm]. The boundary element model of the overcoring are shown for some cases, in Fig.5, 6, 7 which respectively correspond to the cases L=7.3,21.9 and 36.5[mm]. The degrees of freedom of the final case (L=73[mm]) reached 48384. The released strains at the six measuring points (12 elements) are shown in Fig.3.

  
Fig.2 Boundary Element Model (Side View)
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Fig.3 Measuring Points
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As the numerical aspects for FMBEM analysis, some infinite series are truncated at 10 terms and the maximum number of boundary elements included in a cell are set 200.

As the initial stress field unit stress $\sigma ^\infty _{11}$ and $\sigma ^\infty _{22}$ are considered. The results of simulations by using the FMBEM are given in Fig.8 and Fig.9, which clearly show how strains develop by the progress of the overcoring.

For this geometrical shape, it may be concluded that the released strains converge when the overcoring length reaches about 70% of the diameter of the borehole. This fact means that the reliable data may be obtained if the overcoring is done successfully about the length of the borehole diameter at least.

  
Fig.4 Boundary Element Meshes around a Funnel-end Borehole and Overcoring
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Fig.5 Boundary Element Meshes around a Funnel-end Borehole and Overcoring
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Fig.6 Boundary Element Meshes around a Funnel-end Borehole and Overcoring
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Fig.7 Boundary Element Meshes around a Funnel-end Borehole and Overcoring
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Fig.8 Strain vs. Overcoring Length L for Unit $\sigma ^\infty _{11}$
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Fig.9 Strain vs. Overcoring Length L for Unit $\sigma ^\infty _{22}$
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next up previous
Next: CONCLUDING REMARKS Up: 無題 Previous: FORMULATION OF FMBEM FOR
Toru Takahashi 平成11年10月13日