The study is conducted at PT Cibaliung Sumberdaya Underground Gold Mining which apply cut and fill method. The stope opening is excavated bellow sill pillar as bottom boundary of previous slice.

Excavation below the sill pillar deals with static load that generates from filling material above the sill pillar. Excavation will change the stress distribution and also triggers the deformation in the perimeter of the stope opening. For anticipating the instability problem, monitoring system in the roof and wall of the opening is then applied in order to monitor the deformation.

Commonly, convergence meter is one of the most suitable instruments used in underground mining monitoring system. Convergence meter provides a higher accuracy rate but potentially disturbing the mining activity. Along with the development of technology in surveying, the displacement monitoring can be observed by reflectorless total station unit. Monitoring was observed on twelve points which were placed in the perimeter stope opening. Observation points were placed in active stope opening (sill drift) at depth of 141.2 m below surface. The movements of all points were recorded continuously for eight days period.

The result of observation indicates that the deformation of the stope opening is not perfectly squeezing as expected. This phenomenon is probably cause by the influence of another opening near the observed opening. The stress surrounding the opening is then calculated by using strain value from monitoring data and Young's modulus from rock laboratory testing. In order to obtain the distribution of stress surrounding opening, the numerical model using finite element method is conducted.

Results obtained from the monitoring data calculation shows the behavior of the displacement on the wall moving towards the silldrift while displacement upwards towards the roof of the tunnel. This phenomenon can be caused by several factors, among others:

  1. the influence of the horizontal stress is higher than vertical stress.

  2. the effect of softer material is above silldrift.

  3. the influence of the cavity from previous mining above the monitoring location.

Displacement results of the model with stress ratio k = 1.2 gives the smallest error on the displacement value from the calculation of survey data with error value on the Z axis 0.00406 and the X axis 0.00201. While the model without rock supporting gives the greatest error value on Z axis 0.004840 and X axis 0.008127. the model use 1.8m split-set gives error value on Z axis 0.013676 and X axis 0.002271. The greater error value that means the greater displacement so increase unstable of the stope.

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