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Monitoring of Roof Deformation and Delamination using Deep Hole Multi Point Extensometers

Open Cut » Highwall Mining

Published: June 00Project Number: C9069

Get ReportAuthor: Baotang Shen, Mary Duncan Fama, D Adhikary | CSIRO Exploration & Mining

The work presented in this report is part of the Australian Coal Association Research Program (ACARP) project C9069 "Monitoring of Roof Deformation and Delamination Using Deep-Hole Multi-Point Extensometers". This project is an extension of the ACARP project C8033 "Optimal Design and Monitoring of Layout Stability for Highwall Mining".

Three surface extensometers were installed in Pit 20DU North at the Moura Mine and they recorded the vertical displacements at different depths in the roof strata during the mining of 20 entries and for a period of nearly three months after the completion of mining in this section of the pit.

One extensometer recorded a surprisingly large displacement immediately after mining had passed its location. It is believed that a small scale local roof fall may have triggered this displacement.

Two extensometers recorded increasing roof displacement as mining approached the monitored entry but in a quite opposite manner. One recorded a contraction of the roof strata whereas the other recorded expansion. In both cases, the maximum displacement was about 2mm. It is believed that movement along fault 20DU_R_1 was the major cause of this difference. This fault lay between the two extensometers and was predicted to terminate in the reserve. Movement along this fault could have caused horizontal stresses to increase on one side and to decrease on the other side. The horizontal stress change would have caused the vertical expansion or contraction due to the Poisson effect.

All three extensometers showed some irregularity in displacement variation with depth. The limited accuracy of the extensometers could be the cause of this irregularity. It may also be possible that different horizontal stress change in different roof layers contributed to this irregularity.

Some time dependent vertical displacements were recorded during the three month period after the completion of highwall mining in Pit 20 DU North. The time induced displacements were greater than those recorded immediately following mining. The strong time-dependent effect could be a result of a readjustment of regional horizontal stress in response to exposure to a number of faults in Pit 20DU North. The stress redistribution caused by mining is believed to have been complicated and the readjustment process could take a long period of time to complete.

Apart from the large displacement in Borehole A, no significant displacements in the roof strata were recorded during mining and three months after mining. The extensometer measurements were consistent with field observations and microseismic monitoring results which indicated that the panel in Pit 20DU North was stable during mining and at least for three months after mining.

It needs to be emphasized that the local geology and the rock mass behaviour are often far more complicated than that we have realized. This monitoring exercise was aimed to improve our understanding on the rock mass response to highwall mining. The somehow surprising results appear to have confirmed the complexity of the behaviour of a fractured rock mass. The results are explained based on our best knowledge of the local geology and the rock mass behaviour. However, they could also be caused by other factors which are beyond our understanding and observation.

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