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Underground

System for Early Prediction of Catastrophic Roof Failure

Underground » Strata Control and Windblasts

Published: December 98Project Number: C5022

Get ReportAuthor: Ken Mills | SCT Operations

The project aims to develop a system to measure, on a routine basis, the horizontal pressure acting through the roof strata of underground coal mine roadways. This system dovetails with existing monitoring and numerical modelling techniques to allow early prediction of catastrophic roof falls and timely placement of appropriate support.

A small diameter stress measurement instrument has been successfully developed within the project. This instrument makes high quality, three dimensional, stress measurement accessible to underground coal operations on a routine basis. The instrument utilises proven ANZI stresscell technology but with greatly reduced requirement for heavy drilling equipment, specialist personnel and time.

The ANZI 29 stresscell is 29mm in diameter and determines the full three dimensional stressfield in a single measurement. The instrument is small enough that it can be overcored using a standard TT56 core barrel that gives a 42mm diameter core.

The small size of the ANZI 29 saves drilling and installation time, reduces the weight of drilling and other equipment, and reduces the cost of stress measurement. The instrument carries 18 electrical resistance strain gauges that are cemented directly to the rock under pressure. These gauges provide a reliable measurement of the stressfield as well a measure of the confidence that can be placed in the result.

Laboratory calibration tests and field comparisons with other stress measurement instruments confirm the instrument's reliability. Drilling time for a single instrument installation is reduced from 4-6 hours to less than 2 hours. The weight of specialist overcoring equipment is reduced about tenfold and overall equipment weight will reduce further with additional development.

The ANZI 29 is currently used in commercial applications where it offers particular advantages over existing systems. In due course, confidence in the system will develop to a point where it is widely used for routine measurement.

A datalogger capable of logging the ANZI 29 during overcoring is still in development with three fully operational units suitable for field testing and intrinsic safety certification currently being manufactured. The datalogger is intrinsically safe and fits into a 29mm diameter borehole directly behind, and rigidly fixed to, the stresscell. It is capable of logging all 18 strain gauges together with various temperature sensors at 2 second intervals. This logging rate is about 15 times faster than is possible manually so overcore drilling can be speeded up. Material property tests can be conducted in a few minutes and data can be downloaded directly into a computer for analysis.

The project also focussed on obtaining a fast-curing adhesive for the ANZI 29 stresscell. An epoxy based cement has been developed by a local chemical manufacturer specifically for this application. This adhesive cures in about 6.5 hours compared to existing systems that take 18-30 hours. Ongoing development is aimed to further reduce the cure time.

The final report details the development of the ANZ129 Cell.

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