Abstract

There are various methods to determine in situ stress parameters, but each method is having its own advantages and limitations. Among the methods available, the hydraulic fracturing method is the most commonly adopted procedure for in situ stress measurements because of its simplicity, reliability and economic viability. But, the limitations with this technique are that some assumptions implicit for this method such as that the rock mass is continuous and elastic. Moreover, the reliability and validity of this method becomes questionable in porous rocks that may encounter in underground tunnels. These limitations are experienced ever since the introduction of this method, especially in porous rocks.

In general, the recognition of fracture initiation in the hydraulic fracturing test has not proved difficult. The relatively slow rates of pressurisation have ensured that when fracture initiation occurs, the sudden increase in volume has led to a marked drop in pressure in the porous section, which is easily recognised from the pressure record. But the drop in pressure in a porous rock is far more difficult to recognise. This is due to the fact that pressure cannot be developed if the rate of leakage in the formation is equal to or higher than the flow rate applied for fracture initiation. This problem of non-generation of water pressure can be tackled by use of a high viscosity fluid.

Stress measurement were conducted by hydraulic fracturing method by using high viscous liquid in porous rocks. The stresses evaluated by this technique were correlated with stress measured by overcoring method. The stress measured by overcoring method was used as bench-mark as this method does not suffer from the presence of porosity of the rock. This new technique will be helpful in conducting the stress measurements in porous rocks, which will be highly beneficial to both mining and hydropower related excavation.

1.
Introduction

Hydraulic fracturing method is the accepted technique for measurement of in situ stresses required to orient the long axis of underground caverns, galleries in hydroelectric projects and underground mines in India and abroad. But the applicability of this method in coal mines is limited because of the presence of porous rocks. In situ stresses in coal mining projects are determined using overcoring techniques from already excavated underground openings. Though porosity of the rock does not have any influence on the stress measured using this technique, due to practical limitations, it can be used for shallow depth only. However, the need for the stress data in coal basin sediments from the surface to depths up to 500 m or more has always been there for planning of colliery layouts in advance. Overcoring technique obviously precludes the use of the stress data in the early planning phases. Therefore, it has been widely accepted that hydraulic fracturing technique will be advantageously useful though in the presence of the porous media provided if the practical limitations are overcome.

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