**1. Introduction**

Hydraulic fracturing provides only plane principal stresses, and no information on the other components of the tri-axial stress field is available [1]. In hydraulic fracturing, continuous water pressure is applied in confined area which tends the rock to be tensile and while pressure exceeds the strength of rock, water escapes in weak plane formed [1–15].

Haimson studied on various rock specimens of variable pore pressures. Around 400 specimens have been tested under rational loading conditions. All the specimens failed under tensile manner. He was the one who pointed out the role of water pressure in fracture propagation. His study proved that water pressure increases the pore pressure in turn unable to obtain actual results. The reliability and validity of this method is also questionable when dealing with porous and fractured rocks encountered in underground mines [2–8, 16].

The main objective is to develop a proper and add-on technique for hydraulic fracturing for stress measurement in porous and fractured rocks. Hydraulic fracturing tests were conducted by using different flow rates of water inside the fractured rocks and high viscous fluid in porous strata. The stresses evaluated by this method was correlated with normal flow rate hydraulic fracturing method at the same locations where the rock mass was not fractured, and to circumvent the effect of the porousness, by overcoring technique since porosity does not have any influence on overcoring procedures. The correction factor was introduced during stress evaluation by hydraulic fracturing method in fractured and porous rocks. Normal

flow rate is flow rate of fluid during hydraulic fracturing test ranging from 6 to 8 l/min.

This new technique will be helpful in conducting the stress measurements in porous and fractured rocks, which will be highly beneficial to both mining and hydropower related excavations.

The measurement of the state of in-situ rock stress provides essential data for the rational design of underground excavations based on the principles of rock mechanics [13].

The hydraulic fracturing test to determine the stress tensor is rather simple and robust, and it also gives the required magnitude and orientation of the maximum principal stress [17]. Several techniques and equipment have been developed, and are still being developed, to measure this parameter [8].

The main disadvantage of this hydraulic fracturing method when compared to other methods such as overcoring, flat jack and stress-meter, is its limitation when applied to porous and fractured rocks [14, 18]. Rock mass may contain natural occurring discontinuities, including fractures which dissipate fracturing liquid. Hence, it is more difficult to use the hydraulic fracturing process to determine stress conditions in porous and fractured rocks. Whereas in 'non-fractured rock mass', i.e., rock mass without fractures, this limitation is not there. As rocks in a large number of underground coal mines belong to this category, i.e., porous and fractured rocks, finding a methodology to accommodate such rock conditions is essential.

If a high flow rate of fluid is used, experience has shown that there is a tendency of induced fractures to rotate and change the direction of the initial fracture. As the direction of the induced fracture is one of the input parameters for the evaluation of hydraulic fracturing stress, any change in the direction of fracture due to the influence of some external factor, like the flow rate, will give rise to an anomalous pressure or stress value [15].

If, instead of water, a higher viscosity fluid is used for fracture initiation, pressure can be readily developed inside the induced or pre-existing fractures which can be taken for evaluation of stress, but the influence of viscosity on the evaluation of magnitude and direction of stress is not validated.

The above discussed two points show that the limitation in adopting hydraulic fracturing method in porous and fractured rocks is rather due to non-availability of proper technique than the principle of hydraulic fracturing.
