**9. Conclusions**

The work described here however represents the results of field evaluation programme, in which a very pragmatic point of view is being taken. The opportunity is taken to evaluate the results obtained from hydraulic fracturing with the results acquired from overcoring at the same site. The results acquired from overcoring are deemed to deliver a trustworthy indication of the in situ stress field.

The in-situ state of stress is measured for two principal reasons

a. To predict rock response to changed loading conditions caused by construction or excavation, including new engineering procedures that require use of the in-situ stress field as part of the design, and

b. To further understand the tectonic processes.

The hydraulic fracturing stress measurements had become a broadly used technique for determining in situ stresses at depth. It is a technique for understanding rock mass behaviour in conjunction with stability of the excavations in rock. Because of the rapidly expanding use of this method, the method is still evolving in certain rock mass conditions [1, 3–11].

Hence the key objective of this project is to develop a proper methodology for in situ stress measurement by hydraulic fracturing method in porous and fractured rock media, encountered in some of the coal mines as well as in some of the underground tunnels of hydroelectric projects in the Himalayas.

To fulfil the objective of the project, it was proposed to conduct in situ stress measurements in fractured and porous rock mass areas by two different methods at the same location. The hydraulic fracturing stress measurements were conducted by adopting both high flow rate and normal flow rate method in fractured rocks and high viscosity liquid method, and overcoring methods in porous rocks. The stress results by the two methods were correlated with already recognized or established technique as a benchmark. The results of hydraulic fracturing stress measurement methods were authenticated, so that this method can be implemented for stress measurement in porous and fractured rocks and use them widely in mining and hydroelectric projects. This will aid in producing a data bank for in situ stress, which will be highly advantageous for both mining and hydropower industries wherever the rock mass is fractured or porous and the stress measurements are indispensable for designing the support systems.

In the first part of the project, two sites were selected inside a proposed powerhouse tunnel of one of the hydroelectric projects in the Himalayas where the rock formations are fractured. Boreholes were drilled 10–30 m deep depending upon the requirement and site conditions. In situ stresses were measured inside these boreholes by hydraulic fracturing method using manipulation of flow rate. The stress evaluation was made using latest software. The stresses evaluated by this method was correlated with normal hydraulic fracturing method at the same locations where the rock mass was not fractured.

A total of 24 hydraulic fracturing tests were attempted in different EX size boreholes inside the tunnels of the proposed powerhouse and intake drift areas where the rock mass was fractured. The testing zones were selected at depths between 10 and 30 m. In normal conditions, and in good rock mass, the pumping rates of 4–6 l/min are sufficient to conduct the hydraulic fracturing test, but such pumping rates proved to be insufficient for tests in the fractured zones. As this problem became apparent during testing, a high-pressure pump was used to achieve higher pumping rates of up to 18 l/min.

It was observed that with increasing or decreasing pressure in each cycle, the pressure also declined automatically after certain increment of pressure. It is interpreted that, since the flow of water is affected by the whole fractured rock mass, the pressure changes were due to the opening of fractures at different spatial positions.

The hydraulic fracturing tests in good rock mass exposed, repeatable pumping pressures, with the same fracture. This indicates that we were creating a new hydraulic fracturing in a formation which had less tensile strength. Data was evaluated from preexisting reopened fractures, and the orientation of these fractures was analysed to understand how the instantaneous shut-in pressures during the test are related to the value of normal stress across the fracture.

The most reasonable explanation, however, is that at the fast-pumping rate the pressure gradient was so large that the tensile strength of the rock near the borehole exceeded before the shear strength of the outer part of the rock mass was reached.

After shut-off of the pump, instantaneous shut-in pressure was obtained to get the normal stress across the fracture and to calculate the minimum principal stress magnitude and direction.

Stress measurements were conducted by using high viscous liquid in porous rocks; in the same rock mass, at about 1 m away, overcoring method using CSIRO Hollow Inclusion Cell was also carried out. The stresses obtained from hydraulic fracturing method using high viscous liquid were correlated with stresses measured by overcoring method. The stress measured by the overcoring method was used as a benchmark as this method does not suffer from the presence of porosity of the rock.

The average long term instantaneous shut-in pressure showed reasonable agreement with the magnitude of the near vertical principal stress component obtained from overcoring at the site. This was the case in which a viscous fluid had to be employed specifically to enable a crack to be initiated, and the shut-in pressure used to make estimates of some stress component magnitudes.

The results indicated the effect of test fluid viscosity on estimation of the magnitude of minor horizontal stress components. It was observed that the relative differences between the tangent intersection and tangent divergence estimates for instantaneous shut-in pressure decreased as the viscosity of the test fluid increased.
