**3. Geology and tectonics**

#### **3.1. Geology**

measurements at the site and included the use of 3D numerical methods to understand the impact vis a vis mining to help in the designing of openings below mined out areas (Whyatt-

In this study, in a deep underground copper mine, stress measurements using the hydrofrac‐ ture method were carried out in two stages. At the pre-mining stage, when only few devel‐ opments were available and at the post mining stage from the developments between the

Stress data generated from the stress measurements produced a value for the mining induced stress gradient (post mining) which was found to be totally different from the stress gradients of the area measured in the pre mining stage. The orientation of the Maximum Horizontal principal Stress was found to be perturbed and lying perpendicular to the strike of the orebody as against parallel orientation found during pre -mining stage. To understand the impact of the mining on the stresses a 3-D numerical modeling study was carried out using a boundary element method. The initial stress ratio from the pre mining stage measurement was used with gravity loading to account for the surface topography, which is hilly. Three observation points were monitored for stress change in mining, resulting from excavation effects and this data was found to be in agreement the measured induced stresses. The study results helped in the

Hindustan Copper Limited (HCL), a public sector undertaking under the administrative control of the Ministry of Mines, is engaged in mining, beneficiation, smelting, refining and casting of refined copper metal. HCL maintains focused on its mission and vision which include increasing the ore production by three times over a decade and implementing continuous improvement in productivity. To continue to achieve these goals, it has geared up to tap the resources from the un-mined areas by designing stopes below the mined areas.

The present study was undertaken in Kolihan Copper Mine, an important captive under‐ ground mine of Kolihan Copper Complex of HCL and this mine is situated near the village of Khetri, in the District Jhunjhunu, Rajasthan. The mine plan to develop stope blocks at lower

For the design of stopes, in-situ stress is one of the most important factors which dictates the size of the stopes and the size of the pillars and the sequence of extraction. The main host rocks of Kolihan mines are garnetiferous chlorite quartz schist, quartzite and amphib‐ olite quartzite. The strike length of the ore body is 600 m with a width varying from 30 m to 100 m and the ore dips steeply to almost vertical. The main mining method adopted is Large Diameter Blast Hole Stoping. The mine extends from 486 ML to 0 ML. (Hindustan

A detailed stress measurement programme was undertaken before the commencement of any stoping activity (pre- mining stage) between 486 mL and 184 mL for the determination of stress

levels below the mined out areas to sustain and increase the productivity.

JK, Williams-TJ, Blake. W (1995).

916 Effective and Sustainable Hydraulic Fracturing

**2. Background**

Copper Limited internal notes)

mined out area and the non-mined out area.

design of stopes, mining sequences and rock reinforcement.

The rock formations of the area belong to the Alwar and Ajabgarh series of the Delhi system and are younger than the Aravalli system. Both rock formations are highly deformed and metamorphosed. Rocks occurring at Kolihan mines are Amphibolite quartzite/garnet chloride with principal economic mineral is chalcopyrite. Strike of the formation is N 300 E - S 300 W, dipping 500 - 850 westerly (Fig 1)

**Figure 1.** Geological and tectonic map of the project area

#### **3.2. Tectonics**

Structurally the thick prism of metasediments comprising rocks of Alwar and Ajabgarh series has been deformed into northeast –southwesterly trending longitudinal folds of large areal extent. In the northern part of the belt the simplest structures are represented by Khetri anticlines and synclines with increasing intensity of deformation. The simple structure passes westward into overturned Kolihan syncline which is slightly compressed in the north.

In the central part of the belt the formations show as anticline structures.

The southern part of the belt is separated from the central part by a major transverse fault. The southern part of the fault is marked by anticlines and synclines. The asymmetrically over‐ turned Kolihan syncline which is locally recumbent occupies a narrow zone. It plunged towards the SW and in the southern part the limbs are low dipping but gradually steepen northwards. The syncline is defined by the younger quartzites of the Ajabgarh series of reverse faulting (Dasgupta 1965).
