**7. In-situ stress measurements by overcoring technique in porous rock mass**

Overcoring measurements are common in civil and mining engineering and conducted for design of underground openings [2]. The quality of the measurement depends on quality of drilling, gluing and overcoring, and on the rock characteristics such as anisotropy, discontinuities, and heterogeneity [2, 7, 14, 16, 18, 25].

#### **7.1 Overcoring test procedure**

*Drilling HX size hole—* HX size (150 mm diameter) hole was drilled up to a depth of 7 m in the roof sandstone (**Figure 22**).

*Core retrieval—*The overcored rock is recovered from the hole using corebreaking chisel that is attached to the rods used for wedging the core off the face (**Figure 23**). An intact length of core (>500 mm) free of fractures and joints is ideal for a satisfactory overcore. The recovered core was considered satisfactory, and free of fractures and voids [16].

*Drilling EX size borehole (Pilot hole)—*After removal of the HX size core from the borehole, the EX-size Pilot hole (38 mm) was drilled up to 50 cm exactly at the centre of the HX size bore hole from 7 to 7.5 m (**Figure 24**). This hole was collared concentrically with the large diameter hole. To achieve this, the EX-starter barrel is screwed into a stabilizer and about 60 cm of hole drilled. The drill string is then withdrawn and the Pilot hole is drilled with EX twin tube barrel attached to a stabilizer, to a depth of about 60 cm.

When the EX-hole reached the target depth, water was circulated for an additional 10 min so that the drilling sludge and cuttings could be flushed out. The barrel and drill string are then removed and the EX core is recovered for inspection. *Hydraulic Fracturing in Porous and Fractured Rocks DOI: http://dx.doi.org/10.5772/intechopen.106552*

*Preparation of glue—*It was ensured that the resin and hardener had the correct temperature specification for the expected temperature range. These two were mixed (**Figure 25**) according to the prescribed procedure. Any air pockets remaining were removed by carefully dispersing the glue with a small rod.

*Selection of gauge position—*The recovered E core was closely inspected to locate the best possible position for the strain gauges. The distance from the strain gauges to any likely weakness planes must be maximized. The other requirement was that the gauges should be at least one diameter away, preferably more, from the other ends of the EX-hole.

**Figure 26** show a suitable location of the gauges with respect to the core length. Positioning the gauges too far from the collar can cause problems as the core may break during overcoring and damage the shank and the HI Cell as it tends to rotate in the barrel.

*EX size hole measurement—*The range of depths that the HI Cell can be placed is limited by the requirement to be approximately beyond two over-core diameters from end of the overcore hole and before the E hole end, minus the stub left when the core is broken out of the hole. Typically, the stub length is up to 150 mm.

**Figure 23.** *Retrieval of HX size core.*

**Figure 24.** *Drilling of EX size borehole (Pilot hole).*

The strain gauge position was measured and it was decided to keep it at 60 cm from the collar of the Pilot hole. The depth to the position where the strain gauges are to be glued, was recorded. The installation rods were marked with tape, which indicates the depth to the end of EX size hole (Pilot hole). The tape was made to coincide with the edge of the collar of the over-core hole.

*Hydraulic Fracturing in Porous and Fractured Rocks DOI: http://dx.doi.org/10.5772/intechopen.106552*

**Figure 25.** *Preparation of glue.*

**Figure 26.** *Selection of gauge position and fixing the pin.*

Having determined the distance of the gauges from the EX-size hole (Pilot hole) end, the piston was mounted in the shell at the glue extruded position. The piston rod was then cut to length and taped onto the end of the piston.

*Piston attachment—*The piston was sprayed with a silicon-based releasing agent to prevent it from bonding to the inside of the gauge shell. The piston is aligned in the Cell as indicated by the scribed lines on the piston and upper rim of the Cell (**Figure 27**). Each of the four holes was lined up, and lead shear pins were placed through the cell wall into the piston.

*Installation of HI cell—*The completed Cell assembly was inserted in the orienting tool, attached to the trolley (**Figure 28**). The installation tool containing the HI Cell was screwed into the installation rods and the whole assembly pushed up the hole.

Each rod and coupling connections were firmly tightened and the cables were also kept taut. The rods were pushed up the hole until the first tape mark was reached. This indicated that the tip of the piston rod was about to enter the E hole. The Cell was then pushed slowly into the EX-size hole (Pilot hole).

When the second tape mark was reached, the piston rod tip was resting against the end of the Pilot hole. Some extra force was required to break the shear pins and then the rods were pushed slowly inwards so that the glue could evenly distribute itself between the rock and the gauge surface.

In this way, the installation was completed. Once the epoxy glue had gelled and curing was reasonably advanced, the installation rods and the trolley were recovered. Overcoring was commenced after 24 h of installation.

*Overcoring—*The Cell cable was passed through the centre of each rod, and the rod string was held with a slight tension to ensure that it was not cut by the overcoring barrel (**Figure 29**).

**Figure 27.** *Piston attachment for CISRO HI cell.*

*Hydraulic Fracturing in Porous and Fractured Rocks DOI: http://dx.doi.org/10.5772/intechopen.106552*

**Figure 28.** *Installation of CISRO HI cell.*

**Figure 29.** *Overcoring test at KTK 8 Incline.*

**Figure 30.** *CISRO HI cell-strain gauge configuration.*

The cell contains three strain rosettes 120 degrees apart. The gauge configuration is as follows: two axial, three tangential and four gauges inclined at 45 degrees in the 9-gauge cell. The 12-gauge cell has one additional 45 degrees and two additional tangential gauges (**Figure 30**). The gauges are 10 mm long and are located 0.5 mm below the outer surface of the cell.
