A borehole deformation gauge (BDG) is a gauge that measures the in-situ stresses in rock using the overcoring method. The BDG is used in three differently oriented boreholes to get the three-dimensional stress field. The measurement is important in designing sub-surface structures such as tunnels and pipeline routes. Overcoring is one of the most popular methods for stress measurement.
Overcoring is a proven technique and is commonly used to measure in-situ stresses in sub-surface excavations. The strains and changes in the diameter of the borehole resulting from stress relief during the overcoring operations are measured using different instruments. The most popular device for this purpose has been the borehole deformation gauge of the United States Bureau of Mines (USBM).
The gauge is inserted inside an EX diamond drill hole with the plungers depressed. The cable is connected to a strain indicator and the initial deflection of the cantilevers is measured. Further procedures are carried out to form a stress/strain relationship to calculate the elastic modulus of the rock.
In-situ stresses released during a trenchless operation can cause ground control problems such as rockburst, buckling, or spilling. Some factors that affect the magnitude and orientation of in-situ stress include the weight of overlying rock and soil strata, tectonic forces, and residual stress.
It is important to determine these stresses as they dictate essential factors in wellbore design including the selection of the drill bit and drilling fluid constituents.
The borehole deformation gauge (BDG) measures in-situ stresses in a rock mass from the measurement of changes in the diameter of a borehole during overcoring.
The stresses in the plane perpendicular to the borehole axis are calculated by combining the elastic properties of the rock and the deformation measurements. The elastic properties of the rock are determined using rock cylinder samples obtained during the overcoring operation by conducting biaxial pressure tests on them.
Tools and Equipment
For conducting the overcoring operation and in-situ stress measurement, specialized equipment is required.
- A diamond drill machine.
- Drill rods and drill bits.
- Overcoring bit.
- Centralizers.
- Adapter subs.
- Short EX core barrel.
- Water swivel.
- Core retrieving equipment like a core-breaking wedge, core shovel, and core puller.
- Borehole deformation gauge (BDG).
- Signal cable and readout box.
- Placement and orientation rods.
- Calibration rig.
- Biaxial modulus chamber.
Overcoring Procedure
Overcoring should begin at least one opening diameter away from the opening unless rock stress is to be measured close to the underground opening. The method involves drilling a large diameter borehole (60 – 150 mm) at the end of which a small pilot hole (300 to 500 mm in length) is drilled as concentric to the larger hole as possible. Drilling of the larger hole is resumed relieving stresses and strains partially or totally within the cylinder of rock.
Operation of the BDG
The BDG consists of three pairs of strain gauged cantilevers each oriented at 120° from each other and held within a stainless steel housing. The tips of the cantilevers are deflected by means of tungsten-carbide-faced plungers. The BDG is inserted inside an EX diamond drill hole such that the plungers are depressed. The initial deflection of the cantilevers is measured using the readout.
The core is removed from the hole after each overcore operation using core removal tools. It is then placed inside a biaxial modulus chamber and the change in the EX borehole diameter is measured again.
The procedure is repeated in three differently oriented boreholes. The measured stress/strain relationship is used to calculate the elastic modulus of the rock using which rock stress can be computed from the measured strains during overcoring.