A 3-point borehole extensometer is installed to identify the location of a single discontinuity plane in a hanging wall rock by measuring deformations at three locations as shown in the figure. The absolute readings of deformations measured on two different dates are listed in the table. Based on the measured data the most likely inference is
Show Hint
- Discontinuity between Anchor-1 and Anchor-2
- Discontinuity between Anchor-2 and Anchor-3
- Discontinuity between Anchor-3 and excavation boundary
- No noticeable discontinuity
The Correct Option is B
Solution and Explanation
Compute deformation increase from May 2 to June 1:
Anchor-1: $40.56 - 34.52 = 6.04$ mm
Anchor-2: $34.67 - 29.04 = 5.63$ mm
Anchor-3: $44.78 - 43.11 = 1.67$ mm
A sudden drop in deformation between two anchors indicates a discontinuity (movement stops across the plane).
Between Anchor-1 and Anchor-2: deformation change difference is $6.04 - 5.63 = 0.41$ mm → small → continuous.
Between Anchor-2 and Anchor-3: deformation drops from $5.63$ mm to $1.67$ mm → a large reduction → indicates a discontinuity plane blocking displacement transfer.
Thus, the discontinuity is located between Anchor-2 and Anchor-3.
Final Answer: (B)
Top Questions on Stress Measurement Techniques
In a bord and pillar panel, a square pillar of size 35 m \( \times \) 35 m (centre to centre) is extracted to form four equal square-shaped stooks as shown. The width of each gallery and crosscut is 5 m. The height of the working seam is 3 m. The reduction in safety factor after pillar splitting by using Bieniawski’s pillar strength formula, in %, is _______ (rounded off to 2 decimal places).
Bieniawski's pillar strength formula is given by \( S_p = S_1 \left( 0.64 + 0.36 \frac{w}{h} \right) \), where \( S_1 \) is the strength of a 0.9 m\(^3\) coal block, \( w \) is the pillar width, and \( h \) is the mining height.- GATE MN - 2025
- Geomechanics and Ground Control
- Stress Measurement Techniques
In a bord and pillar panel, a square pillar of size 35 m \( \times \) 35 m (centre to centre) is extracted to form four equal square-shaped stooks as shown. The width of each gallery and crosscut is 5 m. The height of the working seam is 3 m. The reduction in safety factor after pillar splitting by using Bieniawski’s pillar strength formula, in %, is _______ (rounded off to 2 decimal places).
Bieniawski's pillar strength formula is given by \( S_p = S_1 \left( 0.64 + 0.36 \frac{w}{h} \right) \), where \( S_1 \) is the strength of a 0.9 m\(^3\) coal block, \( w \) is the pillar width, and \( h \) is the mining height.
- GATE MN - 2025
- Geomechanics and Ground Control
- Stress Measurement Techniques
- A triaxial test on a sandstone sample is conducted at a confining pressure of 10 MPa. The elastic axial and volumetric strains at axial stress of 50 MPa are recorded to be \( 4.2 \times 10^{-3} \) and \( 2.0 \times 10^{-3} \) respectively. The modulus of elasticity, in GPa, and Poisson’s ratio of the sample, respectively are closest to:
- GATE MN - 2025
- Geomechanics and Ground Control
- Stress Measurement Techniques
In a Brazilian test, the front view of a homogeneous rock sample is shown in the figure. The failure initiates at the point, where the rock sample is subjected to stress. The failure initiates at the point marked as:
- GATE MN - 2025
- Geomechanics and Ground Control
- Stress Measurement Techniques
- The major (\( \sigma_1 \)) and minor (\( \sigma_3 \)) principal stresses and the maximum shear stress (\( \tau_{{max}} \)) are related as \( |\tau_{{max}}| = \sigma_1 = - \sigma_3 \). The magnitude of normal stress on the plane where \( \tau_{{max}} \) acts is:
- GATE MN - 2025
- Geomechanics and Ground Control
- Stress Measurement Techniques
Questions Asked in GATE MN exam
Reciprocal levelling is performed for points P and Q by placing the same levelling instrument at A and B. The observations of staff readings are tabulated as below.
If the Reduced Level (RL) of P is 115.246 m, then the true RL of Q, in m, is _______ (rounded off to 3 decimal places)
- GATE MN - 2025
- Surveying
- SO$_2$ is emitted at a rate of 20 kg/s from a 10 km $\times$ 10 km airshed in an industrial area. Wind blows at a speed of 4 m/s from one direction in that area. Radiation inversion restricts the mixing height to 1200 m. Neglect SO$_2$ concentration in the incoming air. Assuming emitted SO$_2$ to be conservative, the steady state SO$_2$ concentration in the airshed, in $\mu g/m^3$, is _______ (rounded off to 2 decimal places).
- GATE MN - 2025
- Environmental Engineering
- In a longwall panel, air flows at a velocity of 1.2 m/s through a 900 m long gate road of 2.5 m height and 3 m width. The coefficient of friction is 0.022 Ns$^2$m$^{-4}$. The frictional pressure drop, in Pa, between two ends of the gate road is _______ (rounded off to 3 decimal places).
- GATE MN - 2025
- Fluid Mechanics
The information of a mining project for a life of three years is given below:
Additional data: Applicable tax rate = 30%
Discount rate = 10%
Depreciation method: Straight line with zero salvage value- GATE MN - 2025
- Mine planning and its components
Data from a borehole log with collar elevation at 590 mRL are given below. Composite grade is calculated using cores of 5 m above and below the reference bench at 580 mRL. The composite grade, in %, is:
- GATE MN - 2025
- Mine planning and its components