Question:
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
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
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A discontinuity lies between the two anchors where deformation magnitude drops sharply.
Updated On: Dec 17, 2025
- Discontinuity between Anchor-1 and Anchor-2
- Discontinuity between Anchor-2 and Anchor-3
- Discontinuity between Anchor-3 and excavation boundary
- No noticeable discontinuity
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The Correct Option is B
Solution and Explanation
To locate a discontinuity, we examine the incremental deformation at each anchor.
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)
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)
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