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GATE 2024 CE (CE2) – Question 45

Geotechnical Engineering · Permeability and seepage: flow nets, uplift pressure, piping, capillarity · 2 marks · Multiple select

Consider the statements P, Q, and R.
P: Compacted fine-grained soils with flocculated structure have isotropic permeability.
Q: Phreatic surface/line is the line along which the pore water pressure is always maximum.
R: The piping phenomenon occurring below the dam foundation is typically known as blowout piping.

Which of the following option(s) is/are CORRECT?

  1. Both P and R are TRUE
  2. P is FALSE and Q is TRUE
  3. P is TRUE and R is FALSE
  4. Both Q and R are FALSE

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Correct answer: (C) P is TRUE and R is FALSE; (D) Both Q and R are FALSE

Explanation

Q is false: along the phreatic line the pore water pressure is atmospheric, that is zero gauge pressure, which is the minimum, not the maximum. This rules out B. R is false: the piping that occurs under a dam foundation is subsurface erosion (a pipe forming from the downstream end back to the upstream), while blowout is the heave of the soil at the exit. So D (both Q and R false) is correct. P is true: a flocculated structure has randomly oriented particles, so the permeability is nearly the same in all directions, whereas a dispersed (parallel) structure gives a strongly directional permeability. With R false and P true, C is also correct. A needs R to be true, so it is wrong.