Section 4: Soil and Water Conservation Engineering
Soil and Water Erosion
Subtopic: Earthen dams
Question Statement:
Which of the following statements regarding the hydraulic design and stability of earthen dams are correct?
Options:
(A) The phreatic line is the top streamline of seepage through an earth dam along which pore water pressure is atmospheric.
(B) Casagrande's method determines the base seepage line through a homogeneous earth dam as a basic parabola.
(C) A horizontal rock toe or chimney filter is provided to prevent the phreatic line from day-lighting on the downstream face.
(D) The upstream slope of an earth dam is subjected to critical destabilizing shear stress during rapid reservoir drawdown.
Answer Key:
A, B, C, D
Step-by-Step Solution & Explanation
• The phreatic line is the upper free surface of the saturated seepage zone with zero gauge pressure (A is correct).
• Casagrande developed the analytical parabolic solution for seepage in homogeneous earth dams (B is correct).
• Downstream filters lower the seepage line to protect against piping and sloughing (C is correct).
• Sudden drawdown removes stabilizing water pressure against the upstream face while pore pressures remain high, causing critical sliding risk (D is correct).