Seepage through earth dams – definition of the phreatic line: Within an earth dam section, the phreatic line (top flow line) is the locus below which which pressure condition exists?

Introduction / Context:
The phreatic line, also called the seepage line, is critical in earth dam design. It separates saturated soil below from the zone of negative pore pressure (capillary zone) above, and its position governs stability and potential for sloughing on the downstream face.

Given Data / Assumptions:

• Steady-state seepage conditions through a homogeneous dam.
• Flow net analysis applies (flow lines and equipotentials are orthogonal).
• Pore pressure sign convention: positive below water table; negative (suction) in capillary zone above it.

Concept / Approach:
The phreatic line is the topmost flow line where pore pressure equals atmospheric at the surface and becomes positive immediately below it. Hence, below the phreatic line the soil is saturated with positive pore-water pressure; above it, capillary tension may produce negative pore pressures (suction).

Step-by-Step Solution:

Verification / Alternative check:
Piezometric head measurements (standpipe piezometers) plotted across a section show heads above datum translating to positive pore pressure below the phreatic line.

Why Other Options Are Wrong:

• Equipotential lines are geometric constructs, not “positive” or “negative.”
• Negative hydrostatic pressure characterizes the capillary zone above the phreatic line, not below.

Common Pitfalls:
Confusing the phreatic line (flow line) with an equipotential; assuming zero pressure below the line instead of positive pressure.

Final Answer:
Positive hydrostatic (pore-water) pressure exists