The elevations of pressure gauge and porous cup of the tensiometer installed in unsaturated zone are 145.8 m and 144.2 m, respectively. Pressure measured at the gauge is −19.62×10³ N m⁻². The specific weight of water is 9810 N m⁻³. The estimated pressure at the porous cup is ________ N m⁻².
Official Answer Key:
3924
Step-by-Step Solution & Explanation
Pressure head at the gauge: h_p = p/γ = −19620/9810 = −2.0 m.
Total (hydraulic) head at gauge = elevation + pressure head = 145.8 + (−2.0) = 143.8 m.
The water column inside the tensiometer is continuous and static, so total head is the same at the porous cup.
Pressure head at cup = 143.8 − 144.2 = −0.4 m.
p_cup = −0.4 × 9810 = −3924 N m⁻².
The negative sign indicates soil-water tension (suction), as expected in the unsaturated zone.
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