Question

Select your answer in the boxes, and provide your supporting explanation briefly. a) A concrete dam with an upstream sheet pile cutoff is shown below. How does the installation of the sheet pile affect the flow rate, g, under the dam, when compared to the dam without the sheet pile? (10 points) 2011 60 ft 20 30 40 50 60 80 90 100 110 120 130 140 150 70 Feet 9 (choose one) increase / decrease / doesn't change Why? b) If the sheet pile in Part a) was moved to the downstream toe of the dam, what effect would it have on the pore water pressures, w, acting along the bottom of the dam? (10 points) u (choose one) increase / decrease / doesn't change Why?

Answer 1

a.) q decreases

Explanation

When there is no sheet pile then the path required by the water is less as to reach the downstream leading to more hydraulic gradient as I= H/L, H is fixed and L is less so I will be more. So q = kiA will be more.

But when there is sheet pile installed as shown it increases the creep length due to which hydraulic gradient decreases i.e I= H/L decreases as L increases as compared to previous case so overall q= kiA decreases as compared to previous case.

b.) Pore water pressure increases

Explanation

When sheet pile is moved to downstream end then it will be ineffective in reducing the uplift pressure i.e pore pressure as water seeps easily and exerts upward pressure on the base floor with full potential as there is no vertical cut off on the upstream side.

If there will be a vertical cut off like sheet pile at upstream end then it negates the water to seep directly below the base floor by increasing it's creep length which reduces pore water pressure.

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