If the groundwater head doubles from 7.15 ft to 14.30 ft, the upward force would be approximately which value?

• A. 858,000 lb
• B. 1,000,000 lb
• C. 2,000,000 lb
• D. 1,716,000 lb

Answer: (D)

Hydrostatic force grows linearly with the water head. The upward force on a submerged surface is F = gamma × h × A, where gamma is the unit weight of water and A is the area. If the groundwater head doubles, the pressure at every depth doubles, so the total upward force doubles as well. In this problem, doubling the head from 7.15 ft to 14.30 ft makes the force roughly twice what it was before. Since the force at the original head is about 858,000 lb, doubling gives about 1,716,000 lb. That matches the option showing twice the initial value, so the approximate upward force is 1,716,000 lb.