In addition to the infiltration determined in Problem 16-21, ground water seepage due to gravity also occurs. Determine the thickness of a suitable drainage layer required to transmit the net inflow to a suitable outlet.

Thickness of subgrade below drainage pipe = 12 feet
Coefficient of permeability of native soil = 0.35 ft/day
Height of water table above impervious layer = 21 feet
Slope of drainage layer = 2%
Permeability of drainage area = 2,000 ft/day
Length of flow path = 60 ft

What will be an ideal response?


Calculate the amount of drawdown.
H – Ho = 21 – 12 = 9 ft of drawdown
Determine the radius of influence.
Li = 3.8 (H – Ho)
Li = (3.8) (9) = 34.2 ft
Compute the average inflow rate due to gravity drainage:
((Li + 0.5W) / Ho) = (34.2 + ((0.5) × (44))) / 12
((Li + 0.5W) / Ho) = 56.2 / 12
((Li + 0.5W) / Ho) = 4.68
W/Ho = 44/12 = 3.67
From Figure 16.34, determine:
k × (H – Ho) / 2q2 = 1.7
q2 = k × (H – Ho) / (2 × (1.7))
q2 = (0.35 × 9) / 3.4
q2 = 0.93
qg = q2 / 0.5W
qg = 0.93 / (0.5 × 44)
qg = 0.042 ft3/day/ft2
Calculate the net inflow.
From Problem 16-21, qi = 0.25 ft3 / day / ft2
qn = qi + qg
qn = 0.25 + 0.042
qn = 0.292
p = qn / kd
p = 0.292 / 2,000
p = 1.46 × 10–4
From Figure 16.38:
L / Hm = 220
Hm = L / 220
Hm = 60 / 220
Hm = 0.27 ft = 3.3 in; use 4.0 inches
Therefore, the suitable drainage layer thickness should be 4.0 inches.

Trades & Technology

You might also like to view...

Which of the following is true regarding the cabin air filter?

A) It is mounted in the incoming air stream from the cowl intake B) Filter restrictions can reduce the volume of air entering the passenger compartment C) Both A and B are correct D) Neither A nor B is correct

Trades & Technology

Conductors that become too hot ________

A) Could be the result of a short to ground before the load B) Create excessive resistance C) Both A and B D) Neither A nor B

Trades & Technology

Ground wires are usually which color?

A. green B. white C. black D. red

Trades & Technology

Any time an air conditioning system is opened it must have the __________ replaced before evacuating the system.

A. Orifice tube B. Thermostatic expansion valve C. Receiver drier D. Compressor oil

Trades & Technology