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.

Question

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?

Answer 1

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.