331/3338–12FE Reference Handbook 10.4 · Civil · Construction · Dewatering

Handbook formula

Wellpoint / Cofferdam Dewatering Head

Lower the water table below excavation subgrade with wellpoints (shallow, suction ~15–18 ft practical) or deep wells. Flow from Thiem/Dupuit. Cofferdam must resist Ka/Kp and seepage (exit gradient).

kb transmissivity
Radius of influence
Well radius

Step-by-step solved example

Want 4 m of drawdown at the well in a confined T=0.01 m²/s, R/rw=200. Q?

h₂subgradeQr
Wellpoint line: overlapping drawdown cones keep the trench dry.
  1. 1. ln

    ln200=5.30.

  2. 2. Q

    Δh=Q ln /(2πT) → Q=Δh 2π T / ln =4×2π×0.01/5.30=0.047 m³/s.

Answer: Q ≈ 0.047 m³/s per well (steady Thiem)

10 practice questions

0/10 correct

1.Practical suction wellpoint lift is about

2.Exit gradient i_e at a cofferdam toe must stay below

3.A cutoff wall lengthens

4.Unconfined Dupuit Q uses

5.Sumps with pumps are OK when

6.T=kb= (1e-4 m/s)(10 m)=

7.OSHA Type C soil needs

8.Recharge wells are used to

9.ln(R/rw) for R=100 m, rw=0.1 m is

10.Wellpoints along a trench are spaced so that