Cofferdam Seepage & Uplift Checks

Multi‑mode tool for seepage and safety checks: single‑wall cofferdam, double‑wall cofferdam (two flow nets), and a simplified Khosla‑style floor uplift check. Includes optional uplift at cofferdam base and quick flow‑net templates.

Inputs

Units

Current units: SI — heads in m, k in m/s, γ in kN/m³, stresses in kPa.

Analysis mode

Single‑wall cofferdam: seepage, exit gradient and FS vs piping using N_f, N_d.

Hydraulic heads (single‑wall)

Use a common datum at the excavation base (or any level). Only Δh = H_up − H_in matters.

Upstream H_up m (outside cofferdam)

Inside H_in m (inside excavation)

Flow channels N_f from flow net

Drops N_d from flow net

Flow‑net template (optional)

Templates just pre‑fill N_f, N_d and L_exit. You can still edit them.

Hydraulic conductivity k m/s

Exit element length L_exit m (height of last element)

Saturated unit weight γ_sat kN/m³

Soil thickness below base H_soil m (optional uplift check)

Critical gradient: i_cr = (γ_sat − γ_w) / γ_w. If H_soil is given, a base uplift FS is also computed.

Hydraulic heads (double‑wall)

Treats outer and inner cofferdam walls as two separate flow nets with an intermediate head H_mid (water level between walls, often controlled by pumping).

Outside head H_out m

Between walls H_mid m

Inside head H_in m

Outer wall flow net

N_f,outer

N_d,outer

L_exit,outer m

Outer wall template (optional)

Inner wall flow net

N_f,inner

N_d,inner

L_exit,inner m

Inner wall template (optional)

Hydraulic conductivity k m/s

γ_sat (soil) kN/m³

Soil thickness below inside base H_soil m (optional uplift check)

Heads & floor geometry

Simplified “Khosla‑style” uplift: assumes linear head variation from upstream H_up to downstream H_down along a horizontal floor of length L.

Upstream head H_up m

Downstream head H_down m

Floor length L m

Floor thickness t m

γ_c (concrete) kN/m³

γ_sat (soil) kN/m³ (info only)

This mode does not reproduce full Khosla curves; it assumes a linear head profile along the floor and checks uplift vs floor weight at many points.

Summary

Select units and analysis mode, fill in the relevant inputs, then click Compute to see seepage rates, gradients and safety factors here.

Single‑wall cofferdam with flownet: N_f flow channels, N_d potential drops, exit gradient at the inside toe.

Profile, table & exports

Cofferdam / floor seepage profile

Index	Value 1	Value 2	Value 3	Value 4
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Theory & formulas

1) Single‑wall cofferdam (flow‑net based)

Upstream and inside heads: H_up, H_in   ⇒   Δh = H_up − H_in

Flow net: N_f flow channels, N_d potential drops.

Seepage discharge per unit length:
    q = k · Δh · (N_f / N_d)

Head loss per drop:
    Δh_drop = Δh / N_d

Approximate exit gradient at inside toe
(L_exit = characteristic length of last element at toe):
    i_exit ≈ Δh_drop / L_exit = (Δh / N_d) / L_exit

Critical gradient from saturated unit weight:
    i_cr = (γ_sat − γ_w) / γ_w

Factor of safety vs piping:
    FS_piping = i_cr / i_exit

Optional base uplift check (cofferdam bottom):
 - Let H_soil = soil thickness below base.
 - Approximate head at inside base: h_base ≈ H_in + Δh_drop (conservative).
 - Pore pressure at base: u_base = γ_w · h_base
 - Effective overburden: σ'_v = (γ_sat − γ_w) · H_soil
 - Factor of safety vs uplift: FS_uplift = σ'_v / u_base

2) Double‑wall cofferdam (two separate flow nets)

Heads: H_out (outside), H_mid (between walls), H_in (inside excavation)

Outer wall:
    Δh_outer = H_out − H_mid
    q_outer  = k · Δh_outer · (N_f,outer / N_d,outer)
    i_exit,outer ≈ (Δh_outer / N_d,outer) / L_exit,outer
    FS_outer = i_cr / i_exit,outer

Inner wall:
    Δh_inner = H_mid − H_in
    q_inner  = k · Δh_inner · (N_f,inner / N_d,inner)
    i_exit,inner ≈ (Δh_inner / N_d,inner) / L_exit,inner
    FS_inner = i_cr / i_exit,inner

Optional base uplift at inside toe:
  Use inner wall Δh_drop, H_in and H_soil (thickness of soil below inside base)
  to check uplift similarly to the single‑wall case.

The smaller FS (piping or uplift) usually governs safety at the excavation.

3) “Khosla‑style” floor uplift (simplified linear profile)

This mode does NOT reproduce the full Khosla method with φ‑curves and charts.
Instead it assumes a linear variation of hydraulic head under a horizontal floor:

Upstream head H_up, downstream head H_down, floor length L:

 at position x along floor (0 ≤ x ≤ L):
    h(x) = H_up − (H_up − H_down) · (x / L)

Pore pressure under floor:
    u(x) = γ_w · h(x)

Floor weight per unit area:
    W_floor = γ_c · t

Approximate factor of safety vs uplift at each x:
    FS_u(x) = W_floor / u(x)

The tool reports FS_u(x) along the floor, including the minimum FS. For detailed design,
true Khosla analysis with curves and corrections should be used.