Groundwater seepage describes how water moves through soil pores—important for uplift, piping, and excavation safety.

Scope (groundwater & seepage)

This page covers steady seepage through porous soil: hydraulic head and gradient, Darcy's law, order-of-magnitude flow rates, and the link to pore pressures and uplift in excavations, embankments, and retaining systems. In South Africa, geotechnical limit states involving groundwater actions are framed in SANS 10160-5; permeability and field testing are covered in the SANS 3001 geotechnical test series. Use the project's ground investigation for k and boundary conditions—this page is educational, not a substitute for a seepage model signed off for construction.

Seepage check (concept)

  1. Establish hydraulic head boundaries (phreatic surface, upstream/downstream, cut-off depth).
  2. Estimate hydraulic gradient i along the controlling flow path (1D path, flow net, or FE mesh).
  3. Select permeability k consistent with soil type and test method (lab vs field).
  4. Apply v = k i and Q = k i A for continuity checks; compare with seepage software for 2D/3D problems and exit gradients.

Geotechnical Engineering — Groundwater & Seepage

Flow of water through soil pores; controls pore pressures, effective stress, uplift, and piping risk.

Introduction

Groundwater movement in saturated soil is often described by Darcy's law: discharge velocity is proportional to hydraulic gradient. That yields seepage flow rates for dewatering estimates, uplift under slabs, and exit gradients at toes of dams or sheet piles—where piping and erosion must be checked.

Two- and three-dimensional problems use flow nets or numerical models; the calculator below is a 1D continuity check only.

Schematic of Darcy flow through a soil sample: discharge proportional to hydraulic gradient
Darcy's law: discharge through a soil column. See licence on Wikimedia Commons
Diagram of elevation head, pressure head, and total hydraulic head in groundwater
Components of hydraulic head (elevation and pressure). See licence on Wikimedia Commons

Flow nets and 2D seepage

For plane strain conditions, equipotentials and flow lines form orthogonal families; the number of flow channels and head drops sets discharge and local gradients. Numerical codes extend this to layered soils, wells, and transient drawdown.

Flow net for a pumping well near a stream and impermeable boundary
Example flow net (well, stream, barrier). See licence on Wikimedia Commons

What seepage analysis produces

  • Pore-pressure field or phreatic surface for effective-stress and uplift checks.
  • Flow rate Q for dewatering pumps, cut-off performance, or leakage through dams.
  • Exit gradient and path length for piping, filters, and drainage design.
  • Input to slope stability and retaining-wall water loads when linked to the ground model.

Standards (South Africa)

Geotechnical design basis and groundwater-related limit states: SANS 10160-5. Permeability and laboratory/field tests: SANS 3001 (parts as specified). Align assumptions with the geotechnical investigation report, monitoring, and any dam, mine, or environmental permits.

Darcy (1D):   v = k i ;   Q = v A = k i A   (consistent units; i = −dh/ds along flow).

Notation (common)

  • k — coefficient of permeability (hydraulic conductivity), often m/s.
  • i — hydraulic gradient (head drop per length along a streamline).
  • v — Darcy (discharge) velocity; Q — volumetric flow rate.
  • A — gross cross-sectional area normal to flow (includes solids and voids).
  • h — total head; u — pore pressure (linked through h = z + u/γw in simple setups).

Examples in practice

  • Excavation beside a river: estimate inflow and pump capacity; check base uplift/heave.
  • Embankment dam: seepage through core and foundation; filters and drainage blankets.
  • Sheet-pile wall: flow under the toe and exit gradient on the excavation side.
  • Well field: drawdown and interference between wells (superposition in linear aquifers).

Calculator — Darcy's law

Uniform flow normal to area A; gradient i and k positive. Output: Darcy velocity v and flow rate Q.

v = k i ;   Q = k i A

Seepage velocity and flow

Darcy velocity v = k i and flow rate Q = k i A through a gross cross-section area A.

Key terms

Hydraulic gradient (i)
Head loss per unit flow length along the path.
Permeability (k)
Soil property relating Darcy velocity to gradient (laminar flow regime).

Software and seepage analysis

Two- and three-dimensional seepage, transient drawdown, and coupled analyses use finite-element or finite-difference groundwater tools. Examples (official sites):

  • GeoStudio — SEEP/W and related geotechnical groundwater modelling.
  • PLAXIS — 2D/3D geotechnical analysis including flow.
  • Itasca FLAC3D — coupled flow–mechanical modelling where required.
  • USGS MODFLOW — regional groundwater flow (public-domain reference model).
  • gINT — borehole and test data supporting parameter selection.

No product endorsement—calibrate models to site tests and monitoring. The calculator above is not a substitute for checked seepage analyses for dams, mines, or safety-critical excavations.

Diagram sources

Educational schematics. Files in Images/groundwater-seepage/ were downloaded from Wikimedia Commons into this repo (not copied from other topic folders). Confirm licence on each Commons file page before reuse.