Engineering & Technology · Geotechnical Engineering & Ground Systems
Effective stress, groundwater, seepage and drainage
Water in pores can carry pressure while the grain skeleton carries the remaining effective stress. Changing groundwater therefore changes more than the amount of water present: it can change the forces governing deformation and friction.
Review force balance, normal and shear stress, strain, density versus unit weight, fluid pressure and hydraulic head. Read graph axes and convert kN/m² to kPa, metres to millimetres and seconds to days. Ground below the surface is spatially variable; the lessons introduce every soil-specific symbol and keep saturated, hydrostatic and loading assumptions explicit.
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Use saturated effective stress and hydraulic head to explain how groundwater changes forces and flow.
Partition stress under declared conditions
Use compression as positive. For saturated soil in this elementary model, effective normal stress is σ′=σ−u, where σ is total normal stress and u pore-water pressure, all in pascals or kilopascals. Effective stress describes the skeleton-scale mechanical state; it is not an extra applied force added to total stress.
Total vertical stress from self-weight is the sum of layer unit weight times thickness. Unit weight γ is weight per volume, in kN/m³, unlike mass density in kg/m³. Under hydrostatic conditions u=γw h below a water surface connected to the point, with pressure zero at that surface in the gauge convention.
Pressure head is part of the flow account
For slow flow in a saturated porous medium, hydraulic head h is elevation head plus pressure head u/γw; velocity head is neglected here. Water flows in response to head difference, not pressure difference alone. Two points at different elevations can have different pressures without flow when their total heads are equal.
Darcy’s relation Q=kAi uses hydraulic conductivity k in m/s, cross-sectional area A normal to flow, and head gradient magnitude i=|Δh|/L. It models a suitable laminar porous flow. Q/A is discharge per total area, not the average velocity through the water-filled pore space. Layering can make conductivity depend strongly on direction.
Seepage changes pressure distribution and can change grain forces. Upward flow may reduce effective vertical stress; excessive gradients and erosion can undermine a soil system. A drain changes a boundary condition and needs a maintained outlet and compatible material system. A low water level at one point does not prove drainage everywhere.
Do not extend the saturated model indiscriminately
Unsaturated ground introduces air pressure, capillary suction and changing saturation. Rock fractures can form preferential paths. A single saturated scalar conductivity may therefore miss important behaviour. Distinguish a convenient calculation from evidence that its material and boundary assumptions hold.
Reducing pore pressure under unchanged total load can increase effective stress and cause compression. Groundwater abstraction can thus affect structures far from the water-removal point. Drainage and ground movement need a coupled explanation rather than the assumption that less water always means no geotechnical consequence.
Check a pressure partition and a separate seepage volume
Use original saturated uniform ground with its water table at the surface, γsat=20 kN/m³ and γw=10 kN/m³. At depth 4 m, σv=80 kPa and hydrostatic u=40 kPa, so σ′v=40 kPa. The plotted profiles show this specific case.
In a separate confined-pressure scenario at that same total stress, let u increase to 60 kPa. Then effective stress falls to 20 kPa. The hydrostatic surface-table profile no longer represents the new pressure boundary. No change in overlying weight was needed for this change in skeleton stress.
Separately assume a saturated specimen length 2 m, area 0.50 m², k=10⁻⁵ m/s and head drop 0.40 m. Gradient is .20 and Q=10⁻⁶ m³/s, or .0864 m³ over 86400 s. This is 86.4 litres/day under steady supplied conditions. It does not establish an aquifer yield or a drainage design.
Try the reasoning
At fixed total stress, what does increasing u do to σ′?
Show the answer and reasoning
It decreases effective stress by the same amount in the stated saturated model.
Does higher pressure always indicate the flow direction?
Show the answer and reasoning
No. Elevation and pressure combine into hydraulic head; hydrostatic pressure varies with depth without flow.
Why can groundwater lowering cause settlement?
Show the answer and reasoning
Under suitable compressible-ground conditions, reduced pore support increases effective stress and the skeleton compresses.
What to carry forward
State the pressure reference, saturation and boundary conditions before interpreting head, flow or effective stress.
References
Teaching resources and research supporting this lesson’s explanations and examples.