Groundwater and Subsurface Engineering: Cut-Off Walls, Dewatering, and Caisson Safety

Practical Engineering explains how groundwater influences the stability of subsurface structures, using a hydraulic model and real world examples. The video covers uplift pressure under foundations, seepage and piping risks, and how engineers mitigate these effects with cut-off walls and drains, as well as dewatering strategies around caissons. It introduces flow nets and Darcy's Law, and shows how subsurface conditions are analyzed and observed before construction to prevent failures. Through animations and a BC caisson incident, viewers learn why groundwater is a critical but often hidden factor in civil engineering projects and how robust design helps keep structures and people safe.

Introduction

Groundwater plays a pivotal but often invisible role in civil engineering. The video frames groundwater as a fundamental factor that can undermine structures if not properly accounted for, particularly in subsurface works like dams, retaining walls, and caissons. Using simple yet powerful models, the host demonstrates how water pressure from above and below interacts with foundations, joints, and soil, and how engineers translate this complex subsurface behavior into workable design strategies.

Groundwater and Dam Stability

The episode begins by relating groundwater flow to hydrostatic pressure and the forces that stabilize structures such as gravity dams. A key point is uplift pressure, which arises when groundwater below a foundation experiences a high hydraulic head and pushes upward. This uplift reduces the effective weight of the dam and can drive destabilization if not anticipated. The Saint Francis Dam collapse of 1928 is cited as a historical warning where uplift considerations were neglected, underscoring the high cost of ignoring groundwater in design.

Flow Nets and Darcy’s Law

Flow nets are introduced as practical tools for estimating seepage and pressure in the subsurface. The viewer can think of the flow net as a 2D representation of groundwater flow, with equipotential lines indicating pressure and flow lines showing direction. Darcy’s Law, which links flow rate to soil permeability and hydraulic gradient, explains how changes in subsurface pathways affect uplift and seepage. The host emphasizes that as gradients change, seepage volume and the potential for soil weakening also change, influencing stability margins.

Mitigation: Cut-Off Walls and Drains

Two major strategies for managing seepage and uplift are discussed: cut-off walls and drains. Cut-off walls create an impermeable barrier below the structure, extending the flow path and reducing the hydraulic gradient, thereby decreasing both seepage volume and uplift pressure. The video demonstrates this concept with an acrylic dam model and a vertical wall inserted below the upstream face, showing how flow lines bend and uplift measurements drop slightly.

• Cut-off walls can be as simple as pre-construction foundation excavation, or involve deeper trenches filled with grout, impermeable clay slurries, or concrete to block seepage paths.
• Sheep piles or grout curtains are practical options for smaller or temporary structures, while injection grouting can form an impermeable curtain below a dam.

Drains perform a complementary role by filtering seepage and providing a low-resistance path for water to escape, reducing uplift pressure while sometimes increasing the hydraulic gradient by shortening flow paths. The model shows a drain draining seepage to the upstream side, illustrating how pressure distribution shifts even as total seepage changes. Many concrete dams incorporate vertical drains or relief wells to depress groundwater levels and improve stability.

Caissons, Dewatering, and Sinkhole Risk

The host turns to caissons, large watertight chambers sunk into the ground to hold back soil during construction. Dewatering surrounds the caisson to create a dry working area, but this reduces the distance groundwater must travel, increasing hydraulic gradients and the risk of soil liquefaction or piping. The model depicts sand boiling up at the caisson bottom as seepage exits, a visual cue for how elevated pressure and high flow velocity can destabilize soils. Solutions include well points to dewater surrounding soils, driving caissons deeper, or reaching impermeable layers to maintain a better seal against seepage.

Practical Takeaways

Across the video, the underlying message is that groundwater issues are often invisible until they cause problems. Engineers use a combination of cut-off walls, drains, and dewatering strategies to manage seepage and uplift. The same principles apply to dams, retaining walls, and temporary structures, reminding practitioners to anticipate groundwater effects early, design for uplift, and verify subsurface conditions before construction begins. The episode concludes with the idea that responsible groundwater management protects both structures and the people who rely on them.

Conclusion

Although groundwater can seem to lie out of sight, it is a central factor in the stability of civil engineering projects. By leveraging cut-off walls, drains, and controlled dewatering, engineers can reduce uplift pressures and seepage, improving the resilience of dams, caissons, and other subsurface structures.