Groundwater and Subsurface Structures: Uplift Pressure, Cut-Off Walls, and Dewatering in Civil Engineering

Practical Engineering explains how groundwater affects subsurface structures, using a real-world caisson incident in British Columbia and a set of acrylic sand models to visualize seepage paths and uplift. The host introduces the concepts of hydrostatic pressure, uplift pressure, and piping, and demonstrates how engineers predict and manage these forces. The episode then outlines practical mitigation techniques such as cutoff walls, drains, and relief wells, showing how extending flow paths and removing groundwater can reduce uplift and improve stability. By illustrating the interaction between groundwater and foundations, caissons, and dams, the video emphasizes that safety depends on thoughtful design even when the subsurface remains hidden from plain sight.

Introduction

Groundwater plays a crucial yet often unseen role in civil engineering. Practical Engineering uses a real incident near a caisson at a construction site in British Columbia to motivate a deeper look at how groundwater can destabilize subsurface structures and how engineers mitigate these risks through design and construction practices.

Groundwater in Civil Engineering: From Hydrostatics to Uplift

The episode explains that groundwater pressure interacts with a structure differently than surface water. Unlike a dam facing a reservoir, groundwater exerts uplift pressure at the foundation interface, potentially reducing the effective weight of the foundation. This uplift, together with seepage, can lead to pipe flow and soil loosening if not properly managed. The host emphasizes that predicting this uplift is complex because groundwater flow depends on soil properties, geometry, and subsurface features, and cannot be measured directly after construction.

Modeling Seepage: Flow Nets and Darcy’s Law

The video showcases flow nets as a graphical tool engineers use to estimate seepage volume and pressure. A flow net is a curvilinear grid that pairs potential pressure with flow paths, illustrating how differences in pressure create hydraulic gradients. Darcy's Law underpins this approach: flow rate is proportional to soil permeability and hydraulic gradient. Through the model, viewers see how increasing the flow path length, or creating barriers to flow, reduces uplift pressure downstream of the structure.

Cut-Off Walls: Reducing Seepage and Uplift

A key mitigation strategy is the cut-off wall, a vertical barrier that slows or blocks groundwater under the dam or foundation. The host demonstrates with an acrylic wall placed beneath the upstream face of the dam in the model. Seepage paths bend and lengthen, and uplift pressure slightly decreases downstream. Multiple options exist for cut-off walls, including shallow foundation excavation, deep trenches filled with impermeable materials, grouting, slurry walls, or driven sheet piles. The overall effect is to reduce uplift even when seepage continues beneath the barrier.

Drains and Relieving Mechanisms

Drains below the dam or foundation serve two purposes: they filter seepage to prevent piping and they relieve uplift pressure by removing water. The model shows seepage diverting into a drain, reducing pressure at the base. Drains can increase the hydraulic gradient by shortening seepage paths, but with appropriate filtration and maintenance, the risk of piping is lowered and the structural stability is enhanced. The episode notes that concrete dams often incorporate vertical drains and even pumps to lower groundwater levels, further improving stability.

Dewatering Caissons and Construction Hazards

The discussion returns to the construction context with caissons, watertight chambers sunk to hold back soil during construction. Dewatering a caisson creates a high hydraulic gradient across a short distance, which can milk soil particles away and trigger liquefaction or sinkholes at surface. The video demonstrates how well points and other dewatering methods outside the caisson help prevent instability at the surface, protecting workers on site.

Broader Applications: Retaining Walls, Shoring, and Pumps

Although the BC incident centers on a caisson, groundwater management applies to retaining walls and shoring as well. The same principles—cut-off walls, drainage, pumps, and drainage design—are employed to ensure stability in a range of subsurface structures. The host emphasizes that while groundwater problems are often hidden, engineers systematically address them through a combination of observation, modeling, and well-planned subsurface modifications.

Takeaways for Engineers and Public Safety

The episode concludes with a practical reminder: groundwater can destabilize foundations, but with the right design strategies, drainage systems, and cut-off barriers, structures can remain safe. The video encourages engineers to plan for groundwater effects from the outset and to use modeling tools and field investigations to inform decisions before construction begins.

Final Thoughts

Groundwater challenges are a fundamental part of subsurface engineering. The video reinforces that responsible design, monitoring, and implementation of drains and cutoff walls help safeguard both infrastructure and people around it, turning an invisible problem into a manageable one through engineering practice.