The uplift of in‑ground swimming pools is a recurring phenomenon in Québec, particularly in connection with the activities required for winterization and during freeze–thaw cycles. This vertical movement generally results from a loss of equilibrium between the mass of the structure and the pressures exerted by the surrounding environment.

In this context, the following is a summary of the physical mechanisms and hydro‑geotechnical conditions typically involved in pool uplift, as well as the installation and maintenance parameters that influence the structural response of swimming pools and must be considered in the context of a legal expert assessment.

1. Fluid Mechanics and Archimedes’ Principle

The uplift of a swimming pool is not a random movement, but a direct physical response to a breakdown of static equilibrium.

Archimedean buoyancy acting on the soil

Any buried structure displaces a volume of soil. When this soil becomes saturated, groundwater exerts an upward pressure on all submerged surfaces. This is Archimedes’ principle—the same phenomenon that allows a boat to float on water. In engineering, the stability of a swimming pool is assessed by comparing its total weight (structure and the volume of water contained inside) to the force exerted by groundwater (based on the volume of groundwater “occupied” by the pool).

The buoyancy threshold

An in‑ground pool therefore behaves like a boat hull. As long as the weight of the pool (the ballast) exceeds the pressure exerted by groundwater, the structure remains stable within the soil. However, if the soil becomes saturated and the water level inside the pool is too low, the upward force (uplift) becomes dominant, leading to a vertical displacement—often sudden—of the pool.

2. Influence of the Québec Geotechnical Context

The geology of the Saint-Laurent Valley imposes potentially severe constraints on buried structures.

The issue of clay soils

A large portion of Québec’s territory is underlain by clay soils at the surface. These soils are characterized by very low infiltration capacity. Unlike sandy or gravelly soils, which allow water to drain freely, clay retains moisture or water within the soil mass and promotes the buildup of hydrostatic pressures on structures built in these soils, such as in‑ground swimming pools.

Freeze–thaw effects

During winter, water retained in clay soils around the pool may freeze, generating lateral and/or vertical pressures on the pool structure. During spring thaw, which does not necessarily occur uniformly, the upper soil layer thaws first and becomes saturated with meltwater, while deeper layers may remain frozen and impermeable. This situation creates a “bathtub effect” in which meltwater is trapped around the pool, increasing localized hydrostatic pressure against the structure.

3. Pool Design and Installation Parameters

The long‑term performance of a swimming pool depends largely on effective groundwater management around the structure.

Peripheral drainage systems

During pool construction, peripheral drainage is critical and must be installed at the base of the slab (pool floor) and connected either to a functional outlet (for permanent drainage) or to a sump well (for intermittent drainage needs), depending on the depth of the groundwater table being managed. A crushed, clogged, or improperly elevated drain loses its decompression function, allowing pressure to accumulate beneath the structure.

Backfill materials

The choice of backfill material is also a determining factor. The use of clean stone (¾‑inch crushed stone) creates a high‑permeability zone around the pool. Sand backfill can also achieve a similar objective by allowing water to migrate quickly toward the drainage system rather than stagnating against the pool walls. Conversely, the use of excavation spoil (often clayey) promotes saturation and water retention.

4. Pool Maintenance and Winterization Activities

When analyzing the cause of a loss event, the operational condition of the site at the time of occurrence is a major technical parameter.

Water level management in the pool (ballast)

The water contained in the pool is the simplest and most important stability factor. Lowering the water level for winterization or seasonal cleaning reduces the overall mass of the structure. If this action coincides with heavy rainfall or snowmelt, the safety factor may drop below unity, resulting in pool uplift if drainage is inadequately designed or used. During winterization, the procedure involves lowering the water level to a specific height relative to the pool coping or to the return nozzles of the filtration and sanitation system. This reduction assumes that drainage around the pool is adequate to prevent groundwater uplift forces from exceeding the combined weight of the pool and its contents. Compliance with the winterization procedure—generally provided by the pool contractor—is therefore essential.

The sump well: a safety valve

The sump well is a critical drainage component, allowing monitoring of groundwater levels around the pool and, more importantly, enabling mechanical lowering of these levels using a submersible pump as needed. This is particularly important during spring snowmelt, during periods of intense or prolonged rainfall in summer, or for various maintenance activities requiring temporary lowering of groundwater levels or partial or complete draining of the pool.

5. Cause Investigation

When movement or deformation of an in‑ground pool occurs, the engineer will look for specific physical evidence to confirm the magnitude and origin of the movement. This includes signs of differential vertical displacement, deformation of the pool walls or floor, abnormal tension in the liner, or misalignment of pool equipment. Determining the cause also relies on temporal correlation between environmental conditions and the appearance of damage, such as winterization dates, other maintenance activities, the timing of damage discovery, or specific weather events. Lastly, examination of the pool and its equipment can help reconstruct the sequence of events and distinguish an unforeseeable accidental occurrence from a chronic drainage deficiency.

Conclusion: Balancing Natural Forces and Design

The uplift of an in‑ground swimming pool results from the interaction between powerful hydraulic forces and the structural characteristics of the pool. While Québec’s climate imposes severe environmental constraints, the long‑term performance of a pool depends on three key pillars: drainage‑oriented design, rigorous installation, and proper maintenance.