Soil and Geotechnical
Can Pools Cause a Surcharge Load on a Retaining Wall?
We have been asked this question many times, from contractors, city plancheckers, buildings inspectors, almost everyone in the pool construction process. Does building a pool on the high side of a retaining wall impart a surcharge load to the wall?
Most people would think “well yeah, a pool is super heavy, so of course, it will cause a load on the wall”, but the reality is that an in-grade pool (or even a pool that is mostly in-grade) doesn’t cause a surcharge load on a retaining wall for one very simple reason; water weighs substantially less than soil.
The average unit weight of water at average temperature and pressure is about 62.4 pounds per cubic foot (pcf), whereas the average unit weight of in-situ soil may be between 110 and 120 pcf. That is a large difference in weight.
That means that when building a pool in the ground you are removing 110 lbs for every cubic foot of soil you excavate and replacing it with around 63 lbs (neglecting the thin concrete shell at the perimeter that makes up the structure of the pool). Thus when building an in-grade pool on the retained side (high side) of a retaining wall, you are effectively reducing the net overburden surcharge to the wall, not increasing it.
In addition, it is generally accepted engineering practice when constructing a pool on the retained side of a retaining wall that the pool walls are designed as freestanding (that is not relying on support from adjacent soil). Therefore, the outward water pressure from the swimming pool would not be transmitted as additional lateral loading to the adjacent retaining wall.
Based on these two facts it is clear that constructing a pool on the retained side of a retaining wall will not increase or impart additional loading to the wall.
Be advised, however, that oftentimes a retaining wall is backfilled with compacted fill therefore be cognizant of a pool’s subgrade conditions when building behind a retaining wall so as to avoid constructing the pool over a cut/fill transition due to a possible wedge of compacted backfill behind the wall. Observation by a licensed geotechnical engineer is highly recommended in these cases.
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