Building A Pool Near A House Or A Boundary

Most pool problems are not pool problems. They are excavation problems next to a structure that was relying on the soil being there.

The zone of influence

A footing spreads load into the ground below it, and that spread extends outward as well as downward.

Excavating inside that zone removes part of the support the footing depends on. How far the zone extends depends on the footing depth, the soil type and the load, and a conservative rule of thumb is that it extends outward at roughly forty five degrees from the base of the footing.

Which means a pool three metres deep, three metres from a house footing, is almost certainly inside it.

Being inside the zone does not mean the pool cannot be built. It means the design has to account for it, and a pool designed without that consideration is how a house ends up with new cracks after a pool goes in.

What the options are

Move the pool. Cheapest and least popular.

Design the pool wall as a retaining structure for the adjacent footing load, which means a heavier shell, a structural connection to the pool floor and frequently piers.

Underpin the adjacent footing so it is founded below the pool excavation before digging starts. The approach described in [underpinning adjoining structures](/blog/underpinning-adjoining-structures-explained).

Pier the pool shell so it is founded independently and does not rely on the ground the house also uses.

Stage the excavation in panels rather than open cutting the whole pool, which limits how much support is removed at once.

Which one applies depends on the soil, the depth and the load, and that means a [geotechnical investigation](/blog/understanding-geotechnical-investigation-reports) before the design rather than after the first crack.

On the boundary

A pool close to a boundary brings the same issue with someone else's building.

You have obligations to support adjoining land and structures, and they are not discretionary. Practically that means establishing what the neighbour's footings are and how deep they go, before designing the pool.

It also means a [dilapidation report](/blog/importance-of-dilapidation-reports) on the neighbouring property before any excavation. Photographed, dated and issued. Without it, any pre-existing crack becomes your crack the moment the excavator arrives.

And it frequently means the pool wall is designed as a retaining wall carrying their surcharge, which changes it from a pool shell into a structural element. The whole set of issues that apply to [building on the boundary](/blog/building-on-the-boundary-and-party-walls) apply here.

Reactive clay makes it harder

On a reactive clay site, the pool introduces a large rigid object into soil that moves seasonally.

Two consequences. The pool itself is subject to ground movement and needs to be designed for it, which usually means piers to stable material. And the pool changes the moisture regime around it: the paving, the coping and the pool itself shed water differently from the garden they replaced, which can dry or wet the clay under the adjacent footings.

That second effect is slow and it is a real cause of movement in houses that were stable for decades. It sits alongside [trees and footings on Sydney clay](/blog/trees-and-footings-on-sydney-clay) as a moisture-driven mechanism rather than a load-driven one.

On a sloping site

A pool cut into a slope is a retaining structure whether or not anyone designed it as one.

The uphill wall retains the slope, and it may retain a surcharge from a structure above. The downhill side may have very little ground in front of it, which affects both the pool's own stability and the stability of the slope below.

Both need engineering, and on a steep site the pool frequently becomes the most substantial structure on the property.

Emptying the pool

Worth knowing before it happens.

A pool is designed with water in it. The water inside balances the soil and groundwater pressure outside. On a site with a high water table, emptying the pool can allow it to float, or allow the walls to be pushed inward.

Which is why the engineering advice on some pools is never to fully empty them without a specific procedure, and it is the same [hydrostatic uplift](/blog/basement-tanking-and-hydrostatic-uplift) mechanism that applies to a basement.

What an engineer provides

An assessment of the excavation against the adjacent structures, a shell design suited to the actual soil, footing or pier details where they are needed, and the retention design where the pool is holding ground.

The [structural design of the pool](/blog/swimming-pool-structural-design) is the straightforward part. The interaction with everything around it is the part that produces the claims.

ACSES provides geotechnical engineering and structural engineering for pools and excavation across Sydney. Talk to us about a site.