Swimming Pools

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Choosing a Swimming Pool Professional

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Guide to Swimming Pool Construction

Swimming Pool Site Preparation and Excavation

Swimming Pool on Problem Soils

Swimming Pool Special Soil Conditions

Swimming Pool on Fill and Rock

How a Contractor Build a Swimming Pool

Swimming Pool Finishes and Trims

Swimming Pool Accessories

Special Soil Conditions

Underground Water Pressure

The problem of underground water pressure exists with all pools with rigid floors. The swimming pool is basically a giant saucer and can be pushed upward if enough water pressure is allowed to collect beneath the pool. This is particularly dangerous when the pool is empty, since there's no weight to counteract the pressure.

To avoid this problem, most swimming pool builders install hydrostatic valves at the main drain of concrete, fiberglass, and hybrid fiberglass swimming pools. These valves open either automatically or manually to relieve the pressure by allowing ground water to enter the pool.

Manual valves remain closed while the pool is full and there is little danger of bouyancy. But when the swimming pool is either partially or completely drained, a long forked pole is used to open the valve, releasing the hydrostatic pressure. When the pool is refilled, the valve is closed.

Hydrostatic valves are equipped with a float that's raised by water pressure from below. This prevents water from leaking out of the swimming pool. Only when the pressure beneath becomes great enough to lift the float will the valve open.

If your swimming pool doesn't have a hydrostatic valve, the pool should never be drained without first contacting the swimming pool contractor or an engineer. They will advise when the water table is low enough to avoid a dangerous pressure build-up.

Where valves are not used, a sub-grade drainage system should be installed. The swimming pool can be ringed with a line of drain tile, placed on a definite grade for good drainage. The floor of concrete and fiberglass pools should rest on a bed of crushed rock, if surrounding soils are normally slow-draining.

Expansive Soil

Generally, expansive soil resists water, but when it finally does absorb moisture, the earth expands a lot. As a result, heavy pressure builds up against the swimming pool walls and my crack them.

Swimming pool builders generally have two sets of steel plans, one for normal soil and one for expansive soil. The latter calls for more reinforcing steel, thicker walls, or both. But since the walls are rigid, there is still a danger of cracking if the soil expansion is uneven or greater than anticipated.

An expandable water stop can be installed to prevent water from seeping into the soil at the joint between the coping-bond beam section and the deck. Made of rubber or plastic, the stop can take several forms. The usual type, called a compressible expansion joint, is poured as a liquid into a gap prepared for it between the deck and coping when the deck is laid out in sections, the joints must also be sealed for several feet around the swimming pool.

Your contractor can take additional preventive measures, often used together to insure protection.

A 15-centimeter lip can be poured on the back edge of the surrounding walkway to provide extra support for the bond beam.
The top 1-meter of clay around the swimming pool can be replaced with clean, compacted fill; this lessens the pressure by removing the cause. In some cases, as with fiberglass, the backfill is reinforced cement grout to give the shell even more rigid support.
A trench, 1.5 meter deep and 30 centimeters wide, can be dug around the swimming pool, 1.5 meter to 3 meters from the walls. This is filled with loose material that can absorb the soil expansion; the top of the trench can be covered with decking later.