It only happens rarely, but sometimes when digging inground swimming pools you may discover a high water table. Or, there can be issues with surface ground water, flowing towards your future pool. Don't let this scare you, there are solutions for these that you can take care of yourself.
Dealing with Surface Run-Off around your Inground Pool:
If your pool area is lower than the ground around it you will need to provide a route for the water to go around, not into or under, your swimming pool. Small retaining walls with french drains can be installed, or you can dig a 18" deep trench next to the pool deck and cover with a few inches of 3/4 in. stone. Top this with a perforated drain pipe and then another layer of stone before backfilling the trench, slightly lower than the surrounding ground. Now, when the water flows towards your pool it will hit your drainage area and flow around to the [lower] end of the pool. Another way to combat surface runoff water is to cut a swale into the ground before your pool patio. What this is would be a V shape cut in the ground with a pitch away to one end of the pool. The water would hit this and flow down and out the pitched end.
Dealing with High Water Table around your Inground Pool:
Water issues from a high water table, encountered during excavation, is the main topic of this blog post. It usually happens while digging an 8' deep pool, and at 6 or 7 feet - you hit water. Usually you can pump it out and it doesn't come back, but sometimes the water seems to come back in as quickly as you can pump it out. (Make sure that while pumping out this water that you pump at least 50' away from the pool, to prevent "recycling" of the water back underneath the pool.)
If the water keeps coming in, even though the weather has been reasonably dry, here's what you do. Keep digging! Over dig your deep end about 2 feet deeper than the pool depth. A trench should be dug, from the deep end floor, up the slope to the shallow end - also 2 feet lower. Then have your gravel delivery company bring you enough ¾” stone to fill in your over dug areas to within 2” from your finished floor level. If the soil is very sandy and loamy you will need more gravel than if you have a tight clay soil, to keep the water flowing.
After the gravel is in place, we will use 1 ½” flexible pvc pipe to create a conduit for the water, while you continue to build your own inground pool. On the pool-end of the pipe, attach a check valve and a grate and run the pipe from the lowest part of the pool, out the bottom side of the pool, into the trench. Dig a trench outside of the pool, to the depth of the over dig. The trench will rise in level to about 1 foot below the ground. Continue to run the pipe to an area where you can connect it to a self-priming pump like an inground pool pump. Connect another pipe or hose on the discharge side of the pump, prime it up and turn it on. Shortly the pump will start pulling your underground water out of the hole.
In some cases, you may find areas above your water table, flowing through the sidewalls, into the pool dig area. Keep digging! What you do with these is dig a trench on the inside of the pool, to connect to the gravel pit on the pool floor. Dig deep and fill with 1-2 feet of gravel. Once you have the water under control you can use a roll of tar paper, roofing paper or a large piece of plastic to cover the gravel areas. The reason for this is when you install your pool bottom material you don’t want it to mix with the stone and eventually clog the dewatering pipe.
Now you can continue with building your pool and during the pool installation project the pump should keep running. Once you have the pool full of water then you can turn off your pump and the water weight above should hold back the water table.
How to Test for High Water Table:
I'll admit it, there are some locations that make it difficult to build an inground pool. Low, topographically depressed areas, or shoreline areas that are actually below sea level. In severe cases, those who really want an inground pool can truck in enough fill dirt to raise the pool area up a few feet, so that ever-present ground water won't be an issue. In most cases, however - the dewatering approach described above will solve the problem.
One way to test your ground water level is to use an auger drill to drill down to the level of your pool's deep end in several spots around the proposed pool site. Do this when the soil has a relatively normal water content - that is, not after a 3-day rain. Instead of the auger, you can dig a 4' deep hole large enough to stand in, and then use a post hole digger to dig down further to check for water. You may encounter some water at this depth, but it doesn't mean that your inground pool project is going to be difficult or impossible to construct!
We've never seen a water situation that couldn't be dealt with somehow. Even in the worst cases, the pool can still be built, with just a little more time (and money) spent on over-excavation and extra gravel.
SPP Pool Expert