GROUND WATER TABLE PROBLEMS
During the construction of a swimming pool or in-ground water retaining structure, one of the most important considerations is the ground water level adjacent to the structure.
In many places, the ground water level may impact certain aspects of the structural and shell design. We have been involved in a number of consultancy projects where the ground water datum level, during certain periods, provides a very real concern to the structure of the water retaining structure.
During the project construction, the site may well have a de-watering pit set up to combat the ground water level, however the ground water level has to be considered beyond just this phase. Often a full swimming pool/water body will not be effected by any high water levels surrounding the structure although the worst case scenario should always be considered and that is with a high water table, and an empty pool.
You may have seen or heard of horror stories arising from such a situation. We were in fact involved recently in a project where a completely new equine pool had to be constructed as the previous equine pool structure never made it past the commissioning phase - causing much furore between the previous contractor, main consultant and an understandably upset client.
So, what is the worst case scenario?
Well, given that your swimming pool, water feature or other such water retaining structure is designed to keep water in, it will naturally keep the ground water out too. Should a situation arise where no precautions have been taken to consider the effects of a high water table, then you face the very real possibility of your structure suffering from flotation if it were to be emptied. In many cases this will cause severe structural damage, or more likely complete demolition resulting in considerable repair costs.
With some careful forethought, this situation could have been avoided.
What can be done?
There are a few options that can be considered to specifically target this problem, such as:
Permanent De-watering
As with the construction phase, a dewatering pit can be installed (generally adjacent to the swimming pool), with a submersible pump installed within a suitable chamber that allows the flow of ground water. This can be used permanently, or during maintenance periods where the swimming pool may be drained. There are certain other factors to consider with this, as the repeated removal and reinstatement of ground water may change the characteristics of the substrate, thus potentially effecting nearby structures.
Hydrostatic Valves
This is a system that we have proposed in a number of projects, which offers a safeguard on your structure. The hydrostatic valves will activate when there is a reverse pressure differential between the pool interior and the exterior conditions. Whilst the pool is full, the valves remain closed and sealed to prevent water escaping. If the pool is emptied and there is sufficient pressure differential, then the valves will open to relieve this pressure by allowing the flow of ground water into the swimming pool. This generally results in a dirty swimming pool, but at least you still have an intact swimming pool.
The hydrostatic valves are generally installed within the main drain sumps such that they are at the lowest point of the structure and are discretely hidden from view.
The valves themselves are inexpensive, and we would advise be installed, even with a permanent dewatering system (as you never know if the submersible pump(s) will fail).
Structural Design Considerations
This method is quite simple, and involves the design of the structure itself to counteract the potential uplift arising from the ground water. The over-sizing of the concrete structure to provide this effect will result in a noticeable increased expenditure in the cost of construction, however if it is designed correctly, will provide peace of mind in future.
As with most aspects of bespoke builds, each project is different and has it's own hurdles to overcome. If you are concerned about potential problems caused by high water levels, then please do not hesitate to get in touch, where will will be happy to talk you through the options available that best suit your requirements.