With water levels fluctuating more erratically in recent years due to an increase in extreme wet weather, it is vital that water table levels are reviewed when considering building structures.
What is a Water Table?
The water table is the surface of a body of unconfined groundwater at which the pressure is equal to that of the atmosphere.
The water table can be viewed directly by drilling an observation well into the groundwater body and waiting for the water level to stabilise. Repeated measurement of the water level in this well over a year will reveal that the water level fluctuates, perhaps by a metre or more.
What is a high-water table area and how could this affect where you build?
A high water table area is defined as any area where the water table is within 1.8 metres, or 6 feet off the ground surface during the frost period up until the end of August or within 2.4 metres or 8 feet of the ground surface during the rest of the year.
Having high water tables can have negative impacts, including:
- Adversely affect how well a sewage disposal system can function, which can lead to groundwater or surface water contamination.
- Make land unusable for building work, or construction of roads or buildings.
How to undertake a water table test hole?
One of the basic methods for determining water table levels is the water table test.
This test consists of drilling a hole to a depth of 3 metres, allowing the water to stabilise, then measuring the distance from the groundwater to the water level.
What are the conditions for a water table test?
- Optimal time for testing – the optimal time for testing is during the spring after the frost is out of the ground. This is where water tables are generally at their peak level.
- Location of test holes – as a general rule one water table test is needed for an area less than 3 acres and 3 table test holes for sights between 3 to 10 acres. If this is a test for one residential home, then a test hole at the proposed building site and another within the drainage disposal field will be sufficient.
- Drilling the test hole and keeping a soil log – drill the hole to a minimum depth of 3 metres. In sandier soils where the contamination of groundwater is of concern, then a deeper test hole may be needed. As the hole is being drilled record the soil texture.
- Preparing the test hole for water level measurements – after drilling the hole, insert a perforated plastic pipe that reaches the bottom and protrudes above the ground surface at least 30 cm. This pipe needs to have a lid places on it to prevent debris entering. The excavated soil should be backfilled around the pipe, and soil should be mounded around the standpipe to prevent the entry of surface water.
After all these conditions are ensured – it is time to measure your water table – this is by measuring the distance from the groundwater to the water level in your 3-metre hole once the water level has stabilised.
Knowing your water table level is very important to assess the flood risk in any area of construction.
We supply, and if required can also install sewage treatment and stormwater attenuation solutions. Please feel free to call us for expert advice on 0121 351 3230.