This insight into groundwater dewatering is written by Ines Epari, an Environmental Engineer with over 12 years of experience in hydrological modelling. Ines has extensive experience in groundwater and surface water numerical modelling having successfully delivered projects for industry and government around the world.
Many Australian suburbs are currently experiencing an upgrade of inner-city industrial areas to residential living areas. Limited space makes apartment buildings a preferable option for development. These apartment buildings require parking spaces, which are built underground. A construction pit for one, two or even more basement levels might intercept groundwater. The most common approach for a dry basement construction is to install a retention or cut-off wall and to remove the groundwater out of the pit.
Groundwater – the big unknown
Basement constructions intercepting groundwater create challenges to the developer (or contractor) during all stages of the project. Groundwater by its very nature is not easy to assess for quantity and quality. In addition to this, groundwater is a valuable resource and is protected under the Water Acts of the states, which has to be considered in the planning process. So how can developers deal with the groundwater related challenges during dewatering?
Internal and external impacts
Dewatering of groundwater can create large amounts of water that need to be disposed of. A disposal permit may be a regulatory requirement depending on the expected groundwater volumes and quality. The expected groundwater inflow is also a key in designing the appropriate dewatering system, i.e. wall type, number of pumps, optimum pump size and pump depth for the project specific ground conditions. If the dewatering option is over-engineered, this can result in unnecessary expenditure. On the other hand, if the dewatering option is unable to cope with the influx of groundwater, this can result in lengthy and costly delays. Therefore, it is important to estimate inflow rates prior to construction, so that the appropriate dewatering system can be designed.
Figure 1 – Modelled groundwater inflow rates and cumulative volumes for two dewatering scenarios over 200 days of dewatering. Scenario 1 is the preferable option for this site, because the results show less groundwater inflow, cumulatively lower than the license threshold.
The dewatering of the construction site may also have impacts on the outside of the retention walls. Due to the lowering of the groundwater levels ground settlement can occur. This can lead to damage on close-by buildings. Depending on the regulatory framework, there also might be a requirement of assessing the impacts of the dewatering system on the surrounding aquifer. If a larger area is developed, the combined impacts of all projects in the might add up and the regulator needs information to assess and approve the combined impacts.
Preparation ensures success
It is important to have a reliable estimate of the quantity and quality of groundwater to be extracted during basement construction. This can be accomplished by:
- Installing boreholes at the construction site;
- Conducting field testing of the boreholes to assess the permeability of the soil / bedrock;
- Sampling the groundwater and analysing its quality;
- Modelling the groundwater inflow to the pit, using the field testing results; and
- The model can also be used to estimate groundwater level decreases near the adjacent buildings.
An accurate estimate of groundwater inflows is a key component of the construction process, and it can prevent costly project overruns. Furthermore, it is important to choose the right dewatering option to reduce settlement issues on the surrounding buildings. A dewatering assessment will remove a layer of uncertainty, allowing for a trouble-free construction phase. As with most successful ventures, preparation is the key to a success.
Figure 2 – Vertical section through a groundwater model for a dewatered site. The permeability of the aquifer is plotted in yellow, brown and grey. The black structure is the retention wall and the red structures the dewatering wells. The water level after 200 days of pumping is plotted in blue.
Talk to our experts to find out more
Are you planning a site that might dewatering? Has your geotechnical investigation report shown that groundwater might be intercepted? Contact us and we will find the best and most cost effective solution for you. This will give you the peace of mind that you have addressed the necessary requirements and are minimising internal and external impacts.