Tunbridge Wells Modelling of Groundwater Flow

Richard Ball

Technical Director

Tel: 01483 310600

Email Richard

Client

Tunbridge Wells Borough Council

Services

  • Ground Investigation
  • Basement impact assessment
  • 3D Finite Element Analysis

3D finite element analysis to assess the impact of a large basement development on groundwater flow and levels

CGL recently undertook three dimensional groundwater modelling to evaluate the impact of the construction of a large new basement in Tunbridge Wells.

The proposed development includes excavation up to 12m into an existing slope. The excavation will be supported by a secant-piled wall providing a cut off to groundwater flow, which could potentially cause a change in groundwater flow rate and direction, increasing groundwater levels around the site.

With the development situated in a built-up area including neighbouring properties with existing basements, it was critical to understand how the new basement would affect the local groundwater regime and establish whether specific groundwater control measures would be required for the development. Understanding the impact on groundwater level was also important to design the retaining structure.

A steady state groundwater flow analysis was carried out using 3-dimensional finite element software (PLAXIS 3D) to assess the impact of the basement on the flow direction and groundwater levels. The model was based on a detailed review of the geology, ground and groundwater conditions derived from historical borehole records, geological memoirs and CGL’s extensive site investigation.  Additionally, LIDAR data was obtained for the site, allowing 3D study of the local topography and catchment area to inform the assessment of both surface and groundwater flow onto the site.

Using this information, the FE model was set up with accurate stratum levels, groundwater levels and hydraulic gradients, including input of geotechnical and permeability parameters. The PLAXIS 3D model included anisotropy within the soils, a factor considered to significantly impact the behaviour of the groundwater flow around the basement.

The model was run in an initial state with no basement to determine the pre-existing flow direction and flow rate. The results of this model were compared to levels and hydraulic gradients derived from the ground investigation data. The basement was then activated in the model using impermeable plate features to represent the basement retaining walls. With the impermeable basement installed, the model was able to analyse the impact on groundwater flow and level. The analysis outputs included the change in pore pressures, change in groundwater head level and flow rate.

Through this assessment, CGL was able to demonstrate that the change in groundwater level would not significantly impact neighbouring basement properties and that the groundwater flow would dissipate around the development, saving the need for complex and costly
drainage strategies to be considered in the scheme. Additionally, this analysis shows economies for design of retaining walls by deriving realistic and accurate short and long term design groundwater levels around the basement.