Basement Impact Assessment

Challenges

TRC was commissioned to undertake a Basement Impact Assessment (BIA) for the planning application of a new six-story residential building. A single-story basement approximately four meters deep was included in the proposal. The purpose of the BIA was to consider the effects of the proposed basement on the local hydrogeology, land stability and hydrology and any impacts to neighboring structures and infrastructure that could violate the policies of the local authority.

The development is situated within a densely built urban area with significant structures and infrastructure in the vicinity, including a six-story grade II listed hotel, the London Underground exclusion zone of the Central line, several buildings up to nine-story high with up to three-story basements, local highways and underground utilities. In addition, the site-specific ground investigation recorded the presence of non-engineered Made Ground locally up to 5.40m thick, associated with past developments.

Solutions

A desk study was undertaken that included a site walkover, a review of historical maps, a review of public geological, hydrological and hydrogeological data, and planning records for the site. A screening and scoping assessment was undertaken, leading to a tailored ground investigation which was assessed to the latest Eurocode and British Standards.

In liaison with the structural engineer, ground engineering properties and geotechnical recommendations were provided for the foundations and basement design. The proposed basement perimeter contiguous piled retaining wall and excavation were modelled using specialist software. A damage assessment of the nearby buildings and infrastructure was carried out.

Results

The BIA report concluded that there will be no impact to the wider hydrogeological environment and that the groundwater flooding risk to neighbouring properties will not increase because of the proposed development. Similarly, the overall impact to the site’s hydrology will be negligible.

Further, the anticipated damage to surrounding structures was assessed in accordance with the Burland Scale and was found to be within the limits set by the local authority. Overall, the cumulative impacts have been screened and identified in respect to hydrogeology, stability of surrounding properties, and surface water flows. Effective measures have been proposed, as required, to mitigate these impacts.