Basement Impact Assessments

Given the relatively high land and property values in London (and its outer suburbs), basement construction has become increasingly popular in recent years. Several local authorities have recognized that the construction of basements has the potential to impact on flooding, groundwater, land stability and the structural stability of adjoining/neighbouring properties.

Therefore, a number of London Boroughs, notably Camden, Westminster, Kensington & Chelsea require a specific Basement Impact Assessment (BIA) at the planning stage to assess the hydrogeological, hydrological and geotechnical impact of basement constructions on neighbouring properties.

In 2009, the London Borough of Kensington and Chelsea produced their own planning guidance document entitled “Subterranean Development”. The London Borough of Camden adopted a similar approach for their use in planning applications involving basements (Camden development policy DP27). The Camden document (CPG4)presents a structured approach which identifies the potential risks in a five stage Basement Impact Assessment (BIA) process. The overall process has now also being adopted by a number of other London Boroughs as a template for their own assessment of basement construction applications.

The level of technical information will vary according to the type of development and planning authority and is likely to include:

- Site specific screening and scoping of existing geological, hydrological and hydrogeological conditions of the site and the wider area in order to identify areas susceptible to instability from ground and/or water movement, localised flooding and lost rivers.

- Detailed intrusive investigation to assess local ground and ground water conditions, including, through the use of boreholes, potential impacts on water movements and on adjoining/nearby properties.

- Identification of suitable temporary and permanent construction methods and mitigation measures for safe basement developments including monitoring of adjacent properties.

-Devising a method for monitoring local ground conditions, water movement, subsidence and drainage.

-Ground movement calculations according to CIRIA to assess potential ground movement in line with Burland’s scale of damage.

BIAs follow the guidelines produced by the various Borough Councils. The requirements generally include some or all of the information below:

Screening and Scoping Stage - Collection and assessment of:

-Archival Maps

-Geology and hydrogeology

-Hydrology and lost rivers

-Flood Risk from rivers/canals/reservoirs/groundwater

-Critical Drainage Areas

-Environmental data

-Landfill and Mining Data

-Site Inspection and walk over survey

-Review of adjacent buildings

-Checking for signs of subsidence

-Checking for underground services

Impact Assessment Stage - Ground Investigation:

-Undertaking boreholes and/or trial pits for ground investigation

-Collecting soil samples for geotechnical/environmental testing

-Installing standpipes for monitoring groundwater and/or gas levels

Ground Movement Calculations

-Ensuring that the scheme will maintain structural stability of the building and neighbouring properties. Calculating, according to CIRIA guidelines, levels of any potential ground movements that may affect adjacent properties, according to Burland’s scale of damage.

Structural Design:

-Structural Method Statement

-Construction Method Statement

-Design of temporary and permanent support for excavation

-Design for monitoring buildings during construction

Additional reports/Risk Assessments:

-Site specific flood risk assessment if required

-Geology and slope stability risk report if required

-Hydrology and hydrogeology report if required

-Construction management plan if required