At Earth Environmental and Geotechnical our Basement Impact Assessment (BIA) involve three key stages:
Stage One: Desk Study and Screening & Scoping Report
Initially a desk based screening and scoping report will be completed alongside a Phase I Desk Study. At this initial stage, the proposed development is reviewed in the context of the site’s history, its geological and hydrogeological conditions, potential for contamination and any risks posed by surface and groundwater flooding.
A Flood Risk Assessment may also be required if the proposed development resides within a defined flood zone area. If the development site is in a Flood Zone 1, this may be of limited scope, however if the property is in a more vulnerable flood zone, detailed further investigation is likely to be required.
The desk study and screening and scoping report will conclude by outlining any areas of concern which require further investigation.
Stage 2 : Site Investigation
The second stage of a BIA is to undertake a Site Investigation / Ground Investigation tailored to investigate the potential issues highlighted within stage one and to provide design information to allow the safe construction of the basement.
The site investigation will help to highlight any potential hazards beneath a site which are unknown and determine the engineering properties of the underlying soils. Ground investigation works also allow the installation of ground gas and groundwater monitoring standpipes and subsequent recording of ground gas and groundwater data. This data will in turn be used to determine if waterproofing / dewatering will be required as part of the development and if ground gas protection measures will need to be installed.
The type of ground investigation techniques used will be dependent on the proposed basement design and the access constraints present at the assessment site.
Given that basement developments are often required on sites with restricted access, specialist limited access investigation techniques may be required.
The following is a list of intrusive investigation techniques often used as part of a BIA.
- Hand Pits (often used to expose current foundations and the presence of known underground utilities)
- Trial Pits (used to visually inspect shallow ground conditions to depths of around 3.5m)
- Boreholes (used to inspect ground conditions and undertake insitu testing on underlying soils and rock)
- Modular / Limited Access Boreholes (used on sites where access for a standard drilling rig is not possible. Several specialist drilling rigs are available with rigs being able to be carried into position by hand)
Once the ground investigation works are complete, a factual and interpretive report will be produced. This report will provide detailed of the works undertaken, alongside geotechnical design information for the basement’s construction. Other influencing factors such as contamination will also be discussed within the ground investigation report and any mitigating measures required will be produced.
Stage 3 : Basement Impact Assessment Report
Finally, after the Site Investigation stage, a detailed Basement Impact Assessment report will be produced.
This report combines the information from both stages 1 & 2 and provides a complete assessment of the risks and mitigation measures required to successfully complete the basement development.
An example of the topics covered within Stage 3 include:
- A summary of the ground conditions identified beneath the assessment site.
- Recommendations on the type of construction methods required, including recommendation on foundation depths and types.
- Recommendations Regarding Structural Monitoring.
- The Potential for Hydrological Impacts.
- The Potential for Hydrogeological Impacts. And
- The Potential for Flood Risk Issues