In this Advantage Home Construction Insurance Ltd new publication we give information on elements that are inspected to current guidelines and British Standards. November’s issue will cover Site Investigation reports – what they cover and why we need them.
Approved Document C (Site preparation and resistance to contaminants and moisture) advises that a Site investigation report is essential to determine the substrata properties on a proposed site location. The engineering make-up of the substrata will govern how a building can be safely constructed including the foundations/substructure in addition to preventing potential geoenvironmental (contamination) hazards affecting the occupants.
There are a variety of influences affecting the scope for each site investigation such as the previous use of the site. Therefore site-specific situations will define the techniques and procedures required, ordinarily the process is as follows:
- Phase 1, Desk study.
- Phase 2, Intrusive investigation.
- Phase 3, Remediation strategy.
- Phase 4, Validation.
“The terms ‘site investigation’ and ‘ground investigation’ are frequently confused. A site investigation involves the collation of desk study information; appraisal of the data; assessment of the ground conditions; and the provision of an interpretative report. A ground investigation is broadly a more restrictive phase of specialist intrusive geotechnical investigation with associated site monitoring, testing and factual or interpretative reporting” (rsa-geotechnics.co.uk).
PHASE 1, DESK STUDY:
Site Description – The first section, should provide the exact location of the site and describe the surroundings areas (vegetation/watercourses etc.) Any invasive plants or potential contamination needs to be reported with photos where possible.
Site History – Historical ordnance survey maps identify previous use of buildings and sites whilst highlighting potential buried impediments such as old foundations usually within a 250m radius of the site. Local Authority libraries are an additional source for information.
Geology/Mining – The British Geological survey Geo-Index delivers existing logs and reports. All potential geological faults, bedrock and weathering that may disturb the site should be incorporated. If the proposed site is situated within an area of mining, surface stability should be established corresponding to the Coal Authority report.
Hydrogeology and Flooding – Flood risk is essential and should be highlighted within the assessment of the site to identify the flood risk, any pollution of groundwater and drainage. The Local Authority and Environment Agency regulate flood risk data.
Environmental Setting – dependent upon the site circumstances as discussed above, the environmental setting will establish if the site is at risk from pollution or/and poses an environmental risk. Generally, historical activates pose a higher risk of pollution on a site in comparison to current industrial processes.
Radon – The Health Protection Agency risk maps should be used as reference concerning radon protection measures.
Geoenvironmental Risk Assessment – Under the Environmental Protection Act 1990 – Part IIA, a quantitative health & environment risk assessment should be undertaken to cover pollution linkage. This will identify the source between a contaminate pathway and the receptor.
PHASE 2, INTRUSIVE INVESTIGATION:
The Investigation – an intrusive ground investigation is used to minimise the risk on site and produce thorough data subsequent to the phase 1 desk study if required. They decrease the risk of possible unforeseen conditions and full supervision should be provided by a Chartered Engineer or Geologist. The site-specific circumstances will determine the processes required for the investigation, many comprise of:
- Rotary Drilling,
- Window Sampling,
- Shell and Auger Boring,
- Trial Pitting.
Strata Profile & Soil Description – Both visual & manual identification should be completely defined including the origin, consistency, colour, weathering etc. The boreholes/trial pits samples described should correspond with the applicable British Standards.
In-Situ and Laboratory Testing:-
- Soakaway Testing (undertook in trial pits to intersect permeable soils inside the bedrock).
- Geotechnical Laboratory Testing
- In-Situ Gas Monitoring (undertook in boreholes/side of trial pits or using standpipes, methane and carbon dioxide levels can then be measured using a portable meter).
- Contamination Laboratory Testing (a UKAS recognised laboratory should produce a preliminary assessment to conclude the environment/health risk related).
- In-Situ Strength Testing
Geoenvironmental Risk Assessment – A Detailed Quantitative Risk Assessment can now be produced using the ground investigation results and previous desk study. It should encapsulate the pathways, receptors and hazards relating to the site in addition to consequential recommendations and remediation’s.
Recommendations – A site location plan is necessary, outlining key features and borehole/trial pit positions. The recommendations should be in relation to the following: change of use, stability & retaining walls, contamination, ground water, existing services, landfill & radon, nearby trees, piling/footings, soakaways, mining and any further considerations.
It is important to note that during the construction phase, if any unforeseen conditions are encountered then the Developer should notify the appointed consultant immediately and undertake any further instruction considered necessary. The report should make recommendations and state measures to be taken concerning foundation design, any and all radiation, water table issues, and drainage concerns i.e. if not suitable for soakaways, past chemical use still identified within the soils and any flora or fauna which may cause instability to the foundations.
Please note, computerised risk assessments are not always deemed satisfactory by regulators. AHCI Ltd recommend that a comprehensive Site Investigation report and/or Desk Top Study is undertaken by a chartered professional. If the desktop does identify any underlying issues, a further phase 2 investigation may not be required.
Look out for next month’s publication which will focus on Foundations and the different types for different purposes.