Tech Update: Site Investigation Phase 2 Geoenvironmental Assessment (Intrusive Ground Investigation)
Before you begin your intrusive ground investigation it’s important that you’ve completed your desk study. Check out our earlier blog on the desk study for more details and follow the flowchart above for direction on what to do and when to do it.
Once your desk study has been completed, it’s time to begin your Intrusive Ground Investigation.
The purpose behind your Intrusive Ground Investigation is to provide detailed information for the safe and economic development of your site. Obviously, no absolute guarantee can be given that each and every pertinent condition will be identified fully, but the investigation should be aimed at reducing risks to acceptable levels.
Spending time and resource on a detailed site investigation can avoid unforeseen conditions during the course of your build. Professional judgement and experience is also required as not all forms of investigation will be required for every site and what is required should be carefully assessed for each individual site you investigate.
You must design your investigation to provide the amount of information appropriate for the ground and ground water conditions on your site as well as identifying potential areas of contamination. Your investigation should be carried out in accordance with the principles of:
- BS EN 1997-1: Eurocode 7 – Geotechnical design – Part 1: General rules
- BS EN 1997-2: Eurocode 7 – Geotechnical design – Part 2: Ground investigation and testing
- BS 5930 and BS 10175
It’s also important that your investigation has the full time supervision of a Chartered Geologist or Chartered Engineer. The dates in which your investigation takes place as well as the methods used should be stated and any exploratory hole positions should be shown on a drawing/map of the site.
Your intrusive investigation may involve the following steps:
- Trial Pitting
Normally these should be at least three times the depth of the foundation or sufficient to prove competent bedrock. Where possible, they should be excavated outside of proposed foundation positions. On completion, the excavations are generally backfilled.
This method enables soil conditions to be examined closely at any specific point across the site and samples to be taken as needed. It also gives useful information on the stability of excavations and water ingress. In-situ gas, strength and California Bearing Ratio (CBR) tests can also be carried out.
- Window Sampling
Window sampling consists of driving a series of 1 metre and 2 metre long tubes into the ground using a dropping weight. When each run is completed, the tube is withdrawn. The next tube is then inserted and the process is repeated to provide a continuous profile of the ground. On each run, the tube diameter is reduced in order to help in its recovery. When complete, the borehole is generally backfilled. It’s also possible to carry out standard penetration tests (SPT) by using the window sampling equipment.
- Shell and Auger Boring
This technique uses a tripod winch and percussive effect with a variety of boring tools, where disturbed and undisturbed samples can be taken. This method is the most suitable for use with soft ground as it enables the maximum amount of information to be obtained. However, minor changes in lithology may be overlooked unless continuous undisturbed sampling is used.
Disturbed samples of soils can be taken for identification and classification purposes. In cohesive soils, ‘undisturbed’ samples 100 millimetres in diameter can be taken by an open drive sampler for laboratory testing of strength, permeability and consolidation characteristics.
‘SPT’ are used in granular as well as in cohesive materials and in soft or weathered rocks. The resulting ‘N’ value can then be compared to empirical data on strength and relative density. Difficulties in obtaining true ‘N’ values mean they should only be used as a guide and not as an absolute value in foundation design.
- Rotary Drilling
There are two primary types of rotary drilling which can be carried out in rock. Rock coring using a diamond or tungsten carbide tipped core bit provides samples and information on rock types, fissuring and weathering.
- Open-hole drilling only produces small particles for identification purposes and the information gained is therefore limited. The latter is, however, useful as a quick method detecting major strata changes as well as the location of any coal seams or old workings within the grounds of the site. Water, air, foam or drilling mud may be used as the flushing medium in either case.
- Rotary Open-Hole Drilling is carried out to determine the existence of any voids or broken ground that could affect surface stability. Due to the risk of combustion, the drilling is normally done using a water flush. On completion, the boreholes are backfilled with bentonite cement. A Coal Authority License is required in advance of any exploratory work intended to investigate any possible coal workings within the site bounds or close by.
Useful in certain situations, especially where significant anomalies exist within the ground. Ground-Penetrating radar is likely the most commonly used for defining near-surface features. The results from geophysics can be variable and, combined with the relative high cost, should be used advisedly.
Full strata descriptions should be given based on visual identification and in accordance with the requirements of:
- BS EN ISO 14688-1Geotechnical investigation and testing – Identification and classification of soil – Part 1
- BS EN ISO 14688-2Geotechnical investigation and testing – Identification and classification of soil – Part 2
- BS EN ISO 14689-1Geotechnical investigation and testing – Identification and classification of rock – Part 1
It’s important that you fully describe samples taken from boreholes or trial pits in accordance with the latest guidance from the British Standards and Eurocodes. They should include the colour, consistency, structure, weathering, lithological type, inclusions and origin. All descriptions should be based on visual and manual identification as per recognised descriptive methods.
In-Situ and Laboratory Testing
- In-Situ Gas Monitoring
Methane is the dominant constituent of landfill gas and can form an explosive mixture in air at concentrations of between 5% and 15%. For this reason, 5% methane in the air is known as the Lower Explosive Limit (LEL). Concentrations less than the LEL will not generally ignite. Carbon dioxide can also be a problem when it occurs in concentrations of greater than 1.5%. You should carry out In-situ gas tests within boreholes on completion and in probe holes made in the sides of your trial pits. You can test with a portable meter that measures the methane content and its percentage volume in the air. The corresponding oxygen and carbon dioxide concentrations are also measured. Care is needed with this, since the rapid mixing and dilution of any gasses within the atmosphere can occur very quickly.
A more accurate method which is used to monitor over the long term, consists of gas monitoring standpipes which are installed in boreholes. These are generally made up of slotted UPVC pipework surrounded by single sized gravel. The top 0.5m to 1m of pipework is usually not slotted and is surrounded by bentonite pellets to seal the borehole. Valves are fitted and the installations protected by lockable stopcock covers normally fitted flush with the ground. Monitoring is again with a portable meter and is usually done on a fortnightly or monthly basis, with at least six visits being appropriate for most sites.
You should consider the risks associated with the gasses in accordance with documents such as:
- BS 8485Code of Practice for the characterisation and remediation from ground gas in affected developments
- CIRIA Report C665 Assessing risks posed by hazardous ground gases to buildings
- In-Situ Strength Testing
Hand vane and MEXE cone penetrometer tests can be carried out in trial pits so as to assess the strengths and the CBR values of made ground, soils and heavily weathered bedrock materials.
- Soakaway Testing
If sustainable drainage is being considered, soakaway testing should always be carried out. This should be done in trial pits with the aim of intersecting permeable soils or naturally occurring fissures within the bedrock.
Soakaway testing involves the filling of your trial pits with water from a bowser or similar and measuring the fall of water over time. Where it is possible, two tests should be carried out so as to allow the immediate surrounding ground to become saturated. By knowing the dimensions of your trial pit, the permeability and/or rate of dissipation can be calculated.
Soakaway test results obtained from small hand-dug pits or shallow boreholes should be treated with utmost caution.
- Geotechnical Laboratory Testing
Soil testing should be carried out to BS 1377Methods of test for soils for civil engineering purposes, and the laboratory used should be recorded and conducted by an approved UKAS Laboratory. Normally the results are summarised and the full results appended.
- Contamination Laboratory Testing
As with your investigation, the sampling should be under the full-time direction of either a Chartered Engineer or a Chartered Geologist. All the recovered soil samples should be screened on-site for any visual or olfactory evidence of contamination, including the presence of Volatile Organic Compounds (VOC’s). You should select samples from your trial pits and boreholes based on those most likely to be contaminated and those that will give the most appropriate indication of the spread of any contaminants. The samples should be stored in either glass or plastic containers and where necessary kept in cooled conditions.
Testing should always be carried out by a UKAS accredited laboratory in accordance with the Environment Agency’s Monitoring Certification Scheme; MCERTS performance standards.
The aim of the testing is to make a preliminary assessment of the level of contaminations (if any) on sit, so as to determine if there are any significant risks associated with contaminants in respect of both human health and the environment, including controlled waters. In addition to the soil, ground water samples should be tested where appropriate.
Geoenvironmental Risk Assessment (Conceptual Site Model)
Your qualitative health and environmental risk assessment carried out as part of the desk study should be revised, based on the findings of the ground investigation and the results of the contamination testing, to produce a Detailed Quantitative Risk Assessment (DQRA)
The DQRA is again based on the conceptual site model, and might look similar to the following examples summary of hazards, pathways and receptors. On sites with known contamination, further investigation and testing may be necessary, together with recommendations for remediation and its validation.
Table: Example of a detailed quantitative risk assessment
If unforeseen conditions are encountered during the construction process, you should seek additional advice from the consultant as to whether the new conditions will affect the continued development of the site and whether any additional investigation or testing is necessary.
The report must include a site location plan and a plan showing any special features plus borehole and trial pit locations (factual reports will describe the work carried out and will include borehole/trial pit logs and the results of all in-situ and laboratory testing, but there will be no interpretation of the date and no recommendations).
The interpretative report should make recommendations in respect of the main points or issues related to design and construction:
- Normal strip or deep trench footings
- Vibro replacement
- Raft foundation
- Building near trees
- Landfill and radon gas
- Existing drains and services
- Road construction
- Sustainable surface water drainage (soakaways)
- Excavations and ground water
- Reuse of materials
- Capping mine shafts
- Site soil reuse
- Slope stability and retaining walls
- Further geotechnical considerations
- Change of use
For further information on Contamination or site investigations you can download Chapter 4 of our Technical Manual:
The information used in this article is taken from Version 8 of the LABC Warranty technical Manual and is provided as guidance in meeting our technical standards. If working on an LABC Warranty site please check which standards apply.
Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. Any written guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.