Site Investigation Phase 1 Geoenvironmental Assessment (Desk Study)
The aim of the Geoenvironmental Assessment is to identify and assess the potential geotechnical and geoenvironmental (contamination) hazards on the site. Since all sites are fundamentally different, it’s imperative to identify the scope and purpose of your desk study. This will include who commissioned the work, the development proposals, relevant procedures followed and the objectives. Issues specifically excluded should be noted if these would normally be expected as part of the desk study
Your site description should define the exact extent of your site. This should include your site address, grid reference, the elevation of the site and the boundaries/ topography of the site. It’s important to remember when inspecting your site, that you should be inspecting the immediate surrounding areas as well.
This needs to include information which is not apparent on the map and describe what currently occupies the site e.g.
- Hard standing
- Trees and any particular features.
The types of vegetation on your site can indicate soil and ground water conditions and note should be made of any invasive plants e.g. Japanese Knotweed and Giant Hogweed. Features and uses of land adjacent to your site should also be reported if there is likely to be an impact on your development. It’s fairly common for features such as tanks to be known about but unrecorded.
You should take note of any potential sources of contamination and geotechnical hazards e.g.
- Land slipping
- Ground subsidence
- Soft ground
- Desiccated/shrinkable soils.
You should ensure that all structures on your site are inspected both internally and externally for any evidence of structural damage such as tilting, cracking or chemical attack. Any evidence of underground features should be noted.
Local residents can often be a valuable source for information, where practical, remember to use caution in respect to their ‘memories’. Local place names often give useful indications of former uses e.g. Gas Works Lane, Water Lane, Tannery Road etc. Aerial photos and their interpretation can also be very useful.
A photographic record of the site as well as any specific site features should be included with your report.
When assessing the potential for contamination or geotechnical hazards on your site the history of both the site and the surrounding areas is extremely important. Historical Ordnance Survey maps date back to the mid-19th century and often specify the actual industrial use of particular sites or buildings. They can show areas of quarrying and of infilling, often indicating where buried obstructions such as tanks or old foundations can be expected.
Whether off-site past industrial use will have any influence or impact on your site will depend on the type of industry that it was as well as the underlying geography/topography of the area. However, consideration should generally be given to any such features within a 250m radius of the site (or further where appropriate) with the potential to affect it.
Historical maps are available from libraries as well as commercial providers such as Ground Sure or Envirocheck. Issues regarding possible breaches of copyright are also avoided by using licenced products
Please remember that historical maps only provide a snapshot in time. Care must be taken when interpreting what may have occurred in the intervening years. For example, a quarry may be shown on one map and infilled on the next. However in the intervening period, it could have expanded prior to infilling; similarly, industrial uses may not always be recorded while many military or sensitive uses may have been omitted.
Other sources of information may include an internet search and any historical aerial photos. Additionally it may be necessary to search Local Authority Libraries and Local History departments.
Geology / Mining
The geology of your site should be recorded by reference to published geographical maps which most commonly exist at 1:50,000 and 1:10,000. The British Geological Survey Geo-Index also provides existing ground investigation records including logs and reports. It’s important to note that these logs and reports don’t just relate to the surrounding area but could also include any previous investigations of the site itself. The information on the geological maps can also be supplemented with British Geological Survey tech reports, flood risk appraisals and memoirs.
Bedrock geology and any overlying superficial deposits as well as the effects of weathering should all be described along with any geological faults that may affect the site. You should also give an explanation of the likely ground conditions together with references to any other mapped geological features, particularly if there are likely to be any natural cavities or solution features.
If you’re site is located in an area formally coalfields or another area of mineral extraction, maps may not always record the presence of old or active workings. The chance of shallow coal workings affecting surface stability should be established in conjunction with the Coal Authority Report. Reports like this also record areas that have been affected by the extraction of brine, which is particularly prevalent in the Cheshire area. Other forms of mineral extraction will require site-specific research.
Any potential of mine working or mine entries within an influencing distance of your proposed site should be addressed by an engineer with suitable experience and qualifications prior to the commencement of works and in accordance with CIRIA SP 32: Construction Over Abandoned Mine Workings, 2002. Reference should be made to reports on geological hazards such as Ground Sure or Envirocheck reports, both on site and locally.
Solution Features in Chalk
Solution features like pipes, swallow holes and solution cavities (sometimes loosely infilled with drift deposits) are commonly found in chalk. These are caused by water dripping through the chalk and dissolving it. The risks caused by solution features should be addressed in your Site Investigation Report generally from a Ground Sure or Envirocheck report on geological hazards.
Hazard maps, with different coloured areas representing different levels of risk are available for such areas. Where risk is moderate or high, special precautions should always be taken. For strip foundations these special precautions would include careful inspection of the excavation, probing and use of reinforcement to span potential voids.
If you’re using piled foundations in your build, CIRIA PR 86 recommends that ‘a CPT (cone penetration test) is undertaken at each pile location at sites identified during desk studies to be prone to dissolution’.
Alternatively, in some instances it can be appropriate to design the pile for shaft friction alone, assuming the pile has no end bearing due to a solution feature below it.
In extreme circumstances, where a site investigation borehole has encountered an extensive solution feature, the shaft friction may also be reduced to take account of this.
Potential effects of soakaways, leaking drains, run off etc. on the chalk on your site will need to be considered and addressed in your design.
CIRIA C574: Engineering in Chalk, 2002 gives the following recommendations:
‘Concentrated ingress of water into the chalk can initiate new dissolution features, particularly in low-density chalk, and destabilise the loose backfill of existing ones. For this reason, any soakaways should be sited well away from foundations for structures or roads, as indicated below:
- In areas where dissolution features are known to be prevalent, soakaways should be avoided if at all possible but, if unavoidable, should be sited at least 20m away from any foundations.
- Where the chalk is of low density, or its density is not known, soakaways should be sited at least 10m away from any foundations.
- For drainage systems, flexible jointed pipes should be used wherever possible; particular care should be taken for the avoidance of leaks in both water supply and drainage pipe work.
- As the chalk is a vitally important aquifer, the Environment Agency and Local Authority must be consulted when planning soakaway installations where chalk lies below the site, even where it is mantled with superficial deposits.’
Hydrogeology and Flooding
It’s important that your assessment includes the flood risk and hydrogeology of your site, particularly focussing on whether your site lies on the Principle Aquifer and/or Source Protection Zone, which are both susceptible to pollution of groundwater.
The presence of surface water features and drainage should be described and the overall risks of flooding to the site should be determined primarily with reference to the Environment Agency flood map data and Local Authority-commissioned Strategic Floor Risk Assessments. Flood risk data is constantly being updated by the Environment Agency as well as the Local Authority.
Any ground / surface water abstraction points downstream of the site (particularly any drinking water abstraction points) should be recorded as these may have liability implications should the development cause any pollution.
The Environmental Setting of your site will determine whether it poses an actual or a potential environmental risk or is at some external risk from pollution. This will all depend on the topography, geology, hydrogeology and hydrology of the site amongst other site-specific considerations.
Remember to consider other potential sources of contamination too:
- Pollution Control Licenses
- Discharge consents
- Hazardous Sites (COMAH, NIHHIS)
- Pollution incidents
- Waste treatment sites
- Past/current industrial sites
Modern day industrial operations rarely provide a risk of pollution to a site. Pollution is more likely to have been caused by historical activities and processes that were often deemed normal practise at the time but which are considered unacceptable today. In this regard the past history is invariable highly significant in respect to the possibility for ground pollution on your site.
Your site should also be considered in relation to any designated environmentally sensitive sites, such as Special Areas of Conservations, Special Protection Areas, Nature Reserves and Sites for Special Scientific Interest. In particular, could contamination on the site be affecting such sensitive areas whether these are on or adjacent to the study site?
Use of these supplies, as with historical maps, is generally considered the most cost-effective method of data gathering. The issue with historical maps and datasets is that they generally exist with little or no interpretation of the mass of information that they provide. It is therefore essential that the data is interpreted by an experience and qualified individual. Automated Risk Assessments do not include appraisal by qualified staff, and should therefore be viewed with caution and not usually acceptable to Regulators.
Whether or not you need to incorporate radon protection measures should be determined by reference to risk maps produced by the Health Protection Agency. Such information is also usually included within commercially available datasets.
Geoenvironmental Risk Assessment and Conceptual Site Model
As part of your assessment you should carry out a quantitative health and environmental risk assessment. The process to take for a risk assessment is set out in Part IIA of the Environmental Protection Act 1990, and amended in subsequent legislation. This act introduces the concept of pollution linkage, which consists of a pollution (contaminative) source or hazard and receptor together with an established pathway between the two. For land to be contaminated a pollution linkage (hazard-pathway-receptor) must exist. This forms the ‘conceptual model’ for your site.
Pathways of potential contaminants
Examples of pathways and the effects of land contamination (after PPS 23) are shown in Figure 2: Pathways and potential contaminants.
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 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 on 10.4.2018. 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.