Phase 2 Site Investigation Derbyshire

Phase II Site Investigation Derbyshire

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A Phase II Site Investigation comprises intrusive ground investigation works and can vary significantly in scale and scope, from the collection of a few near-surface soil samples for laboratory analysis to a full-scale, multi-technique investigation.  The long industrial history of Derbyshire means ground investigations are often large and complex. Phase II generally refers to the second stage of contaminated land assessment after the Phase I Desk Study (where one exists), although geotechnical investigations should also be based on some initial desk-based research. The desk study should inform the scope of the ground investigation as the locations of the exploratory holes and the investigation techniques will be a function of the anticipated ground conditions and location of potential contamination sources, as well as any site development proposals. Access constraints can influence the methods and equipment used during the investigation. The nature of the site, including factors like geology and specific contaminant linkages, also influences the methodologies used. Depending on the objectives, the ground conditions and any site-specific constraints, a Phase 2 site investigation may comprise a selection of the following elements:

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• Hand Dug Pits – Suitable for inspecting and sampling near-surface soils. It is important to examine shallow soils to assess potential contamination sources.

• Trial pits – Excavated using a JCB or similar, are ideal for inspecting and sampling soils in the upper 3m.

• Window(less) sample / mini-boreholes – Drilled using a small, tracked-mounted or hand-held percussion rig, allowing multiple boreholes to be drilled up to 6m deep over a short space of time.

• Cable percussion (shell & auger) boreholes – Larger percussion rigs that can reach depths >20m in favourable ground conditions.

• Rotary boreholes – Powerful drilling rigs that can penetrate dense soils and rocks reach depths well over 50m.

• Cone Penetration Testing (CPT) to provide near-continuous data through the soil profile, including strength and porewater pressure

• All drilling techniques can be used for in-situ tests, such as standard penetration testing (SPT) and facilitate the installation of gas/groundwater monitoring standpipes.

• Permeability Testing – can be undertaken in both boreholes and trial pits.

• Laboratory Testing – chemical and geotechnical analysis of soil and water samples. Geotechnical testing is crucial for providing critical data on soil characteristics, assisting in foundation design and other infrastructure considerations.

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Each ground investigation technique has its advantages and limitations. For example, a tracked mini-borehole rig can be used to drill up to approximately 8 boreholes in a day and be used to undertake in-situ strength tests (Standard Penetration Tests), but can generally only reach 5-6m depth and may be unable to penetrate gravels or very stiff clays. Cable percussion borehole rigs can reach greater depths, allow in-situ strength testing and provide undisturbed samples for laboratory testing, but are slower (commonly one 10m borehole per day) and may be unable to penetrate dense gravel or rock. Rotary drilling can penetrate most strata, provide rock cores for logging and is relatively quick, but is expensive, and the quality of samples is reduced.

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All boreholes can be installed with monitoring wells. Depending on the objectives of the investigation, these can be used for groundwater monitoring to establish the groundwater level and allow the collection of samples for laboratory analysis, or can be targeted to provide gas monitoring data for strata that have been deemed to be a potential source of hazardous gases.

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The collection of representative samples of soil, rock and water for laboratory analysis is an essential element of a ground investigation. All samples should be representative of the material from which they were taken and free from any cross-contamination. Laboratory test results should be used alongside the information gathered in the field during the investigation (such as logs of the encountered strata, visual or olfactory evidence of contamination, in-situ test results, etc) to provide an overall characterisation of the physical and chemical properties of the ground.

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Carrying out the Phase 2 Site Investigation efficiently and thoroughly is key to ensuring the identification of any geo-environmental risks and liabilities, and for the development of cost-efficient solutions. 

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What is a Ground Investigation?

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Ground investigation is essentially another term that means the same as (Phase II) site investigation.  The main difference is that Phase II site investigation is a specific term used primarily in the assessment of contaminated land, which follows on from the Phase I Desk Study.  Ground investigation is a more general term that covers investigations undertaken to characterise the ground for a variety of reasons that include determining the geotechnical properties of the ground to inform foundation design and investigating groundwater conditions for drainage and / or water supplies, but may also include the investigation of potential contamination. Ground investigations are often multi-purpose as it is nearly always most cost effective to cover all project elements with a single ground investigation.

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What is the Purpose of a Phase II Site Investigation?​

 

​The purpose of a Phase II site investigation is to determine the extent of contamination identified during the initial desk study and to determine whether any significant risks to human health or the environment are present. This stage of the site investigation process involves intrusive investigation methods, such as trial pitting and borehole drilling, to collect soil and groundwater samples for laboratory analysis and testing.

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The results of the Phase II site investigation help to refine the risk assessment and the Conceptual Site Model, by confirming whether any significant source-pathway-receptor pollutant linkages are present.  This allows for the development of effective remediation strategies that either address the contamination sources or break the pollutant linkages.  Geotechnical information collected during the site investigation is used to inform the development design in terms of foundations, pavements, floor slabs, drainage and concrete classification.  By undertaking a Phase II site investigation, developers can ensure compliance with regulatory requirements such as planning conditions, and minimise the potential risks and liabilities associated with land contamination and ground instability.

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Phase 2 Site Investigation Methods

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Phase 2 site investigation methods include a range of techniques, as described above, which include hand dug pits, trial pits, percussive drilling, and rotary boreholes, to collect data about the site’s subsurface conditions.  The methods used will be dependent on the anticipated ground conditions, the development proposals and the investigation objectives.  Shallow methods such as trial pits, hand pits and mini-boreholes may be suitable for low-rise developments with a low contamination risk, whereas deeper investigation techniques, such as rotary or cable percussive boreholes, may be required for high rise developments or where contamination may affect soil or groundwater at depth.

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In some cases, more specialised techniques may be required such as vacuum excavated trial pits, often used to expose or avoid damage to sensitive underground services or infrastructure, or sonic drilling, used where ground conditions and investigation requirements are not satisfied by other drilling techniques.

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​Contaminated Land Assessment

 

The Phase II site investigation is a key component of the overall assessment and remediation of contaminated land.  The Phase 2 investigation follows on from the Phase 1 Desk Study and provides the information required to develop an options appraisal and Remediation Strategy, which defines the remedial solutions required to address the identified contamination and ensure it will not present a significant risk for the future development.  ​​​

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In some cases, the Phase II investigation will be supplemented by further stages of investigation (Phase 3), usually to provide more detailed information on specific contamination sources / risks identified by the Phase 2 investigation.  This may, for example, comprise the delineation of the extent of area of contaminated soil, or additional groundwater analysis to input into the risk assessment.

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At the Phase II investigation stage, the risks are usually initially determined by undertaking a Generic Quantitative Risk Assessment (GQRA), which is a simple assessment based on comparing the contaminant concentrations recorded in the soil with conservative published benchmark concentrations, known as Generic Assessment Criteria (GAC). Should this assessment identify potentially significant risks, which is common for industrial sites in Derbyshire, then a more complex Detailed Quantitative Risk Assessment (DQRA) may be required.​  

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​Geotechnical Assessment

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The results of the ground investigation are used as the basis of assessments of the physical characteristics of the ground. ​​In terms of geotechnical properties, the ground investigation results can be used as the basis for identifying suitable founding strata for buildings, establishing the bearing capacity for road and pavement construction, determining the suitability of infiltration drainage and providing recommendations to protect from aggressive ground and potential volume change. On some sites, the ground investigation will also provide information to inform future earthworks or allow an assessment of slope stability.

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​G&J Phase II Site Investigation Costs Derbyshire

 

​G&J’s geo-environmental and geotechnical engineers utilize their wealth of experience obtained through years of site investigation work throughout Derbyshire, the Midlands and across the UK, to design ground investigations that are specific to your site and your development, whether that be a single residential development or a multi-unit commercial build, ensuring cost-effective and robust development solutions.​

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The cost of an intrusive ground investigation is highly dependent on the size of the site and the specific scope and objectives.  For example, a very simple investigation on a one or two-plot residential development comprising the collection of a few near-surface samples for laboratory analysis for a suite of common contaminants would start at around £1K plus VAT, including all siteworks and reporting. From this starting point, costs may rise to over £100K for intensive investigations on very large or complex sites. Our clients have consistently provided positive feedback on the success of our Phase 2 Site Investigation reports, highlighting how our investigations have helped them gain planning permission and meet regulatory requirements.  For further information about site investigations and their processes, please reach out to us.​