Phase 2 Ground Investigation Services

Ground investigation is a crucial stage in the site investigation process, aiming to collect and analyse data about the site’s subsurface conditions. This process involves various techniques, including trial pits, trial pitting, rotary drilling, and boreholes, to gather information about the soil, rock, and groundwater conditions. A thorough ground investigation helps identify potential risks, such as contamination risks, and informs the design of foundations, remediation work, and other construction activities. By understanding the site’s geology and potential sources of contamination, developers can ensure a cost-effective and environmentally responsible project.

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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. 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 ground investigation may comprise a selection of the following elements:

• 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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Carrying out the Phase 2 Site Investigation efficiently and thoroughly is essential for managing site works. The collection of samples during the investigation is crucial for assessing ground conditions and determining potential risks to human health and the environment.

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

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The results of the Phase II site investigation inform the risk assessment and help identify potential sources of contamination, allowing for the development of effective remediation strategies and foundation design. By undertaking a Phase II site investigation, developers can ensure compliance with regulatory requirements and minimise the risks associated with land contamination.

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Phase 2 site investigation methods include a range of techniques, such as trial pits, rotary drilling, and boreholes, to collect data about the site’s subsurface conditions. Trial pitting involves excavating shallow trial pits to examine the soil profile and identify potential contamination risks. Rotary drilling is used to collect deeper soil and rock samples, while boreholes provide information about the groundwater conditions and potential sources of contamination. Machine-excavated trial pitting is also used to collect soil samples and identify contamination risks. These methods are targeted to the specific site conditions and are carried out by regulatory requirements to ensure a cost-effective and environmentally responsible site investigation.

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Contaminated land assessment is a critical component of the site investigation process, aiming to identify potential sources of contamination and assess the risks to human health and the environment. This assessment involves a thorough review of the site’s history, geology, and environmental setting to identify potential contamination risks. The contaminated land assessment also includes the analysis of soil and groundwater samples, as well as the evaluation of ground gas and surface water risks. By undertaking a contaminated land assessment, developers can identify significant risks and develop effective remediation strategies to mitigate the impacts of land contamination. This assessment is essential for ensuring compliance with regulatory requirements and minimising the risks associated with contaminated land.

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The results of the ground investigation are used as the basis of assessments of the geo-environmental risks and/or geotechnical properties of the ground. In terms of land contamination, the Phase II Site Investigation results are used to refine the Conceptual Site Model and confirm which of the potential pollutant linkages identified by the Preliminary Risk Assessment are present and may be significant.

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At this stage, this is usually 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, then a more complex Detailed Quantitative Risk Assessment (DQRA) may be required.

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If the preliminary risk assessment indicates potential issues, further investigation is necessary to ensure comprehensive analysis and informed decision-making.

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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’s geo-environmental and geotechnical engineers utilize their wealth of experience obtained through years of site investigation work throughout 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. 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 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. For further information about site investigations and their processes, please reach out to us.​