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. 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.

• 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 favorable ground conditions.

• Rotary boreholes – Powerful drilling rigs that can penetrate dense soils and rocks reach depths well in excess of 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.

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Each ground investigation technique has its own 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 again 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 standpipes. 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 have been taken from 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 characterization of the physical and chemical properties of the ground.  

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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 potential pollutant linkages identified by the Preliminary Risk Assessment are present and may be significant. 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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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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Costs:

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.