The findings of the desk study research and site reconnaissance, together with knowledge of the development proposals are used to design the intrusive investigation. Depending on the objectives the intrusive investigation can be designed to cover geotechnical or geoenvironmental aspects, or both.
Typically the fieldwork will involve exploratory holes to establish the ground conditions beneath the site and recovery of samples for geotechnical and geoenvironmental analysis.
Depending on the anticipated geology, site access and purpose of the investigation the exploratory methods used may involve a range of techniques including:
Cable percussion boring
Window sampling and probing
Plate bearing testing
California bearing ratio (CBR) testing
The ground conditions encountered on site are recorded and samples are collected from appropriate depths, from changes in strata, or where visual or olfactory evidence of contamination is recorded. Recovered samples are sealed into appropriate containers and transported to the testing laboratory under controlled conditions to preserve their integrity.
In-situ testing of the soils is appropriate for the majority of investigations. The tests tend to cover the geotechnical aspects of the ground investigation, and include standard penetration testing (SPT), dynamic cone penetrometer (DCP) testing and dynamic probing (DP).
Samples recovered during the intrusive investigation are inspected, and where appropriate selected samples are subjected to a suite of geotechnical and/or environmental laboratory testing. The scope of the testing will take into account the findings of the conceptual site model, the ground conditions encountered, and the proposed development plans.
Geotechnical test data is used to make recommendations regarding foundation and structural design. The geotechnical testing will typically include: soil classification data (moisture content and plasticity index for cohesive soils or particle size distribution for granular soils); chemical properties for buried concrete design (pH value and soluble sulphate content); undrained shear strength to calculate bearing capacity or for pile design; and compressibility or swelling parameters to estimate settlement or heave, where required.
Geoenvironmental test results are used to compile risk assessments for site receptors identified by the outline conceptual site model (CSM). These receptors include end users of a development, controlled waters, ground workers, off-site human and property receptors, building materials and vegetation. The findings of the environmental laboratory analysis and assessment enable refinement of the CSM, so active linkages between contamination sources and site receptors can be determined.
The refined CSM summarises the findings of the assessment identifies which contamination sources pose an unacceptable level of risk to site receptors and therefore require remedial measures. Depending on the site’s history and the level of contamination, several phases of ground investigation and refined risk assessment may be required