Who Should Carry Out the Work
An AGS (Association of Geotechnical & Geo-environmental Specialists) registered firm should be engaged to
conduct the investigation, prepare the fieldwork schedule, commission the testing and report on the findings.
The laboratory work should be carried out by a suitably accredited laboratory. The report should be made in
accordance with BS5930 (1999), and data supplied in AGS format.
What Methods Should Be Used
Shell & Auger boreholes are the most commonly adopted deep investigation technique. Consistency between the ground conditions encountered and those anticipated from geological references and between individual borehole locations should influence the extent of the works undertaken during the fieldwork phase.
Variability of ground conditions encountered may lead to alternative investigation techniques being considered. Trial pitting, window sampling, dynamic and cone penetrometer testing can all be used to supplement the information derived from shell & auger boreholes. Sulphate and pH testing should also be carried out on both soil and ground water samples. Appropriate testing should also be undertaken where contamination may be present.
Information obtained includes:
- Visual assessment
- Stratification
- Groundwater
- Bore instability
- Strength data
- Samples for laboratory testing
Where unexpected ground conditions are revealed during the fieldwork, further investigation should be undertaken. For the purposes of this guidance note we assume shell and auger techniques are used.
Other Considerations
During U100 sampling, blow counts should be recorded in order that the designer has additional information at their disposal when assessing soil strength characteristics. Excessive blows during the sampling of hard clays can yield unreliable and low triaxial test results, which if presented alone can lead to an uneconomic design line.
Groundwater conditions must be accurately recorded. Blowing or piping in sands/gravels can cause low SPT results and the absence of accurate reporting can lead to an uneconomic design line. The addition of water during drilling can mask natural groundwater inflows therefore must be recorded in order that a correct assessment of soil densities can be made.
Should drilling refusal occur before the intended depth of the investigation has been reached, the actual nature of the refusal should be determined. It may be the case that bedrock stratum has been reached and the layer may be impenetrable by normal piling techniques. Rotary drilling may be required to prove the thickness of the layer. It is certainly the case that further boreholes should be installed on the site to check the consistency of the stratum encountered.
Site Investigations
The investment in a professionally conducted site investigation will reduce the developer's risk. At project
feasibility stage the viability of a development can be assessed.
Design parameters are obtained from the fieldwork and during subsequent laboratory testing. The better the information gained from the investigation, the more cost certainty can be ensured. With complete, reliable data at their disposal, the designer can offer
economies.
Without a comprehensive site investigation, assumptions are made, conservatism will prevail and value engineering is impossible.
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