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A desktop study is the first step in the geotechnical investigation process. Use of secondary data can help inform choices as to whether a site investigation is required. In its most basic form a desktop study might be considered in part of the tendering process for a site investigation, viewing of geological maps or historic borehole records is vital when choosing the correct equipment, for advancing bore holes.
Secondary data can also throw up risk ratings around issues such as subsidence of shrink swell, and this will dictated whether a site investigation is required or not. For example a high shrink swell risk (plasticity) would lead to the inclusion of plasticity index testing in the site investigation, whilst a low risk (gravels for example) may indicate these tests are not required.
British Columbia for example give the following guidance:
If there is no existing study of the area but one is required, the District Development Technician, at the direction of the Approving Officer, sends the applicant a letter asking for an initial geotechnical study.
If a blanket study of the area has been done and shows evidence of natural hazards, the District Development Technician, at the direction of the Approving Officer, sends the applicant a letter of preliminary non-approval suggesting that a qualified Professional Engineer be hired to do a site-specific study. If the blanket study is a public document, it should be referred to in the letter.
After receiving the letter from the District Development Technician, the applicant may engage a qualified Professional Engineer to do a geotechnical study of the site. Its goals would be to: Determine if there is a hazard Determine extent of any hazard Identify building sites free from hazard, or where risk could be rendered acceptable, a tipical scope as requested by TRCA (by example) might be:
1. Carry out a visual inspection to assess the slope conditions including the slope features, structures close to the slope crest, erosion features, past slide features (e.g. signs of instability, tension cracks, toe erosion and undercutting) and other features such as vegetation cover, drainage, previous fill placement, etc.
2. Obtain the soil stratigraphy (without drilling boreholes), based on a visual inspection, and/or previous or nearby subsurface investigations, and/or geologic mapping, and/or hand augering and/or test pits to determine shallow depth soil types and fill thicknesses.
3. The slope should be surveyed and the slope profile should include the height and inclination of the slope, the location of the top and toe of slope, any slope features and structures close to the crest, the locations of past slope instabilities, erosion and drainage features, and the location of the watercourse (if close to the toe of slope).
4. Undertake a slope stability analysis, using a slope stability software, of critical cross-sections to calculate the safety factor based on the above information and measurements. Since boreholes are not required to be drilled, conservative soil strength parameters should be utilized for the analysis. The Long-term Stable Slope Allowance will need to be calculated for the critical cross sections using a minimum Factor of Safety of 1.50. The Toe Erosion Allowance associated with the watercourse will also need to be determined.
5. Using the above referenced Toe Erosion Allowance and Long-Term Stable Slope Allowance, determine the location of the Long-Term Stable Top of Slope (LTSTS) line on the tableland. This line will need to be shown on the site plan and cross section.
A site investigation is the "practical" part of the process, with primary data collection being the main goal. A drilling rig is typically used to collect soil samples and to take samples. An excavator is sometimes uses instead of a drilling rig, or in addition to a drilling rig to form trial pits, that may be observed for various geotechnical features.
Broadly a site investigation can be split in to two camps:
Geotechnical Site Investigation is rock are typically more costly than those carried out in soils. This is because of the equipment required, and in some cases the relatively slow rate of progress.
Soil is typically soft than rock (although not always) and so this means that smaller drilling rigs can be used and progress is quicker.
Data from the labs and from in-situ testing is used to design foundations. For simple structures it may be appropriate to use basic loads bearing calculations, but for larger projects modeling may be required.
Please contact SWEL if you would like us to undertake a Geotechnical Assessment of your Site.