Lecture 10 and 11 - X1 SITE EXPLORATION(Your site is...

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X1 SITE EXPLORATION (Your site is probably not homogeneous, elastic, or isotopic.) Purpose – gather data for foundation design and construction methods o Identify soil type, especially difficult soils such as soft clay, expansive clay, or muck o Determine layering (“stratification”) and variability o Obtain samples for identification and lab tests to measure soil properties o Determine depth to rock (if present) o Perform insitu field tests for soil properties o Observe site drainage conditions o Observe nearby structures possibly affected by construction effort o Measure depth to water table (effective stresses ...... ) Planning an Investigation o Obtain information about site usage and design of structure (often preliminary). Compile best information about anticipated foundation loads (moving target). Consider building code requirements (can be restrictive). Anticipate construction difficulties (excavation, fill requirements, drainage, cofferdams, shoring). o Collect existing information: square4 Geologic survey maps – type and age of soil/rock formation, faults square4 Geodetic survey maps – land form, elevation, topography, site access square4 Specialty maps – seismic potential, frost heave potential, flood zones square4 Aerial Photographs – land form, sinkholes, lineaments, a picture is worth ..... square4 Soil survey maps – information about surface soils (<10’ depth) square4 City, county, state maps & documents – radon potential, previous soil investigations, title history (previous owners - environmental hazards). square4 Adjacent site information – geotechnical reports, permit applications o Field trip – visit site, observe current usage, nearby structures/problems, type of vegetation, nearby cuts, accessibility o Detailed site investigation - develop information for foundation design square4 Start with a few preliminary strategically located borings/soundings. square4 The final number and depth of borings/soundings, and the amount of lab testing is based on the results obtained – a “work in progress”. Include client in these decisions. Actual site usage may depend on results of the investigation. Some sites just aren’t worth the effort. square4 Minimum number of borings: one in center of each structure (building, pier, tower, etc.) Prefer 4–5 borings for buildings, 2-3 for large bridge piers. square4 All borings/soundings should penetrate to at least depth of 10% stress increase (2B). A few may be deeper, especially for large or heavy structures. For deep foundations penetrate 1B-2B (width of pile/shaft group ) below tips. square4 No hard rules. Geotechnical engineers have high professional liability rates because early mistakes may affect the entire project “from the ground up”. The “art” of engineering combines economy of effort with practical and reliable design – tradeoff between construction costs and investigation effort.
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X2 Geophysical Methods (non-invasive, quick, characterize soil mass, seldom definitive) o Seismic Refraction square4
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