Site Investigation Methods
Soil testing and site investigation are critical first steps in foundation design. The investigation program determines soil stratigraphy, groundwater conditions, and engineering properties of each soil layer. A typical investigation begins with a desk study reviewing existing geological maps, aerial photographs, and nearby project records. This is followed by field exploration and laboratory testing.
geotechnical site investigation.
Test pits are the simplest exploration method, providing direct visual observation of soil conditions. They are limited to depths of 10 to 15 feet depending on soil stability and groundwater. Boring methods including auger drilling, wash boring, and rotary drilling reach greater depths and sample soils at regular intervals. Standard Penetration Tests performed during drilling measure soil resistance and provide samples for classification.
Soil Sampling Techniques
Disturbed soil samples collected during drilling are suitable for classification tests but do not preserve the natural soil structure. Undisturbed samples are required for strength and consolidation testing. Thin-walled tube samplers pushed into the soil collect samples that retain natural density, moisture content, and structure. Piston samplers provide the highest quality undisturbed samples in soft soils. soil compaction testing. excavation protective systems. groundwater control methods.
Groundwater observations during drilling provide initial information about the water table depth. Piezometers installed in boreholes measure groundwater levels over time and allow sampling for water quality testing. Monitoring over multiple seasons may be necessary to determine the highest groundwater level that will affect construction.
Laboratory Testing Program
Laboratory tests determine the engineering properties needed for foundation design. Classification tests include grain size analysis, Atterberg limits, and natural moisture content. Strength tests include unconfined compression, triaxial compression, and direct shear tests. Consolidation tests measure the compressibility and settlement characteristics of clay soils.
Chemical tests assess soil corrosivity, sulfate content, and organic content that affect concrete durability and foundation material selection. The test results are compiled in a geotechnical report that provides foundation recommendations including allowable bearing pressure, estimated settlements, and construction considerations for excavation, dewatering, and earthwork.