A subsurface investigation is one of the fundamental assessments performed at the beginning of engineering projects. The subsurface investigation is the primary method for gathering the data to generate a report of geotechnical engineering recommendations. Subsurface investigations include the sampling and examination of the below surface materials including soil, rock, groundwater and any manmade materials. The results of this investigation provide a “picture” of what is beneath the ground which is critical for designers and construction teams alike to help assure the successful completion of any construction project.
Often subsurface investigations are performed in a phased approach as the project evolves from conceptual to final. As design changes such as building type, number of stories, basement, grade changes or special features evolve, so may the geotechnical recommendations.
Why Are Subsurface Investigations Important?
When subsurface investigations are being performed, geotechnical engineers and geologists are looking to provide recommendations such as allowable bearing capacity and estimates of settlement for structure foundations, earthworks, retaining walls & slopes, and to identify potential problems with the stability of the underlying soil and rock or the presence of groundwater which may cause difficulties during design and construction. Armed early on with the knowledge of any potential problems, the design team can engineer appropriate, economical solutions prior to construction which results in reducing delays and cost over-runs while a project is under construction.
Subsurface conditions are a primary cause of project delays, cost over-runs and dispute. The right geotechnical engineering firm providing the right guidance throughout the entire design and construction phases of the project is paramount to the managing these risks.
Methods of Subsurface Investigation
Conventional Soil Borings
- Small diameter soil and rock cores are obtained using a drill rig.
- Provides Standard Penetration Test (SPT) values which correlate to soil strength and can be used to determine soil bearing capacity and estimate settlements for foundation design.
- Groundwater data can also be obtained and small diameter groundwater monitoring wells can be installed following the completion of the soil and rock sampling.
- Test borings can extend to depths of 200 feet or greater depending on the drilling equipment used and can obtain intact samples of soil and rock making them a versatile investigation method.
- Unlike other testing methods test pits allow for visual inspection of a greater area of the subsurface which pairs well with more technical information gained from soil borings. Test pits are normally <12 feet in depth and provide investigators with visual information of the subsurface including:
- Better visualization and characterization of fill materials (e.g. Demolition debris, orgaincs…), existing below grade structures such as foundations
- Groundwater information
Cone Penetrometer Testing
- Cone penetrometer testing (CPT) allows for rapid exploration of the subsurface materials. Soil and rock samples are not obtained. Instead different sensors gather information on the soil as a cone shaped tip is pushed through the soil. The information gathered includes:
- Tip resistance which is the force required to push the tip of the cone through the soil.Sleeve friction which is the force required to push the sleeve through the soil.
- Pore Water Pressure
- Seismic shear wave velocity
- The ratios of the above CPT parameters provide additional information regarding the physical soil properties and pore fluids.
Earth Engineering conducts subsurface investigations for PA, NJ, and Delaware. Contact one of our project team at any of our 4 locations to discuss your subsurface investigation needs and how Earth Engineering can help.