Geotechnical Services for Solar Foundations

Both geotechnical testing and pile testing are required to develop a solar project. Test pit excavations, test borings, in-situ field testing, and load testing are generally the most common practices prior to performing a geotechnical investigation for a solar project development.

Depending on the scope and budget of the project, the geotechnical investigation will consist of:

1. Soil type classification
2. Identifying Atterberg limits
3. Particle size distribution of soil
4. Friction angel or cohesion
5. Resistance factors for the various layers of soil
6. Corrosion considerations
7. Adfreeze (frost heave) and frost penetration depth (including historic weather correlation studies, percolation, and water table)
8. Pit excavation to find possible constraints during construction (soil stability, rocks, etc.)

A pile testing and design program is also necessary to determine how to best install the right solar racking foundation in the proposed site. The most important factors for a pre-production load testing and pile design testing program are the tension/pullout capacity and the lateral capacity. The parameters considered in a pile testing investigation include:

1. Testing loads
2. Testing piles
3. Axial compression test (only needed in very large, heavy solar systems)
4. Lateral test (not that necessary but optional)

Yes, there are tailored geotechnical investigation programs for solar projects. This is because soil and subsurface conditions vary significantly based on geographic location. All civil engineering and public works projects require geotechnical investigations to ensure the safety and reliability of the structure. Depending on the size and other factors identified in the preliminary site survey and test plan, the scope of the geotechnical investigation program for the solar project will vary.

A geotechnical report for a solar project usually includes results from the geotechnical investigation, the pile testing and design recommendations, and the pre-production load testing. The report begins with a basic background review of the site. It will then include information on the geotechnical investigation procedures such as the field investigation program, piezometer installation, laboratory testing program, and field resistivity testing.

Subsurface conditions are also detailed in length including information on the overburden, bedrock and other obstructions, and groundwater and borehole stability observations. Most if not all geotechnical reports conclude with a recommendations section which identifies general recommendations such as site preparation, excavations, groundwater control, and material reuse, backfill, and compaction.

The geotechnical recommendations section also includes frost considerations (such as soil frost susceptibility, frost penetration depths, and water availability), seismic site class, and recommendations for foundation design (including which pile is recommended such as driven piles or helical piles).

Other information that is found in a geotechnical report for a solar project includes the groundwater and soil stability, corrosivity analysis, thermal resistivity testing, liquefaction susceptibility, and other hydrogeological considerations such as an evaluation of impacts from sediment erosion control, contamination of aquifers, and/or the drawdown of water wells and water features. Construction monitoring and design review are also included.

Each geotechnical report will vary based on the scope of the project, which is why the initial site survey and test plan are so important. Depending on what is found in the initial screening, the geotechnical report will have different information and recommended designs.