Why Use Test Pits For Ground-Mount Solar In New England?

At GZA’s northeastern offices, we get many requests for proposals from ground-mount solar developers from other parts of the country that request just borings for their geotechnical studies. We prefer doing test pits as well as borings as explained below.

Typical foundation systems for ground-mount solar projects are either short driven piles or screw piles. The piles are generally designed by a design-build contractor mainly based on multiple site-specific pile load tests.

We find borings supplemented with test pits a cost-effective approach to characterize sites for pile foundations by addressing these construction impact priorities:

  1. Are there boulders and/or shallow bedrock?
  2. Are the subsurface conditions consistent over the site? To answer this question, we need to explore as many locations as possible.

For bidding projects, designers need to know the soil type and potential for shallow obstructions, such as boulders and bedrock. For ground-mount solar projects we prefer to supplement borings with test pits because:

  1. Test pits allow observation of the bedrock surface, whereas for borings, a drilling refusal may be due to a boulder or bedrock. The only way to confirm bedrock in a boring is to core the bedrock over about 5 feet, which is time consuming and requires drilling water. For remote sites, boring locations can be difficult to access and to bring water to the drilling location.
  2. Test pits allow boulder counts, and evaluation of ease of excavation, soil types, groundwater, and layering. Test pits also allow for collection of bulk samples for laboratory thermal resistivity and corrosivity testing.
  3. Many more test pits can be performed in a single day, compared to borings.

Solar developers from other parts of the country may prefer solely borings to evaluate subsurface conditions because shallow bedrock and boulders are less of an issue than in the northeastern USA. Borings provide the designers with blow counts (N-value) from the Standard Penetration Test (SPT), which is a commonly used indicator of soil density and consistency. But if a designer mainly uses site-specific pile load tests, SPT N-values may be less important than evaluating the potential presence of boulders and/or bedrock in as many exploration locations as possible. Hence, GZA prefers to supplement borings with test pits for geotechnical studies for ground-mount solar in the northeast.

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