Interpreting Geotechnical Results: What to Look For and How to Best Leverage the Data

July 21, 2017


Commissioning a geotechnical investigation is an essential part of ensuring that your utility-scale or commercial ground mount solar installation is successful and bankable over the long term. That said, how do you interpret the reports you receive to ensure you’re leveraging the findings to your best advantage?

Site Observations and Conditions

First, carefully review what the geotechnical team observed on site and look for potential red flags in connection with qualitative assessments of slopes, surface water, vegetation and rocks:

  • Steep slopes – If slopes exceed the maximum terrain following capacity of your chosen mounting system, your site might require earthwork before you are able to proceed.
  • Surface rocks, trees and vegetation – Natural site obstructions will likely require clearance.
  • Surface water – Water should trigger warning bells related to potential ponding around posts, which in turn can lead to corrosion.
  • Slopes + water – Taken together, these site conditions forecast likely erosion of the surface of your site, which has the potential to compromise the structural integrity of your foundations.

General Investigations

When it comes to a general investigation document, pay special attention the standard penetration (SPT) borehole data and related soil properties. These data and properties will provide important context for the foundation design, as well as valuable information for the installation:

  • Relative soil density – This measure tells you first and foremost if this is a good site for direct drive foundations. If the soil contains too much rock, driving piles may be difficult or the conditions may require an alternative foundation. Secondly, the relative density provides qualitative information regarding foundation size and installation times for pile. Dense conditions will have shallower foundations with longer drive times, while loose conditions will have deeper foundations and shorter drive times.
  • Corrosive properties – As discussed in depth in our recent blog post “Don’t Let Your Soil Eat Away Your Profits,” you should examine findings related to resistivity, salts, water and pH.
  • Expansive properties – Soils that exhibit plastic properties might require larger foundations to accommodate seasonal changes in soil conditions.

Pile Load Testing Methodology

Understanding the qualifications of the team conducting the testing, as well as the methods employed during testing, puts the data in your reports in context. It’s important to ensure that the appropriate set ups, hold durations, equipment calibrations and displacement thresholds were used.  Methodologies in general accordance with ASTM 3689 for testing in tension, and ASTM 3966 for lateral testing should have been employed. Importantly, the methods and any deviations from the standards should have been approved by the licensed engineer of record.

Pile Load Testing Results

When it comes to load testing results, look closely at the depths tested and the loads achieved. Depth findings will reveal the range of embedment depth you will be installing. At each depth, you should find an associated achieved load – for example, at shallow depths, testing achieved a load of X, while at deeper depths, testing achieved a load of Y. This data will confirm the appropriate design for the solar system and provide context for the recommendations.

 

Among all the important qualitative and quantitative data included in geotechnical reports, prioritizing these four sections in your report review will improve understanding of your site, leading to both installation and long-term project success.

 

 

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