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We deliver real-world energy solutions today to build a sustainable tomorrow. - See more at:
We deliver real-world energy solutions today to build a sustainable tomorrow. - See more at:
We deliver real-world energy solutions today to build a sustainable tomorrow. - See more at:

Determining Pile Depth for Utility Scale Solar Farms

June 15, 2015

Providing optimized utility scale solar engineering designs almost always involves performing an actual field loading and testing of piles at the site during the design phase. Pile load testing is important because if you can save 1 foot of post embedment across thousands of posts, a substantial labor and material cost savings may be achieved.


Empirical techniques usually require some type of validation and testing during construction to be in accordance with many building codes, such as the California Building Code (CBC). Additionally, we recommend actual field load testing as the accuracy of empirical estimations is reduced with shallow pile embedment depths as expected for this project. It is our professional opinion that performing actual load testing for design tends to reduce design conservatism (and thus material costs) and can potentially reduce “changed conditions” claims during construction.

We usually propose driving test piles to provide information for the design phase, recording such information as the total depth driven, time to drive each pile, and depths of abrupt changes in driving resistance and refusal. We have assumed mechanical pile driving equipment (with vibratory driving methods) will be an option for installation of the piles during construction and we typically base our proposed technical services around this method.


As part of this scope we will perform uplift and lateral load testing of piles a minimum of 24-hours after they are driven. We expect to perform the uplift test first, and then apply a lateral load on the same pile. Since the amount of movement associated with the uplift test is very small, the potential for an adverse effect on the lateral load test should be acceptable.

Vertical pile load testing Lateral pile load testingVertical and lateral pile load testing

Pile uplift and lateral deflection testing shall be performed by using a 12-foot steel load frame with a 10,000 lb. capacity. A calibrated 30,000 lb. capacity hydraulic hollow-ram load device manufactured by Enerpac will be used for load measuring. A reaction frame equipped with 2 digital indicators accurate to within 10 thousandths of an inch will be used for vertical and lateral deflection measurements. The testing will be in general conformance with ASTM D 3689-07, Standard Test Method for Deep Foundations under Static Axial Tensile Load and ASTM D 3966-07, Standard Test Method for Deep Foundations under Lateral Load, as modified by our sub for the small piles being tested. The testing apparatus will be transported to the project site by trailer and will be positioned at the desired test location using a backhoe and/or skid steer loader. Once the load testing is complete, we will remove and dispose of the piles.


Our typical deliverable when optimizing pile embedment depths for a site will be a brief narrative of our testing observations and an Excel spreadsheet containing the test results. Our report will include the following:

  • Project identification and location, including test site locations.
  • Descriptions of test, including pile installation equipment, reaction systems, load and deflection measurement devices, weather, unusual events during pile installation, and other factors which may have an influence on the results.
  • Site plan showing approximate pile test locations.
  • Type and dimensions of test piles, length of test pile during driving, final bottom elevations relative to ground elevation, embedded length of test piles, and tested length of test pile. Driving records (date installed, rate of pile penetration in feet/second for driving, cause and duration of interruptions in pile installation, if any, and notation of any unusual occurrences during installation).
  • Tabulated and graphed measurements of load and deflection for each test, with the test number and embedment depth indicated.


Although there is some effort involved in pile load testing for solar projects, we have witnessed this advance planning resulting in site optimization and expense reduction. If Blue Oak Energy may be of service to assist with pile load testing, our civil and mechanical engineering team is a great resource for developing the test plan, deploying the pile load test crews, and summarizing the results in a meaningful format for your utility scale solar facility.

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