Project Overview
The Assembly Solar project consisted of construction of a 200 MW solar field in a 1,200 acre site. A preliminary geotechnical engineering report had been previously performed for the site by another consultant for the project owner, Assembly Solar, LLC based a Brooklyn, NY company. McCarthy Building Companies based in Phoenix, AZ, was selected by the owner to design and construct the project. McCarthy selected NTH to provide geotechnical engineering and pile load testing services to complete the project design through a request for proposal solicitation process. NTH was selected for the project based on their long history of geotechnical engineering knowledge of the area and our ability to deliver the engineering services in a quick and efficient manner due to our local presence. Once the design was completed, McCarthy hired NTH to observe testing of production piles and to perform field density testing of utility trench backfill at the site.
Project Scope
The scope of work for NTH consisted of performing preconstruction soil boring explorations and resistivity testing, as well as pile load test programs for PV solar arrays and supporting structures. The initial effort consisted of limited number of test borings and pile load tests throughout the site. The pile load tests included compression, vertical uplift (tension) and lateral loading and were performed in accordance with procedures that were prepared by McCarthy specifically for this project. NTH engineering staff witnessed load testing of the steel W6 piles and documented the results in daily field reports and associated load test forms. NTH issued an engineering report documenting the field geotechnical exploration, and pile load testing report for the initial round of testing. Following discussions with the team, and in order to provide more location specific geotechnical information and pile load tests and to better account for adfreeze forces in the design, NTH was requested to provide additional test borings and observe additional pile load testing in order to tailor the results to specific locations within the large site. The additional test boring exploration was completed and the results were documented in an engineering report. The report provided exploration results and geotechnical pile foundation design recommendations. McCarthy installed additional test piles throughout the site and used predrilling to 3.5 feet in some of the piles in order to discount the contribution of the upper soils on uplift resistance.
NTH monitored preconstruction load testing of the additional W6 pile foundations and provided the results to the team in a written report. Other scope items included interaction with the client and participation in meetings to discuss the geotechnical parameters and pile load testing findings during final design of the project production piles. The client performed additional uplift testing through a third party to quantify the adfreeze forces for inclusion in the final design of the production piles. At the client’s request, NTH also provided pile load testing for 110 production piles and field density testing of the trench backfill at selected location during the project construction and provided daily field reports of the activities.
Client Benefit
Although literature related to this issue is limited in this area, NTH witnessed additional testing and interacted with the client in order to provide input to account for adfreeze but still avoid unnecessary over design of the production piles. The client performed additional special uplift testing and was able to design and successfully construct the project.
