Harnessing the sun can be a daunting task, yet photovoltaic (PV) solar installations are expected to triple around the globe within a matter of years. A single site can span miles, with tens of thousands of driven pile foundations supporting the panels. How can we expedite the process without compromising quality?
Design and construction of functional, cost-effective PV structures requires a thorough understanding of local soil, rock and groundwater conditions. Terracon has developed two systems that are improving solar panel foundation design and installation for our clients. With Pile Driving Performance Monitoring, we are able to remotely capture not only how deep each pile is driven, but also the time and energy expended in the process. Once it is in the ground, we are collecting accurate vertical and horizontal load-deflection data using our Tripod Solar Pile Load Test System, without the use of heavy equipment (previously a standard requirement).
Pile capacity correlates better with drive energy than pile depth
On large solar sites where typical boring coverage is one per 40 acres or less, the potential for encountering soil conditions not represented by the borings is highly likely. The risks (to Terracon and our clients) associated with this level of coverage is what drove Terracon to design a system to obtain production pile driving resistance records so that anomalous areas could be identified and addressed.
How much time and energy it takes to drive a pile to design depth is directly related to soil strength. We are able to detect when production piles drive too easily and may not have adequate capacity, by establishing benchmarks for both pile depth and driving energy in the initial design phase load test program. Conversely, we can tell when stronger soils are encountered and design pile depths can be reduced. In effect, every pile driving record provides both quality control and additional soils information.
Terracon’s new Pile Driving Performance Monitoring (PDPM) system (patent applied for), delivers enhanced quality control and added pile design optimization capabilities. The electronic sensing, data-logging and communication system can be installed on various types of solar pile drivers. The autonomous system collects data continuously during pile driving and transmits the data every night to Terracon. Each driving record is mated by GPS coordinates to its pile and checked against established depth and driving energy acceptance criteria for both vertical and lateral load capacities. Acceptance reports are generated the next day listing all the piles successfully installed. If any piles fail to meet either vertical or lateral acceptance criteria, they are listed in an exception report and Terracon provides corrective recommendations. This typically involves performing a simplified proof test that can be done quickly by the contractor using his construction equipment and methods developed by Terracon. If the pile passes the proof test, it is added to the acceptance report. If it does not pass proof testing, Terracon coordinates with the contractor to determine an effective means of bringing the failing pile into compliance. As the project progresses, proof test results are reviewed and considered in adjustment of the ongoing acceptance criteria (pile design optimization).
The PDPM system is currently being used by Mortenson Construction on Alamo V, a 100 megawatt solar project in Uvalde, Texas that will require about 27,000 five-inch pipe piles. On this project, application of the PDPM system combined with pile design optimization consultation resulted in a substantial reduction of foundation cost and schedule, combined with enhanced quality control, compared to conventional alternatives.
Tripod load testing unit covers more ground
Ensuring that the piles supporting PV panels subjected to 90 MPH wind loads or heavy snow loads don’t yield is a critical element of construction. The sheer volume of pile testing within the solar industry creates a logistical challenge that Terracon engineers have resolved with our portable tripod testing system. Not only is the system easily transported and assembled, but it also allows real-time test data to be obtained digitally into a laptop computer. This avoids transcription and human reading errors and keeps people out of harm’s way not having to read gauges close to a loaded pile. Two operators can set up, test both vertical and horizontal loads, and tear down the unit within an hour. This expedites more accurate and cost effective load testing for large solar installations.
Saving time and money while ensuring good quality control can translate into substantial benefits on a large PV project. Application of the PDPM technique and tripod testing unit will enable the owners, financiers, consultants, and contractors building solar developments to have an optimized design and a complete record of pile installation, an important advancement in this emerging industry.