When it comes to energy storage projects, having the right foundation involves careful planning upfront. But each site is different, requiring careful consideration for details like the types of equipment being supported, site location and geologic factors. An integrated engineer-procure-construct (EPC) team provides a comprehensive approach to solve complex site challenges with solid foundation solutions to create a seamless project outcome.
An initial geotechnical investigation reveals soil conditions and can supply the design parameters needed to minimize risk and support a proposed foundation type, such as a shallow, pier, or pile foundation. This specific geotechnical investigation and subsequent report are imperative for the design engineer to move forward safely and thoughtfully with the project design using investigative research and reliable information.
Slab or Mat Foundation
A slab foundation, also referred to as a mat foundation, is a type of shallow foundation that bears directly on the soil, or prepared surface, below grade. This type of concrete foundation is typically reinforced for strength or to minimize cracking in the concrete due to shrinkage and temperature fluctuations. With this option, the bottom of the foundation sits below the frost line to minimize seasonal foundation movements due to freeze-thaw effects. Such positioning is especially important in areas with colder climates.
When compared to individual footings, grade beams and other types of individual or non-monolithic foundations, slab foundations have the advantage of being able to spread loads over a wider area. This may be particularly useful for sites with subsurface soils that have low allowable bearing pressures or when there are concerns of inadequate compaction or soft soil areas under the foundation.
A pier foundation, often referred to as a drilled pier/shaft, involves drilling a cylindrical hole into the ground and filling the void with concrete. Reinforcement cages are typically installed prior to placement of the concrete. Piers can be used to support a variety of equipment sizes and the size/length of the piers may vary from site to site. However, depending on the equipment type and site soil conditions, piers may be installed at relatively shallow soil depths. Pier foundations are typically designed as end bearing, side friction or a combination of both.
The cost of pier installation can vary from site to site depending on size, length and site soil conditions. A thorough geotechnical investigation can determine whether the site soil conditions could require special installation considerations. Sites with adverse subsurface conditions, such as collapsing soil, rock or high groundwater levels, will most likely require special installation methods to navigate the soil conditions when installing a pier foundation.
Common pile types are driven steel H-piles or pipe piles. Piles can be used for most applications but are commonly used when a weak layer of soil is present near the surface and insufficient bearing capacity exists. At sites where weaker soils are present, piles are typically driven to a depth where more competent soil — or rock — is, with the loads transferred to that stronger layer. Piles are typically designed using side friction, end bearing or a combination of both.
Another pile type becoming more common in the energy storage market is helical piles. Such helical piles are made up of a central shaft with helical bearing plates welded to the shaft. Loads are transferred from the shaft to the soil through the helical bearing plates. This type of solution can be pre-engineered and is screwed into the soil with minimum disturbance. However, helical piles are limited to sites that have soils free of rocks and cobbles. Sites with hard soils may require pre-drilling.
Similar to a slab or mat foundation, a gravel foundation supports equipment on a prepared slab/mat, but instead of using concrete, the foundation is composed of compacted gravel. This type of solution has been used to support compatible battery enclosures and is typically selected based on the lower costs associated with it. However, extensive due diligence is required to confirm that the site has the right conditions for a secure and reliable gravel foundation. This type of foundation could be used at sites where environmental — wind and seismic — loads are low and the soil conditions are right for the application. A comprehensive geotechnical investigation can determine if a gravel foundation would be acceptable given the site conditions as well as outline design and installation requirements for making the approach a success.
Gravel foundations are more susceptible to erosion and washout over time, and therefore are not often recommended for just any battery storage site, despite the potential upfront construction cost savings. The edges of gravel foundations are typically sloped away from the equipment to provide proper drainage and help reduce the chance of erosion and washout of the foundation. Also, routine maintenance will typically be required throughout the lifetime of the foundation to maintain required slopes and to check for any voids or ponding in other portions of the foundation. There is also a higher probability for non-uniform bearing and differential settlement with the use of gravel foundations. Corrosion of the equipment base skid may also be a concern, particularly if the equipment base is made of steel. Non-uniform bearing, excessive differential settlement and corrosion can ultimately lead to damaged equipment on gravel foundations.
Many considerations must be taken into account before executing a battery storage project. Discussing all foundation options can help determine what makes the most sense to make a project successful based on geological, budget, infrastructure and time factors. Regardless of the type of foundation chosen, having an integrated EPC team is imperative to progressing the project quickly and efficiently for a seamless project process and solid, long-lasting outcome.
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