Utility Scale Solar - Environmental and Geotechnical Considerations

The growing development of solar photovoltaic (PV) facilities across the U.S. leads to geographically varied site challenges and considerations. Site selection is a crucial component of developing a successful solar PV project. The site selection process considers many factors that impact the cost of the electricity generated such as grid connection, financial incentives, solar resource, and local zoning regulations and policies. Early in the site selection project teams must consider environmental and geotechnical conditions.

Utility-scale solar environmental considerations include land disturbance and land use impacts; potential impacts to vegetation, wildlife, wildlife habitat, and sensitive species.

Challenge. Obtaining preliminary due diligence data on historical site use, wetlands, geology, surface, and near-surface soils, threatened and endangered species, geotechnical conditions, accessibility, topography, and shading.

Solution. A low cost (<$900) modified Phase I Site Assessment (Phase I ESA) with a report from services like EDR Lightbox[1] can provide desktop information on wetlands, subsurface geology, near-surface soils, historical features, floodplains, hydrology, topography, aerial photos, and groundwater, as well as protect the owners and operators from joint and several liabilities under CERCLA, should any exist.?

Challenge. Avoiding sensitive or critical habitats and species. Construction and operation of solar PV power plant sites and ancillary infrastructure (access roads, transmission lines) leads to clearing of existing habitats and disturbance to fauna and flora.

?Solution. A desktop study can provide preliminary information on sensitive or critical habitats and species. Later, field research by experts can provide critical information and management solutions for potential impacts. In many cases, a consulting expert can perform an ecological baseline survey and avoid sensitive habitat or develop habitat mitigation measures. Mitigation measures can include careful site layout and design to avoid areas of high ecological value or translocation of valued ecological receptors.

?Some examples include:

·??????In the northeastern U.S. the Northern long-eared bat is listed as threatened. Trees that present habitat for roosting between March and November cannot be cut without a special permit from the US Fish and Wildlife Service (FWS). In some cases, a consulting expert can develop a plan for incidental take (defined as unintended impacts to the species from otherwise legal activities), without the need for a federal permit.

·??????In the Southeast, the gopher tortoise is listed as threatened by the FWS in Alabama and Louisiana, and a candidate species for federal protection in GA, AL, FL, and SC. Both the tortoise and its burrow are protected under state law in several states. Gopher tortoises must be relocated before any land clearing or development takes place, and property owners must obtain permits before capturing and relocating tortoises. The relocation cost of tortoises can be as much as $1,000 per tortoise with possibly dozens per acre. Furthermore, the burrows can be very deep, burrows average 15 feet long and 6 feet deep, and they often use multiple burrows throughout their lives. The burrows can present significant adverse geotechnical issues. A consulting expert can prepare the proper management plan for relocation or develop a management plan for construction methods to protect and/or enhance the habitat.

·??????In the western U.S. the Mojave Desert tortoise has been listed as threatened under the Endangered Species Act. Impacts to tortoises and other species can be managed following a site assessment under a habitat conservation plan.

Challenge. Solar foundations present unique geotechnical challenges due to the very high number of PV panel mount locations and variable near-surface conditions over areas of 20 acres to more than 1,000 acres. Geotechnical site assessment must gather enough data about site characteristics like soil composition, moisture content, bearing capacity, soil corrosivity and resistivity, and surface water runoff to optimize foundation design.

Obtaining the subsurface geotechnical data necessary to design optimal solar PV foundations can be very expensive, requiring more time, and more planning than budgets allow.

Solution. A phased design approach or sequenced data evaluation can help focus data needs to avoid unnecessary data collection and provide a steady accumulation of information. An iterative process helps pick-up data gaps. Seasonal work windows should be factored into project schedules so work can be reasonably performed to obtain needed information.

?Challenge. Collect sufficient geotechnical data for site characteristics to optimize foundation design.

Solution. A well-designed solar foundation needs to be cost-effective without sacrificing reliability. The investigation methodology should be designed for the types of near-surface deposits as identified in the desktop assessment. Boreholes are appropriate for sands, silts, and clays. In other types of near-surface deposits, such as glacial till, drilled boreholes can miss or misidentify important soil features, such as the percentage of rocks and cobbles. In these cases, test pits can help the geotechnical engineer identify the percentage of rocks and cobble. Test pits can provide multiple uses when used for both geotechnical and cultural inspections.?Handheld explorations utilizing a dynamic cone penetrometer (DCP) can be performed to allow for collection of subsurface in situ density measurements in areas that are otherwise difficult to access with traditional exploration equipment.

Challenge. Improper design and installation cost overruns for solar panel mounting foundations.

Solution. Subsurface conditions account for some of the most significant risks on a solar installation. Foundation design should begin with a desktop study long before land acquisition and field work. Publicly available information on land features is available from the USGS, US Soil Conservation Service, state environmental agencies, and local GIS mapping in most areas of the U.S. A Phase I ESA report can provide preliminary information on near-surface soils and geology. For example, surficial geology maps provide information on glacial deposits and non-glacial landforms such as faults, lineaments, landslides and escarpments.

For example, the presence of glacial tills can mean a heterogeneous mixture of clay, silt, sand, pebbles, cobbles, and boulders that can present significant pile driving and boring costs. Geotechnical engineers can use desktop resources to learn what to expect and how to design the geotechnical investigation before conducting field studies to further evaluate foundation design parameters. Since most solar installations rely on driven or pre-drilled piles, proper design typically requires geotechnical data on compression, tensions and lateral loads, groundwater information, lateral and axial design parameters (i.e., L-Pile or A-Pile), and frost depth and how to address ad freeze stresses for colder climates. Thorough reports greatly reduce the risk of costly change orders that can sometimes result in seven-figure cost differentials.

Challenge. Optimize foundation design.

Solution. Appropriately spaced test locations, along with soil sampling and in-situ testing, at a depth appropriate for the foundation design, will provide the necessary geotechnical data to optimize foundation design. Allow for field adjustments that may reduce installation time and compensate for inaccuracies in placement of foundations. The topographic conditions of the site and information gathered during the geotechnical survey will influence the choice of foundation type. This, in turn, will affect the choice of support system design as some designs are more suited to a particular foundation type. Near-surface soil conditions can vary extremely over short distances, e.g., bedrock to soft sands.?For driven piles and earth screws, empirical results from test piles and pull testing (lateral and axial) to agreed load levels will assist the geotechnical engineer with the optimum design.

Foundation options for ground-mounted PV systems include:

·??????Driven piles – Soft to medium penetrable ground.

·??????Earth screws – Soft to medium penetrable ground without cobbles and boulders, suitable for slightly uneven or sloping terrain.

·??????Concrete piers cast in-situ -Uneven and sloping topography.

·??????Pre-cast concrete ballasts – Sites with rock outcrop, flat laying, or impenetrable ground.

·??????Bolted steel baseplates – flat laying, hard surfaces.

A successful solar project begins with setting goals, developing a framework based on experience, focus on key decision points early in the process, and is iterative through a phased approach.?

[1] https://edrnet.com/prods/lightbox/