Project Geologist Terracon Sterling, Virginia, United States
Background, Description, Delivery and Learning Objective Construction in karst terrains can present a challenge to development, and the solar energy industry has become increasingly aware of the impact, both to the site infrastructure and to human health and the environment, that may result from mismanagement of construction activities at solar sites where karst terrain is present. These impacts include development of new karst features or acceleration of existing features, short term and long-term damage to water supply wells from adjacent or nearby homeowners, and the potential to negatively impact Federally listed endangered species that reside in the karst terrain. To address these concerns, Terracon has developed a site karst construction plan specifically tailored to solar field construction and divided into three main parts: (1) Pre-Construction Survey and Risk Assessment; (2) Construction Monitoring; (3) Conservation.
The Pre-Construction survey and risk assessment phase consists of desktop data review and subsequent field reconnaissance to verify and locate features identified in the data review. After the survey is completed, risk is determined based on variables including: 1) the presence of an open throat, 2) parapet characteristics, 3) degree of soil raveling, 4) drainage leading to the karst feature, and 5) presence and quality of vegetation. Each karst feature is assigned a risk category where the risk recommendations detail approaches for each karst risk level. Upon completing the survey phase, Terracon works with project engineers to assist in designing appropriate erosion and sediment control (ESC) measures, feature buffering and setbacks to minimize the impact to the karst aquifer. The field reconnaissance may be followed by specific subsurface investigations of high-risk features that may be impacted by construction, including (but not limited to) geophysical investigations (e.g. electrical resistivity investigation, seismic investigation, etc.) and “ground truthing” employing soil and/or rock borings.
While conducting the karst survey as detailed above reduces the risk of karst issues during construction, it only identifies karst features that are present and visible at the ground surface. Regardless of the number of karst features identified at the ground surface, when working in Karst terrain, there is always the possibility that karst features may become exposed or formed during construction. Changes to the surface water flow caused by grading operations and blasting are well-documented to cause acceleration of existing karst features and forming new previously undetected karst features. Terracon has developed a more systematic approach to implementing Construction Monitoring measures during construction to reduce the risk of karst issues occurring during construction. The measures include determining the frequency of on-site karst feature inspections during construction based on the risk assessment made during the karst survey completed during design, training of on-site personnel to identify the presence of karst features, and the immediate steps to be taken in the event a karst feature is encountered.
The conservation phase runs simultaneously with construction, but also for the life of the facility. It outlines specific avoidance and minimization measures designed to protect the karst aquifer, thus minimizing impacts to human health and the environment. The karst aquifer is the primary source of potable water for the majority of people in karst regions, whether accessed through individual private wells and springs, or municipal water supply systems. In addition, the karst aquifer serves as the habitat for many rare, threatened and endangered species (RTES) across much of the United States. This phase includes specifications for feature buffering, surface water control measures, SWPPP (stormwater pollution prevention plans), and rock reduction or removal.
