Development and Construction in Karst Terrains Meeting the Challenge

What is “karst,” and why is the construction and development industry taking a greater interest in it? Simply put, karst is a landscape type that is characterized by the presence of sinkholes, caves, sinking streams, areas of soil subsidence, and an irregular “pinnacled” soil to bedrock interface. The features which characterize the karst landscape are all a consequence of the presence of soluble bedrock, typically carbonate rocks such as limestone, but karst terrain can also develop in so-called “evaporite” rocks, such as gypsum. Widespread residential, commercial, and industrial development in karst terrains in recent years has led to increased interest in karst geohazards and the concurrent impacts to human health and the environment that can result from their mismanagement. Poorly handled construction management in karst terrains not only can result in damage to site structures but also can impact the underlying karst aquifer that often provides the sole source of drinking water for both municipalities and private homeowners. In addition, the subsurface karst system serves as the habitat for many rare, threatened, and endangered species (RTES), and their protection has become the subject of numerous federal and state-level regulations and requirements. In this article, we describe some approaches Terracon’s karst specialists have taken to address both potential risks to infrastructure, as well as impacts to human health and the environment.

Pre-Construction Site Survey

Terracon’s karst specialists have developed an approach to pre-construction site surveys that supports the creation of karst management plans for a wide variety of development projects. One of the challenges that are difficult to overcome is when there is a high density of features, most often sinkholes, present at a site. For instance, we have worked on solar sites where within 1,000 acres, there have been hundreds of sinkholes, and in order to assess the risk these sinkholes present, we have developed a three-stage approach for site surveys and risk assessment.

The first step is to perform a desktop review. This involves the identification of suspect surface karst features remotely using various resources, including (but not limited to): federal and state sinkhole location databases, aerial photographs, geological mapping, digital elevations models and LiDAR.This process significantly reduces the amount of time spent on field verification and survey tasks, thus saving the client both time and money. Among these resources we use, perhaps the most powerful and informative is LiDAR. LiDAR data is now available for much of the continental United States. It is a particularly powerful tool for surveying large land areas or inaccessible sites and is capable of “seeing” closed depressions, sinkholes, and other karst features not easily imaged by standard methods. LiDAR also has so-called “bare earth” capability, which strips away vegetation and allows the identification of sinkholes and closed depressions in forested areas.

Upon completion of the data review, the second step is the field survey and reconnaissance. Specifically, the field survey involves locating and verifying all of the surface karst features identified in the desktop review, as well as uncatalogued or previously unidentified features. The field survey also documents feature characteristics necessary for the development of risk assessments, as karst features vary in their potential impact on buildings, infrastructure, human health and the environment; accordingly, it is important to understand what their relative impact is and how best to minimize the risk.

The risk assessment is the third step in the process. This involves creating a data matrix of specific characteristics that are then compiled to create a risk “ranking” for each feature identified in the desktop and field reviews. The risk levels of each feature are then presented in a colour-coded figure which provides to the client an easy to understand the overall view of the risks that the karst features may present.

Risk ranking results for a high density of karst features at a proposed solar site.

This allows the client to decide if site development is feasible, and if so, how to manage karst during construction. Terracon’s karst specialists then work closely with the client to develop a construction management plan with emphasis on effective sediment and erosion control measures that maximize protection of the karst resource while simultaneously protecting the planned site infrastructure.

Protecting a Drinking Water Resource

Terracon was the primary karst consultant for a major natural gas pipeline project that crossed over 70 miles of karst terrain in Virginia and West Virginia. Of particular concern were areas where pipeline construction might impact karst aquifers that provided drinking water for local communities. Among the most notable of these was the potential impact of pipeline construction on the water supply of the small community of Deerfield, in Augusta County, Virginia, provided by a municipal well and karst spring.

Well at Deerfield showing artesian behavior after a 0.5” rainfall event.

A study conducted in March 2017 by the county suggested the water providing recharge to the system was originating from several sink points (swallets)along the bed of the Hamilton Branch. Because pipeline construction crossed several tributaries of the stream, there was a concern that runoff might result in the infiltration of sediment-laden water into the swallets. 

This had the potential of severely damaging the Deerfield water treatment plant’s filtration system or even rendering it inoperable. As a result, Terracon’s karst specialists planned a dye trace to verify the connection between the sink points and the treatment plant. 

The Deerfield monitoring sites were equipped with charcoal dye trap packets in late November and monitored for the presence of the selected tracer dyes (Rhodamine WT and Fluorescein) for two weeks prior to dye injection. Neither dye was detected in this background sampling. Subsequently, Fluorescein dye was placed in the channel of a small tributary/drainage ditch that entered the Hamilton Branch channel from the north. Rhodamine WT was subsequently placed in the Hamilton Branch, approximately 0.2 miles above the streambed swallets. It was verified visually that the dye reached the swallet and was carried into the subsurface by the streamflow.

Rhodamine being introduced to the channel of the Hamilton Branch. It traveled approximately 0.5 miles downstream to the swallet where it went underground.

Rhodamine was detected at the well and water supply spring24 hours after injection. The fluorescein dye took slightly longer, with the first arrival occurring between 24 and 48 hours. These data suggest the fluorescein was only partially captured by the recharge to the well and spring.

The rapid travel time of both injections (i.e. 48 hours or less) suggested that there was a potential impact to the Deerfield public water supply if sediment or contaminants resulting from the pipeline construction impacted the Hamilton Branch or its tributaries. Based on these results, a monitoring and emergency response plan was developed for the Hamilton Branch, which would be used during construction, with periodic monitoring after construction continuing until the pipeline alignment was revegetated, stabilizing the soil.

Conserving Potential RTES Habitat

In May 2013, a natural gas pipeline was being constructed in Warren County, Virginia, to supply fuel to a new power plant that was taking the place of the former coal-burning power plant facility. During trenching for the pipeline, the operator of the trenching machine felt an unusual vibration, and looking behind, saw that an opening had developed along half of the base and up to one wall of the newly cut trench. In compliance with the management plan for the project, work was stopped immediately. It was determined that the opening led into a sizable chamber beneath the alignment that was at least 10 feet in height. The pipeline company called the project karst specialists at Terracon to conduct a detailed examination of the opening, and it was determined that the trencher had “unroofed” a cave. Terracon’s karst specialist knew that this cave was of considerable importance, as it was located within the habitat range of an RTES called the Madison Cave Isopod (Antrolana lira), or “MCI” for short. An inspection of the cave’s interior would be necessary to determine if it was inhabited by the MCI. This was particularly critical, as specimens of the MCI had been found in pools located at the bottoms of two caves located less than a mile from the newly unroofed chamber.

Cave unroofed during trenching operations.

Terracon contacted the Federal Energy Regulatory Commission (FERC), the U.S. Fish and Wildlife Service (USFWS) and the karst program of the Virginia Division of Conservation and Recreation, Natural Heritage Program (DCR-NHP) and explained the situation to them. With the agreement of the regulators, two experienced cavers and specialists in the conservation of the MCI were sent to the site from DCR-NHP. Once it was determined that the cave entrance could be accessed safely, the cavers descended into the opening and carefully surveyed and mapped the interior of the cave. The cave was a single chamber, and there was no evidence of a pool of water that would be the required habitat for the MCI. As a result of the inspection, the USFWS requested that the cave entrance be permanently sealed. A roof repair was planned and submitted to FERC, USFWS and DCR-NHP, all of whom approved the repair plan. The roof of the cave was sealed as specified, and the pipeline construction was completed through the section successfully.

Detail of the cave roof repair that was approved by the regulatory agencies (right).

Summary

This is just a limited summary of the types of challenges that Terracon’s karst specialists have overcome prior to and during construction in karst terrain. Unlike the challenges presented to geologists and engineers by geohazards such as landslides, soft soils, or seismic issues, the management of karst requires a multidisciplinary approach, with the engagement of specialists who have the expertise not just in engineering geology but in hydrolo