October 2001

Applying the Risk Analysis Process to the Aggregate Industry

Applying the Risk Analysis Process to the Aggregate Industry

By William H. Langer

Editor’s Note: This article is the tenth in a 12-part series focusing on how geology can lessen the “surprises” and help overcome the challenges posed by nature during the process of aggregates extraction.

In last month’s column, I defined environmental risk and demonstrated how environmental risk relates to the aggregate industry. I also mentioned Pliny the Elder, who made an unwise risk assessment and, consequently, was killed by toxic fumes during the eruption of Mt. Vesuvius. Now, I will describe one approach for applying the process of risk analysis to the aggregate industry. If Pliny had followed this process before visiting the active volcano, he might have had more time to spend with his nephew and ward, Pliny the Younger.
Some approaches to risk assessment are linear, assume only one causal factor and tend to overemphasize stability. However, natural systems should be studied holistically because they change over time and are composed of numerous diverse elements linked by strong interactions. A holistic approach to risk assessment emphasizes natural processes, focuses on multiple interactions among the elements of a system and integrates time, feedback and uncertainty.
The holistic method is iterative and begins with the creation of conceptual models that provide a preliminary understanding of how various systems behave at the site and off-site areas that might be impacted. Conceptual models are tested and revised using scientific principles, existing data, new field or laboratory data and experience. Each iteration of the process increases the understanding of the total system and, thus, reduces uncertainties. Because the understanding of each part of a system depends on an understanding of the others, analyses of the individual natural systems are integrated with one another. The process results in the characterization of the entire environmental system.
An approach to extracting and processing aggregate is selected based on the characteristics of the site. The method then determines how aggregate mining will impact the entire environmental system, including the atmospheric, land surface, geomorphic, subsurface and ground-water systems, which requires learning how the various parts of the natural system (including the human part) create and transmit impacts and the resulting condition of the impacted system. It may be necessary to rerun the site characterization process using new parameters created by the mining. By comparing the “before mining” and “during mining” scenarios, it is possible to determine how the systems will be impacted by, and respond to, mining activities. If necessary, the process can be restarted at any stage using an alternate mining approach.
The process continues by identifying initiating events and consequences. Human initiating events include drilling, blasting, excavating, dewatering, transporting of material, crushing, screening and washing material. Human activity outside a quarry or pit can also be initiating events. For example, a change in land use from a natural area to a paved area could create increased runoff and cause an aggregate operation to become flooded. Many human-initiating events are neither planned nor started at the time an environmental analysis is being conducted, thus making prediction very difficult.
Natural initiating events include climatic events (droughts, heavy precipitation and precipitation during critical periods), seismic activity, landslides, natural ground-water level changes and natural fluvial processes. Most natural initiating events are difficult to predict.
One consequence may be the initiating event for a subsequent consequence (a cascading environmental impact). For example, excavating rock (initiating event) in a karst area might intercept a natural conduit (consequence), which may become the initiating event that results in flooding the quarry (consequence), which may become the initiating event that results in dewatering and ground-water lowering (consequences). A drought may be a natural initiating event that results in further ground-water lowering (consequence), which may become the final initiating event that results in a loss of buoyant support to the overlying soil, resulting in sinkhole collapse (consequence).
The next step of the process, environmental risk evaluation, combines the outputs from the consequence and likelihood analyses to create an estimate or indication of the likelihood of defined adverse outcomes. Risk estimates can be described in many ways, such as the likelihood of a specific consequence occurring per unit of time, per unit of area mined or per unit weight or volume of material produced. Sometimes, such as with a rare or endangered species or with a sensitive ecosystem, the relevant measure may be a probability of occurrence of the consequence within the life of the operation.
Environmental risk management is a continuing process. After all systems have been analyzed and all risks have been characterized, an assessment of the adequacy of process is conducted. If the process is judged to be incomplete or inadequate, the process is repeated at whatever step or level is required. If the process is judged to be complete, aggregate extraction can begin. Information learned while excavating aggregate is plugged back into the site characterization, thus further reducing the uncertainty. And reducing uncertainties reduces the risks associated with aggregate production. 

William H. Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey.