GYPSUM KARST AND POTENTIAL PROBLEMS IN SITING WIND TURBINES IN NORTHWEST OKLAHOMA

Gypsum is a highly soluble rock, and therefore is readily dissolved to form karst features identical to those associated with limestones and dolomites. Gypsum karst is widespread in the United States, and has locally been associated with highway subsidence, dam failure, and foundation cracking. Therefore, care must be exercised in construction within or above gypsum-bearing bedrock.

Investigations are underway in northwestern Oklahoma to evaluate the potential hazard that gypsum karst may pose for a proposed wind-turbine project several miles east of Watonga. Catastrophic failure is clearly unacceptable. However, even seemingly minor differential settlement due to subsidence could be a risk. Differential settlement on the order of one inch across a 50-foot-wide turbine foundation could lead to the turbine tilting out of tolerance, and require remedial repairs.

Gypsum beds of the Permian Blaine Formation underlie all parts of the project area, at depths ranging from 40 to 140 feet below ground level. The Blaine Formation here is about 95 feet thick; it consists of four gypsum beds, each 2 to 12 feet thick, interbedded with red-brown shales. The Blaine Formation is overlain by Permian Dog Creek Shale and by unconsolidated Quaternary sands, clays, and gravels that may obscure karst features. Field studies, aerial-photo analysis, and literature study show that, as yet, there is no direct evidence of gypsum karst in the project area. Lacking direct evidence of karst, additional risk mitigation can be achieved by placing wind turbines at sites where the gypsum beds are deepest, and we believe gypsum buried 80 feet deep, or deeper, would pose a low risk related to gypsum karst. Placing them at sites where gypsum beds are at depths of less than 80 feet would pose a medium or high risk.

Combining structure-contour data and surface elevations (USGS topographic maps), we have mapped areas of low, medium, and high risk, and thus the developer can select those sites where wind turbines can be placed with a low risk, related to gypsum karst. We recommend that additional subsurface data be acquired from petroleum tests and core borings. Also, a survey of landowners in the project area, while not proving the negative, is prudent to try to establish if there is any history of sinkholes or collapses that would indicate undetected karst features.