The significant geologic formations at the site are the St. Louis and Sainte Genevieve limestone formations, both highly susceptible to solution weathering. For those not familiar with the formation of sinkholes in Karst Topography, you may wish to start with the Wikipedia article on the topic. 10 sinkholes were identified initially with an additional 5 identified during construction. The initial geotechnical investigation consisted of soil borings and test pits that indicated a variable depth to bedrock of approximately 11 to 20-ft. Later, the design team hired the author’s firm, Hagerty Engineering, Inc. of Jefforsonville, Indiana (Hagerty) to perform additional work based on their experience with mitigation of sinkholes on similar projects. A portion of the project sitemap is shown below.
Sitemap for new Harrison County Hospital, Kentucky, showing sinkhole locations (from Duffy (2008b), Civil Engineering Magazine)
Sinkhole Mitigation Measures
Additional excavation was performed to locate the throat of the sinkhole in selected locations. Not all sinkholes were excavated because of fears of altering subsurface drainage patterns in the months between the investigation and the start of construction. Hagerty designed a graded filter detail for the mitigation of sinkholes, including those under the hospital structures. The open graded rock was to ensure that subsurface drainage features were not altered, and the compacted clay above the filter served to minimize the amount of water that reaches the rock. For pavement structures, a modified version of the detail was used to allow for use of on-site material given the non-critical nature of deformations to pavement structures and the fact that the owner accepted the reasonable increase in risk. (Detail at left: Graded filter for sinkhole mitigation, designed by Hagerty Engineering, Inc. [Duffy, 2008b])
Using Sinkholes as Surface Drainage Facilities
Several of the existing sinkholes were already draining a significant portion of the surface drainage at the site. One sinkhole was estimated to be receiving runoff from approximately 5 acres. The design team decided to use this to their advantage, and 4 of the sinkholes were selected to be used as drainage features. The “Chimney Structure” detail shown below was developed by Hagerty to convert the sinkholes into drainage structures while minimizing the potential for more rock to be eroded. Once the contractor excavated to the bottom of the sinkhole and exposed the throat, pump testing was performed to confirm that no obstructions were present to obstruct flow prior to construction of the chimney structure. Oil-water separators were used to attempt to prevent contaminants from reaching the subsurface.
Chimney structure for storm water, designed by Hagerty Engineering, Inc. (Duffy, 2008b)
Structure Foundations In Karst Topography
For the structure foundations, drilled piers socketed into sound limestone were used as well as footings bearing on rock for the basement structure. Because of relatively low lateral loads and moments, 1-ft of rock socket was determined to be sufficient. However, to confirm that the rock beneath the socket was sound, a visual inspection of the socket was performed by the geotechnical engineer in the cased hole and a 5-ft probe hole was cored beneath the socket.
The article is highly recommended reading, as it contains much more information that what is given in this summary. Of particular interest to me were the author’s discussion of the problems with determining bid quantities for the mitigation of the sinkholes and the description of a number of additional challenges faced during construction. The footer for the article indicates that it was based on a paper presented at the 11th Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst in Tallahassee, Florida in September 2008 (Duffy, 2008a).
- Duffy, Peggy Hagerty (2008a), “Foundation Design and Site Development for a New Hospital over a Complex Karst System”, Proceedings of the Eleventh Multidisciplinary Conference held in Tallahassee, FL, September 22-26, 2008, Sponsored by Geo-Institute of ASCE, by Lynn B. Yuhr, E. Calvin Alexander, Jr., and Barry F. Beck (editors), pp. 555-564, Reston, VA: ASCE/Geo Institute of ASCE, 978-0-7844-1003-5, 2008, 761 pp.
- Duffy, Peggy Hagerty (2008b), “Karst and Complications“, Civil Engineering Magazine, V. 78, No. 11, November 2008, American Society of Civil Engineers, Reston, VA., pp. 70-75, 78-79. [Editor] I beleive a member login is required to access the online version of Civil Engineering Magazine. [/Editor]