Successful Grout Curtain Installation - Means and Methods, Lake Needwood Dam, Rock Creek State Park, Derwood, Maryland

Injection grouting, or pressure grouting, has been in practice in various forms since the 1800’s. Means and methods have continuously evolved over the past 2 decades and continue to evolve today. Currently, there are a variety of “accepted” systems in practice that have been successfully implemented for dam foundation grouting all over the world, but to date, no rigid “rules” have been established for the performance of this technology. This paper is intended to illustrate means and methods successfully used to install a grout curtain at Lake Needwood Dam in Derwood, Maryland. This information should be of value to owner/operators of similar problematic dam structures that may be evaluating grout curtain installation as a means of seepage control. Lake Needwood Dam was built in 1964-1965, and has a 65-foot high earthen embankment constructed on a fractured rock foundation. The Dam is a flood control structure that captures floodwaters within the 12.8 square mile Upper Rock Creek watershed, and is designed to discharge floodwater through a 42-inch principal spillway at a controlled rate that remains consistently manageable for the channel and stream banks downstream from the dam. The MDE Dam Safety Division classifies Lake Needwood as a High Hazard Dam Structure. In late June 2006, a heavy rainfall event with total measured accumulation in excess of ten inches caused a 23-foot rise in the water level in Lake Needwood, resulting in observed uncontrolled seepage from the downstream slope of the dam. According to initial evaluations, the dam had essentially developed a leak and was potentially on the verge of failure. Within a matter of hours, 2,400 people were evacuated from the downstream communities, including part of the City of Rockville, Maryland. Following inspection and a comprehensive investigation by MDE Engineers and county managers, it was determined that the earthen embankment was intact, and the observed seepage was a result of the increased head pressure forcing water through open joints and fractures within the underlying bedrock. Instrumentation showed that the water pressures within the foundation rock continued to increase, even after the storm had passed and the lake began to recede. Geo-Con, a specialty Geotechnical Contractor, was contracted by Maryland National Capital Parks & Planning Commission (MNCPPC) to work with URS Corporation to install a grout curtain intended to reduce seepage through the underlying fractured bedrock. The work began in April of 2007, and was successfully completed during October of the same year. This paper will detail means and methods used to install the grout curtain. Topics will include: Drilling Water Testing Grout Mix Design Foundation Grouting Quality Control, Reporting, and Instrumentation Site Specific Problems and Solutions Completion and Verification. 12 pp.