Derwood, MD: Lake Needwood Dam Restoration, Grout Curtain Installation
Geo-Solutions Inc. was contracted by the Maryland-National Capital Park and Planning Commission (Parks) to install a grout curtain in the rock foundation beneath the earth fill embankment dam at Lake Needwood in Rockville Regional Park. The grout curtain was installed in response to seepage observed on the downstream face of the dam during past high-water events.
Borings were installed in two phases:
- Drilling and casing through the dam embankment material (soil overburden) was accomplished by the sonic drilling method and was done without the use of water.
- Drilling through the foundation rock was accomplished by a combination of high-speed coring and diamond destructive NQ-sized drilling.
Two curtain lines were installed (upstream and downstream). The borings were installed at 15-degree angles and in a split-spaced pattern. The grout curtain also included "fan" borings at each abutment that ranged from vertical to 60-degrees to provide a tie-in to the existing natural rock surface. Borings were drilled up to 80-feet below the surface of the dam.
Water pressure tests were conducted in 10-foot ascending stages to assess the permeability of the rock. Water passed through a header system configured with flow control valves, and flow/pressure transducers, prior to delivery to the test stage. The pressure and flow transducers were linked to a specialized real-time monitoring system. Information was transmitted from the header to a central computer system which displayed, in real-time, the pressure and flow readings in graphical format. The system also recorded and logged the pressure/flow readings in a file archive. The test stage interval was isolated using a water packer apparatus that consisted of two straddle-style pneumatic packers separated by perforated pipe. "Houlsby"-style (multi pressure) water tests were required for primary borings. Secondary and subsequent borings were water tested with single pressure five-minute tests in 20-foot ascending stages. Water flow rates and total takes were recorded and used to generate total Lugeon values for each stage.
Grouting of the bedrock progressed in 20-foot ascending stages. Geo-Solutions and URS established a very stringent grouting program that included progressive grout viscosity adjustment and flow rate limitations in response to real-time assessment of grouting parameters. The goal of the program was to achieve efficient refusal/closure of fractures while maintaining a limited grouting reach in each boring. Grout passed through the header system prior to delivery to the target grout stage. Stages were isolated with a grout packer apparatus that consisted of a single pneumatic packer attached to the bottom of a steel pipe. Difficult, or high-take stages were addressed with a medium mobility grout containing Portland cement, bentonite and fly ash. Grout flow rates, pressure and total gallons of grout pumped were recorded and displayed by the real-time monitoring system.
Dam movement monitoring was also performed during all drilling and grouting operations. The real-time movement monitoring system consisted of an automated total station and a series of permanently mounted prisms located along the upstream and downstream faces of the dam. The total station continuously performed X, Y and Z readings on the prisms which it transmitted to the central computer system which displayed the readings real-time in graphical format.
The performance goal of the project was to achieve a residual Lugeon value of five (5) or less. To accomplish this goal, Geo-Solutions worked closely with URS and the Parks to respond to individual boring Lugeon values and grout takes and make real-time decisions to reduce the split spacing by adding additional boring locations. To keep the project moving forward in this constantly changing work scope, Geo-Solutions had to actively manage the drilling and testing/grouting crews to ensure all crews remained productive and efficient.