Franklin County, AL: Bear Creek Dam, Grout Curtain Installation
Geo-Solutions Inc. was contracted by the Tennessee Valley Authority (TVA) to install a grout curtain in the rock foundation as a portion of a dam restoration project at Bear Creek Lake in Franklin County, Alabama. The grout curtain was installed in response to seepage observed on the downstream face of the dam during past high-water events.
Geo-Solutions had to install borings in a variety of manners dependent on their location along the dam alignment. Borings in the spillway were installed in three phases:
- Core through spillway concrete slab with a concrete core machine.
- Drill through the overburden material and advance a 4" PVC casing 1 foot into the bedrock layer.
- Coring through the rock foundation was accomplished by high speed HQ-sized coring for exploratory borings.
Primary borings and subsequent borings were drilled with a production rig utilizing a destructive diamond bit. Primary borings and subsequent higher order borings were drilled with a rotary percussion rig utilizing a destructive diamond bit with water-only flush.
The remainder of the alignment was completed from top of exposed bedrock or dental concrete layers allowing borings to be installed using the rotary percussion rig with a destructive diamond bit.
Two curtain lines were installed (upstream and downstream). The borings were installed at 15-degree angles and in a split-spaced pattern. Borings were drilled up to 90-feet below the surface of the dam.
Water pressure tests were conducted in 20-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 exploratory borings. Primary 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 Rizzo Associates 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 an aluminum pipe. Difficult or high-take stages were addressed with a medium mobility grout containing Portland cement, bentonite and fly ash. Grout flow rates, apparent lugeon, 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 mounted prisms located along the grout curtain alignment. 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 Rizzo Associates 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.
Geo-Solutions also worked closely with TVA; Heavy Equipment Division, who was concurrently working on other dam restoration activities, to coordinate work crews and tasks to minimize conflicts.