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Seepage Barriers/Grout Curtains


Derwood, MD: Dam Foundation Pressure Grouting, Lake Needwood Dam

dam-seepage-barriers-grout-curtains-derwood-md

In late June 2006, heavy rains resulting in over ten inches of rainfall caused a 23-foot rise in the water level in Lake Needwood, resulting in uncontrolled seepage from the down stream slope. 2,400 people were evacuated from downstream communities.

Geo-Con 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. The overall objective of the grout curtain was to reduce the permeability of the underlying bedrock formation to a residual Lugeon value of 5 or less.

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-Con 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.



 

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