Wolford Mountain Reservoir

Project Description

The outlet conduit at Ritschard Dam, is constructed primarily of an 8-foot diameter, 786-foot long steel liner encased in reinforced concrete. Steel stiffener rings are present along the pipe length at 7-foot spaces. Additionally, twenty feet of the outlet conduit is constructed of reinforced concrete and provides the transition from square to circular cross section. An initial inspection has located voids in the reinforced concrete encasing the steel liner. The voids are predominantly indicated at the liner invert along an estimated 491-feet of its length. Some voiding in the crown has also been identified. Initial void mapping of the outlet conduit was done by sounding with a hammer, a technique commonly used in finding delaminations in concrete.  Olsen Engineering was contracted by Restruction Corporation to perform a non-destructive investigation to confirm voiding behind a steel lining at Ritschard Dam outlet works. A variety of test methods were used during the investigation, including: Impact Echo (IE), Spectral Analysis of Surface Waves (SASW), Slab Impulse Response (SIR), and Ultrasonic Thickness (UT) testing as well as some experimental micro-seismic reflection surveys. Olsen Engineering was also contracted to re-test portions of the structure post repair as a quality assurance method. For Void detection, each of the methods (SIR,SASW, and IE) provided good results at locating existing voids behind the steel lining. Additionally, some (UT) testing was performed at locations thru out the length of the pipe to check for any corrosion damage in the form of section loss, which may have occurred due to water filling any voided or debonded areas. Cement grouting of the voids at the invert of the outlet conduit is the recommended repair solution. Delivery of a high-quality grouting program for this project will require optimization of several major variables. Time to completion, grout material flow properties and grouting method to deliver the material. The District is obligated to deliver water to Muddy Creek either through the outlet works or over the dam spillway. All are important elements of the construction problem.

Construction Schedule

Our construction schedule was  based upon work completion during water run-off season, we are expecting that our construction schedule must be completed within a 5 week time period. This is based off historical snow-pack and run-off data, and the requirement of the Colorado River Conservation Districts Obligation to deliver water to Muddy Creek. This 5 week period allows work on the outlet pipe in a closed condition, as water will flow over the spillway.

Grouting Method

Holes will be drilled in the steel liner using magnetic drills. Initially, one hole will be located at the invert spaced @ 3-4 feet of void running along the liner length. Hole diameter was estimated at 1-1/2 inch. One additional hole will be drilled at the upper boundary each side of the invert in the radial direction (along the circumference). Spacing of these radial holes are also estimated @ 3-4 foot along the void length. This will be a vent hole(bleed hole) estimated diameter was ½ inch. A pipe bushing internally threaded will be inserted into the drilled hole and welded to the steel liner. This will allow a 1-inch pipe nipple to be threaded into the bushing with a ball valve attached to lock off the grout. The same would be true for the vent holes. Grouting will begin at the lowest elevation of each steel stiffener ring, grouting predominantly through the 1-inch ports at the invert and grouting upstream and using the high radial holes as air bleed or vent holes. The low viscosity of the grout and shallow slope of the pipe will eventually require grout to be pumped through the radial ports. Pumping of grout in the radial holes will be timed to coincide with the grout set time to ensure filling of voids to the spring line and higher.

Grout Material and properties

The grout material and properties are instrumental in satisfactory completion of the project. Installation of 2500 cubic feet of material over a 5 week period will require a 20 g.pm. grout plant. This larger grout plant dictates that the plant be located outside of the outlet conduit. Due to the lack of access into the outlet conduit, the grout will likely have to be pumped the entire length of the conduit. This long length of grout hose will cause grout pump pressures too be to high for the steel liner acceptable grouting pressures. So an additional 5 g.p.m. grout pump will be used to pump the grout into the voids. The 20 g.p.m will relay the grout to the 5g.p.m. The grout pumps will be fitted with pressure gauges and re-circulation manifolds for line control. High grouting pressures are not expected unless water head is encountered or filling of poorly connected voids is required. Back pressures will be measured to determine the preset gauge pressure. Grout will be mixed and proven to have uniform consistency. Samples will be taken by the engineer. The grout will consist of 25 # of Type I/II Portland cement, 8.8 # of potable water and 1# of BASF flow additive trade name “Meyco Fix Flow cable”.

The Flow characteristics of this grout will allow for longer distance pumping. The grout will easily pass a flow cone test in 19 seconds. Additionally important, the grout is low bleed with a shrinkage value of 0.13. Compressive strength  of the grout is acceptable, 4,000-psi at 28 days. Grout set time will be documented and can be controlled with temperature of mixing water and will be somewhat slowed by the moderate temperatures within the liner.

Sanded grouts, modified with plasticizers in order to pump 400-800-feet run the risk of sand bridging inside the hose. Our recommended grout will  grout all void sizes including very thin voids near the liner crown. Changing grout mix designs to accommodate different void sizes will consume labor and time and likely create wasted material residing in the pump hose or mixer. The grout recommended for this project optimizes the grout plant size, allowing for maximum production (shortest construction schedule) without compromising installation method and quality. Records of grout take will be documented  and a daily grout log will be reported. Olsen Engineering will use non-destructive testing for quality assurance to verify the success of the grouting operation.

Coating the Steel Liner

Through the grouting process it was identified that the protective coating on the steel liner would be severely compromised and that the welded bushings would become a permanent fixture in the outlet conduit. These bushings would have a plug inserted into the internal threads and would be capped and welded leaving a ¼” profile at all the injection points along the conduit. The coating process would involve using a dehumidifier to dry the moist air. Pumps and hoses used to divert water from a leaking head gate seal. All of this to create an optimal environment to apply the selected coating. Each injection port would have to be hand coated and sealed individually prior to rolling on the coating for the entire conduit. This is to ensure that the coating would be a complete system with no vulnerabilities. The conduit was then dehumidified and sand-blasted in preparation for the steel coating. The material was roller applied due the V.O.C of the coating it could not be spray applied in a confined space for health/ safety reasons.

This was a very demanding and technical design-build project with several  team members, structural engineers, geotechnical engineers, state dam engineers, water resource engineers, experienced structural repair contractor, experienced coating/lining contractor. This team provided a very successful project for the owner.

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