Microtunneling under lake filled with twists, complications.
Standley Lake has served residents of north central Colorado for almost 100 years as a source of water for agricultural irrigation and providing water for growing suburbs north of Denver. It is also the centerpiece of a regional park.
Located about 10 miles northwest of downtown Denver, the 4-million-cubic-yard embankment that created the lake was one of the world's largest earthen dams when it was completed in 1912. Steam shovels filled rail cars with soil from land near the site, and the loaded cars were hauled across temporary wooden trestles where the loose fill was dumped to form upstream and down stream slopes of the dam.
This method of construction had been successfully employed to build other darns, but the clay and shale used here proved to be poorly suited for such a project, and the upstream slope of the dam failed the following year. Following repairs, the lake remained in use for irrigation, but the earthen structure was beset by on going problems, necessitating almost continuous repairs. As water demands increased, so did concerns about the structure, and in the mid '60s the dam was enlarged and reinforced.
But only a few years later, a longitudinal crack in the crest of the dam required construction of a stability berm across a portion of the dam's toe. Then, shifting of the unstable structure caused elongation of the four original outlet conduits and the downstream movement of the lake's valve house. Leakage from separated joints in conduits was temporarily stopped by installing internal joint seals. Foundation anchors were put in place to secure the valve house.
In July 2002, work began on the current project which is intended to rehabilitate the dam to meet current needs and anticipated demands through the end of the century. Work is scheduled to be completed by this fall.
Construction is a joint venture of ASI RCC, Buena Vista, CO, and R.E. Monks Construction Co., Colorado Springs, CO.
"The Standley Lake Dam improvement Project is unique in several aspects--we are rehabilitating an aging dam without affecting the water level of the lake or the water supply of neighboring communities," says Project Manager Lee Schermerhorn, ASI. "A significant design and construction constraint has been the need to keep Standley Lake in service during the project. The lake is a vital component of the water supply for 250,000 municipal customers, as well as downstream farms, and could not be taken out of service."
Key elements of the project include:
* Constructing a stability berm embankment containing 1.2 million cubic yards of material;
* Building a new spillway composed of 6,000 cubic yards of reinforced concrete that will release water into a new down stream channel;
* Adding eight grade control drop structures containing 28,000 cubic yards of roller compacted concrete; and
* Constructing two new 72-inch-diameter wet lake taps beneath the lake.
"The new outlets work to draw water from the lake and involved construction of a large, deep shaft; a 1,000 foot-long outlet tunnel 12 feet in diameter; and the microtunneled intakes--one 650 feet in length and the other 1,250 feet, long. The microtunnel intakes were beneath the lake at depths of 60 and 90 feet below the active reservoir with underwater retrieval of the tunneling equipment required after the completion of each tunnel," explained Schermerhorn.
Michels Pipeline Construction, a division of the Michels Corp., Milwaukee, WI, completed the microtunnel portion of the project in fall of 2002. Akkerman Inc., Brownsdale, MN, designed and manufactured a microtunneling machine specifically for the Standley Lake installations.
One of the important features of the custom machine was placing jacks on the rear of the machine to sustain the force necessary to cut through the clay stone soil, says Bill Weltin, Michels Pipeline vice president.
"As the length of a tunnel increases," he explains, "face pressure is lost because the cutting head is extended so far. So by placing the jacks at the back of the unit where they thrust off the lead pipe, we could maintain constant face pressure. Akkerman also designed the configuration of the cutting heads specifically for the clay stone type material at the lake."
ASI constructed a 35-foot inside diameter shaft 100 feet deep to accommodate the microtunneling operation and ultimately to serve as a valve vault. The shaft was reinforced with shotcrete and inside, two-foot thick walls were finished with concrete. Reinforcement was designed to provide jacking pads behind the microtunneling machine's set up positions to resist pressures of up to approximately 1,200 tons.
Both tunnels were initiated from the shaft, one directly above the other. The shaft was excavated to the maximum depth required for the longer of the two tunnels, then backfilled to the higher elevation of the shorter tunnel which was made first. After the first tunnel was completed, equipment was removed and that shaft was deepened for the second tunnel.
The microtunneling procedure was basically the same for both installations. once equipment was installed in the shaft, the tunneling machine's rotating cutting head was advanced through the ground by thrust transferred to the head through steel pipe advancing behind the head.
Tunneling moved forward, adding 20-foot sections of the 72-inch pipe as the cutting head progressed. Cuttings were collected in slurry which was pumped to the surface, cleaned of solids and recirculated to the shaft.
"The steel intake pipes have a three-quarter-inch wall thickness," says Schermerhorn. "This relatively thick pipe was selected to accommodate the high jacking pressures expected because of the swelling properties of the clay stone. The pipe provided anticipated jacking load requirements, plus a safety factor. The pipe provided both the initial and final support for the microtunnel intakes."
The 650-foot tunnel required approximately seven weeks to complete. The 1,250-foot installation was completed in approximately 13 weeks.
Each tunnel exited into the lake, followed by underwater recovery of the microtunneling machine and related equipment.
Despite the complexity of the project, Weltin says work on each tunnel proceeded without major problems, but the abrasiveness of soil conditions prompted changes to the cutting head before the second drive was begun.
"After the first drive," says Weltin, "wear of the cutter head was a major concern and our people and Akkerman decided to reposition the teeth, use different types of picks, and reposition the rakers. We made the changes on site, and they proved to be a tremendous help during the second, longer, drive."
Recovery of the tunneling machine after the completion of each drive was itself a challenge.
"These were typical underwater retrievals," says Weltin. "An air lock is prepared behind the machine and all umbilicals to operate the unit are removed. That can be the trickiest part of the job--you don't want the machine to come off the end of the pipe before we are ready. Once the machine is totally enclosed or encapsulated, it is pushed into the water; divers connect cables, and a crane lifts it out of the water."
Final lining for the outlet tunnels is welded steel conduit 102 incites in diameter with wall thicknesses of one-half inch. External stiffeners add hoop strength to the steel liner pipe. Annular space between primary and final supports were filled with low-density cellular concrete.
The 1,250-foot drive is one of the longest ever made to install 72-inch pipe. "It went very smoothly," concludes Weltin. "Had the project required it, we could have gone much farther."
Standley Lake Dam Rehabilitation Project At A Glance
Project Owner: Cities of Westminster, Thornton, Northglenn, CO, and the Farmers Reservoir and Irrigation Co.
Engineers: CH2M Hill, Denver, CO; GEI Consultants, Englewood, CO; and Tetratech RMC. Longmont, CO.
General Contractor: Joint venture of ASI RCC, Buena Vista, CO, and R.E. Monks Construction Co., Colorado Springs, CO
Microtunneling Contractor: Michels Pipeline Construction, Milwaukee, WI
Type of Dam: Earthfill
Height: 80 feet
Crest Length: 8,500 feet
Akkerman Builds Special Machine For Dam Renovation
Word of the Standley Lake Dam rehabilitation project was making the rounds among contractors and suppliers in the microtunneling industry long before plans were completed and it came out for bid.
"We had heard the job would require a large diameter machine with long drives through rocky conditions with the added challenge of having a 'wet' retrieval of the microtunnel boring machine (MTBM)," says Carl Neagoy of Akkerman Inc., Brownsdale, MN. "There were a lot of interested parties, and Akkerman was tracking the project like everyone else."
Michels Pipeline Construction, Brownsville, WI, got the contract to make two MTBM installations of 72-inch diameter intake microtunnel lake taps.
"Michels is a good customer and owns Akkerman microtunneling machines and operating systems, and we were pleased to be asked by Michels to provide a machine for this project," Neagoy continues.
Neagoy and Akkerman President Maynard Akkerman met with Michels' Pat and Kevin Michels and Bill Weltin to discuss MTBM requirements for the project.
"The original idea was to up size a previously-built model to accommodate the Colorado project," says Neagoy. "Bur plans often change and this one soon did, too. Michels personnel decided that the machine should be more powerful. So we up-sized the motor from the originally specified 150-horsepower, water cooled, electric main drive motor, to 200 horsepower, and ultimately to 250 horsepower. Now the MTBM had become a new design and was still evolving."
The next challenge was changing the overall length of the machine. Ordinarily, Neagoy says, a MTBM is a two-section machine Because an exit shaft would have to be constructed in the lake bottom, at depth, the plan called for as small a hole as possible. This led to the next design change in the MTBM.
"Instead of two sections of machine measuring approximately 25 feet in total length, Neagoy continues, "there would now be only one shorter section MTBM in order to fit it into the small reception shaft and be able to pull it out of that 20-foot exit hole."
Another issue, primarily relating to concerns about the geology and behavior of the anticipated ground conditions, was that the MTBM should have an independent form of advancement control at the heading.
"The geology indicated a type of clay stone that could exhibit squeezing tendencies, especially when exposed to water," explains Neagoy. "Because one of the drives was long, there was concern that the main jacks might not be able to be controlled with enough finesse over the long length of tunnel, because the squeezing ground might impede progress and lead to a surging motion, possibly plugging the cutting face or damaging the cutting wheel.
"A decision was made, to put a 'jacking can' (basically a sealed intermediate jacking station), in line directly behind the MTBM, on the front end of the tunnel. This would allow the operator more precise control of advance rates of just the MTBM, and the tunnel could be jacked up behind it using the main jacks. The jacking can would also require a hydraulic power source in the heading, to avoid the need for long oil lines up the tunnel from the shaft and the heat and potential environmental mess that they could cause."
To eliminate the second trailing section portion of the MTBM, and also to be able to position a hydraulic power pack close to the MTBM jacking can, Akkerman engineers built some of the components onto two sleds that would ride inside the jacking pipe behind the MTBM. The tunnel booster pump was mounted on one of the sleds, along with some of the control electronics, and the hydraulic power pack was mounted on the second sled. At the end of each drive, the sleds were simply pulled back out of the tunnel when the slurry lines and power cables were stripped out. Then all that was necessary was to close off the bulkheads to permit the wet retrieval of the MTBM."