The Upper Stillwater dam, located in Duchesne County, Utah, US, was completed in 1987 after a four year construction period, using 1.1Mm3 of roller compacted concrete (RCC). The dam, operated by the US Bureau of Reclamation (USBR), was constructed with no transverse contraction joints and the RCC began to exhibit cracks at stress points throughout the dam. The cracks were continuous and irregular from the upstream to the downstream face of the structure. Table 1 shows the recorded crack data.
The cracks had been grouted previously with hydrophilic urethane materials, but this method had failed. As a result Mountain Grout Ultra (Ultra), a moisture activated/cured polyurethane grout, was used to repair the transverse cracks.
The project consisted of remediation of leaks through a 70m high existing RCC dam via polyurethane injection using 95,466 litres of Ultra. The general contractor was ASI RCC Incorporated, while the sub-contractor was FEC (Nicholson). Ultra is a single component (with accelerator) hydrophobic polyurethane that possesses the physical properties much the same as traditional hydrophilic systems. Ultra was successful in stopping the leaks and this portion of the project was completed well in advance of schedule.
Hydrophobic polyurethane system
Mountain Grout Ultra, manufactured by Green Mountain International, Inc. was chosen for this project because of its unique properties that combine the benefits of a hydrophobic and a hydrophilic chemical grout. Ultra is technically a hydrophobic system in that it takes very little moisture to initiate the chemical reaction and the resulting elastomeric material will maintain its form for the life of the product. It will not shrink during or after cure regardless of the presence or absence of moisture. Ultra has up to 400% elongation making the system as flexible as most hydrophilic systems. It is also considered environmentally safe to apply and completely inert when cured. All Mountain Grout Polyurethane Systems are solvent free and are shipped as non-hazardous per USDOT guidelines for ground, sea and air.
Ultra is within the chemical family of Polyisocyanate (Synonmous with Methylene Diphenyl Isocyanate (MDI)). Previous experience by the USBR using a TDI type grout and the MSDS GM7312 criteria developed concerns for the levels of exposure the MDI product may present during the mixing and grouting operations within the 1.8m wide, 2.7m high gallery of the dam structure.
ASI implemented an individual monitoring plan to monitor and assess the levels of risk presented during the grouting operations in accordance with USBR requirements. ASI provided supplemental ventilation in accordance with ventilation plans to provide additional airflow in the grouting areas. Technical representatives from Green Mountain International were also present during the initial startup and grouting operations within the gallery.
The supplemental ventilation enhanced the existing ventilation within the dam, substantially increasing the velocities and volumes of air passing through the gallery. Two additional fans were installed to increase the air flow three fold.
The objective of the ventilation and personnel monitoring programme was to maintain exposure levels at or below the PEL mandated byOSHA of .005ppb on an eight hour average work shift The difficulty of this requirement is polysiocyanates are odorless and do not register or impact a P100 vapor/mist respirator cartridge regales if the cartridge has an end of life indicator.
The additionally required that the company needed to adopt a regular change out schedule for changing the cartridges even if they were still able to function on an adequate level.
On 29 October 2003 ASI-RCC and FEC began grouting at the Upper Stillwater dam gallery at station 42+85, using the Ultra two component mix (grout and accelerator). Because Ultra contains the chemical diphenylmethane diisocyanate, concern was specially focused on the potential for the vaporization of the grout mix during both the mixing process and the addition of the accelerator. An additional concern was the exposure levels to grouting personnel within the limited confines of the dam gallery when grout discharged from the cracks within the structure into the gallery.
The concern was more acute when the process was performed within the dam gallery’s confined space, based on the information witin the Moutain Grout MSDS. A key parameter to observe was the air quality within the confined space which was addressed by hiring a ventilation engineering company to conduct a through study of the existing facilities ventilation system. Considerable additional ventilation of the gallery was required based on the parameters identified within the materials MSDS.
Co-ordinaton with Green Mountain resulted in the use of MDI monitoring badges (safe air) as manufactured by K&M Environmental, attached directly to the employees involved in the mixing and injecting the product into the crack zones via drilled holes from designated stations along the axis of the dam. The Safe Air monitoring badges provided the means to establish a baseline MDI exposure for the project.
During the initial grouting activities, the mixing station was located outside the dam and materials were hand carried to the pump/injection area, while concurrently directing the interior airflow from the grout station, pumps and employees. A suction fan was placed directly over the grout pumping equipment. This combination of outside mixing and ventilation management provided a further reduction of employee exposure due to possible MDI levels anticipated or possible within the gallery area.
ASI/FEC worked the initial grouting shift from 0830 to 1600. Only one Safe Air badge registered any MDI results and that particular badge had a finger print right on the area of the badge reserved for colour comparison. It was obvious that during the grouting procedure the employee had touched the badge with his glove which contained mixed grout. The badge registered a 2.5PPB colour comparison which is one half of the eight hour PEL of 5PPB/hr
The second day of grouting, because of the low to non-existent exposure levels of the day before, the mixing station was moved into the inspection gallery at the area of the adit/inspection gallery intersection. Again, all personnel were fitted with with half-mask respirators and MDI monitoring badges and checked on a regular basis for any indicators from the colour indicating badges. The air was monitored as before for velocities and quality prior to employees beginning the task.
Procedures
From within the gallery, a series of bore holes were drilled in a fan pattern to intersect the existing cracks. Injection ports were especially designed for the large amounts of grout that needed to travel through the cracks. The angle of the ports, when staggered, produces the fan pattern.Drill holes ranged in depth from 15.5m to 54.6m.
Drill holes were flushed with water prior to pumping the hydrophobic polyurethane. The flushing provided several benefits:
• Flush out residual mud and organic particles
• Provide moisture to fully react the grout.
• Flush out previous grouting materials (hydrophilic)
• Verify intersection of cracks
The Mountain Grout Ultra was pumped through a Graco Bulldog unit. Pumping pressure was limited to 900 psi. Each drill hole was pumped until the Ultra was observed on the face (both sides) or until the backpressure on the pump reached 1000 psi.
Material handling
The average temperature in Duchesne, Utah for January is 19ºF. The elevation of the dam is 2438m. The Upper Stillwater dam is 64km northwest of Duchesne with surrounding mountain elevations in excess of 3962m. The weather at the site is harsh from November to May. In consideration of these conditions special handling and storage arrangements were implemented by ASI-RCC. Typical quantities shipped monthly between November and April were 1500 to 2000 gallons or 30 to 40 55-gallon drums. The Accelerator was packaged in five-gallon containers. Materials shipped from the Southeast were in heated trailers maintained at 55 to 70ºF. The drums were then transferred to an electrically heated 40-foot shipping container insulated with Styrofoam sheeting one inch thick. The temperature within the container was maintained at 60 to 70ºF. Materials were transferred to the grout pump station within the gallery as needed. When pumping was outdoors, insulated blankets and electric band heaters were utilized to maintain proper temperature.
Polyurethane resin systems are easier to pump at temperatures above 60∞F, which keeps viscosity lower and promotes reactivity.
Results and testing
On-site BOR Quality Control Personnel conducted extensive monitoring. Daily logs of grouting quantities and quality were maintained. FEC/Nicholson conducted extensive coring for verification purposes. Below is a photograph of a sample core.
Large fractures were observed in many of the cores. The pale yellow material in the photo is the polyurethane grout. The polyurethane varied in width reaching 1-_ inches (38mm) within this sample. The sample proved the superior bond strength of the hydrophobic system with no observable shrinkage. Early core samples contained latent mud, organics and “chunks” of previously installed hydrophilic grout material. The material supplier recommended flushing each drill hole extensively prior to pumping the Ultra grout to wash out as much of this latent material as possible. This proved to be appropriate advice.
Multiple cores were taken from each injection point to verify the presence, bond and physical form of the reacted polyurethane material. Upon return of the water to the reservoir, no leaks in the grouted zones appeared.
Author Info:
For further information, contact:
Green Mountain International Inc., 235 Pigeon Street, Waynesville, NC 28786. Tel: +1 800-942-5151 Fax: 888-632-5360
ASI RCC, Contractors/Engineers, 28221 County Road 319, Buena Vista, CO 81211
TablesTable 1 Table 2 Table 4 Table 3 Table 5
