A leaking 48-in., ductile steel force main that has already been rehabilitated with a cured-in-place (CIP) liner starts to get interesting. Add a jobsite right next to a community park and you have a public works department adding urgency to the situation. This was a problem the City of Hollywood, Fla., could no longer live with.
The force main had areas where holes developed, allowing wastewater traveling between the liner and the host pipe to escape. The situation called for quick and creative problem solving.
What could be done to cost-effectively remediate this problem without service interruption? Only one answer came to mind — grouting the annular space between the liner and the host pipe with a low viscosity polyurethane resin. Lanzo Trenchless Technologies was the contracting organization and they had to prove to the City of Hollywood this would work. Of course, the solution had to be cost-effective.
Grout selection was critical. Lanzo contacted Avanti International for consultation and described the project requirements. A super low-viscosity, flexible and slow-curing grout was recommended as the best solution. The annular space was confirmed to be 1/16 in. or less, which was the reason for the low viscosity. Any movement within the annulus requires a highly flexible material to maintain its seal. Lastly, an important requirement was the grout needed to “travel” a good distance so slowing down the set time was a critical factor.
The urethane grout with all the right properties was AV 248 LV. It is a hydrophobic grout that allows for adjustment of its set time with the use of a catalyst. At 150 centipoise (about the same as motor oil SAE30 or maple syrup), it would be thin enough to penetrate the annular space. Lastly, this product has the flexibility to respond to any movement. AV 248 LV is also nontoxic, inert, non-biodegradable and contains no VOCs.
Real World Prototype
Once the product selection was complete, a mockup was required to simulate field conditions. The two main concerns of the mockup were grout penetration and the effects of the grout expansion on the liner during cure process. Lanzo Trenchless Technologies lined a pipe and tapped in a garden hose in between the liner and the host pipe to simulate the force main’s pressure. The contractor then carefully cut holes in the pipe, making sure not to penetrate the liner. Holes were cut at 12, 3, 6 and 9 o’clock to help verify full 360 degree coverage. At the other end, a very large rectangular hole was cut into the pipe just to see the effect.
To enable pressure grouting, a fitting with a pressure gauge was tapped to the pipe. A Graco 395 airless paint sprayer was fitted with a ball valve and connected to the fitting. The AV 248 LV was mixed with minimal catalyst to achieve a longer set time (target: 3 minutes).
For the proof of concept, the City of Hollywood had numerous officials onsite. Lanzo Trenchless Technologies and the field services manager from Avanti conducted the demonstration in Lanzo’s warehouse. The demo went well and yielded some valuable points of information.
Point 1: Although pressure grouting was to be limited to 5 psi, Avanti tested and discovered it took about 100 psi to deflect the liner. The grout’s expansion during curing had no effect on the liner. Cured grout between the host pipe and liner could hold back at least 100 psi, far in excess of any field pressures that would be experienced.
Point 2: Due to the long set-time and excellent penetration of the grout, Avanti lost a lot of grout from the holes. This caused Avanti to use more grout than anticipated for the proof of concept. This was not a concern for the field work because the 48-in. force main was buried underground. Thus, the grout would form a barrier in the soil preventing excessive loss of grout. In the demonstration, the grout spilled onto a tarp because it lacked containment. All the holes were sealed with the exception of the large rectangular one. The flow of water from the rectangular cut-out was slowed considerably, but too much grout escaped to seal it completely. Among those at the demonstration, there was consensus that if the pipe had been encapsulated in soil, even the large rectangle would have fully sealed. The City of Hollywood and Lanzo personnel were encouraged with the product’s penetrability and performance.
Point 3: The grout could be successfully pumped against the flow of water at low pressures. This was identified by observing grout at both ends of the demo pipe.
On the jobsite, the pipe was exposed at two locations about 75 ft apart. The first location is where a 12-in. pipe connected to the force main. This area was thought to be the source of the leak. The second location was at the area where the holes in the ductile steel pipe were leaking. Pressure was monitored at the point of injection and in various locations along the pipe. Throughout the grouting process, pressure never exceeded 5 psi. Grouting started at the first area after the dye test verified connectivity to the leak.
Avanti saw indications of grout downstream at the leaking areas. Fine ribbons of grout and bubbling were both observed. The bubbling effect was the carbon dioxide released during the curing of the grout. Even with those indicators, Avanti was not shutting down the leak and verified that the grout traveled more than 10 ft to test ports installed on the pipe in hole number one.
Avanti then moved to hole No. 2 and tapped the pipe about 5 ft up the line from the leak. Grouting at this location yielded immediate results, causing the leak to be shut down very quickly.
This project was completed in November 2012 and there have been no further leaks. The work was performed during normal working hours with no interruption of service. AV 248 LV cured into a thin, highly flexible, hydrophobic foam as expected. This is an example of a contractor working with a manufacturer to solve an urgent problem for a public works department, with an open mind for creative problem solving, and a “will” to think outside the box.
Charlie Lerman is field service manager at Avanti International.