From stabilizing carrier pipes, to refurbishing water lines, to trench backfill, pipe abandonment and shaft backfill, cellular concrete is changing the tunneling industry.
When grouting relined or new pipelines, tunneling professionals face the risk of pipe collapse, as well as the challenges associated with pumping long distances and uplift force. Over the last few years, pipeline professionals have installers have looked to the unique characteristics of cellular concrete to help solve these issues.
Using Cellular Concrete
Cellular concrete contains type I-II-V Portland cement, water and preformed foam to make a “flowable” grout. Specialty contractors can create grout with a density varying from 25 to 65 lbs/ft3 with a 28-day compressive strength of 40 to 1,000 psi. Using a mobile batch plant to mix the material at the job site, they produce a base neat cement slurry.
“We then mix it with a liquid preformed foam, which has a similar consistency to shaving cream. The foam is injected into the base slurry and varied in volume based at the desired density for the project,” said Louis Fisher, President of Cell-Crete Corp. Based in Monrovia, California, Cell-Crete is a leader in the poured cellular concrete market in the western United States.
This foam creates a cellular matrix of air bubbles in the neat slurry that is stable enough to withstand the hydration period. Once the cement hardens a uniform matrix of air voids is left behind which provides excellent compressive strength and a relative low density.
“The foam mixed with the cement-water paste is thixotropic – it’s thick, or viscous when static, but will thin, or become less viscous, and flow when shaken. It’s a bit like ketchup – you have to shake that bottle up so it can flow,” Fisher said.
The important aspect of air entrained grout like this, aside from unit weight, is the time the grout is available to stay fluid and mobile. Once material starts to set, pressures can spike, applying excess stress on the carrier pipe and potentially damaging it. As long as an external stress is applied, the grout will remain fluid.
This cellular grout mixture has near self-leveling characteristics. The annular space fills almost like water would fill a glass. This makes it easy to predict how the grout will behave and ultimately easier to account for buoyancy.
“Because cellular concrete has a long working time before set and easily pumps long distances, due to the absence of sand/aggregate in the mixture, it is ideal for many annular grout projects,” said Diego Villegas, Engineered Fill Manager at Cell-Crete Corp. “Its low density greatly reduces the buoyant force and risk of damage on the sliplined pipe due to high pumping pressures when grouting through grout ports.”
Much of the waterline infrastructure in Southern California built beginning in the 1930s used pre-stressed concrete cylinder pipes (PCCP). This helped minimize the use of steel and prevent corrosion, plus PCCP provides superior inner pipe strength.
With that said, after 85 years of service in some cases, the concrete pipes have started to show wear and are near their full design life. Knowing this, many municipalities are taking steps to reinforce, or reline, existing infrastructure and build new pipes to accommodate population growth.
The following projects illustrate the varied and successful cellular concrete applications as a backfill material to grout the annular space between the existing and new carrier pipes.
San Diego County Water Authority
Like a majority of the water pipeline infrastructure in Southern California, the San Diego County Water Authority’s system utilized pre-stressed concrete cylinder pipes (PCCP). Relining the pipeline, involves inserting new steel liners into an existing pipeline.
This sliplining method is a faster, more cost-effective alternative to excavating, removing and replacing an entire pipeline. This project required a collapsible steel liner that was brought into the existing pipeline in pieces and welded together inside of the pipe.
Adding complexity, this pipe can only withstand 3 to 5 psi of pressure at the grout ports and the annular space is minute, roughly 1.5 to 3 in. all around. In addition to this, the pipe elevation also varies from 2 to 20 percent, potentially causing the pipe to become buoyant during grouting. The elevation changes also affect the head pressure of the grout and adding too much surcharge load could cause the pipe to collapse entirely.
Cellular grout provided the perfect solution to these challenges. It has a much lower density than conventional grout, thus reducing the buoyant force acting on the carrier pipe. It also has better pumpability due to the absence of sand/aggregate in cellular concrete, thus reducing the frictional resistance pumping through the concrete hose and overall reducing the pumping pressure. And finally, cellular grout can go the distance as there is less risk of material separation and plugging the lines as with conventional grout.
To date, the authority has relined more than 33 miles of PCCP within its service area and is anticipating rehabilitating the remaining 58 miles of PCCP by 2027.
River Supply Conduit Improvement
The Los Angeles Department of Water and Power is building new infrastructure to boost capacity and meet updated state standards for water pressure. The new infrastructure will also enhance system reliability, minimize air entrainment to improve water flow, and widen operational flexibility within the distribution system.
Cellular concrete is being used to grout the annular space between the new tunnel, which has a 108-in. diameter, and the new steel carrier pipe, which has a 78-in. diameter. There are several tunnels that vary in length, with the longest tunnel a total of 6,400 lf.
What makes this job particularly interesting is that the tunnels will be grouted through the inside of the tunnel, through grout ports, while the stulls remain in place.
“With cellular concrete we can keep the pressure low at the ports, and grout the line in fewer lifts since the lower density of the material exerts less uplift force on the pipe when grouting,” Villegas said.
In addition to this, DWP elected to use a 1,000 minimum compressive strength cellular concrete material for this project with a shrinkage less than 0.03 percent as a requirement.
“It is crucial to reduce the shrinkage of the annular grout material to ensure intimate contact between the tunnel lining and the host material. This will prevent voids from forming after grouting due to shrinkage,” Villegas continued.
“We grout in two lifts per 200-ft section through 2-in. grout ports inside of the pipe, which never exceed the maximum allowable injection pressure of 5 psi,” he said.
“As an added benefit, due to the very minimal shrinkage in cellular concrete there was no need to go back through the pipe and contact grout after the cellular concrete material had set,” Villegas finished.
Flowing into the Future
“All of our offices across the western United States have seen an uptick in this type of work. We are using cellular concrete across the nation in ways we never imagined 20 years ago,” Fisher said.
“Take trench backfill for example. Using soil, backfill can only occur in 6-in. layers which then needs to be compacted afterward. With cellular concrete, you can backfill 4 to 5 ft at a time and use small batch plants onsite to follow the trench,” Fisher said.
Villegas and Fisher also suggested pipe abandonment as another area cellular concrete can and should be used more often.
“The lower density material combined with the flowable texture, can safely and effectively stabilize an abandoned pipe to withstand the load on top – ultimately avoiding disasters like a road or structure collapsing. It also prevents the lines from transferring harmful contaminants, or soil subsidence,” Villegas explained.
And finally, it’s the ideal material for shaft backfills.
With its lower density, strength, flow ability and low cost, cellular concrete is replacing soil and other sanded slurry mixes as the ideal solution for many challenges in the tunneling industry.
This article was submitted by Cell-Crete Corp.