The problem: Replacing a segment of failing sanitary sewer pipe without disturbing other buried utilities in an already-crowded easement or damaging streets, driveways, and other surface improvements above the old pipeline.

It is a challenge public works agencies across the country face every day, and frequently the solution is using one of the several trenchless technologies which permit replacement or rehabilitation of old pipe without extensive excavation.

In California, the Central Contra Costa Sanitation District turned to an innovative pipe reaming system to replace 920 linear feet of deteriorating sanitary sewer pipe in an area where conventional cut-and-cover construction could not be used.

The InneReam® pipe replacement system employs a horizontal directional drilling (HDD) machine and special downhole reaming tool to grind up and remove old pipe while simultaneously pulling new pipe into the ground.

A Different Approach

Instead of making a pilot bore and pulling new pipe into the bore hole—the usual procedure for making a HDD installation—the InneReam method uses the directional drilling machine to pull the pipe reaming tool and new pipe through the host pipe. The method can be especially effective for gravity-flow systems because grade already is established by the segment of pipe being replaced.

Another advantage of the procedure is that pipe in a system can be upsized. In Contra Costa project located in the city of Walnut Creek, 14-inch diameter HDPE pipe replaced existing 12-inch clay pipe. Pipe depth ranged from 8 to 12 feet.

Allison Sierra, Inc., Mariposa, California, was contractor on the project.

“The InneReam system was chosen for the project because pipe that had to be replaced is in a developed area beneath yards and fences and driveways,” explains Steve Allison, president of Allison Sierra. “The old pipe to be replaced also was within two feet of an 18-inch PVC sewer line. For these reasons, open-cut construction was not an option.”

Proximity to the other sewer pipe and the presence of nearby surface improvements also ruled out conventional pipe bursting. “The InneReam method,” continued Allison, “is similar to pipe bursting in that old pipe is broken up as new pipe is pulled into place behind it.”

Pieces of Old Pipe Removed

However, Allison explains that there are significant differences.

“A primary and very important difference between the procedure and conventional pipe bursting is that cuttings of the old pipe are removed during an InneReam installation,” Allison says. “As the old pipe is pulverized into small pieces, the fragments and cuttings are collected by drilling fluid being pumped through the pipe by the drilling unit and are carried by the fluid through the old pipe to a manhole or receiving pit where it is removed for transportation to a disposal site.”

Conventional pipe bursting leaves the pieces of old pipe in the ground around the new pipe.

Removing old pipe cuttings can be critical on some jobs, says Allison, especially when a system is being upgraded with pipe larger than the pipe being replaced and on projects where there are other utilities in close proximity to the installation. Displaced fragments of old pipe, along with the larger size of the new pipe, could push subsurface soils against adjacent utilities and damage them.

When installations are beneath streets, the pressure can "heave" upward, causing paving to buckle or crack, disrupting vehicular traffic, and requiring costly repairs.

Preparations for placing the new pipe in the ground included locating laterals and electro-fusing plugged or bypassed laterals. A bypass pumping system carried sewage flow during construction.

Step-by-Step Process

The Allison Sierra crew used a Ditch Witch® JT4020 directional drilling unit to push a string of drill pipe through the old segment of clay pipe, entering the pipe through a manhole. To monitor progress, a closed-circuit television camera was pulled through the pipe in front of the first joint of drill stem. The drill pipe exited at another manhole, the reaming tool was attached and new pipe connected to the tool, and tool and pipe were pulled into the ground through the host pipe. Remnants of the old clay pipe, flushed through the old pipe into the entry manhole, were removed with portable vacuum excavation equipment.

New pipe then was connected to the existing sewer line, and laterals were reinstated.

The pipe reaming tool is similar to a compaction reamer equipped with a special cutter head and a mandrel at the tail with a diameter the same size or slightly larger than pipe to be installed. Reamers are sized in accordance with new pipe to be installed.

The InneReam system can grind and remove pipes made of vitrified clay, polyvinyl chloride, asbestos cement, and non-reinforced concrete pipe and replace it with new pipe in sizes as large as twice the diameter of the old pipe. Fused lengths of HDPE pipe and assembled sections of restrained-joint PVC and ductile iron pipe can be installed by the process.

The JT4020 directional drill is powered by a 153-horsepower turbocharged diesel engine. The unit develops 40,000 pounds of pullback, 5000 foot pounds spindle torque, and spindle speeds to 250 rpm. The self-contained track-mounted drilling unit has the power to pull in main-line lengths of sewer and water pipe and bundles of duct.

The InneReam system was developed and patented by Nowak Pipe Reaming, Goddard, Kansas. Allison Sierra is a general engineering contractor specializing in the installation and replacement of underground water, natural gas, electric, and telecommunications utilities using open-cut construction, rock trenching, horizontal directional drilling in both rock and dirt, and pipe reaming and pipe bursting.