The TCH 80 and colleagues ready for work at the San Diego Metro site.

TCH Scales Retaining Walls For
San Diego Metro Station
From our May 2002 newsletter

Under the auspices of the Metropolitan Transit Development Board, a new 5.8-mile light rail transit (trolley) line is under construction in San Diego. Estimated cost for the project, which includes 4 new stations and new vehicles, is over $400 million. Each station will include public art as an integral part of the design: four artists, one for each station, have been commissioned to develop themes that reflect the history and character of the area. Completion is expected in late 2004.

Clark Construction is building the only underground portion of the new transit line—the tunnels and station at San Diego State University. Excavation work began almost a year ago and it will take at least two years to complete construction.

Digging Down One Level at a Time
We spoke with Denny Potter, Senior Superintendent of Clark, who told us that excavation work is being done in three stages—one level at a time—to a final depth of approximately 60'. To build the retaining walls, drills are being used to create overlapping shafts, and alternating shafts are fitted with H-beams. A slurry mix is poured over the opening, resulting in a seamless concrete wall.

The next step is to scale back the excess concrete to the face of the steel beams, creating a smooth surface. A Cat 320, equipped with a Tramac Rotary Cutting Head (the TCH 80) is "doing the job well and quickly", according to Potter.

The TCH gives precise control over the scaling process, important in a situation such as this where too much removal would result in voids in the surface and a weaker retaining wall. Another benefit, Potter told us, is that the fine material produced by the cutting head is easy to cart away. Using a hydraulic hammer would not only result is a less than satisfactory finished wall but would require special hauling to remove the large debris.

The Quiet One
Asked if there were special considerations on this project, Potter gave us an absolute "yes". San Diego State University is a busy college campus with classroom buildings adjacent to the construction site. This meant severe noise restrictions—another reason why the cutting head is preferable to a hydraulic hammer, as the cutting head produces much less noise and vibration.

The TCH is an easy attachment to operate. After an hour or two of instruction, the operator is able to scale a wall with great proficiency. Installation is fast and simple. The TCH is powered by the excavator’s hydraulic circuit and is interchangeable with other attachments.