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The Final Plan

We submitted a revised plan to the city engineer calling for 100 feet of 6-inch HDPE (high-density polyethylene) pipe running the same path where the swale was supposed to be, with a single 12-inch by 12-inch catch basin on the low spot between the two properties. According to our engineer, that system would handle the water from a 50-year storm event. That seemed reasonable to us, since even if such a storm occurred there was no danger the water would flood into either house — excess water would simply run across the yard of the lower house before reaching the street.

Revising the revision. The plan was rejected. The city engineer wanted the system to be designed for a 100-year storm instead, which has a 1 percent likelihood of taking place any given year. Although that’s a typical standard for dams, levees, and storm sewers, it seemed like a lot to expect of a yard drain. But because I’ve found that maintaining a good relationship with city officials usually pays off in the long run, I agreed to try again. Our second plan was for two 12-inch by 12-inch catch basins and a fusible 8-inch HDPE drain tile; it was approved.

Not your father’s drainpipe. Fusible HDPE drain tile is heavy-duty stuff that has little in common with ordinary corrugated drain tile. The 8-inch pipe we needed has walls nearly an inch thick, which is necessary to allow it to withstand the tremendous pressure of being pulled through the soil. It came in 50-foot lengths that cost $400 apiece, and even after I had them cut in half at the plumbing supplier so I could bring them to the site on my ladder rack, each 25-footer weighed 200 pounds. While I was at the supplier, I also picked up the fusing machine needed to weld the sections together.

Boring the Hole

When I showed up at the job site, the excavator’s two-man crew had already set up the boring machine and was making good progress (see slideshow). For this job, Goldade brought out a Case 6030, which is considered medium-sized. The 6030 can push or pull at 30,000 pounds of pressure and is capable of pulling a 14-inch pipe more than 1,000 feet.

While running this equipment efficiently takes plenty of experience, the basic concept of the boring machine is straightforward: A drill head is attached to the end of a steel drill rod (2 3„8-inch, in this case). The rotating rod is pushed through the ground until it reaches the end of its length, then another rod is attached, and the process continues. Water from a tank mounted on a vacuum truck is pumped through the rod to loosen the soil, and the excess water and mud are sucked into a tank on the rig.

Steering the bit. The spinning drill head tends to go in a straight line until it hits a large rock or other obstacle. If the obstacle isn’t too big, the bit will often spin it aside and keep going, but this can cause it to veer slightly off course. A transmitter built into the drill head allows a crew member with a hand-held monitoring device to determine its precise location at any point along the way.

If the bit does start to wander, the solution is to turn off the rotation and drive the bit forward without it. Because the drill head is shaped at an angle, advancing it without spin will cause it — and the drive shaft behind it — to move right, left, up, or down, depending on its orientation. The positioning information is relayed back to the boring-machine operator, who adjusts the controls as required to “steer” the drill head toward the target. (More sophisticated machines used on public works projects and other big jobs are sometimes computer-guided by means of GPS coordinates.) On this job, the drill head surfaced within a foot of the marker stake.