Gluing the Dowels

After drilling the holes, we used compressed air to blow out the dust before setting the dowels in epoxy. We used an epoxy cartridge system from Covert Operations (1940 Freeman Ave., Longbeach, CA 90804; 310/986-4212) to meet the strength requirements called out by our engineer. We found both the gun and cartridges easy to work with. The epoxy did not generate heat as it set up, and did not emit the foul odor I’ve encountered with other brands. (This last point may not seem like a big deal, but the smell from some epoxies I’ve used stuck to me like glue. When I started noticing that people were looking funny at me when I was in a crowd, I decided to switch brands.) We injected an inch or so of epoxy into the hole, coated the portion of the dowel that would be embedded, and inserted the dowel in the hole, working it up and down to get a good coating on both the dowel and the walls of the hole. Then we removed the dowel, injected another inch of epoxy in the hole, and reinserted the dowel. Properly embedded dowels would show signs of epoxy squeeze-out. We averaged one tube of epoxy for every 20 dowels. After the dowels were in place, we tied a continuous run of #4 rebar to the dowels. We used a hand-held rebar bender to shape the bar to the contour of the rock before tying it off 4 inches above the ledge. With the footing bars in place, we began building the inner wall forms in the same manner as the outer wall forms, installing form ties as the inner form went up. Form tie spacing varied between 18 inches and 4 feet, depending on the height of the wall. Bracing the Forms

Bracing our formwork to the surrounding rock called for a little ingenuity. Drilling holes for angled bracing stakes proved too troublesome, so we created a bracing system using MST framing straps, 2x4s, and expansion bolts (Figure 4).

Figure 4. Simpson MST framing straps and 2x4 strongbacks, anchored with expansion bolts, braced the forms to the irregular rock outcroppings. The 2-inch-wide framing straps are manufactured by Simpson Strong-Tie (4637 Chabot Dr., Suite 200, Pleasanton, CA 94588; 800/999-5099) from 10-gauge galvanized steel; they’re typically used to resist lateral movement. We nailed one end of the framing strap to the form, and placed the other end on the surface of the rock, pounding it with a hammer to shape it to the configuration of the rock. We drilled a 1/2-inch-diameter hole 21/2 inches deep in line with the framing strap hole and inserted a Rawlplug tap-in expansion bolt (Rawlplug Co., 200 Petersville Rd., New Rochelle, NY 10801; 914/235-6300). To add rigidity and compressive strength, we nailed a 2x4 strongback to the strap through the prepunched nail holes. After stripping the forms, we were able to straighten the straps and reuse them for the framing. Preparing the Piers

This project also had 13 pier footings that needed to be doweled to the ledge. Our engineer specified that a flat surface be jackhammered into the sloping rock where the piers were located, and that two dowel pins be provided per pier (Figure 5).

Figure 5. For pier footings, a flat surface was jackhammered into the ledge and two epoxied dowels inserted.

We scribed the forms to the rock and held them in place in the same manner as the wall forms. Utilities and Drains

Care should be given to protect pipes and wires from abrasion when bringing utilities into a foundation that is built on top of rock ledge. We wrapped all water lines, sewer lines, and conduit with Flex Wrap (WrapCo, P.O. Box 8043, Citrus Heights, CA 95621; 916/723-2121), a closed-cell foam that protects against puncturing or cracking (Figure 6).

Figure 6. To protect against puncture and cracking, all utility lines were wrapped with a closed-cell foam wrapping (at left in photo).

We used 4-inch perforated drain pipe in a bed of 3/4-inch stone for seep water, and placed a 6-inch rigid drain line on the high side of the house to pick up runoff water from the hillside above and direct it completely around the house. Swales at critical points directed runoff to intakes in the 6-inch drain line, and all drain outlets exited onto rock slabs to prevent erosion problems. Added Costs

We drilled more than 300 holes to prepare for a foundation that required nearly a 100 yards of concrete. Forming to a rock ledge can increase costs by 30% or more when compared with a typical "dug" foundation. In our case, the excavation costs were lower but labor was 30% higher and materials about 20% higher. I found building on rock ledges as satisfying and rewarding as the time I spend climbing on them. It’s not that we conquered the mountain; rather that with a little vision, ingenuity, and patience, we achieved a goal worthy of standing back and saying, "Been there, done that."