The mountainous terrain of northeastern Utah is home to a small earthfill dam which impounds water for local agricultural purposes. When potentially damaging underground water seepage was discovered, a geotechnical consultant was retained to investigate the problem and offer a solution. Because the site geology consisted of layered and weathered rock formations, a drilling-based method was chosen to create a cutoff wall beneath the dam.

The final design recommendation was for a secant pile wall: a series of 20” diameter overlapping shafts which, when filled with concrete, would provide a continuous 8” thick wall. Drilling depth was designated at 60 ft. to insure imbedment into a competent/sound rock formation.

Work on site began by scraping and cleaning the exposed rock face along the 470 ft. wall alignment and then placing a 12” thick concrete work pad. A 6” x 24” trough was formed in the middle of the pad to serve as a guide for the drilling. At the high end of the dam abutment, a winch and cable system was installed to assist and support the drilling rig as it moved up and down the 3.5:1 slope.

The drilling rig components consisted of 36” box leads, hydraulic spotter frame and a rotary top drive unit, all of which were mounted on a 120 ton crawler crane. The down hole hammer was connected to an 18” diameter x 100’ long drill pipe to allow hole drilling in a single pass by using direct air circulation. A bank of air compressors, stationed on site, provided sufficient air volume and pressure to maintain uphole velocity. A horizontal diverter pipe at ground level channeled the flow of cuttings directly into an adjacent spoil pile.

The drilling process started with alternating “primary holes” drilled to the design depth. Verticality was measured at 10 ft. intervals by an inclinometer mounted on a custom-built, non-magnetic sled. After circulating high pressure air for hole cleanout, 3,000 p.s.i. concrete was placed and allowed a pre-determined cure time. Next, the “closing holes” were drilled, overlapping the primary holes by 12” minimum. A special down-hole camera was then used to view the exposed concrete surface. With verification that the minimum 8” continuous overlap (i.e. final wall thickness) would be achieved, the closing holes were then filled with concrete. A total of 353 holes were drilled to complete the 28,000 sf. installation.

Keys to the success of this project were:

• Control of hole/wall alignment by concrete guide trench.
• Control of drilling verticality by use of fixed leads.
• Verticality verification by inclinometer.
• Wall thickness verification by remote video camera.
• Continuous support of rig travel on the sharply-sloping work platform with cable and winch system.
• Use of down-hole hammer and full length drill pipe for optimum drilling performance.