2014 Issue

26 2. 120” Olmsted & 90” Alpine-1 Pipes at Slow Moving Landslides During 1999-2000 planning and predesign of Olmsted Pipe Reha- bilitation, the 1000-foot wide activeWicks Landslide (Figure 5) was identified and studied. Maintenance records and photos showed ongoing shear at both edges of the slide through the pipe’s 90- year life. Inclinometers found Wicks Slide slip-plane 95 ft down, shearing at 0.2 to 0.3 in/yr. In 2000, 3 miles downstream, the Al- pine-1 Pipe broke for the third time in its 19 year life. A city- owned, parallel 20- inch pipe, 1000 feet uphill had also bro- ken 4 times due to ground movement in the same period. The Alpine-1 breaks occurred 4, 8 and 19 years after construction, each at a different welded joint, each break 80-feet apart. Because the first two breaks occurred in severe wet periods, they were deemed hydrology- induced, one-time events. But when the last break occurred in a dry period, additional investigations were made. Inclinometers found the Alpine-1 Slide slip-plane 75 ft down shearing at 0.2 in/ yr. This landslide was mapped about 280-feet wide. Because the low head pipe on girders and skids already had a 50 year success record on Wicks Slide, when the pipewas upsized, it was rebuilt with flex, rubber joints (photos below). Thrust harnesses restrained joints axially andbumper plates restrainedgirder feet laterally. The low-head (12 psi) Alpine-1 replacement pipe used a similar design. Alpine-1 90” Pipe Break Findings. This zero slope, straight steel pipe broke four places in the 280-ft wide slide: at a west-edge lap weld, at two center lap welds, and at an east-edge Figure 5. Wicks Slide outlined in photo. Figure 6. 120 inch replacement pipe at Wicks Slide. Figure 7. Expansion joint at Wicks Slide. Figure 8. Failed mortar coatings and linings focused corrosion at stressed welds. Figure 10. Talus buildup was jacking pipe off skids. Figure 11. Soil nail wall uphill of 120-inch buried replace- ment pipe Figure 9. The above section shows Wicks Slide as a small active area on a ½-mile deep x 2-mile wide inactive (stable) slide matrix. Figure 12. Several options were considered to reduce system risks in crossing Blacks Slide. Figure 13. The shallow slide, hard rock outcrops, and options to reroute water all reduced system risks. flex-joint that pulled apart. When the last single lap weld broke, the pipe was dug up and surveyed. It had moved 6.5 inches at the slide’s center - bent like a beam with the west end “fixed” and the east end “pinned.” If the pipe were above ground, the 6.5-inch deflection would have caused only one or two weld breaks, but the uneven ground forces caused four breaks. Lessons learned: 1) uneven forces that bend buried pipes warrant large safety factors, and 2) bending disbonds mortar linings/coatings and accelerates corrosion at stressed welds (see Fig. 8). 3. 120” Olmsted Pipe at 2-miles of Inactive Slides and Talus Slopes The Olmsted Pipe crosses 2-miles of inactive landslides, (see Fig 9) which in an extreme (500yr) wet period, were stable except for ta- lus buildup on the pipe which, com- bined with thermal movements, was jacking the pipe off its supports (Figure 10) right). Both tun- neling and horizontal realignment options were cost-prohib- itive, so CUWCD chose to rebuild the pipe below ground. In order to perma- nently stabilize slopes and to allow for space for construction, the steep 1.5:1 and 2:1 cross-slopes were soil-nailed 30-feet deep uphill of the pipe (Figure 8) for two miles. 4. 96” Spanish Fork Canyon Pipe at Recently Active Blacks Slide CUWCD AQUEDUCT | continued from page 25