A. SPECIAL TREATMENT 23. SILTSTONE ZONES. During excavation of the keyway for the left abutment of the dam, a zone of poor-quality siltstone was uncovered. The fractured siltstone zone covered an area between elevations 3635 and 3650, and terminated about 15 feet downstream from the toe of the dam. To prevent further deterioration of the siltstone and protect the toe of the dam, a concrete slab pinned to the abutment with 1-inchdiameter anchor bars, spaced on 4-foot centers, was placed over the area. The protective slab was warped to blend in with the concrete fillet at elevation 3635. 24. ANCHORAGE OF STRESS RELIEF JOINTED ROCK. Rock conditions, revealed while excavating the abutment keyways, indicated that stress relief joints on both abutments immediately downstream from the toe of the dam would require remedial treatment to eliminate potential rockfall hazards. Relief joints on the right abutment were controlled by installations of anchor bars to pin large masses of rock to sound abutment rock. Rock bolts to stabilize surface rock were also required. Anchor bar and rock bolt installations were made after loose or badly jointed rock was first removed by blasting and scaling. Subsequent to these installations, B-hole grouting was performed to seal all joints with grout. Three-inch-diameter drainage holes were then drilled to minimize hydrostatic loads that might develop behind the rock mass. Stress relief joints located in the left abutment between elevations 3590 and 3640 were stabilized by installing rock bolts, where required, during progress of the work. 25. RIGHT ABUTMENT SLIDE AREA. In November of 1961, shortly after the access road to the crest of the dam (right abutment) had been excavated, a minor slide was noted in the overburden above the road cut. The slide was located on an area above and downstream from the dam, and developed primarily because of the removal of toe support through construction of the roadway. A larger landslide developed in July of 1963 and in May of 1964. The slide in May of 1964 involved an area 400 feet long and 200 feet wide, and had a height of about 75 feet between the uppermost scarp and the toe of the movement. These latter slides were initiated as the exposed rock formation (basal Amsden) on the roadway cut, gradually deteriorating through air and water-slaking effects, began to slough and initiate massive movements higher up in the embankment. Additional movement, if permitted to continue, would have involved the highvoltage and control cable shaft and cut-and-cover section, the access road, and the visitors' center complex. Progressive movement could also have undermined a portion of the switchyard. Remedial treatments involving subsurface drainage, general resloping, and construction of a toe support retaining wall were developed to stabilize the area. The first step in stabilization of the slide area was to construct the subsurface drainage system shown on figure 14. Two 5- by 7.5-foot drainage tunnels were excavated beneath and slightly beyond the slide area. Three-inch-diameter drainage holes, spaced on about 10-foot centers, were drilled upward from the roof of the tunnels to within about 10 feet from the top of the rock surface. The drainage holes were designed to intercept most of the ground water flow before it could enter the main mass of the slide. The second phase of corrective treatment consisted of general resloping of the slide area to as flat a slope as practicable, placement of a 12-inch impervious soil layer below the toe of the resloped surfaces to prevent surface water from entering the main mass of the slide, and construction of drainage ditches to collect surface water. Survey lines to monitor movements were installed on the flattened slopes of the slide. No movement along any of the survey lines has been recorded to date. Although the main mass of the slide remained stable, localized sloughing of the toe continued, and if untreated could have conceivably initiated large movements in the main body of the slide. Toe stabilization was achieved by constructing the concrete walls and rock buttresses shown on figure 15. Most of the toe was stabilized with reinforced concrete panels held in place by a series of grouted rock bolts anchored in the Madison limestone. The panels and their rock bolts were designed to resist a rectangular loading pattern of 16,700 pounds per linear foot, acting along the total height of the panel. The load was developed by assuming a Swedish slip circle type of failure on the toe of the slide, and determining the net driving force along the failure plane. The basal Amsden material in which the slip circle failure plane was assumed to have developed was assigned the following properties: |