Figure 44. --Truck-mounted drilling jumbo operating at the diversion tunnel inlet portal. P113-129-79A, November 12, 1958. Figure 45.--Diversion-outlet tunnel upstream from station 6+38 during excavation operations. P113-129-88, December 23, 1958. 45. Spillway. Excavation of the spillway and stilling basin (which would serve both spillway and outlet works) was started on December 16, 1958, using a 2-1/2-cubicyard crawler-mounted power shovel and two 25-cubic-yard motorized two-wheel scrapers. After most of the overburden in the stilling basin was removed, the contractor moved in two crawler-mounted wagon drills (fig. 46) and an air compressor and started to drill rock on December 22, 1958. As excavation progressed toward invert grades, material consisting of interflows and soft breccia was encountered. From approximately station 10+00 to the end, many soft seams of flow breccia were encountered. The contractor was directed not to go below grade in order to secure better foundation, but this was difficult to do in the soft material and most of the excavation was below subgrade. The excavation from the beginning of the open cut at the inlet end to station 8+50 was in more competent flow breccia material and through this section there was little overbreak in the subgrade. The height of the rock cut above the spillway outlet works stilling basin was in excess of 100 feet. To make it safe for men working under these high cuts, a wire mesh fence was placed along the upper slopes; this caught any material loosened by wind action. Figure 47 shows the spillway excavation during construction operations. All suitable rock excavated from the spillway was stockpiled and used in the dam embankment. 46. Access Road. Excavation of the access road was started on October 23, 1958, soon after brush and trees were removed from below the road. Excavation of the road was completed to rough grade before the spillway excavation was started in December. Since most of the access road was on very steep slopes, practically all excavation after the rock was shot was accomplished by 191-horsepower crawler tractors with dozer attachments pushing the material over the shoulder. The rock rolling down the long steep slopes had a screening effect in that all the larger rocks rolled to the toe of the slope. Final grading of the access road and excavation for the gutter was not completed until November 1960. 47. Dam Foundation. Stripping of the right abutment for the dam was started on October 23, 1958, but work was intermittent until December 1958 when excavation again was started on the right abutment for the cutoff trench. A stratum of vertical and overhanging columnar rock was encountered on the right abutment, which required sloping back on a 1/4 to 1 slope. Excavation for the cutoff trench on the left abutment was started on May 27, 1959, and continued through June. This excavation went deeper and was more extensive than expected. Where the walls were vertical or overhung and proper foundation for the dam abutment could not be obtained, the rock was drilled and blasted back to approximately a 1/4 to 1 slope. Stripping in the riverbed area was carried on intermittently during June. On June 29 the first river gravel was excavated and placed in zone 2. The original ground elevation in the riverbed area averaged approximately 3080. Excavation progressed well until the latter part of July when several rock isolations were encountered in the left portion of the foundation approximately at elevation 3070. In the central portion of the channel, rock was encountered at a fairly uniform elevation of 3060. Through the center of the river channel, a narrow, deep, very irregular eroded channel with vertical and overhanging walls was encountered. The contractor chose to remove these overhangs as they were exposed, so that the material could be removed with the minimum of expense even though as the excavation progressed additional laying back of the walls to the 1/4 to 1 slope would be necessary. The maximum amount of water encountered in the deep gorge was less than 1 secondfoot, and this had percolated through the river gravels on both the upstream and downstream ends of the cutoff trench excavation. Only a few very small water seeps actually occurred through the solid rock consisting of a very closely jointed hard hackly basalt. The river sands and gravels in the deep, narrow gorge above elevation 3050 were fairly loose and could be easily dislodged with a dozer ripper. The gravels below this elevation in many places were tightly cemented together and required heavy ripping to Figure 46. --Crawler-mounted wagon drills used during construction of Prineville Dam. P113-129-77D, November 20, 1958. Figure 47.--Spillway excavation on the right abutment. In the foreground, gravel is being placed over the drain pipe and the subcontractor has started placing forms for concrete. P113-129-155, May 13, 1959. loosen them. Many large boulders were embedded in the sands and gravels and had to be blasted before removal. The minimum elevation of bedrock was 3022.0 and the lowest point of foundation excavation was at elevation 3019. By September 13, the upstream portion of the old river gorge was cleaned, springs and seeps collected into one sump, and a sump pump installed (fig. 48). Placing of zone 1 embankment started on September 13. A map of bedrock in the cutoff trench was prepared as final cleaning was completed. By September 19, the downstream portion of the channel had been cleaned and a sump and pump installed at the downstream end of the cutoff trench. These two pumps were operated continuously until September 30 when the fill was approximately at elevation 3080. Considerable slow and expensive excavation was required in the many potholes of the old river gorge, as well as in the left side of the gorge. The minimum elevation in these depressions was 3042.5 and the elevation of the surrounding rock points ranged from 3063 to 3073. Cleanup in this area was done while placing of embankment progressed in the gorge. A large crevice appurtenant to the old river gorge is shown in figure 49. Before backfilling was started, the contractor sluiced the right abutment rock from approximately elevation 3200 down the slope. This material was removed with the keyway material. The right abutment was entirely stripped before excavation in the keyway was started, and final cleanup was accomplished by using air as the embankment height increased. The cleanup was kept 3 to 15 feet above the embankment. Three distinct types of rock were encountered in the dam foundation. From the bottom of the old riverbed channel up to an average elevation of 3165 on the right abutment and 3155 on the left abutment was a hard, hackly-type basalt. When shot, this rock contained numerous hairline fractures and broke down into small irregular-shaped pieces with rough jagged edges. Above the hackly basalt on both abutments was a layer of columnar basalt. This columnar rock lay in vertical cleavage planes and broke into octagon-shaped pieces 1 to 2 feet in dimension. Above the columnar basalt on the right abutment from approximately elevation 3220 to the top of the dam was a flow breccia material (fig. 50). This is an interflow material which is very soft and decomposes easily. Above the columnar basalt on the left abutment from approximately elevation 3210 to the top of the dam was a semihackly-type basalt with traces of flow breccia. The hackly basalt formed an excellent foundation; however, the columnar basalt was badly broken and had large vertical seams (fig. 51). It was therefore decided to grout the columnar section on both abutments. On the right abutment the grout cap extended from elevation 3154 to elevation 3219 and on the left abutment from elevation 3145 to elevation 3219 (fig. 62) Order for changes No. 1 covered this work. In addition it was decided to puddle zone 1 material adjacent to the columnar basalt on both abutments in lieu of hand tamping, in order to get better penetration into the rock crevices (fig. 59). Order for changes No. 2 covered this work. A total of 600 cubic yards of zone 1 material was placed by puddling. This proved very satisfactory and it is believed an excellent job was obtained. 48. Borrow Areas. (a) Stripping and Irrigating. --Borrow area A had many juniper trees, and clearing was started in November 1958 and continued intermittently as fill-in work until completed in February 1959. This borrow area was stripped during February 1959, from 3 to 4 feet of roots and grasses being removed in the operation. In April 1959 the contractor started irrigating borrow area A by the ponding method, and this was continued until August when excavation was started. Borrow area B had been farmed many years before, so there were no juniper trees, only sagebrush and grass. Stripping in this area was started in May of 1959 and completed in June. Only the top few inches was removed, just sufficient to remove sagebrush and grass. Sprinkling and pond irrigation was carried on in July, August, and September 1959. (b) Borrow Areas A and B. --The contractor started excavation in borrow area A on August 20, 1959. After passing through the separation plant, zone 1 material was stockpiled just west of the plant. This operation continued until zone 1 placing operations were started on September 13. |