Technical Record of Design and Construction

contribute to possible instability of rock masses if the support of the underlying shale zones were removed. To safeguard against weathering of certain of the shale zones and consequent undercutting of rock masses it was decided to construct concrete protective walls.

In all, six protective walls were constructed for the protection of the following shale

Zones:

Numbers 9L, 10L, 6R, 7R, 9R, and 14R

All of the walls, with the exception of the 9L wall, were designed and constructed for the protection of the shale zones in the areas downstream of the toe of the dam. The 9L wall is located upstream of the spillway intake structure on the left side of the intake channel, extending upstream until the shale zone dips below the floor of the channel.

All of the walls were designed to a minimum thickness. Anchor bars, 1-3/8 inches in diameter, were utilized wherever possible to increase the stability of the walls. Drainage systems were installed to take care of any seepage in the rock that might tend to build up hydrostatic head behind the walls. All of the walls were of nominal size except for shale zones 6R (fig. 6) and 14R. At these locations the walls not only covered the shale zone but were extended a considerable distance above the tops of the shale zones to provide additional stability for the overlying rock.

16. Anchorage of Rock Masses Adjacent to Shale Zones. There were several areas adjacent to the downstream toe of the dam where rock masses might have a tendency to slide, due to the jointing system and the dip of underlying shale zones or seams toward the river. These areas, with the exception of an area adjacent to the upstream portal of the spillway tunnel, were anchored in place by 1-3/8-inch-diameter reinforcement bars, grouted into the rock. The bars were embedded to various depths, depending on the locations of the cracks and seams that might affect the stability of the rock masses.

The area adjacent to the upstream portal of the spillway tunnel required more elaborate anchorage than was designed for other locations. A pattern of 1-3/8-inch-diameter reinforcement bars, 94 in number, were grouted in place for the rock slabs on the left side of the portal structure. Subsequent excavation and cleanup revealed the need for additional anchorage, principally on the left side, and at a localized area on the right side. In order to induce a downward weight component to the rock slabs, prestressed 1-1/2-inch-diameter anchor bars were installed, 59 on the left side of the portal and 5 to the right of the portal. The prestressing was obtained by embedding and grouting the bars a minimum depth of 10 feet below the questionable bedding plane on which the rock slabs might slide, then stressing the bars by applying a torque through a nut and plate arrangement at the rock surface. Load cells were installed on three of the anchor bars to obtain a correlation between applied torques and induced tensions (sec. 43). It was desired to obtain induced tensions in the bars of approximately 18, 000 pounds per square inch.

The torquing and consequent tensioning of the bars equipped with load cells furnished the basis for torquing the remainder of the bars and also furnished valuable data for any future similar installations. Information obtained is listed as follows:

(1) With the threads clean and dry and with no lubricants it was not possible to obtain the desired tension with a reasonable torque. A torque of 550 foot-pounds induced an average tension in the bars of approximately 6, 000 pounds per square inch. (2) The use of hardened steel washers to reduce the friction losses at the nut and bearing plate increased average tensions in the bars to about 10, 000 pounds per square inch.

(3) To obtain the approximate desired tensions of 18, 000 pounds per square inch it was necessary to lubricate thoroughly and torque to approximately 700 foot-pounds. A temporary lubricant of engine oil was satisfactory, but for a more permanent type of lubricant a mixture of white lead, graphite, and engine oil was used.

(4) Some relaxation was noted in the tensioning of the bars equipped with load cells in the first few weeks. After an elasped time of about a year and retensioning, very little if any reduction in tensioning has been noted for the subsequent 2 years.

17.

B. Grouting

Basic Grouting Plan. The grouting plan as formulated by the specifications was very similar to standard Bureau procedures for foundation grouting that have been established through past practices. The plan was to consolidate the foundation rock beneath a specified upstream portion of the dam by drilling a pattern of relatively shallow holes and grouting under comparatively low pressures prior to concrete placement in the area. This program was identified as B-hole drilling and grouting. Later, as specified, a highpressure grout curtain would be obtained by drilling deep holes from the foundation gallery and grouting under high pressures. These holes were identified as A-holes. See sections 173 through 185 for foundation drilling, grouting, and drainage.

Four lines of B-grout holes were specified, with the lines on about 20-foot centers and the upstream line directly under the upstream heel of the dam. Grout holes in the lines were to be on about 20-foot centers and drilled normal to the rock surface to approximate depths of 25 feet.

The spacing of holes for the high-pressure A-hole curtain was to be determined to a certain extent by the grout take. The specifications indicated a spacing of 5 feet, with depths of holes ranging from a maximum depth of about 200 feet in the river channel to a depth of approximately 50 feet near the top of either abutment. It was assumed that the maximum depth of hole should bear a direct relation to the maximum reservoir head at the location of the hole. This relation, roughly, is that the depth of hole should be 40 percent of the hydraulic head. At and near the bottom of the canyon, the holes had been planned to extend to curtains 75, 150, and 200 feet in depth where applicable.

18. Additional Consolidation Grouting Requirements. Owing to the condition of the jointing system of the rock after excavation for the dam had been completed, the entire

foundation of the dam required consolidation grouting rather than the upstream portion only, as shown in the specifications. Eight lines of B-grout holes spaced at 20-foot centers were drilled and grouted from the foundation between shale zones 14L and 14R in the central portion of the dam. The depths of holes were increased to 50 feet, twice the specifications recommended depth. Concern was felt about obtaining satisfactory consolidation grouting on the steep abutments by grouting from the foundation. It was therefore decided to incorporate a series of transverse adits at approximately 50-foot intervals in elevation in the gallery system from which consolidation grouting could be accomplished after mass concrete had been placed and contraction joints grouted. Seven grouting adits were formed in each abutment from which B-hole grouting was performed, as shown on drawings No. 591-421-1246 and 591-421-1247*.

The addition of the grouting adits to the gallery system was also beneficial in permitting auxiliary foundation grouting and drainage to reduce seepage near the downstream toe of the dam. These adits can also be used for the above purpose should future additional grouting or drainage be required. Also see section 25.

19. Additional High-Pressure Grouting Requirements. As drilling progressed and water was being stored behind the dam, seeps developed at and beyond the downstream toe of the dam, indicating seepage either through or around the already grouted foundation. Additional holes were drilled and depths were increased along the A-hole curtain to try to intercept the water passages. Auxiliary and exploratory curtains were also drilled and grouted downstream from the main curtain until favorable reductions in seepage become evident. The downstream curtains were drilled from the transverse grouting adits. The main A-hole curtain was drilled from the foundation gallery through block 24 and was extended to the right abutment by drilling from the top of the dam and along the downstream shoulder of the access road. See sections 173 through 185 for additional grouting information.

A grout curtain, beginning at the right end of the dam and extending in a westerly direction in the upstream portion of the parking area, was drilled and grouted to provide a barrier against the possibility of reservoir leakage through the narrow saddle into the switchyard area.

Figures 7 through 10 and drawings No. 591-421-1248* through 591-421-1250* show some of the high-pressure grout holes "as drilled" indicating locations, lengths, and grout

takes.

C. Drainage

20. Basic Drainage Plan. The drainage plan as shown in the specifications was based in general on standard Bureau practice. A change from most previous jobs was the drilling of the drainage holes on the abutments sloping toward the gallery whenever possible-thereby making the holes self-draining. The depth of the drainage curtain at the foundation of the dam was established at 60 feet, varying to 30 feet near the top of the dam. Metal pipe and fittings were to be embedded in the floor of the galleries or the vertical gutters in the stairway shafts so that the drainage holes when drilled would be on approximately 10-foot centers at the bottom of the curtain.

21. Additional Drainage Requirements. It became evident during the construction of the dam and initial reservoir storage that the control of seepage downstream of the dam would require a combination of both extensive grouting and drainage programs (see secs. 173 through 185). The drainage program as formulated for the specifications was followed to approximately elevation 5675, to which a drainage curtain about 60 feet deep was drilled. The curtain above elevation 5675 was deepened to about 100 feet in order to be more effective in intercepting seepage.

After completion of grouting, including the auxiliary and exploratory programs, it was decided to drill downstream drainage curtains on both abutments from the transverse grouting adits to try to intercept the increasing seepage as the reservoir rose behind the dam. Drainage holes were also drilled into areas where seepage was evident on the right abutment, from locations in the diversion tunnel downstream of the tunnel plug.

Figures 11 through 14 show the drainage holes as drilled.