LOG OF DRIFT NO. I

SANDSTONE entire drift was driven in this formation
CROSS BEDDING DISTINCT indicates areas where the
rock is moderately hard, friable, coarse to tine
grained, weakly cemented, cross-bedded and red-
brown. The larger grains are segregated in layers
about one sixteenth of an inch apart. The layers
are separated by fine grains. The contact between
the two types of rock is a half-inch layer of red-
purple SANDSTONE Breaks parallel to cross -
bedding
CROSS-BEDDING INDISTINCT indicates areas where
the rock is moderately hard, friable, medium-grained
even textured, weakly cemented, poorly cross-beddec
irregular fracture, red brown, with some white spots.
Rock is easily crushed to sand by blasting.

LOG OF DRIFT NO. 2 SANDSTONE- entire drift was driven in moderately hard, friable, medium to finegrained, weakly cemented, cross-bedded, redbrown SANDSTONE which contains some white spots and thin brown layers. Rock is easily crushed to sand by blasting. CROSS-BEDDING DISTINCT, indicates area where ROCK breaks to cross-bedding planes in roof of drift. CROSS-BEDDING INDISTINCT, indicates area where cross-bedding is not readily noticeable and where ROCK fractures irregularly. STATION O+06 Joint" width. Smaller joints spaced about one foot apart are parallel. Large joint is probably indicative of ex tension of surface slab. STATION O+44-Joint varies in width from hairline to thin CALCITE filling STATION O+50-Joint hairline width, thin CALCITE filling. Tendency for parallel joints to form nearby.

570

Thin shale seams were encountered at elevations 3115 and 3065 at the base of the right abutment keyway. The seam at elevation 3115 varied from one-eighth of an inch to about 4 inches thick and had a waterflow of 2 to 3 gallons per minute. A 5- by 7-foot drift following this seam was excavated near the heel of the dam to a depth of 73 feet into the abutment to a point where the flow of water disappeared. The seam at elevation 3065 varied from a thin shale parting in the sandstone to a shale layer 1 to 2 inches thick and had a waterflow of 75 gallons per minute. A 5- by 7-foot drift following this seam was excavated near the heel of the dam to a depth of 215 feet into the abutment. The flow of water decreased with depth and at the end of the drift was just a small trickle. Both drifts were back filled with concrete and grouted to form a barrier to seepage through the foundation.

Although the Navajo sandstone is remarkably uniform and yields remarkably smooth excavation surfaces, it has two principal characteristics which contributed to design problems. The stress-relief jointing parallel to the canyon walls showed a tendency to open slightly with time and slab or peel off onionskin fashion. The second defect is that the rock has a fairly large percentage of "set" or unrecovered strain occurring during the first loading of the sandstone. Special grouting design was developed to offset this characteristic.

Rock bolts were used extensively as shaped excavations developed; such as, tunnel portals and

reentrants in the powerplant foundation adjacent to the base of the canyon walls. When excavation faces cut through sheeted-type, stress-relief joints, the free ends of slabs or sheets tended to separate and warp. The low tensile strength of the sandstone contributed to fallouts, and bolting was used for safety and to maintain the original integrity of the rock. Experience demonstrated that multiple lines of bolts installed adjacent to the edge of a proposed excavation surface prior to blasting was the most satisfactory procedure. Similarly, edges of rock slabs were "sewed" with bolts to prevent progressive warping and deterioration.

The influence of stress-relief in the rock in the canyon walls was perhaps the most important single geologic problem encountered during the construction at Glen Canyon Dam. These joints increased the estimated quantities of excavation, rock bolting, and wall scaling.

The problem of stress-relief jointing was first noticed during the excavation for skewback No. 2 for the Glen Canyon Bridge on the right canyon wall. A large slab had to be removed which extended from below the skewback at elevation 3570 to the canyon rim at elevation 3828. This overexcavation resulted in considerably more concrete under the skewback.

A second local problem area caused by stress-relief jointing occurred at the downstream portal of the left diversion tunnel. A large rock slab fell during excavation due primarily to warping along stress-relief joints. That part of the slab which did not fall was later removed and the sandstone area stabilized with rock bolts.

Stress-relief jointing, along with undercutting by water discharging from the right diversion tunnel, caused a large rock slab, estimated at 18,000 cubic yards, to fall into the outlet portal of this tunnel. This rock mass almost completely choked the outlet portal, and the tunnel had to be unwatered before the rock could be removed and repairs made to the portal area. This operation involved closing of the gates at the upstream portal and construction of a cofferdam around the downstream portal. The problem was corrected by building a concrete wall to support and protect the sandstone. sandstone. In addition, the area downstream from the outlet portal of the left diversion tunnel was lined with concrete to prevent the occurrence of erosion or undercutting.

Excavations in the powerplant area near the right canyon wall and in the machine shop area near the left

canyon wall were troubled with slabbing along stress-relief joints. The bench-type excavation design was modified to more or less fit the joint attitude.

Zones of less well-cemented sandstone encountered in the keyway excavations, although surrounded by normally cemented sandstone, required considerable local resloping to meet existing design concepts. A program of extensive rock bolting and drainage immediately downstream of the dam was accomplished to securely anchor large slabs of rock which were the result of jointing. This work was done for about 200 feet downstream of the toe from elevation 3190 to elevation 3450 on the right abutment and from elevation 3190 to elevation 3350 on the left abutment. The problem of stress-relief joints in the keyway was taken care of by grouting the joints, one lift at a time, as the dam rose in elevation.

Laboratory tests performed on cores of Navajo sandstone showed a high percentage of "set" or unrecovered strain during the initial loading period. This adjustment is thought to be due to a slight rearrangement of the quartz grains in the interstitial cement. With subsequent loadings, the unrecovered strain decreased and the sandstone became more elastic. The range in "set" under the first loading was 5 percent in drill hole J at a load of 800 pounds per square inch to 29 percent in drill hole H at 200 pounds per square inch. The range under the second loadings were zero percent in drill hole D at 800 pounds per square inch to 9 percent in drill hole H at 400 pounds per square inch. Figures 12 and 13 are graphic presentations of the principal variations in engineering properties of the Navajo sandstone as related to the exploration drilling and typical canyon section.

The correction or adjustment for this characteristic of the sandstone was accomplished by prestressing the dam so as to bring the first loading onto the rock before the reservoir exerted a force. This was done by cooling the blocks of the dam concrete down to 40° F. and grouting the contraction joints. As the dam warmed up, the reservoir load was transmitted to the abutment. This load will be maintained and so becomes the minimum load imposed by the dam on the rock. After this first loading, the sandstone becomes essentially an elastic substance which will deform slightly with loading and will recover this deformation when unloaded.

Transformer circuit towers are located on the canyon rim downstream from the highway bridge. The joints in the area were mapped and examined in detail prior to construction. Three test pits were excavated on joints to determine the dip of the joints and study their character underground. There was no evidence to indicate the joints extended very far below the surface. Similar joints intersecting the canyon wall are visible for only 50 to 60 feet down from the canyon rim. Most of the joints dip away from the canyon, and all of the joints exposed in the test pits tightened with depth and were well cemented.

11. GLEN CANYON RESERVOIR GEOLOGY. The main body of Glen Canyon Reservoir (Lake Powell) occupies a long, narrow canyon of the main stem of the Colorado River for a distance-at full reservoir-of about 186 miles. A number of long, slender arms extend out from the main canyon where tributary streams enter the Colorado River. The largest of these is the arm extending approximately 70 miles up the San Juan River.

The canyon as a whole is long and narrow and is characterized by smooth, steep walls and a gentle stream gradient. The average slope for the first 165 miles upstream is 2 feet per mile. The gradient increases rapidly above this point as the reservoir enters the lower end of Cataract Canyon.

Except at the Waterpocket Fold, where Chinle shale is exposed, the confining walls of the main body of the reservoir basin are the massive sandstone cliffs of the Glen Canyon Group made up of the Wingate, Kayenta, and Navajo formations. Throughout its course in the sandstone formations, the canyon is narrow and straight-sided; but, in the weaker shale, its bottom widens and the sides become gently flaring slopes. That part of the reservoir which will extend up the San Juan River will be largely in Triassic beds made up of the Moenkopi, Shinarump, and Chinle formations.

Porosity of the Navajo sandstone ranges between 20 and 25 percent, so the bank storage or waterholding potential is considerable. The reservoir started filling in 1963; and, since that time, a program has been underway to determine the volume of water in bank storage. This program is continuing, and it will be several years before an accurate quantitative figure on bank storage can be determined.

Minor problems have arisen as the reservoir rises regarding rim stability. Sand dunes and talus deposits slide into the reservoir as the materials become saturated. Most of this material goes into the zone of dead storage so has only a very small effect on the total storage capacity. Rockfalls have only occurred on a very small scale and in areas where the sandstone has been undercut.

A slight movement along an existing joint was observed on the right side of the reservoir several miles upstream of the dam. It is assumed that such slight readjustments are occurring elsewhere in the reservoir area as the reservoir level reaches successively higher levels. This movement is a normal reaction and represents readjustment along existing joints resulting from the increasing waterload of Lake Powell. Future movements will be recognized, as in the case of the other large reservoirs, only as slight shocks on seismograph instruments.