28. Station 70+22 to Station 72+08--Metamorphic Contact Zone of Shasta Bally Batholith. Gneissic to hornfelsic metasediments irregularly invaded by abundant dikes are crossed in this reach. Foliation, where evident, dips steeply northeast and strikes across tunnel.

The dikes vary from granitic to dioritic and are mostly porphyritic--some of typical "birdseye" type. Attitude and widths are highly variable also. They are often displaced by faults with associated gouge, brecciation, and shearing. Waterflows were numerous, with appreciable initial volume.

Rock stability was poor and overbreak high; but excavation characteristics improved significantly and progressively beyond the fault zone. In the following tabulation of steel support, changes from a 2-foot spacing of 8-inch sets to a 4-foot spacing of 6-inch sets will be noted:

29. Station 72+08 to Station 527+46--Shasta Bally Batholith. In this interval the rock is biotitehornblende quartz-diorite. It is medium to coarse grained with a faint gneissic planar structure, trending N. 60° W. to N. 70° W. Composition and texture vary locally and affect the strength and stability. It is stronger where it is finer grained and the quartz content is greater, and weaker where it is coarser grained and the biotite content is greater. Dikes and irregular bodies of contrasting composition and texture are present.

Percolating ground water has caused varying degrees of decomposition; and locally, in concentrated volume, it caused serious problems during excavation.

No steel tunnel support sets were used in 13,201 feet of this 45, 538-foot reach, which is 23. 3 percent of the entire tunnel and 29.0 percent of the granitic reach. Of the unsupported section in the granitics, 98.7 percent is between stations 386+50 (under Bear Creek) and 526+79 (61 feet from the intrusive contact on the outlet leg).

Steel supports of 6- or 8-inch size, spaced from 1 to 7 feet apart according to the local conditions, were used in the remaining 32,347 feet (or 71 percent) of the granitic reach. The size and spacing of the supports can be studied on figure and have been tabulated in table 6 of the Construction Engineering Geology Report, Trinity River Project, Trinity Dam, November 1961 (unpublished).

Slip planes--joints, usually curved or wavy, with indications of minor movement in the form of slick surface and at times slickensides or films of gouge--were found to be very significant as a cause for overbreak and large amounts of steel supports in the quartz-diorite in the upstream section of the tunnel, but not in the downstream adit section.

As indicated above, tunneling was particularly good in quartz-diorite downstream of station 386+50 (south of Bear Creek); and out of the 14, 100 feet of tunnel in this section, steel ribs were used for only 1,483 feet in a few minor zones in which support appeared necessary. Rock bolting, used here to a limited extent, appeared to have been a suitable way to provide the support needed.

In the sound quartz-diorite from station 386+00 to station 515+00, 12-foot rounds, the longest used in the tunnel, with average powder loadings of 550 pounds, were used. Elsewhere, the round generally used was a 4-foot length with 250 pounds of 40 to 70 percent powder.

The rock under Crystal Creek (station 462+00), although providing as little as 60 feet of cover, as was evident at the surface, was particularly good and also very dry. A core drilling program, as mentioned earlier, was nevertheless conducted after this section of the tunnel had been driven, for aid in designing the lining.

Sections in which adverse water and/or support conditions were met will be considered in some detail in the following subsections.

(a) Station 126/80 to Station 130/90. --From about station 113+74 where it swings more to the east (S. 50° E.), the tunnel more nearly parallels the Hoadley fault and the formational contact.

The rock in this section consists of alternate zones of hard, moderately hard, blocky, crushed or soft quartz-diorite. It ranges from the light-colored to the dark, fine-grained type, and from faintly to the distinctly foliated. The planes of foliation, along with prominent joints, faults, and shear zones, trend from about S. 60° E. to N. 80° E., and dip steeply to the north. There are also numerous other joints, slips, and faults with diverse attitudes. Many cross the tunnel at high angles, dip steeply, and have fillings of silica, calcite, or chlorite. Overbreak is prominent at structural intersections.

Rock stability is generally poor and rock pressures locally high. This is indicated by the squeezing of steel ribs on the right wall, near station 126+85, and by the bowing of breast jacks by rock pressure on the heading at station 127+07. Invert struts were required between stations 126+85 and 127+05, and the drift method of tunnel excavation was resorted to between stations 127+65 and 127+99.

Heavy waterflows were encountered from station 126+85 to station 131+54, roughly coinciding with the section of poor rock stability just described. As much as 550 gallons per minute flowed through openjointed, loose rock and through test holes bored into the drift headings. The increased flows forced interruption of excavation for about 30 days, while additional pumps and 20-inch pipe were installed. Six months later, these flows had practically dried up.

At about station 129+80 the stability of the rock had improved to about normal, and at about station 131+54 the last heavy waterflow for this section was met.

(b) Station 147401 to Station 150/440. --The rock is fine-grained, dark quartz-diorite, locally subporphyritic and with random tongues of the normal coarse-grained rock appearing as the secondary type. It is locally blocky, brecciated, and sheared and is cut in various directions by many faults and innumerable slips and gouge and by numerous granitic dikes, often pegmatitic. A prominent zone of faulting is located between stations 148+ 10 and 148+70 and overbreak is considerable.

Rock stability is generally poor. It begins to deteriorate at about station 147+01; the rock becoming quite slabby, with moderate amounts of sloughing. At about station 147+77, crushed and displaced granitic or quartzose dikes were encountered. With the heading at station 148+16 on July 17, 1959, the rock face caved into the tunnel under the pressure of a large flow of water, estimated to be 2,000 gallons per minute. A cavern washed out 20 to 30 feet ahead and above the arch of the tunnel. (See fig. 12.)

Without any arch support it was necessary to resort to the method of drifting. Two side drifts, 5 feet square, were started at the springline. Progress by this method and by the use of breast boards and top heading was very slow, particularly in the left side where the large flow of water was an even greater obstacle.

After the caved-in portion was finally supported, development of a full heading without the aid of breast boards was not possible until the excavation progressed to about station 148+54 on August 12, 1959. Maximum water seepage occurred near station 148+30; and flows gradually subsided as boring progressed beyond this station. Beyond station 149+88 some faults and slips with gouge and sheared rock contacts still persisted but were not significant, and rock stability became more normal.

(c) Station 166/46 to Station 211/14. --Large quantities of water were encountered that required additional pumps and grouting of the water-bearing openings. The greatest concentrated waterflow in the entire tunnel occurred at station 169+99 on November 24, 1959. The flow over the weir at the intake portal increased on that date to 6, 400 gallons per minute, from a previous high reading of 5, 500 gallons per minute taken on November 20, with the heading at same station. Tunneling came to a standstill for 59 days, November 8, 1959, through January 5, 1960. During this period additional pumps to handle the increased water and grouting equipment to seal the water-bearing openings were put into operation.

On January 5, 1960 (7:30 p. m.), after about 4,750 sacks of cement were pumped in as grout, grouting at this station (169+99) ceased. Eleven test holes were drilled into the grouted face to depths from 27 to 40 feet and, although some holes produced flows of 80 to 100 gallons per minute, the first stage of grouting operations was considered to be effective. A 4- by 6-foot crown drift, driven to station 170+21, gave indications that it was possible to excavate on a full heading.

As it turned out, however, excessive concentrated quantities of water encountered intermittently made grouting necessary to station 211+14. Waterflows were sealed off or restricted before excavation was resumed. The method used was to bore exploratory holes into the heading, and to proceed with grouting if the water tapped through these holes was great enough in quantity and/or pressure to hinder excavation. At times the sealing off or checking of large waterflows was not effective. Extreme measures were tried with little success. Additives such as bentonite, sawdust, rice hulls and shredded redwood were tried with the cement. Ratios of water to cement varied from 5:1 to 1:1 and depths of grout holes varied from 20 to 76 feet. At certain stations excessive flows were coupled with unstable ground making tunneling still more difficult; and use of drifts, crown bars, spiling, breast boards, and spading was resorted to as well as grouting. A summary chart of the grouting is on figure 8.

Because tunneling was being carried out from both portals, grouting had to be carried out from both ends of this section (station 166+46 to station 211+14). The "hole-through" occurred on July 28, 1960, at station 194+43.

Grouting to seal off water-bearing zones was not required elsewhere in the tunnel. The grouting was carried out at pressures ranging from 200 to 1,300 pounds per square inch, against ground-water pressures ranging from 175 to 410 pounds per square inch. A total of 74, 145 sacks of cement was pumped as grout.

The large flows did not always coincide with rock of poor stability. Sound rock in some places carried large volumes of water through open joints, while weaker rock was relatively dry.

The main structural features in this part of the tunnel are slip joints, striking either at random or in sets. The most persistent set strikes N. 30° E. to N. 50° E., nearly normal to the tunnel axis, and dips steeply southeast. Many minor faults with steep dips, often associated with shear or brecciated zones and containing silicified gouge, also trend normal to the tunnel axis.

It is thought that many of these structural features branch from the Hoadley fault zone and constitute an aquifer system that contributes to the concentration of water in this section. Some closely spaced slips and joints with various dips roughly parallel the tunnel axis and locally, structural intersections produce loose, blocky or slabby rock and large overbreak. Basic and aplitic dikes are occasionally encountered, generally crossing the tunnel axis at high angles and dipping steeply. Foliation is occasionally apparent, striking roughly parallel to the tunnel and dipping about 75° NE.

(d) Station 369/50 to Station 386/50--Heavy Faulted Ground under Willow Creek-Bear Gulch Divide. --In this very troublesome 1,700-foot section, the rock is intensely crushed and softened in major portions, and brecciated and sheared at others. The tunnel was advanced slowly through this bad ground in the upstream direction after 14,100 feet of good progress through the quartz-diorite with relatively minor localized use of supports. About 4-1/2 months were required to penetrate this 1,700-foot section; whereas average progress downstream from this area had been nearly 1,000 feet per month.

The largest, well-defined gougy fault is at about station 385+10, near the downstream limit of the crushed zone. It strikes N. 12° E. to N. 38° E., dips 40° W., and is in a 100-foot zone of intense crushing, contorted slip planes, and disrupted dikes. Another outstanding fault with a 7-foot wide gouge and breccia filling crosses the tunnel at station 374+60 with a N. 70° E. strike and 65° SE. dip.

Most of the other widespread faults have steep dips and cross the tunnel at high angles. Steep-dipping, closely spaced joints and shear zones are numerous. Some roughly parallel the tunnel, but generally they strike variably across the tunnel. Randomly oriented slip planes are numerous also.

A very remarkable feature of this large weak zone is that, although as noted in the opening sentence of this subsection, much of the quartz-diorite is "intensely crushed and softened," the original granitoidal texture of the rock appears intact. The apparently intact rock can be readily broken down in the hands to a silty sand state. Some chemical alteration, possibly hydrothermal, of the rock is suggested by its more whitish appearance than the normal vitreous-grayish fresh quartz-diorite. Within the crushed and faulted mass, however, are large ribs of hard rock and breccia zones in which blocks of hard rock are embedded in softer material.

Dikes are plentiful, and their frequent disruption and displacement along slip planes serves to indicate intensity of the deformation. The dikes are chiefly light-colored aplites, but pegmatitic and basic types are also present. The usual range in size is from ribbons and stringers to widths of 2 to 3 feet. Most dip steeply and cross the tunnel at high angles; less frequently they trend along the alinement. Some dikes are bordered by brecciated and gougy faults. Two such dikes, each about 7 feet thick, are noteworthy. One at station 370+15 strikes N. 12° E. to N. 15° E. and dips 60° E. to 70° E.; the other, an aplite at station 383+15, strikes N. 67° E. and dips 20° W. and is severely crushed.

The stability of the rock in this section was generally very poor and greatly hindered tunneling. Use of crownbars and breast boards was frequently necessary. Much of excavation was by spading, at times without the use of any explosives. Closely spaced 8-inch steel supports and considerable lagging were necessary to hold the crumbly heavy ground. Particularly noteworthy are the two intervals where the supports were set on 1- to 3-foot centers as listed in the table below:

Excessive rock pressures bent the steel supports and broke the lagging. The most common deformation occurred as a bulging out near the spring line and a pinching in near the footings. After the ground remained open for a period, the addition of ribs was necessary to support the increased pressures that had accumulated on some of the sets. Some ribs were so deformed that they had to be replaced. Many invert struts were installed when the ribs were reset or replaced. Later, in the lining of this section, the reinforced-concrete lining was redesigned to withstand stresses of 5,000 pounds per square inch, rather than 3,000 pounds per square inch as planned originally.

On June 19, 1958, with the excavation progressing from downstream and with the heading at station 377+66, the ground became so heavy in some portions of the section that it was decided to stop the advance and replace deformed ribs and to place additional ribs and lagging to restore or improve stability. Ribs were replaced between stations 378+00 and 378+87. Ribs set at 2-foot centers were replaced by ribs at 8-inch centers. From station 378+19 to station 378+47 progressive increase in pressure over the crown had resulted in increased deformation of the ribs along both walls. Bending was more pronounced along the right wall. The greatest deformation took place at spring line, while the sharpest bending took place about 3 feet above the floor. With some ribs a twisting movement, more noticeable toward the footings, took place. About 11 days were employed in this resupport work. When tunneling resumed, it became part of the work routine for a period to drill exploratory holes beyond the headings. Some produced minor waterflows.

Waterflows were not uncommon but were generally small. The disturbed rock was generally wet and the section seldom completely dry, but at times it was possible to shut off pumps and depend on gravity flow to keep the heading vicinity drained. The areas of more crushed rock, fortunately, were relatively free of waterflows during initial penetration and only became wet after additional advance.

This troublesome zone of faulted and crushed heavy ground was not predicted, as mentioned in the section on preconstruction geology (sec. 23). It is believed, as a result of the special program of surface mapping and trenching at the time of the difficult tunneling in the summer of 1958, that the heavy ground was related to the Willow Creek fault, a large regional fault found to cross Willow Creek heading through a saddle on the divide between Willow Creek and Bear Gulch into the area where the conditions deteriorated about 400 feet after going under Bear Gulch. As shown in the geological map (fig. 7), the Willow Creek fault intersects, or splits from, the Hoadley fault in this area. The Willow Creek fault is believed, from the topographic relations, to have the northwesterly dip indicated on Willow Creek, where a gougy crushed zone similar to the tunnel rock is excellently exposed over a distance of 200 feet (northern limit hidden by alluvium). Thus, after crossing the Willow Creek system, the tunnel was evidently in the wedge of squeezed and shattered rock between the Willow Creek fault and the Hoadley fault.

(e) Station 510/95 to Station 511437--Fault Zone. --The easy going in the early stages of the advance of the outlet heading through the sound rock of the batholith was accented by the interception of a 40-foot-wide soft zone centered around a fault crossing the tunnel at station 511+20 with a transaxial northeasterly strike and 55° NW. dip. This fault, which contained 18 inches of gouge, correlates with the fault inferred from surface mapping west of drill hole B 106. It is noteworthy that no support other than rock bolts with wire mesh was used in this weak interval.

The moderately wet conditions between stations 505+50 and 508+50 is noteworthy, although it did not cause any problem. In this section minor amounts of hydrogen sulfide gas were detectable in association with local heavy drips. This is near drill hole B 105 from which there was a persistent artesian flow (about 10 gallons per minute) of water bearing hydrogen sulfide gas. This artesian flow stopped after the tunnel was driven, but after lining the flow has resumed at a lower rate than originally. A second occurrence of hydrogen sulfide gas was between stations 395+50 and 397+00 near Bear Gulch a short distance before the troublesome Willow Creek fault zone was reached.

30. Station 527+46 to Station 537+25--Gneissic Metamorphics. At P. I. station 519+61. 74 the tunnel curves on a 300-foot radius, and a short distance ahead at station 527+46 it crosses the intrusive contact. The metamorphic rocks are chiefly hard, strongly foliated hornblende gneisses and quartz-mica schists, with narrow amphibolite zones. The foliation strikes N. 55° W. to N. 70° W. and dips 50° to 70° NE.

Chloritic gougy shear zones, minor faults and slip planes parallel to the foliation are common, as are tongue-like quartz-diorite dikes which are sometimes displaced by the minor faults and dips. Closely spaced, random-oriented joints are numerous. There is a persistent dominant N. 30° E. to N. 35° E. joint set with steep east or vertical dips which, in crossing the foliation at right angles, tend to cause blocky breakage.

The favorable relation of the tunneling direction to the foliated structure of the hard rock provided high natural strength to the tunnel section. Widely spaced steel supports (6-inch, 20-pound beams at 4 to 7 feet) were placed, however, for protection from loosened rock in the arch.

31. Station 537+25 to Station 538+00--Hoadley Fault Zone. A sharp fault plane at station 537+25 defines the eastern edge of the gneissic metamorphic belt. This fault plane strikes N. 55° W. and dips 55° to 60° NE., roughly parallel to the foliation of the gneissic rock. In advancing from the inlet portal, before reaching this fault contact, the tunnel crossed a 75-foot-wide, highly disturbed gougy and brecciated interval. The upstream 40 feet is intensely contorted fissile crush-breccia and soft talcose chloritic gouge. The remainder of the fault zone has occasional lenses of the metavolcanic rocks that were crossed downstream. The schistose metamorphic structure in these lenses crosses the tunnel at right angles with a more northerly strike than the attitudes given above and dips 60° to 90° E.