A report summarizing the record of the construction engineering geology by the field forces was prepared in December 1961. Figure 8, showing tunnel geology and engineering correlations, was adapted from drawing No. 416-229-1647 of the project report; and the following section, which discusses the tunneling experience with relation to the reaches in each of the geologic divisions, is substantially drawn from that report also.

In figure 8 the main geologic features--contacts, faults, weak zones, et cetera--are shown on the profile and plan of the line; and in the related charts data on the type and amount of support, waterflow quantities, types of reinforcements used in the lining, overbreak, use of breast boards, grout-takes in certain very wet sections are compiled for comparative study. In addition, typical tunnel wall logs and heading logs are included as figures 9 and 10.

Before entering on the specific experience in the geologically distinctive sections, some general comments on tunnel supports, ground-water inflow and temperature will be made.

(a) Tunnel Support. --Steel sets were used in 76. 7 percent of the tunnel in the form of 6- or 8-inch ribs ranging in spacing from 0.9 to 7 feet. It had been estimated that they would be required in 35 percent of the tunnel. Rock bolts were used extensively only in the Crystal Creek adit junction (station 451+93) area and upstream to station 387+00.

The steel support spacing can be studied in figure 8, and is covered, usually by detailed tabulations, in the following portion of this account.

Factors, other than geological conditions conducive to rockfalls, led to the installation of supports in many cases; some of these factors were the tendency of the contractor to maintain certain patterns, a safety psychology associated with steel ribs, and very high unit prices for steel supports.

(b) Ground Water. --Many types of water conditions were encountered. Wet zones ranged from areas a few feet long to stretches of several hundred feet. The flows varied from drips to high-pressure flows estimated at up to 2,000 gallons per minute. Water temperatures ranged between 57° and 60° F, remarkably low considering the depth of the tunnel where the large inflows occurred.

In some cases the flow was sufficient to tax the available pumping systems and force termination of tunnel operations (in one area for a period of 2 months). In other cases, pressures up to 410 pounds per square inch constantly threatened the stability of the heading and required the installation of relief holes and grout injected under pressures up to 1,300 pounds per square inch. The most severe conditions were met in that part of the Shasta Bally quartz-diorite under the Hoadley Peaks where the tunnel was more than one-half mile below the surface.

Many of the largest flows, including those which were sealed off by grouting, occurred in the tunnel section where cover exceeded 1,500 feet. However, considerable portions of the tunnel with over 1,500 feet of cover had no significant flows. The following conclusions were reached:

(1) All of the large waterflows were the result of interconnected, open fractures and joints in the otherwise impervious rock.

(2) These openings were the result of tensional forces and usually were immediately adjacent to fault zones. It could be said that fault "drag" sometimes pulled the rock apart at fractures and joints to produce the openings.

(3) If the openings described in (1) and (2) above were in a part of the tunnel with greater cover and therefore a greater volume was stored with higher static head, flows were proportionally greater than if the head were less.

In relatively low cover areas, localized heavy flows were encountered but rapidly drained until these areas dried up. From Crystal Creek to the outlet, water was negligible with local exception. After the lining was placed, a progressive buildup of water pressure commenced. Weep holes in these areas, to which gages were connected, supplied information on the pressure increase. There are 1,657 weep holes in the tunnel lining.

The large flow of water from the inlet portal led to the conclusion that the tunnel may have influenced wells and streams in the Hoadley Creek drainage. However, the extent to which this is true could not be ascertained and under operational conditions the matter is, of course, not a problem.

2. Geology of Tunnel

25. Station 1+62 (Inlet Portal) to Station 31+57--Meta-Andesite (Copley Greenstone). The meta-andesite or "greenstone" is fine-grained, greenish gray, hard and tough.

Faults and shear zones are fairly numerous, mostly trending northeasterly across the tunnel. The stronger ones, chiefly spaced 50 to 250 feet apart beyond station 14+50, have a N. 50° E. to N. 75° E. range of strikes, and dip 60° to 85° SE. Joints and slips with more diverse pattern, but commonly roughly parallel to the above, are very numerous.

This reach was, in general, wet or damp. The amount of water varied from very light seepage to heavy flows. In most cases, once tapped, the flow from each source peaked and then diminished until parts of the tunnel become practically dry.

Figure 9. --Clear Creek Tunnel geology--Station 567+00 to station 568+50.

Near station 23+00 there were large flows, from strongly developed systems of joints and slips whose strikes range north-south to N. 15° E. and dips 20° to 35 SE. Copious streams flowed from most cracks. These conditions were most pronounced between stations 22+50 and 24+04 under Jessup Gulch where cover was only about 300 feet. Waterflow increased from about 270 to 480 gallons per minute.

On January 15, 1958, about 800 gallons per minute were released from a relatively large fault with 3 to 5 feet of breccia near station 25+53. The excessive volume of water pouring out of the face and immediate surroundings made attempts at regular tunneling operations ineffective. Additional pumps were installed but tunneling had to be suspended for the day. The flow at the inlet weir had increased from 500 gallons per minute with the heading at station 25+46 on January 14, to a peak of 1,700 gallons per minute with the heading at station 25+53 on January 15. By January 17, with the heading at station 25+69, the waterflow had dropped to 840 gallons per minute.

At about station 31+57 a concentrated flow of about 500 gallons per minute was encountered which streamed from the left wall. The discharge over the inlet weir rose from 1,040 gallons per minute with heading near station 31+31 on February 12, to a peak of 1,325 gallons per minute with heading at station 31+59 on February 13. By February 14, with heading near station 31+90, the flow had dropped to 1,080 gallons per minute. Station 31+57 marks the faulted contact between the Copley meta-andesite and the Bragdon formation. The nature of the rock and the structural condition in the vicinity of the contact is discussed in the following paragraph.

Coinciding roughly with increased waterflow between stations 23+00 and 26+00, a dense, darker colored rock was encountered. Petrographic analysis showed metamorphism of the Copley had advanced considerably beyond the "greenstone" stage to hornblende-biotite hornfels. Also, significant changes in structure took place between stations 26+00 and 31+57, near the Copley contact with the Bragdon. At about station 26+60 the heading was cut in a cemented breccia, with a definite foliated lineation having alinement parallel to the tunnel at a S. 30° E. strike and dip 55° E. This cemented breccia, which was followed 410 feet to station 31+31, appeared to be a result of faulting, since near this station the rock became highly silicified and at station 31+57 was in fault contact with the Bragdon. The Bragdon near the contact was quite soft from numerous random contorted slip surfaces and related shearing. The fault contact dips 60° S. and crosses the tunnel at 55° from the axis.

Overbreak of 1 to 2 feet is general, especially beyond station 10+50, and reached 7 feet at the shear zone near station 14+50. Rock stability is, however, good except locally where open closely spaced steep joints parallel the tunnel.

26. Station 31+57 to Station 66+00--Bragdon Formation. The Bragdon formation in the tunnel is predominantly dark-gray to blackish, slaty meta-argillites. Near the contact with the granitic batholith, they have been metamorphosed further into hornfels.

The bedding was obscured by fracturing and contortion. However, a large number of measurements warrant generalization that the strikes range between N. 25° W. and N. 77° W., changing back and forth through intermediate strikes and, in places, parallel the tunnel. The dips range between 25° and 70° NE. This pattern indicates that the tunnel crosses folds of moderate breadth (from about 100 to 600 feet), the axes of which plunge steeply northeast.

Joints are very numerous. Most strike across the tunnel and dip steeply; some occur closely spaced in intersecting sets which produce fragmented or blocky conditions and contribute to large overbreak. Slips are less numerous and occur more at random, but also trend generally across the tunnel. Occasional silicified zones and rust-stained, water-bearing slips cross the tunnel. Faults and zones of shear, many strongly developed, are quite numerous. In portions of this section several of the above structural features occur together to produce unusually poor stability. Specifically, zones of this sort occur between stations 31+57 and 32+00, and are related to the nearby fault contact at station 31+57.

Two more severely sheared and faulted intervals are described below in subsections (a) and (b).

(a) Station 42400 to Station 43/10. --Under Deadwood Creek, the slates are considerably sheared and occasionally crushed. Bedding strikes parallel to the tunnel and dips northeast. Several faults and slips cross the tunnel at various angles. They are accompanied by shear zones, in some cases up to 4 feet thick, and by soft gouge or hard silicified gouge. Strongly developed, closely spaced joint trends of varying attitudes intersect, causing overbreak and need for close support.

(b) Station 59/30 to Station 61/50. --The rock is soft to moderately hard slate and fine-grained metasandstone. The strike of the beds is nearly parallel to the tunnel axis, and the dip 35° to 45 NE. Most

joints occur in strongly developed trends and are associated with shear zones, some parallel to bedding. Other trends with steeper dips, cross the bedding planes and the tunnel either at low angles or nearly at right angles. Several strong faults, associated with gouge and/or breccia, dip steeply and cross the bedding and the tunnel nearly at right angles. Others, also strongly developed, strike generally parallel to the bedding and the tunnel and dip from 45° E. through vertical to 45° W.

27. Station 66+00 to Station 70+22-- Hoadley Fault Zone. This section of the tunnel, which crosses highly disturbed Bragdon metasediments and some of the higher-grade metamorphic rocks in the Hoadley fault zone, was one of the most troublesome encountered. (See fig. 11.) The bedding structure of the metasediments, chiefly slaty to hornfelsic Bragdon, is almost totally obscured by very closely spaced joints, shears, slips, and faults--most of which cross the tunnel at high angles. Brecciated zones and gouge are associated with the faults.

Waterflows were numerous, with appreciable volume when encountered, but were not abnormally high. Flows were smaller than the loose and fractured conditions of the rock would have indicated.

Rock stability was very poor and overbreak high. Excavation was as difficult as any found in the entire tunnel, being especially difficult near station 69+00, where a change from 6- to 8-inch steel was made. With regard to the heading at station 69+35, the Project Geologic Progress Report of July 29, 1958, summarized conditions as follows:

"The rock at the heading in this end of the tunnel had deteriorated sufficiently to require the use of a 6- by 6-foot crown drift. The process of excavation requires crownoars, breast boards and spading. The heading could be held only by the use of breast boards because of the finely fractured rock condition and numerous slips and gouges. A bedding structure composed of thin bands of slates, usually partially silicified and showing reddish alteration products, is consistently present, striking about 30° east of the tunnel alinement and dipping 50-70° NE. All bedding partings show slip surfaces, thin gouges and a lack of cohesion. Another persistent trend of slip planes and gouge zones strikes almost transverse to the tunnel and is probably a reflection of faulting related to the Hoadley fault. The result of these strong structures is a very unstable rock mass which may be interpreted as either part of the Hoadley structure or the full development of it. Verification of this interpretation should be gained in a very short advance in tunneling from the present heading."

The Hoadley fault contact was crossed at station 69+95 and consisted of 2 to 3 feet of "impervious clay gouge" striking east-west and dipping 60° N. Downstream from the contact the fault zone consists of severely sheared gneisses and igneous dikes.