37. Diversion During Construction. Diversion during construction was accomplished by taking advantage of normal dry seasons to prepare the foundation and to construct a portion of the upstream toe of the dam to be used as a cofferdam, and by routing Trinity River through the 28-foot-diameter diversion tunnel. A 5-year flood would route through this tunnel with a maximum water surface at elevation 2001, requiring about a 65-foot-high cofferdam.

38. Foundation Treatment. (a) General.--Dredged and undredged stream gravels in the amount of some 1,500,000 cubic yards filled a 300-foot-wide channel for the full width of the dam. A third of this deposit, which underlies the impervious core, had to be removed to permit bonding the water barrier with the rock foundation. In the remaining two-thirds, which underlies the outer shells of the embankment, test pits disclosed appreciable silty gravel deposits intermixed with the pervious gravel. The silty materials were present in both undredged material and dredged material. The presence of the silty materials in the undredged material is possibly due to secondary effects of slopewash and landslide activity, while in the dredged material the presence of silty materials is due to the segregating effect of the dredging operation. Compared with the strength characteristics of the proposed outer zone fill material and with the values used in the stability analyses, these silty gravel deposits were submarginal. To be assured that all these low-strength areas were eliminated, it was considered necessary to remove all the streambed gravels down to the general rock surface. However, dental cleanup of pockets, plunge basins, and the narrow inner gorge was limited to a band under the core.

Another area requiring extensive excavations was the foundation of the fill upstream on the right abutment. This appears to be a slump or landslide deposit partially displacing and overriding streambed gravels. The base of this debris, at and below stream level, is a soft silty, gravelly, clayey mass. The top back slope of the slide consisted of a loose rubble. About 1,000,000 cubic yards of excavation was required to remove this slump deposit.

A second similar, but more extensive, landslide area has been identified downstream on the left abutment. Though beyond the limits of the dam, further movement of the mass would endanger the spillway stilling basin. Since the material composing this slide mass was found suitable and economically desirable for use in the embankment, it was decided to unload the slide rather than to relocate the spillway. Approximately 2, 200, 000 cubic yards of excavation was removed to stabilize the area.

The toe of a third potential slump mass is located under the upstream half of the left abutment foundation. This area showed signs of movement during construction and was buttressed by the dam embankment. Since maximum stress in this area occurs during construction, no remedial measures were incorporated in the design.

The remaining fraction of the foundation excavation item, some 1,000,000 cubic yards, consisted of removing the layer of residual clayey soil and decomposed rock covering sound greenstone. The criterion for this stripping was that excavation should reach a material at least as dense as the compacted fill that would be placed upon it. In practice this horizon was generally found to correspond with the limit of excavation for scraper-type equipment.

(b) Foundation Excavation. --The required foundation excavation is divided into two parts, that portion above elevation 1940 and that portion below elevation 1940. Above elevation 1940, the material is primarily weathered greenstone. Near elevation 1940 the condition changes to near saturated or one requiring underwater operation and the material changes from impervious to pervious. The planned utilization of the foundation excavation was as follows:

Above elevation 1940:

To zone 2 ..

To access road

1,070,000 cubic yards

. 6,000 cubic yards

(This material was obtained primarily from the deep slopewash deposit on the
right abutment upstream from the dam axis and from the left abutment.)
To waste

524,000 cubic yards

(Normal stripping and clearing above elevation 1940 was estimated at 124, 000
cubic yards; the remaining 400,000 cubic yards came from the lower section
of the slopewash on the right abutment upstream from the axis--this material
was considered too wet to be used in the dam.)

Below elevation 1940:

To zone 3 ..

(This material, comprised of dredger tailings and screenings and undisturbed
sands and gravels, was obtained primarily from the river channel section of
the foundation.)

4,000 cubic yards

To waste

46,000 cubic yards

(The quantity estimated is from stripping of the area below elevation 1940.)

39. Grouting. The grouting program was based on the nature of the rock in the foundation and the hydraulic pressures acting on the structure. Judging from percolation tests, the rock was expected to be relatively tight in the bottom of the canyon and would be more receptive to grout as higher elevations were reached on the abutments for an overall average take of 1 sack per foot of hole.

In view of the hydraulic height of the embankment, a grouting pattern with holes up to 260 feet deep was considered necessary and a double grout cap and curtain was provided across the lower foundation area below elevation 2200.

40. Miscellaneous Design Details. (a) Crest Details and Camber. --The crest elevation without camber is 2395 and the maximum water surface elevation is 2387, thus providing the 8 feet of freeboard normally used in connection with a glory-hole-type spillway. Above elevation 2395 the crest is cambered in proportion to the height of the fill to a maximum of 4 feet between stations 10+40 and 16+70. Camber is applied to insure that the freeboard will not be diminished by embankment consolidation.

At the time the specifications design was prepared, no road requirements for the dam crest other than for operation and maintenance had been established; consequently, no treatment other than 6 inches of selected surfacing was provided. However, the 40-foot-wide crest could accommodate a local road if at a later date it is decided to route one across the crest.

(b) Access Roads. --Most of the road construction required in connection with Trinity Dam and Clair Engle Lake was accomplished by separate contracts.

The dam contract included only two short lengths of access road. These are the crest of dam access road connecting the dam crest with the relocated county road; and the control house access road going from the county road, across the downstream toe of the dam, and into a parking area at the outlet works control house. Both access roads were designed to the standards required for the relocated county road on the right abutment.

41. Investigations of Construction Materials. (a) General. -- Appendix G presents a summary of earthwork control data for the dam. Studies showed that a nominal embankment of the specified height at the Trinity site would require over 30 million cubic yards of fill. Of this total quantity, up to 40 percent, or about 12 million cubic yards, would be available from required foundation excavations and channel improvements, provided the excavated material was acceptable fill material. The difference, a quantity in excess of 18 million cubic yards, must be borrowed.

Embankment construction materials available within a reasonable haul distance were residual soils from Weaverville, Bragdon, and Copley formations; dredged and undredged streambed deposits; and quarried rock.

(b) Impervious Materials. --A summary of field investigations for water-barrier zone materials is shown in table 2. Preliminary investigations of construction materials for Trinity Dam were made in 1950. The extensive mountainous terrain on both sides of the river in the vicinity of the damsite was scanned in reconnaissance fashion for deposits of the required magnitude. Because of surface indications that thin rocky soils were the rule in the areas of the Copley and Bragdon bedrocks, it was believed that the possibility of developing more than a few hundred thousand cubic yards of relatively uniform materials in the vicinity of the damsite was remote. The most obvious source of impervious embankment soils was the Weaverville formation on the Buckeye plateau.

The Buckeye plateau is a dissected, densely forested tableland located about 4 miles west of the Trinity River in the vicinity of the damsite. It is underlain by deeply to moderately weathered gravels of the Weaverville formation. These gravels were never deeply buried; and the fairly level surfaces above 3000 feet in elevation, extending westward about 4 miles from the river, may approximate the flood plain of the ancient river which deposited them. Protracted weathering in a humid climate, facilitated by the large proportion of easily decomposable, highly basic igneous rock particles, developed a remarkably deep residual soil on the loosely consolidated gravels. To a depth of 100 feet or more, the soil underlying this portion of the deposit is a clayey silt, with minor amounts of relict gravel fragments and sandy layers. Preliminary laboratory tests indicated that this very fine textured soil was highly compressible and, when compacted, of low density. It was estimated that at least 15 million cubic yards of this type of impervious material could be obtained within 5 miles of the damsite.

The Weaverville formation materials on Buckeye plateau, together with a smaller deposit of partially decomposed terrace gravels in an old hydraulic gold mine located 4 miles north of the damsite, were considered as the sources of impervious materials in the preliminary designs and estimates. More detailed investigations of these sources revealed that the engineering properties of the soils were rather poor for a dam of this then unprecedented height. (At the time of its completion in 1962, Trinity Dam was one of the highest, if not the highest, earth dam in the world.) Also, the large hauling costs dictated further exploration of all possible sources of soils closer to the damsite.

A few power auger holes excavated during 1954 in the residual soil formed on the Bragdon formation in the uplands 3-1/2 miles west of the damsite, disclosed the presence of a silt with some flat, friable gravel particles to a depth of up to 80 feet. The engineering properties of this soil appeared to be superior to the Weaverville soil; therefore, the extent of the weathered Bragdon was investigated by drilling power auger holes. During this exploratory program, it was found that the Copley meta-andesite (greenstone) was also deeply weathered for an area of about 400 acres on the east slope of Buckeye Ridge to within 1 mile of the damsite. Moreover, the residual soils derived from the Copley appeared to be

somewhat superior to the Bragdon. An intensive drilling program finally proved the existence of about 30 million cubic yards of weathered Copley meta-andesite and 3 million cubic yards of weathered dacite porphyry, which occurs as a dike within the Copley formation.

On the left abutment of the damsite, just above the downstream toe of the dam, there is a large mass of slide material of weathered Bragdon. This material was explored for the dual purposes of (1) removing a hazard to the spillway and outlet works which are located on that side of the river and (2) obtaining a nearby source of soil for the dam embankment.

Several other potential sources of impervious soils were investigated for Trinity Dam, but could not be seriously considered. A large source of disintegrated and decomposed granite, located about 10 miles downstream from the damsite, was a potential source of borrow for the Lewiston Diversion Dam, but the hauling distance made it prohibitive for use in Trinity Dam. A deposit of terrace gravels, comparable in quality to some of the less deeply weathered Weaverville formation, was investigated on the Stuart Fork of the Trinity River 5 miles upstream from the damsite; but shallow depths to water table, the presence of appreciable quantities of hard cobbles that would require separation, and the limited quantity influenced the rejection of this source. Other small deposits of weathered terrace gravels were briefly explored and rejected. The three sources of impervious materials that warranted serious consideration were the Weaverville formation on Buckeye plateau, the weathered Bragdon adjoining it, and the weathered Copley above the right abutment of the proposed dam. Figure 14 shows the location of these potential borrow sources. Borrow area A is the specifications source.

(c) Pervious Materials. --For purposes of investigating dredge tailings and undisturbed gravels in the river channel, the following areas were arbitrarily selected: Downstream from the damsite--area 1000; damsite, including the abutments--area 1100; upstream from the damsite, except the undisturbed Stuart Fork gravel--area 1200; undisturbed Stuart Fork gravel--area 1300.