Figure 23. --Trinity Dam stability studies for specification design sections.

BRAWN. Ind.O..A.2.

TRAGED. BOR

CHCOMBOL...

DENVER, GOLORADO,

The contractor was permitted to construct the zone 1 to elevation 2050 without placing soil in the other zones, provided the slopes of the zone 1 are 2-1/2 to 1 or flatter.

44. Earthquake Considerations. Because of the size and importance of the structure, and the seismicity rating of California, seismic stability of Trinity Dam was investigated by two methods. The first method that is more commonly used in engineering is the acceleration of gravity applied as mass times acceleration reversed; the second is the seismic resistance method which requires knowledge of the elastic properties of compacted soil.

To determine the proportion of the gravity force to use in the acceleration method of analysis, earthquakes with an intensity of IV or greater at the epicenter (Modified Mercalli sensory perception scale) and with an influence zone within about 75 miles of the Trinity damsite are shown on figure 25. Zero intensity isoseismic lines are shown on figure 25. Nineteen earthquakes were found to be applicable to this problem during the period 1928 to 1953. Only one isoseismic line encloses the Trinity area and that is No. 15 which occured September 23, 1947, and had an intensity of VII at the Punta Gorda light station (near epicenter). From these data it appears that strong earthquakes would affect the Trinity area on the average about once every 27 years.

At the Ferndale, Calif., accelerograph, maximum vertical acceleration of earthquake No. 15 was reported as 0.007 times gravity and maximum horizontal acceleration as 0.015 times gravity. However, the acceleration reported was qualified by the statement: "Possibly preceded by stronger motion. " In view of the qualifying statement and the fact that the trace of this component was moving vertically downward considerably above the zero reference line at the time the pen first made contact with the recording paper, accelerations approximately 10 times as strong as those reported are possible.

Study of the zero isoseismic diagram shows that strong motion was translated inland about 30 miles east of Trinity damsite, suggesting that most of the acceleration may be reasonably transposed to Trinity Dam. Therefore, about 70 percent of the interpreted acceleration at Ferndale is a conservative estimate of the acceleration to be applied to Trinity Dam equal to a vertical component of 0.05 times gravity.

The centerline of the crest of the dam is N. 90° E. which is parallel with the advancing earthquake wave fronts from either the west off the coast of California or from the east in the area north of Reno, Nev. Consequently, the dam is in an advantageous position relative to the seismic wave train, or known epicentral areas. Since the horizontal earthquake component can be expected to advance toward the abutments, it would be less likely to influence stability of a failure circle normal to the centerline of the dam.

No major faults have been located near the site. The foundation is Copley meta-andesite; hence, geological conditions at and adjacent to the site should not contribute to seismic instability.

In the light of the maximum recorded acceleration over a 27-year period, a part of gravity equal to 0.05 was applied horizontally, vertically, and simultaneously in these directions as shown on figure 23. The specifications design section least factor of safety circles for the limiting stress conditions of the construction stage, high-level steady state, and drawdown state were used for the analysis. The results are shown on figure 23. The factors of safety for earthquake less than 1.5 are considered acceptable in view of the facts that complete drawdown is unlikely, apparent cohesion can be expected to increase, the value 0.05 times gravity is considered conservative, and the frequency of strong earthquakes is low.

The seismic resistance method study indicated that the dam can be considered stable under the influence of the transient stresses, since the geometry of the mass precludes segments from vibrating independently. Any longitudinal part of the dam having a tendency to vibrate would be damped by adjoining parts. Values of seismic resistance, Rs, are shown on figure 26 for various embankment heights. This type of analysis is discussed in an intraoffice memorandum.6/

45. Instruments. Because of the then (1957) unprecedented size and the importance of Trinity Dam as well as the use of residual soils in so large a structure, various standard Bureau of Reclamation type instruments were to be placed within the embankment and upon the outer slopes of the embankment. Location and details of the instruments are shown on figures 27 and 28.

The instruments include: (1) twin-tube piezometers to indicate pore-fluid pressures within the embankment, (2) internal vertical movement devices to observe strain in a vertical direction, (3) internal horizontal movement devices to observe strain in a horizontal direction, and (4) surface settlement points to indicate movement of the outer slopes. At the time of installation of the horizontal movement devices the Bureau believed it to be the first installation of its kind.

Since stress conditions within an earth embankment change from the time of construction through the operational life of the structure, these instruments provide a means of obtaining a continuous performance record. The assembled data may be studied with respect to current design and construction control practices, with the object of installing, in the future, more stable and more economical designs from available earth materials. These instruments provide a means of checking the stability at any time, and hence, may be used as controls during construction as well as during operation.

The instrument installations include 72 piezometers; 5 vertical movement devices containing about 100 crossarm units on 10-foot centers; 9 horizontal movement devices incorporated into vertical movement 6/Esmiol, E. E., "Seismic Resistance--Trinity Dam--Central Valley Project, California," February 13, 1957, Bureau of Reclamation (unpublished).