maximum section was assumed to be at elevation 3010. Laboratory tests indicated a value of 750,000 pounds per square inch for the modulus of elasticity of the abutment rock and 0.15 for Poisson's ratio. At the abutments a limiting compressive stress of 750 pounds per square inch for a loading condition, including the effects of earthquake, was tentatively set. In the interior of the dam a compressive stress of 1,000 pounds per square inch was the limiting stress established by design criteria.

(a) Design A-6.-Design A-6, the first of the new layouts, is shown on figure 33. The shortest usable racius to the axis of the damn was found to be 900 feet. By thus reducing the radius, more load is taken in arch action and less in the vertical direction. Consequently, the vertical section could be thinned. To reduce tensile cantilever stresses on the downstream face near the top of the dam, the upstream face was curved in a vertical plane. The crown cantilever analysis in design A-6 revealed the presence of compressive stresses in excess of 750 pounds per square inch at the abutments of the top half of the dam. The need for thicker abutments in the upper portion was evident.

(b) Design A-7.-By reducing the intrados radii in the upper portion of the dam in design A-7, the abutment thicknesses were increased. The crown section of design A-6 was retained. A plan and maximum section for design A-7 is shown on figure 34.

A radial adjustment analysis showed excessive abutment stresses at the top of the dam. By leaving that portion of the dam above elevation 3665 ungrouted, thus assuming no load above this elevation to be carried by arch action, stresses from the complete trial-load analysis were found to be well within the limits set at the time. The arch and cantilever stresses parallel to the faces, along with the loading conditions and assumptions used in the complete trial-load analysis, are shown on figure 35.

In reevaluating the strength of the rock, a limiting compressive stress at the abutments of 600 pounds per square inch was established. To reduce the stresses of design A-7 to an acceptable limit, several steps were considered other than increasing the abutment thickness. Temperature in the dam at the time of grouting had been assumed to be 45° F. at all levels of the dam. By varying this temperature from 40° F. in the bottom to 55° F. at the top, the bottom part could support more of the load, while relieving some of the load in the top. The other measure taken was to formulate a construction program that was realistic and

would force the arches and cantilevers in the lower part

of the dam to support more of the load. This construction program assumed the dam constructed to elevation 3550 and grouted to elevation 3500. As construction is continued, water would be stored in the reservoir, and when the dam is topped out, the water in the reservoir would have been raised to elevation 3500. The grouting would then be completed, and the water in the reservoir would be allowed to rise to a normal level of 3710.

A comparison of stresses including the effects of the construction program and omitting them is shown on figure 36. At the abutments the arch stresses are reduced from 20 percent to 30 percent in the upper portion of the dam. Although these stresses were improved, they were not considered entirely satisfactory.

To further reduce the critical abutment stresses, the abutments had to be thickened. Since further reduction of the intrados radii did not appear to be feasible, the alternative was to add concrete on the upstream face.

28. SPECIFICATIONS DESIGN-DESIGN A-8. In design A-8, as shown on figure 37, 15 feet of concrete was added to the maximum section at the base and extending up to elevation 3300. From here the face was curved in a vertical plane to the axis at elevation 3710. Stresses listed at the bottom of figure 37 are estimated final stresses based on stresses resulting from a Crown Cantilever Analysis. Since these stresses were acceptable at the time, specifications for construction of Glen Canyon Dam were based on this layout.

29. SOME DESIGNS PREPARED SUBSEQUENT ΤΟ SPECIFICATIONS ISSUANCE. After specifications were issued, based on design A-8, a number of additional layouts were made in an effort to reduce the volume of concrete in the dam and produce a more acceptable stress distribution on the abutment rock. Additional tests of the abutment rock resulted in a lower value of the modulus of elasticity, which in turn tended to increase the compressive stresses at the abutment intrados. Stresses in other portions of the dam were conservative and well below the allowable limits. The problem of design resolved into how to increase the abutment thickness while maintaining or reducing all other thicknesses. This could be accomplished by using uniform thickness sections in the central portion of the dam, terminating in short radii fillets on the downstream face.