专业英语资料4

Design of Gravity Dams

Concrete dams may be categorized into three principal types according to their particular physical form and the features of their design. The three types are: arch, gravity, and buttress.

A gravity dam is a concrete structure which resists the imposed forces by its weight and section without relying on arch and beam action.In its common usage the term is restricted to solid masonry or concrete dams which are straight or slightly curved in plan. The downstream face of gravity dam is usually of uniform slope which if extend, would intersect the upstream face at or near the maximum reservoir level. The upstream face is normally vertical excepting for steep batter near the heel. The upper portion is usually thick enough to resist the impact of loading objects and accommodate a roadway. The thickness of section at any elevation is adequate to resist sliding and to ensure compressive stresses at the heel under different conditions of loading.For

a gravity dam to be stable , the following criteria should be satisfied:

1)resultant of static and pseudo-static forces should lie within the

middle third lines or the kern of the dam at all section. This ensures a factor of safety of about 2 against overturning and eliminates tensile stress at the heel and the toe of the dam.

2)The dam should provide adequate factor of safety against sliding at

the construction joints, the base of the dam, and any planes of weakness within the foundation.

3)Maximum stresses in the dam section and the foundation should be

within the permissible stresses of the concrete used in the dam section and the foundation rock respectively.

Gravity dams can be analyzed by the Gravity Methods, Trial-load Twist Analysis, or the Beam and Cantilever Method, / depending upon the configuration of the dam, continuity between the blocks, and the degree of refinement required.

1.Concrete Gravity Dams on Rock Foundations

The designer of any dam must make basic assumptions regarding site conditions and their effects on the proposed structure. Site investigations provide the engineer with much of the information to evaluate these assumptions, the bases for safe dam design. Some important assumptions for small dam design involve uplift pressure, seepage control measures, channel degradation and downstream toe erosion, foundation conditions, and quality of construction. Additional assumptions should involve silt loads, ice pressures, earthquake accelerations, and wave forces.

Safety factors should be considered in the light of economic conditions. Large safety factors result in a more costly structure: however, low safety factors may result in failure, which could also lead to high cost. Proper safety factors result only from an adequate determination of sliding, overturning, and overstressing forces within and acting on the dam.

2.Overturning

Ordinarily，the safety factor against overturning is between 2 and 3. In smaller dams it is often larger. If the computed safety factor falls below 2, the section of the dam should be modified to increase the safety margin.

A gravity dam rarely fails from over turning since any tendency to overturn provides greater opportunity for a sliding force to create the