10-08-2012, 12:56 PM
GRAVITY DAMS
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Purpose and Scope
General
The objective of this chapter of the Guidelines is to provide Staff engineers,
licensees, and their consultants with recommended procedures and stability criteria for
use in the stability analysis of concrete gravity structures. Engineering judgement must
be exercised by staff when evaluating procedures or situations not specifically covered
herein. Unique problems or unusual solutions may require deviations from the criteria
and/or procedures outlined in this chapter. In these cases, such deviations must be
evaluated on an individual basis in accordance with Chapter 1, paragraph 1-4 of these
Engineering Guidelines
Review Procedures
Review by the staff of analyses performed by licensees, or their consultants,
should concentrate on the assumptions used in the analysis. The basis for critical
assumptions such as allowable stresses, shear strengths, drain effectiveness, and loading
conditions should be carefully examined. The consultant's reports, exhibits, and
supplemental information must provide justification for these assumptions such as
foundation exploration and testing, concrete testing, instrumentation data, and records
maintained during the actual construction of the project. Also, the staff engineer's
independent knowledge of the dam gained through site inspections or review of
operations inspection report as well as familiarity with previous reports and analyses,
should be used to verify that the exhibits presented are representative of actual
conditions. Methods of analysis should conform to the conventional procedures used in
the engineering profession.
Conservative assumptions can reduce the amount of exploration and testing
required. For example, if no cohesion or drain effectiveness is assumed in an analysis,
there would be no need to justify those assumptions with exploration and testing. For this
reason, it may sometimes be more beneficial to analyze the dam with conservative
assumptions rather than to try to justify less conservative assumptions. There is however
a minimum knowledge of the foundation that must be obtained. The potential for sliding
on the dam foundation is generally investigated. However, the potential for failure
through a deep surface deep in the foundation should be investigated. Experience has
shown that the greatest danger to dam stability results when critical attributes of the
foundation are not known. For example, in the case of Morris Shephard Dam, 26/ P-
1494, a horizontal seam underlaid the dam, providing a plane of weakness that was not
considered. This oversight was only discovered after the dam had experienced significant
downstream movement.
Forces
General
Many of the forces which must be considered in the design of the gravity dam
structure are of such a nature that an exact determination cannot be made. The intensity,
direction and location of these forces must be estimated by the engineer after
consideration of all available facts and, to a certain extent, must be based on judgment
and experience.
Dead Loads
Unless testing indicates otherwise, the unit weight of concrete can be assumed to
be 150 lb/ft3. In the determination of the dead load, relatively small voids, such as
galleries, normally are not deducted unless the engineer judges that the voids constitute a
significant portion of the dam's volume. The dead loads considered should include
weights of concrete and superimposed backfill, and appurtenances such as gates and
bridges.
External Water Imposed Loads
Hydro Static Loads
Although the weight of water varies slightly with temperature, the weight of fresh
water should be taken at 62.4 lb/ft3. A linear distribution of the static water pressure
acting normal to the surface of the dam should be applied.
Nappe Forces
The forces acting on an overflow dam or spillway section are complicated by
steady state hydrodynamic effects. Hydrodynamic forces result from water changing
speed and direction as it flows over a spillway. At small discharges, nappe forces may
be neglected in stability analysis; however, when the discharge over an overflow spillway
approaches the design discharge, nappe forces can become significant and should be
taken into account in the analysis of dam stability.