Safety of Existing Dams Evaluation and Improvement (1983) / Chapter Skim
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6. Concrete and Masonry Dams
6. Concrete and Masonry Dams
Pages 183-212
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From page 183... ...
A gravity dam depends on its weight to withstand the forces imposed on it. It generally is constructed of unreinforced blocks of concrete with flexible seals in the joints between the blocks.
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From page 184... ...
In addition to the factors mentioned for gravity dams, particular attention must be paid to the quality and performance of the concrete in the face slab. Because of its relative thinness it cannot withstand excessive deterioration, pitting, or spelling that will decrease the strength of the slab and
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From page 185... ...
Arch Dams Arch dams (see Figure 6-3) are relatively thin compared with gravity dams.
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From page 186... ...
~ - at, ~ - = - ~ by-== -= ~ = = 5- ,= ~ \t 1, 1 1 ,;Arch it', il ,' ., ',element ~t it 'I/ 11 ll , __D __ 1~ 1 1 \, ELEVATION (developed ) ~ Maximum cantilever element | Arch el ement PLAN SECTION AT CROWN CANTI LEVE R \_ FIGURE 6-3 Plan, profile, and section of a symmetrical arch dam.
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From page 187... ...
Arch-Gravity Dams In arch-gravity dams imposed loads are carried partially by the foundation and partially by the abutments. These dams are of block construction and have a cross section that has a mass somewhere between that of an arch and FIGURE 6-4 Concrete arch dam under construction; shows keys between blocks.
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From page 188... ...
COMMON DEFECTS AND REMEDIES The following discussions are intended, first, to emphasize the defects and remedies that generally could be relevant to any type of concrete dam and, ~ .
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From page 189... ...
189 Abutments Joints, Fractures, Faults, and Shear Zones The orientation of major discontinuities in abutments is critical in relation to the distribution of stresses from an arch dam but not as critical for a gravity structure. For an arch dam the main consideration is whether the direction of such discontinuities is parallel to or closely parallel to the directions of thrust from an arch (see Figure 6-6~.
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From page 190... ...
Concrete (local) Spalling Stress and concentrations cracking Freeze-thaw action Differential movement Reduction of effective section Increased stresses Loss of weight Increased leakage deterioration Increase leakage Loss of section Stress concentrations Determine concrete qualities by testing.
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From page 191... ...
surfaces concrete Leaching apply and pursue same on Porous concrete Loss of weight remedial measures. concrete Loss of strength Determine depth and Increased leakage extent of cracks and see (C)
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From page 192... ...
Increased uplift Pursue measures similar to Movement Loss of stability (E) Differential Specifically assess hazards movement of associated with slides, dam piping, or sloughing.
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From page 193... ...
Lack of protection Overtopping Increased leakage Inoperable appurtenances Severe cracking Stress redistribution Reduction in stability Anomalous changes in section or plan Increased cracking and spelling Increased leaks Binding of gates and operators Undermining Loss of stability Complete failure of appurtenances Establish survey control system. Monuments for horizontal control— some must be sufficiently far from dam to be out of influence zone.
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From page 194... ...
Reservoir slides (M) Siltation Inability to operate Reduced capacity of spillways/outlets Increased probability of overtopping Unstable geology Saturation High runoff Sloughing Geology Normal or abnormal inflow Cultivation upstream Vegetation removal Sudden high waves with resultant overtopping Siltation Blockage of outlets and spillways Increased loading Reduction of reservoir capacity Increased loads Reduced stability Plugging of outlets Reduction of reservoir capacity Rock bolt blocky or slabby rock.
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From page 195... ...
Review loadings on dam resulting from ice and assure dam can tolerate. Accelerated deterioration Blockage of spillways and outlets Damage to piping and equipment Misoperation of gates Damage to trash racks Parapet damage Increased loading
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From page 197... ...
Particularly critical are the slopes adjacent to spillways; slide blockage of an intake, chute, or spillway basin could be disastrous if it occurs during the operation of a spillway. In the case of an arch dam care must be taken to ensure that stresses developing from the dam have a sufficient mass of stable rock available to accept such stresses without undesirable displacements occurring in the rock mass.
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From page 198... ...
. Deformation may be caused by reservoir load, earthquake forces, arch dam thrust, unfavorable orientation of the cracks in the rock with respect to the load directions imposed by the dam or reservoir, large ranges of temperature, freezing and thawing of water within the cracks in the rock, softening of the contact surfaces between rock blocks caused by water or other weathering forces, an abutment mass insufficient to withstand the overall thrust forces from the arch dam, presence of shear or fault zones that were not contemplated in the design, or excessive uplift forces.
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From page 199... ...
As previously noted, generally the upper part of valley walls are looser than the lower part of the walls; thus, earthquakes can induce much stronger reactions in the upper portion of the abutments. One California owner of a concrete gravity dam with a suspect abutment has used a number of measures to monitor and stabilize an abutment including regrading part of the abutment (upstream)
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From page 200... ...
Thus, it is only reasonable that the evaluation of existing dams consider the influence of actual site features and characteristics. In a recent stability evaluation for an overtopping flood condition (CIaytor Dam, New River, Virginia)
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From page 201... ...
Costly remedial measures might be avoided where foundation drainage, either existing or added, can be relied on to preclude the buildup of uplift pressures to full reservoir magnitudes in narrow cracks of the size comparable to those created by structure rotation under the increased flood loading. In such cases uplift can be represented by a linear distribution from tailwater pressure at the toe to tailwater pressure plus a percentage of the difference between the headwater and tailwater pressures at the line of drains and thence to headwater pressure at the tree!
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From page 202... ...
From the cores, measurements were made of strength, modulus of elasticity, Poisson's ratio, density and thermal diffusivity, also, a careful petrographic examination was made. Correlations between pulse velocity measurements and strength were used to target the areas of generally deteriorated concrete, which by this time had reached strengths as low as 1,400 psi.
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From page 203... ...
For example, the upstream face of Rush Meadows Dam, a concrete arch at high elevation in the Sierra Nevada, was coated in 1977 with a layer of "unite covered by two coats of polysulfide. The first layer of polysulfide was thin, placed over a primer, and was followed by a thicker final layer.
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From page 204... ...
Gravity dams can be analyzed by the gravity method, trial-load twist analysis, or the beam and cantilever method, depending on the configuration of the dam, the continuity between the blocks, and the degree of re
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From page 205... ...
Bureau of Reclamation (1977~. Flood Loading The evaluation of stability of gravity dams during a spillway design flood is necessary in deciding whether modifications, such as added spilling capac
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From page 206... ...
However, concrete gravity dams on firm rock foundations are inherently resistant to overtopping flows provided stability against overturning and sliding are ensured and that the groin and foundation downstream of the dam are capable of resisting erosion and disintegration resulting from impingement of the overtopping water. An analysis to determine stability during great floods should be based on conservative estimates of headwater and tailwater elevations.
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From page 207... ...
Increased positive resistance has been accomplished by stressed tendons anchored in the foundation rock, addition of concrete mass, construction of concrete buttresses, or placing a buttressing embankment against the downstream face. Buttress and Multiple-Arch Dams Slab and buttress and multiple-arch dams built 50 to 70 years ago were designed on principles that may not meet modern standards.
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From page 208... ...
The arches forming the faces of some old multiple-arch dams have small central angles, so that the arches impose considerable thrust on the buttresses normal to their center lines. Such forces must be resisted partly by the adjoining arches if the buttresses are insufficiently braced.
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From page 209... ...
The promise of this new technology may be greatest in construction of large new structures, where the potential economies of scale are obvious. However, it would appear to offer advantages also in remedial work on existing dams.
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From page 210... ...
Experiences at Spaulding Dams Three separate concrete dams form Pacific Gas and Electric Co.'s Lake Spaulding in California. The main dam is a 276-foot-high arch-gravity dam.
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From page 211... ...
(1970) "Earthquake Response of Concrete Gravity Dams," Journal of the Engi peering Mechanics Division, ASCE, Vol.
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From page 212... ...
(1928) "Analysis of Arch Dams by Trial-Load Method," ASCE Conference Proceedings International Commission on Large Dams (1970)
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