INTRODUCTION

The Stewart Mountain Dam located 41 miles (66 km) east of Phoenix, Ariz., on the Salt River, was completed in March 1930. The structure contains an arch dam, two thrust blocks for simulating abutments for the arch dam, three gravity dams, and two spillways. The arch dam measures 212 ft (64.6 m) high at the maximum section, 8 ft (2.44 m) thick across the crest, 34 ft (10.36 m) thick across the base, and 583 ft (177.7 m) in length along the crest. Four keyed vertical contraction joints with copper water-stops separate the arch into distinct concrete sections called cantilevers. The concrete structure has experienced alkali-silica reactions and has exhibited no bond across horizontal construction lift surfaces. In addition, the dam could be subjected to upgraded maximum credible earthquake (MCE) or probable maximum flood (PMF) loadings.
Numerous investigations, field measurements, laboratory test, inspection, and on-site tests have been performed over the years to assess material properties, deformation, and deterioration. Concrete cores were extracted in 1943, 1946, 1947, 1948, 1968, 1977, 1979, 1982, and 1985. The many engineering questions that arose during the investigations and inspections included the following: (1) What caused the poor lift surface bond and what was its extent?; (2) What is the serviceability life expectancy of the existing or deteriorating concrete?; (3) At what rates are the alkali-silica reactions deteriorating the concrete? Have the reactions stopped? Could changing reservoir levels or other conditions accelerate the reaction?; (4) is the concrete more brittle due to micro-fracturing from the reactions?; (5) Why does the upper arch appear more susceptible to the alkali-silica reactions than other areas of the dam?; and (6) Why do deflection measurements of the crest indicate a slowing or stopping of the rate of permanent drift toward the up-stream direction?
绪论：Stewart大坝位于亚利桑那州，凤凰岛东部66Km的盐河上，于1930年3月建成，主体结构包括一座拱坝，两座模拟拱坝拱端的推力墩，三座小重力坝和两个溢洪道。拱坝最大坝高64.6m，坝顶厚2.44m，坝底厚10.36m，坝顶弧长177.7m。四条带键槽的垂直收缩缝用止水铜片连接，将大坝分成不同的混凝土部分，通常称之为悬臂梁。坝体混凝土结构由硅碱反应实验显示在其水平结构的上表面无粘聚力，另外，该拱坝能经受住强化的可能最大震荷载或者可能最大洪水荷载。
多年来，为评估材料特性，变形和恶化过程已进行过无数调查研究，野外测量，实验测试，观测和单点实验。混凝土核心在1943，1946，1947，1948，1968，1977，1979，1982和1985年相继被提取出来。
在调查研究过程中提出了许多工程师的疑问，包括：
1：是什么导致上表面粘聚力的减弱，它的范围是多少？2：现有或恶化中的混凝土可用龄期是多久？3：硅碱反应到何种程度开始破坏混凝土？反应会停职吗？可以改变水库等级或其它条件来加快反应速度吗？4：混凝土是否由于反应中的宏观破碎变得硬脆？5：为什么在硅碱反应中拱坝上部相比其它区域的混凝土更易受影响？6：为什么坝顶朝上游的永久位移观测显示坡率变缓或者停止增长？

LITERATURE REVIIEW

Many dams built before 1945 and located in the southwestern United States, such as the Coolidge, Stewart Mountain, and Parker dams in Arizona and the Riant and Matilija dams in California, have shown signs of alkali-silica reactivity in the concrete. The Matilija Dam showed permanent displacement upstream at the crest ("Railroad" 1984), with concrete cores indicating alkali-silica reactions and deterioration in the uper 25 ft (7.6 m). Modifications made to the Matilija Dam included notching and enlarging the spillway. The Railroad Canyon Dam in southern California has similar horizontal lift surface bond problems (Matilija 1972). The dam, completed in 1928, consists of an arch dam portion with supporting thrust blocks. The dam was stabilized by placing additional concrete on the thrust blocks and installing six 200 kips (890 kN) post-tensioned cables in each abutment.
文献综述：
许多位于美国西南部在1985以前建造的大坝，像在亚利桑那州的Coolidge，Stewart Mountain，Parker大坝和在加利福尼亚州的Riant，Matilija大坝，都显示了在混凝土中硅碱反应的特征。Matilija坝显示在坝顶朝上游的永久位移（“Railroad”1984），同时，混凝土骨料显示硅碱反应和混凝土恶化在顶部7.6m范围内。Matilija坝整修包括开槽和扩大溢洪道。位于加州南部的Railroad峡谷坝有相似的水平上表面粘聚力问题(Matilija 1972)，该坝建于1928年，由拱坝加推力墩组成。坝体稳定由推力墩上的额外的混凝土块和安装在每个拱座上的六根预应力锚索维持稳定。

MODIFICATION CONSIDERATION

Seismic analysis of the dam showed that the arch portion of the dam is potentially unstable during a MCE seismic event. Justification of the decision to modify the structure and the chosen method of modification is based upon the following investigation findings.
1. Inertia forces at the crest of the arch are probably quite large judging from the resulting peak accelerations of 2.32 g at the crest.
2. A linear finite element analysis of calculated tensions indicates that the arch dam pulls apart horizontally with a duration of upu to 0.1 seconds, long enough for concrete blocks to slide.
3. Horizontal construction lift surfaces are laitance filled and exhibit little or no cohesion.
4. Vertical contraction joints are keyed but provide little resistance against sliding of the massive concrete blocks.
5. Uniaxial compression tests on 6-in. (15.2 cm) cores extracted from the dam interior indicate very strong concrete of about 5,400 lb/sq in. (37.21 MPa). Alkali-aggregate reaction has not deteriorated the dam to the point requiring its total replacement.

Seismic analysis revealed that the dam will not perform dynamically as a monolithic unit, because of the unbonded horizontal lift surfaces.
坝体整修考虑的因素：
坝体地震分析显示，在可能最大地震荷载中拱坝起拱处可能不稳定。决定整修该建筑结构的理由和选择整修的方法基于以下的调查发现：
1、 从山顶2.32倍的重力加速度判断，拱顶端的内力极有可能很大。
2、 线性有限元分析计算张力表明，拱坝被水平拉开持续时间达到0.1秒，这个时间足以让混凝土块产生滑移。
3、 水平结构上表面填充水泥浆，很少或者没有粘聚力。
4、 垂直收缩缝挖槽连接，但对于大体积混凝土块来说，键槽只提供很小的抗滑力。
5、 从坝体内部提取的，尺寸为15.2cm的混凝土立方体块单轴压缩实验显示，混凝土块强度达到37.21MPa，强碱实验并没有把坝体破坏到需要完全置换的程度。
地震分析显示，由于水平上表面无黏聚立，坝体并不是像单块单元那样动态发挥作用。

POSTTENSIONING

Post-tensioned tendons increase the normal force on the unbonded horizontal arch lift line surfaces and consequently the frictional component of sliding. Cables also produce three-dimensional stresses throughout the arch section depending on orientation and eccentricities. Post-tensioning induces two equal and opposite loads at the ends of the free length. Load at the top or head transfers through the bearing plate into the concrete. This load can be considered a concentrated, or point, force. Load at the bottom develops through bond along the embedment length of the cable.
后张拉力
预应力钢筋束在未黏着的水平拱上表面增加了普通力，因而增加了滑移的摩擦阻力。锚索还产生贯穿拱区的三维应力，这个力大小取决于力的方向和偏心率。预应力在自由长度尾端引起两个大小相等、方向相反的荷载，作用在顶部或上部通过支承平台传递到混凝土上。这个荷载可以当做作用于一点的力来考虑。底部的荷载通过埋置的通长锚索的黏聚力产生。

CABLE CAPACITY AND CONSIDERATIONS

The aforementioned inertia forces were computed at 15 locations along the crest and substituted into Eq. 3. A design cable load is 700 kips (3,114 kN) pre 10 ft (3.05 m0 spacing along the crest. The cables were positioned within the arch were as close as possible to the centerline of the vertical radial section. Finite element studies showed a beneficial stress distribution within the arch dam created by the cable load during normal operating conditions. Special design considerations and requirements were developed for drilling methods, drilling accuracy and tolerances, tensioning sequence, placement within the arch, corrosion protection, grouting, monitoring, and pre-stressing.
锚索负载和考虑因素
前面提到的内力沿拱冠15个位置计算，分成三个Eq。一条设计锚索应力为3114KN，沿拱冠3.05m分隔。锚索布置在拱中应尽量靠近垂直径向中线。有限元研究显示正常工况下由锚索应力产生的分布于拱坝中的应力是有利的。考虑到钻孔方法，钻孔精度和容许偏差，张拉顺序，坝体内的布置，防腐，坝体灌浆，监测和先压法，应进行专门设计考虑和有必需的条件。

CONCLUSIONS

The Stewart Mountain Dam has been deteriorated by alkali-silica reactions and exhibits no bond across horizontal lift surfaces. In addition, it is now required to be subjected to an upgraded maximum credible earthquake. Trends from historic deflection measurements, concrete coring programs, and laboratory tests indicate that the deterioration from alkali-silica reactions is contained. A system of post-tensioning for arch stabilization was chosen. Ease of design and cable-load control were among the factors in this selection. Post-tensioned cables are a viable solution for the dynamic stability of a thin arch dam.
结论
Stewart大坝被硅碱反应破坏，并且显示在水平结构上表面无黏聚力。另外，现在很可能遭受到一个强化的可能最大地震荷载。从历史偏移测量趋势，混凝土块试验，实验室测验显示其已经包含了硅碱反应导致的变质。一系列为拱端稳定的后张法已经被采纳。优化设计和锚索应力控制在这个选择因素之内。预应力锚索在薄拱坝动态稳定分析的方面是个切实可行的措施。

最后说明一点　　
以上是一篇文章　还有几张图没有上传　中文翻译是我自己翻的　因为对这工程不了解　难免有些不切实际之处　故本文只供学习交流之用　

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