بررسي تأثير درز محيطي و پولوينو بر رفتار بدنه سد قوسي با پي سنگي نرم

Perimetral joint and pulvino effects on the behavior of arch dam built on weak rock foundation

در مطالعه ي پيش رو تاثير پولوينو بر رفتار بدنه سد، در سدهاي با پي سنگي داراي لايه نرم بررسي مي شود و با مقايسه با يك سد معمولي(بدون پولوينو به عنوان شاهد) تاثير پولوينو به عنوان يك راه حل سازه اي بررسي مي شود. رفتار مصالح مورد استفاده الاستيك خطي است. مدل تحت بارهاي وزن و هيدرواستاتيك با در نظر گرفتن مراحل ساخت سد قرار مي گيرد. نتايج حاصل از اين مطالعه نشان مي دهد استفاده از پولوينو باعث توزيع متقارن و يكنواخت تنش در بدنه ي سد خواهد شد حتي اگر پي سد داراي لايه هاي نرم و نامتقارن باشد. تنش بيشينه ي كششي در پولوينو رخ مي دهد و بدنه ي اصلي سد از اين تنش ها مصون مي ماند. گرچه ساخت سدهاي داراي پولوينو گران تر بوده و اجراي مشكل تري دارند اما حجم كمتر اين گونه سدها و الگوي مناسب تري كه از توزيع تنش ارائه مي دهند، باعث مي شود اين سدها هم چنان اقتصادي باشند و به عنوان رقيبي جدي براي سدهاي معمولي باقي بمانند.

Design and construction of a concrete arch dam needs two essential conditions: these are good rock foundation and convenient topography. When these two conditions are satisfied, arch dams would be the most desirable and the most economical types of dams. Sometimes the geometry of the valley is good, but the rock foundation is not appropriate or the rock has good material but the geometry of valley is poor. One important factor in safe design of an arch dam is the rock foundation stability problem when a large part of the external loads is transferred to the foundation by the arches. In arch dams, these forces are much larger than similar forces as compared with other dams. Moreover, the stability of an arch dam also depends on bearing capacity of the rock foundation. The idea of construction of arch dams with perimetral joint and pulvino was introduced by Italian engineers in the 40s to improve stress conditions. It was gradually expanded in the following decades. Pulvino is a thick concrete pad built between the arch dam body and the rock foundation as a strip foundation. Use of this structural component, reduces the uncertainties of the rock foundation, enabling a thinner body for the dam. Thus providing perimetral joints between the pulvino and the dam body; ensures more symmetrical distribution of stresses within the dam body. It also reduces potential tensile stresses at the boundaries of the dam body. In this study, the effect of pulvino is investigated on the behavior of a concrete arch dam body built in a valley with weak rock layers. The results are compared with the case of a conventional concrete arch dam (Control Dam); i.e., without pulvino in the same valley conditions. In order to maintain the same concrete design properties, the volume of the Control Dam had to increase by 40% in respect to the total volume of the dam with pulvino. The foundation has a weak layer in different situations identically for both dams. The only nonlinearity accounted for, corresponds to the perimetral joints. Applied loads include the weight and the hydrostatic pressure load. The dam weight is applied step by step to simulate the staged-construction of a concrete arch dam. The ANSYS 12.1 program is used to create the finite element models of the objective arch dam and its foundation. Results of this study show that use of pulvino causes symmetric and uniform distribution of stresses in the dam body even if the rock layers are weak and asymmetric. Contrary to the Control Dam case, higher tensile stresses occur only inside the pulvino and thus the main body of the dam is protected against such stresses. As pulvino is usually reinforced, the dam with pulvino and its perimetral joint remain acceptable. Thus, despite a rather expensive and harder construction job for such dams with pulvino and perimetral joints, their considerably lower concrete volume may well compensate the problem. Thus this type of concrete arch dams remains still economic and competitive for the future designs.