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原文传递 Drilled Shaft Under Torsional Loading Conditions

题名：	Drilled Shaft Under Torsional Loading Conditions
作者：	Kamal Tawfiq. Ph.D., P.E.
关键词：	Drilled d Shaft Slurry, Deep Foundation
摘要：	This study was directed toward investigating the effect of the combined modes of loading on the stress distribution along the shaft and determining the torsional capacity of the foundation. Additionly, it was decided to include the effect of the method of construction onthe load transfer of the shaft. Using different types of slurry fluids in drilled shaft construction could impact the shear resistance of the soil surrounding the shaft. To achieve our objectives, the study was divided into three phases. The first phase involved thorough laboratory testing on scaled shafts subjected to torsional loads in cohesionless soil. A special testing device was assembled for this purpose. Also the conventional direct shear test was used to determine the shear friction of soil saturated with different types of slurries. Slurry saturation of soil was completed in a testing setup developed exclusively for this study. In this setup the degree of slurry penetration in the soil could easily be determined. The rate of penetration was found to be much higher in polymer slurries as compared with the regular mineral slurries. However, for the mineral slurries, the rate of fluid loss in attapulgite slurry was higher than the bentonite. Also it was found that polymer slurry concentration has a great effect on the rate of fluid loss. High concentrations as 1 to 200 by weight produced penetration rates that were comparable to those obtained from the mineral slurries. This ratio, however, was four times higher than the manufacturers recommended value. Although this finding had been followed in our field testing on a full scale drilled shaft, the outcome performance of the mineral slurry was not satisfactory because of the fact that a regular pump was not able to discharge the high concentration polymer slurry from the boreholes which caused and extended delay when poring the concrete. In the second phase of the study a through numerical analysis was performed to explore the pattern of the stress distribution along a drilled shaft diameter X 20' length). The three dimensional finite element simulation could give an insight on how lateral stresses evolve when torsional loads are applied simultaneously with the lateral loads and overturning moments. The loads applied in the FE modeling were similar to those used by the FDOT on single and double arm assemblies. Using this technique it was possible to simulate cases of orthogonal loads and moment produced by the two arm assembly. Such type of analysis was not possible using the traditional design methods. The third phase involved full scales tests on three drilled shafts (4' diameter X 20’ length). These shafts were constructed using dry method, bentonite slurry, and mineral slurry. The dry hole shaft was used as a control test to compare with the performance of the other methods of construction. A fourth shaft was set between the three shafts to act as a support structure for the applied lateral and torsional loads. The testing arrangement used in this study was the first of its kind, since literature search could produce any full scale torsional testing on drilled shafts. Through the trial and error, it was possible to come up with a loading head for the three shafts and a load supporting unit for the fourth one. Schematics of this setup ate present in this study. Results for the full scale testing supported the laboratory findings that the method of construction has a profound influence on the maximum capacity of the drilled shaft under torsional load. The mineral slurry shaft experienced the lowest torsional capacity; while the dry shaft could sustain much higher loads. The modes of failure in the three shafts were also varied. The dry shaft suffered a structural failure that was noticeable from the 45° concrete cracking which develop^ at torsional load of about 480,000 ft.lb. The bentonite shaft failed at the foundation level and that was evident for the rotational slip of the shaft at torsional load of about 280,000 ft.lb. The polymer slurry shaft failed at a torsional load that was similar to the dry one. As opposed to the dry shaft, some rotational displacement was recorded, but the final failure occurred in the concrete and it was similar to the dry shaft. The field test demonstrated that various types of slurries may produce different magnitudes of torsional edacities in the drilled shaft. Therefore, using bentonite slurry would necessitate using a reduction factor of 0.5 when estimating the soil-shaft frictional resistance. Also, it was found that the existing FDOT methods underestimate d the capacity of the drilled shafts used in the field testing. The ultimate capacities of the shafts were higher than those predicted by the FDOT methods. Finally, to fulfill our main objectives in determining the capacity of the drilled shaft, it was certain that formulating a closed form solution approach would narrow the gap between the findings of this study and the results obtained from the existing simplified methods. Accordingly, the option was to incorporate the actual stress distribution along the drilled shaft from lateral loads and overturning moments in determining the torsional capacity of the shaft. Considering the most complicated case of loading when double arm assembly is constructed, the orthogonal lateral loads and moments were distributed on the shaft according to AASHTO requirements for the wind load analysis on double arm assembly. Using this distribution, it was possible to adapt the subgrade reaction method to develop a lateral stress distributions along the drilled shaft that was similar to those obtained from the finite element simulations. Consequently, the developed method was formulated in a MathCAD add-in routine where variables can easily be input and a quick solution to the problem could be obtained. It should be noted that the main concern of the developed method was to estimate the maximum capacity of the shaft and not to predict the magnitude of the torsional deformation. The reason for this constrain was that the soil reaction was bounded by the passive resistance of the material. Along the drilled shaft. it was not allowed to for the stress distribution to exceed the upper limit value set by the passive resistance of the soil. Using this method it was possible to correspond the shaft capacities obtained from field testing. A comparative study was done to demonstrate the variations in the shaft resistance to torsional loading using all the available methods.
报告类型：	科技报告