بررسي تاثير اشباع شدن خاك دانه‌يي بر مقاومت ونشست ناگهاني آن در آزمايش برش مستقيم

EFFECT OF SATURATION ON THE SHEAR STRENGTH AND COLLAPSE SETTLEMENT OF GRAVELLY MATERIAL USING DIRECT SHEAR TEST APPARATUS

هنگامي كه مصالح دانه‌يي اشباع ميشوند، به‌دليل تغيير در حوزه‌ي تنش، جابجايي ذرات خاك اتفاق مي‌افتد و درنهايت پديده‌ي نشست ناگهاني به‌وجود ميآيد. در اين نوشتار، تاثير اشباع‌شدن مصالح شني بر روي پارامترهاي مقاومت برشي و نيز عوامل موثر بر ميزان نشست ناگهاني اين مصالح با دستگاه برش مستقيم بزرگ‌مقياس مورد بررسي قرار ميگيرد. در ابتدا آزمايش‌هاي مشخص‌كننده‌ي خصوصيات فيزيكي و آزمايش‌هاي وزن مخصوص كمينه و بيشينه بر روي مصالح شني انجام شده است. نتايج آزمايشهاي مقاومت برشي بيانگر كاهش پارامترهاي مقاومتي در وضعيت اشباع نسبت به خشك است. نشست ناگهاني در مصالح نيز با آزمايش برش مستقيم بر روي نمونه‌ي خشك و سپس اشباع انجام و اثر عوامل مختلف بر آن بررسي شده است. نتايج نشان ميدهند كه افزايش رطوبت و چگالي باعث كاهش نشست ناگهاني در مصالح مي‌شوند، در حالي ‌كه افزايش سربار و سطح تنش برشي منجر به افزايش مقدار نشست ناگهاني مي‌شوند.

Dried granular material subjected to saturation under certain vertical stresses, results in collapse settlement, due to sliding in the grain contacts, particle rearrangement and breakage. In this research, the effects of saturation on shear strength parameters and the collapse behavior of clean gravels are investigated via a large scale direct shear test. For instance, first, all physical properties of the gravelly material were evaluated via some specific laboratory tests, including grading analysis, specific gravity of soil solids, unit weight of soils and Los Angeles abrasion tests. Maximum and minimum densities were determined in order to provide samples in a specific relative density. The materials, which were selected for collapse tests were classified as GP and SP, due to their potential for collapse during saturation. In order to compare the shear strength parameters in dry and wet states, large-scale direct shear test apparatus with a sample dimension of 30×30×15 cm was used, and strain control tests were done on the saturated and dried samples. Vertical stresses of 1, 3 and 5 Kg/cm2 were applied to the samples and shearing continued until 4.5 cm shear displacement. Collapse tests were done by the direct shear apparatus on the initially dry samples, which were flooded during shearing at the specific strain or in the specific shear stress levels. Result of laboratory tests showed that in most samples, shear strength parameters decreased with inundating the material, whereas the parameters in collapse and saturated tests were close to each other. An important part of this paper is studying the effect of different parameters on the collapse phenomenon in gravelly material. Specific parameters were selected as vertical stress, soil density, coarseness or fineness of particles, initial water content, and shear displacement or shear stress level. Vertical stress and shear stress levels had a direct effect on the collapse settlement; in fact, an increase in vertical and shear stress levels results in more collapse settlement and more stress release during saturation. Another factor is sand content in the whole sample. It was shown that although sand particles were not prone to crack due to their small size, the collapse potential in the sand samples was much higher than gravelly material. In fact, particle rearrangement was the fundamental cause of collapse in sandy soils, while particle breakage in gravelly material is another reason for the collapse. Also, experimental results indicated that addition of moisture content, even in small amounts, would cause a high reduction in collapse settlement. Furthermore, the effect of soil density was studied in the collapse settlement and it was observed that collapse decreases with an increase in relative density. Thus, in order to decrease collapse settlement in the compacted material, soil mass must be well compacted and prepared in a wet state.