تاثير مسير تنش بر رفتار ترشدگي خاك هاي غير اشباع

Stress path effect on wetting behaviour of unsaturated soils

يكي از جنبه هاي مهم رفتار خاك‌هاي غيراشباع، تغييرشكل ناشي از افزايش درصد رطوبت يا به عبارتي ترشدگي است. عمده كارهاي صورت گرفته در خصوص مطالعه اثر ترشدگي در مسير اديومتري است. آزمايش اديومتري توانايي مطالعه مسيرهاي مختلف تنش را ندارد، بنابراين به منظور بررسي مسيرهاي مختلف تنش استفاده از آزمايش سه محوري ضروري ‌است. در اين مقاله با استفاده از دستگاه سه محوري غيراشباع، رفتار خاك در فرايند ترشدگي در سه مكش بافتي مختلف و در هر مكش بافتي براي دو تنش انحرافي متفاوت مورد بررسي قرار گرفته است. دو مسير تنش متفاوت براي حصول وضعيت تنش قبل از ترشدگي به نمونه‌ها اعمال شده ‌است. مسير اول با ثابت نگه داشتن سطح مقطع نمونه طي بارگذاري متناسب با شرايط اديومتري است و در مسير دوم نمونهها ضمن تجربه يك تنش تحكيمي همسان، با ثابت نگه داشتن تنش محصور كننده تنش محوري افزايش داده شده است. نتايج آزمايش‌ها نشان داد كه در يك وضعيت تنش انحرافي و مكش بافتي اوليه يكسان، رفتار نمونه متاثر از تاريخچه تنش يا مسير تنش طي شده قبل از ترشدگي است. همچنين، تغيير‌شكل‌هاي به‌وجود آمده ناشي از تر‌شدگي خاك غير‌اشباع، در يك مسير مشخص، اساساً به مكش اوليه كه در آن خاك در معرض افزايش رطوبت قرار مي‌گيرد، بستگي دارد. به‌طوري كه، نمونه‌ها با مكش اوليه بيشتر كه در معرض ترشدگي قرار گرفته‌اند، تغيير‌شكل‌هاي بزرگتري را تجربه كرده‌اند.

One of the most important behavioral aspect of unsaturated soil is, engendered deformation due to wetting. Numerous research works exist in the literature concerning wetting behaviour of unsaturated soils. However the majority of them have been done in oedometric conditions. The oedometric apparatus, despite its simplicity, presents a certain limitations. In fact in this apparatus lateral deformation is zero, Also in the its classical form, confining pressure cannot be controlled and pore water pressure and matric suction measurement are not possible. Therefore, the effect of stress path cannot be studied. These limitations are not relevant in the triaxial apparatus. On the other hand, there is a few study concerning stress path on wetting behaviour of unsaturated soils. In the present study, using a special triaxial apparatus, influence of stress path on wetting behaviour of an unsaturated sand-fine mixture, for two level of initial deviator stresses and different initial suctions, was studied. The selected soil is composed of 60% sand, 25% clay and 15% silt. This soil is non plastic and according to Unified Soil Classification System, it is classified as silty sand (SM). The specimens were prepared using the moist tamping method. The purpose of using static wet compaction as opposed to dynamic compaction is to obtain a more homogenous specimen in terms of density and shear strength. Before applying wetting process, two types of conventional stress paths were induced on soil specimens,. The first stress path was coincided with k0 lines. For obtaining the oedometric condition using triaxial apparatus, the area of specimen was kept constant by augmentation of confining pressure incrementally. In the second stress path, first, specimen was isotropically consolidated, then axial stress was increased, while confining stress was maintained constant during loading process. Two groups tests including low and high levels of initial deviator stress state were considered in this study. For every group, the level of initial stress exerced on specimens, before wetting step, was similar. All of specimens were in equilibrium regarding to pore-air and pore-water pressures. Pore-air pressure was kept constant equal to 250 kPa for all of specimens. During wetting process external loadings, applied on specimens, were constant, while matric suctions was decreased incrementally due to change in water content. There are several conclusions deduced from this study. Difference between observed deformations in consolidation phase (before wetting) for two stress paths depends on the level of deviator stress, so that for low level of deviator stress this difference is insignificant. However for high level of deviator stress, this difference is considerable. Engendered deformation during wetting for a given stress path depends essentially on the levels of matric suction and deviator stress at the start of wetting, so that, deformation due to wetting is increased with increasing in initial suction or initial deviator stress. Based on the measured axial and volumetric strains during wetting phase, in the axisymmetric triaxial conditions, lateral strain was calculated. The obtained results indicate that lateral strain is considerable, therefore, evaluation of wetting effect in oedometric conditions underestimates deformation field