تاثير افزودن سيمان بر پارامترهاي مقاومت برشي خاك هاي مسيله دار واقع در سواحل جنوبي درياي خزر

EFFECT OF CEMENT TREATMENT ON SHEAR STRENGTH PARAMETERS OF DIFFICULT SOIL FROM SOUTHERN COASTLINES OF THE CASPIAN SEA

در اين تحقيق تاثير افزودن سيمان در پارامترهاي مقاومت برشي 3 نوع خاك مسيله دار واقع در سواحل جنوبي درياي خزر، شامل لس گرگان، رس رشت، و ماسه‌ي انزلي با استفاده از آزمون هاي مختلف آزمايشگاهي از قبيل حدود اتربرگ، تراكم، تك‌محوري و سه محوري تحكيم‌يافته- زهكشي‌شده مورد ارزيابي قرار گرفته است. نتايج نشان مي دهند كه افزودن سيمان باعث بهبود كارايي و افزايش چگالي خشك بيشينه مي شود. همچنين افزايش ويژه ‌يي در مقاومت فشاري تك‌محوري، مقاومت برشي، و مدول كشساني نمونه ها مشاهده شده است. ارزيابي ها بيانگر اين نكته‌اند در حالي‌كه تثبيت با سيمان منجر به بهبود قابل توجه مقاومت برشي مي شود، گسيختگي نمونه ها از رفتار نرم به شكننده تغيير مي‌يابد. از سوي ديگر مشاهده مي شود كه پوش گسيختگي نمونه هاي سيمان دار به‌صورت غير خطي است و تيوري هايي از قبيل تيوري گريفيت اصلاح‌شده و معيار گسيختگي جانسون در حد قابل قبولي رفتار خاك هاي سيمان دار را تشريح مي كنند.

Many attempts have been made by civil engineers to improve and optimize the use of existing soils. The distribution and extent of problem soil pose many difficulties for construction projects. All improvement techniques seek a solution for an increase in density and shear strength, providing stable conditions and reduction of soil compressibility, in order to control ground water flow and accelerate the rate of consolidation. Experience shows that the use of additives, such as cement, lime, fly-ash, bitumen and polymer stabilizers, leads to an improvement in the engineering properties of natural soil deposits. The choice and effectiveness of an additive depends on the type of soil and its field conditions. Nevertheless, knowledge of the mechanical behavior of treated soil is as important as selecting the appropriate stabilizer. This study has made a comprehensive examination of the effectiveness of cement treatment on the shear strength parameters of soil encountered in the southern coastline of the Caspian Sea, including Gorgan Loess, Rasht Clay and Anzali Sand. Cement was added in percentages of 2.5, 5, and 8 by dry weight of the soil. A series of laboratory tests comprised of Atterberg limits, standard proctor, unconfined compressive strength and consolidated-drained triaxial tests were performed on non-treated soil as well as on cement treated samples. The addition of cement was found to improve the workability and compaction characteristics of the soil. Moreover, significant improvements in unconfined compressive strength and modulus of elasticity were observed. The improvement is dependent on the type of soil. Triaxial test results indicated that while cement treatment improved shear strength remarkably, the type of failure varied greatly from ductile to brittle behavior. Non-treated, 5%, and 8% cement treated soil displayed ductile, planar, and splitting types of failure, respectively. Therefore, while increased strength is achieved by cement treatment, high percentages of cement should be used with caution in field applications. In addition, results showed that cohesion increased significantly by variation of cement content, but internal friction angle increased initially and remained constant at higher percentages. Eventually, it is found that the failure envelope trend of cement treated samples is non-linear, and the use of Mohr-coulomb criteria has led to a number of anomalies. Some failure criteria, such as the modified Griffith theory (1962), and the criterion suggested by Johnston (1985), based on mean squared error (MSE) analysis, can satisfactorily describe the soil-cement behavior.