تعیین مدول برشی حداکثر محصور نشده خاک رس تثبیت شده با آهک بوسیله دستگاه المان خمشی

تعیین مدول برشی حداکثر کرنش کوچک خاک‌های تثبیت شده از اهمیت ویژه‌ای برخوردار است و یکی از اساسی‌ترین ویژگی‌های خاک در زلزله و بارگذاری های دینامیکی است. برآورد دقیق پارامترهای دینامیکی خاک از جمله مدول برشی حداکثر در تحلیل رفتار سازه‌ها نقش تعیین‌کننده‌ای دارد. در این مطالعه با استفاده از دستگاه المان خمشی، تاثیر مقادیر مختلف آهک (5%، 10%، 15% و 20%)  با میزان آب (45%، 65% و 85%)در زمان‌های عمل‌آوری 28 و 56 روزه بر مدول برشی حداکثر خاک رس نرم تثبیت شده با آهک ارزیابی گردیده است. مقدار آب در ترکیب خاک رس و آهک حدوداً به‌ترتیب 1، 1.5 و 2 برابر درصد رطوبت حد روانی خاک رس انتخاب شده است.  نتایج نشان می‌دهد که تغییرات مدول برشی حداکثر خاک رس نرم تثبیت شده، تابع مقادیر آهک و آب است، با افزایش میزان آب بیش از حد روانی خاک، مدول برشی حداکثر به‌طور قابل‌توجهی کاهش پیدا می کند و افزایش میزان آهک تا سطح معینی باعث افزایش مدول برشی حداکثر می‌شود و پس از آن کاهش می یابد. افزودن میزان آهک و آب بیشتر از مقادیر تعیین شده موجب ناکارآمدی فرایند تثبیت می‌گردد.

Evaluation of the unconfined maximum shear modulus of lime-stabilized clay with bender element experiments

Soil dynamic properties play fundamental roles in the analysis and design of various earth structures. Shear modulus and damping ratio are two important dynamic properties. In particular, shear modulus, especially at the level of very small strains typically denoted as Gmax, is a dynamic property of great significance, which is frequently implemented in the seismic design of geostructures against the destructive earthquakes. The shear modulus reveals the resistance of geostructure to the deformations imposed by the external loading. On the other hand, one of the commonly used methods to increase the strength and stiffness properties of soft soils is to stabilize them using either cement or lime. Addition of these stabilizing agents to the parent soil strengthens the bonding among particles, thus resulting in the overall increase in the shear stiffness of the mixture. Therefore, the stabilization technique is always considered as an efficient method of soil improvement. In the current study, the small strain shear modulus of soft clay stabilized with various lime contents is thoroughly evaluated using the results of a comprehensive series of bender elements tests under isotropic stress states. Bender element is a nondestructive experiment commonly used to estimate the velocity of shear waves propagating through the soil samples. Using the shear wave velocity obtained, the small strain shear modulus of the specimens could be easily evaluated with a simple equation in soil dynamics. The applicability of the bender element test to measure the shear wave velocity for the stiff stabilized samples in this study was extended using a new innovative method. The influence of input frequency on the shear wave velocity measurements was also rigorously examined and it was concluded that it barely affects the received signals. Based on the bender elements experimental results, the influence of lime inclusion (5%, 10%, 15% and 20%), water content (45%, 65% and 85%) and curing time (28 and 56 days) on the small strain shear modulus is thoroughly investigated. The amount of water in the soft claylime mixture was selected to be about 1, 1.5 and 2 times of the liquid limit moisture of the parent clay. According to the experimental results, it is observed that the small strain shear modulus decreases dramatically with the increase in the water content over the liquid limit. The addition of lime to the clay, up to a particular level, leads to a considerable increase in the small strain shear modulus. However, beyond this optimum value, the shear modulus shows a declining trend with the increase in the lime content, which is an indicative of the inefficiency of the stabilization process. Thus, it is a common practice to limit the lime content to a specific percentage so as to obtain the maximum possible value of shear stiffness. The general trends of shear modulus variation for the samples stabilized at different curing periods are also observed to be quite similar. In general, the small strain shear modulus increases with the increase in the curing time, as more chemical reactions could occur within the mixture. Finally, a microstructural analysis was also conducted using the scanning electron microscopy (SEM) images of the treated specimens so as to somehow justify the trends of variation in small strain shear modulus obtained from the bender element experiments. Utilizing the results obtained in the course of this study, useful information is provided for the prediction of the small strain shear modulus of the clays stabilized with lime using deep mixing or grouting methods. Indeed, the results of this study could be effectively used in different geotechnical construction projects to improve the parent soil strength and stiffness properties and ensure about the serviceability and efficient performance of the underlying soil deposit.