مقایسه مقاومت بیرون‌کششی استاتیکی و پساتناوبی ژئوگرید در خاک ماسه‌ لای‌دار

در این پژوهش با انجام آزمایش های بیرون کششی استاتیکی، تناوبی و پساتناوبی، رفتار بیرون‌کششی استاتیکی و پساتناوبی یک نوع ژئوگرید تک‌سویه تولید داخل ایران تحت نام تجاری gpgrid80/30، در خاک ماسه ای و ماسه لای دار بررسی شده است. آزمایش ها بصورت بزرگ مقیاس با ابعاد90ˣ50 ˣ50 سانتیمتر و بصورت جابجایی کنترل با سرعت ثابت و چندمرحله ای تناوبی بر روی سه نوع خاک مختلف شامل ماسه یکنواخت و ماسه لای‌دار، حاوی 10 و 20 درصد ریزدانه لای و در سه تنش موثر قائم 20، 40 و kpa60 انجام شده است. نتایج حاکی از افزایش مقاومت بیرون‌کششی استاتیکی ژئوگرید با افزایش تنش موثر قائم در هر سه نوع خاک مختلف است. همچنین افزایش ریزدانه لای در خاک ماسه، منجر به افزایش مقاومت بیرون‌کششی استاتیکی بیشینه ژئوگرید در تنش موثر kpa20 گردید، حال آنکه در تنش‌های بیشتر، افزایش درصد لای تاثیر چندانی بر مقاومت بیرون‌کشش استاتیکی بیشینه نهایی ژئوگرید نداشته است. با مقایسه مقاومت پساتناوبی و استاتیکی، مشاهده شد که در تنش موثر kpa20 مقاومت پساتناوبی ژئوگرید در هر سه نوع خاک، نسبت به مقاومت استاتیکی متناظر آن کاهش داشته است، لیکن در تنش های 40 و kpa60 اعمال بارگذاری چرخه ای تاثیر معناداری بر مقاومت بیرون کششی پساتناوبی ژئوگرید در هر سه نوع خاک نداشته است.

Comparison of static and post-cyclic pullout strength of geogrid embedded in silty sand

The interaction between soil and geosynthetics has great importance in engineering work, especially in design and stability analysis of geosyntheticreinforced geotechnical structures. In recent decades, several laboratory methods have been performed to properly understand the interaction between soil and geogrids, including pullout test, largescale direct shear test. Although factors such as the geometry of the reinforced soil system and its construction process may affect the interaction properties between the soil and the geosynthetic, these properties are strongly influenced by the physical and mechanical properties of the soil and the geometrical and mechanical properties of the geosynthetic. Pullout test determines the geosynthetic pullout resistance, which is an important design parameter in relation to the internal stability of geosyntheticreinforced geotechnical structures, and allows the measurement of displacements throughout the specimen during the pullout testing. Pullout force refers to the tensile force required to create an external sliding of geogrid embedded in soil mass. The tensile strength of the reinforcement consists of the frictional resistance on the surface of the longitudinal and transverse members of the geogrid and the passive resistance that is mobilized against the transverse members. Although finegrained soil is recommended in the design of geosyntheticreinforced soil structures, many geosyntheticreinforced soil structures are constructed using soil containing a fine percentage. Therefore it is important to investigate the effect of fine grains on the stability and performance of such soil structures under different loading conditions. Geosyntheticreinforced soil structures are sometimes affected by cyclic loads due to traffic and train crossings, vibration of industrial machinery, wave and earthquake. In this study, by performing static and multistage pullout tests, the static and postcyclic pullout behavior of a uniaxial geogrid manufactured in Iran under the brand GPGRID80/30 is presented. The tests were carried out on a large scale pullout box with a dimension of 90 × 50 × 50 cm and with a constant rate and multistage procedures on three different soil types including clean sand, sand containing 10 and 20% fine silt and three effective vertical stresses of 20, 40 and 60 kPa. Results show that geogrid static pullout resistance increases with increasing effective vertical stress in all three different soil types. Also, the increase of silt in the sandy soil resulted in an increase in the monotonic maximum pullout resistance at effective stress of 20 kPa. The geogrid behavior in all three soils for 20 kPa vertical effective stress was strain softening and for the 40 and 60 kPa vertical effective stress ​​the geogrid pullout behavior was strain hardening. However, 10% increase in silt content leads to a slight decrease in monotonic pullout resistance and a 20% increase resulted the slight increase in monotonic pullout resistance of geogrid at vertical stress of 40 and 60 kPa. As the amount of silt content increased, the effect of cyclic loading on postcyclic resistance increased, especially in vertical effective stresses of 40 and 60 kPa. Also, at effective stress of 20 kPa, the geogrid postcyclic resistance decreased in all three sands, sand containing 10% silt and sand containing 20% silt relative to its corresponding monotonic pullout resistance.