ظرفيت باربري پي هاي مسلح شده با ريزشمع

Bearing Capacity of Foundations Reinforced with Micropiles

ريزشمع، يك شمع جايگزيني حفاري و تزريق‌شده با قطر كمتر از 300 ميلي‌متر است كه عموماً مسلح بوده و براي اجراي آن ابتدا چاه شمع حفاري شده و مسلح‌كننده در داخل آن قرار مي‌گيرد و سپس ملات به داخل چاه تزريق مي‌شود. از اين تكنولوژي براي بهبود ظرفيت باربري پي‌هاي موجود نيز استفاده مي شود. تحقيق حاضر به بررسي ميزان تاثير ريزشمع در بهبود ظرفيت باربري پي تحت اثر بارگذاري استاتيكي قائم مي‌پردازد. تعدادي آزمايش بارگذاري بر روي مدل آزمايشگاهي كوچك‌مقياس از پي مسلح شده با ريزشمع كه بر روي ماسه شل قرار گرفته انجام گرديد. براي مسلح نمودن پي صلب دايره‌اي با قطر 10 سانتي‌متر، از ريزشمع‌هاي فولادي آج‌دار به قطر 6 ميلي‌متر و طول 20 سانتي‌متر استفاده شد. با تغيير زاويه نصب و تعداد ريزشمع‌ها، تاثير نحوه چيدمان ريزشمع‌ها بر رفتار ژئوتكنيكي پي مطالعه شد. براي سنجش كمّي ميزان افزايش ظرفيت باربري پي مسلح شده با ريزشمع، ضريب شبكه‌اي(R) تعريف مي‌شود كه عبارتست از نسبت ظرفيت باربري پي مسلح شده با ريزشمع به مجموع ظرفيت باربري پي و ريزشمع‌ها به صورت مجزا. نتايج آزمايشها نشان مي‌دهد كه مقدار ضريب شبكه‌اي با افزايش تعداد ريزشمع‌ها بالا مي‌رود. همچنين در صورت كاربرد 6 يا 8 ريزشمع، با افزايش زاويه نصب ريزشمع نسبت به امتداد قائم، از مقدار R كاسته مي‌شود. در حالت استفاده از تعداد زيادي ريزشمع‌هاي داراي زاويه نصب كوچك ، ظرفيت باربري به مقدار قابل توجهي افزايش مي‌يابد. مقدار ضريب شبكه‌اي در صورت كاربرد 8 ريزشمع با زاويه نصب 15 درجه، برابر 1.997 مي‌باشد.

Improvement of bearing capacity of existing foundations is of great significance. There exist many methods for practical purposes. Micropile is one of the most promising methods. Micropile is a replacement pile of small diameter (usually less than 300 millimeter) which is frequently reinforced using steel elements. To construct a micropile, steel reinforcement is placed in the borehole after drilling the borehole and subsequently the grout is injected into it. Micropiles transfer the structural loads to the deeper and stronger layers of the ground and confine settlement (similar to conventional pile foundations). They also improve the mechanical properties of soil layer such as density, bearing capacity, permeability and compressibility. Owing to their advantages, micropiles are widely used as foundations of new structures construction and also for reinforcing the foundation of existing structures. This research aims at experimental investigation of bearing capacity of foundations reinforced with micropiles under the condition of static loading. A smallscale physical model of a foundation reinforced with micropiles was developed and a series of static loading tests were carried out on. The model micropilefoundation was located on loose sand. Density of the underlying soil was kept almost uniform throughout the tests. The foundation model was circular and 100 mm and 5o mm in diameter and thickness, respectively. It was made from polyamide and considered to act as a rigid foundation during the loading owing to its material and thickness.This foundation was reinforced with a group of micropiles with 6 mm and 200 mm in diameter and length, respectively. These model micropiles were made from threaded steel bars. In order to mobilize friction, sand grains were glued to the surface of the micropiles. Various arrangements of micropiles including the number and inclination angle of micropiles were tested. From the comparative examination of the observed behavior of micropile foundations, the influence of micropiles’ arrangement on the mechanism and improvement of bearing capacity of foundation was investigated. Number of micropiles used in the group varied from 2 to 8. Micropiles were inclined at different angles (0°, 15°, 13°, 45° and 60°) to study its influence on the behavior of foundation reinforced with these elements. In order to quantitatively assess the degree of improvement in the bearing capacity of surface foundations reinforced with micropiles, an index R called “Network Index” was introduced in this study. The index R of unity means that the bearing capacity of foundations reinforced with micropiles is simply equal to the summation of the individual value of the surface foundation and that of the micropile group. There is an upward trend in the rate of index R when the number of micropiles is increased. On the other hand, in high numbers of micropiles used to reinforce the foundation, index R declines with increasing of inclination angle. In the case of micropiles with lowinclinationangle being implemented, bearing capacity is improved remarkably an index R of 1.997 is achieved in this study where 8 micropiles inclined at an angle of 15° were used to reinforce the foundation.