پديد آورندگان :
مرتضايي، محمد دانشگاه بين المللي امام خميني(ره) - گروه مهندسي عمران - خاك و پي , نائيني، ابوالحسن دانشگاه بين المللي امام خميني(ره) - گروه مهندسي عمران - خاك و پي , اسلامي، ابوالفضل دانشگاه صنعتي اميركبير - گروه مهندسي عمران و محيط زيست، تهران، ايران
كليدواژه :
شمع , خاك غيراشباع , مكش , ظرفيت باربري نهايي , دستگاه فشار همه جانبه مخروطي
چكيده فارسي :
يكي از راه كارهاي مناسب براي بررسي رفتار شمع، مدل سازي فيزيكي آن در آزمايشگاه مي باشد. براي مدل سازي فيزيكي مي توان از محفظه ساده ، محفظه كاليبراسيون و دستگاه سانتريفيوژ استفاده نمود. محدوديت ها و سختي هاي موجود در اين وسايل باعث شد اخيرا وسيله اي به نام دستگاه فشار همه جانبه مخروطي(FCV) براي مدل سازي فيزيكي پي هاي عميق به جهان معرفي گردد. در اين دستگاه كه به شكل مخروط ناقص مي باشد، مي توان با اعمال فشار در كف دستگاه يك گراديان خطي تنش در امتداد محوري مركزي ايجاد نمود تا با آنچه در واقعيت اتفاق مي افتد هم خواني داشته باشد.علي رغم مزاياي زيادي كه اين دستگاه دارد ولي در آن سيستمي براي سنجش مكش بافتي خاك هاي غير اشباع در نظر گرفته نشده است. لذا در اين تحقيق با ابزارگذاري نمونه ساخته شده از اين دستگاه در دانشگاه صنعتي امير كبير(FCV-AUT) و اجراي شمع درون آن، اين امكان فراهم مي گردد تا رفتار شمع در خاك غيراشباع و تاثير مكش بافتي بر نمودار نيرو- تغيير مكان بررسي شود. شمع استفاده شده در اين تحقيق يك شمع انتها باز مي باشد كه با روش كوبشي و در ماسه لاي دار فيروزكوه با رطوبت هاي وزني 0، 5، 10 ، 15 و 20 درصد اجرا و در هر مورد نمودار نيرو- تغييرمكان ترسيم شده است. نتايج نشان مي دهد در اين خاك با افزايش درصد رطوبت از حالت خشك( مكش صفر) به رطوبت 5 درصد (افزايش مكش) با يك افزايش در نقطه اوج نمودار(ظرفيت باربري نهايي) مواجه مي شويم سپس از رطوبت 5 تا 15 درصد با آنكه مكش كاهش مي يابد باز هم ظرفيت باربري افزايش مي يابد ولي در انتها با افزايش رطوبت از 15 به 20 درصد روند كاهشي مي گردد. اين روند تغييرات نشان دهنده يك رطوبت بحراني در اين خاك است كه در آن رطوبت ظرفيت باربري شمع حداكثر مي گردد. مقدار اين رطوبت بسته به دانه بندي و تراكم خاك مي تواند متفاوت باشد.
چكيده لاتين :
Deep foundations are often used in a typical engineering project to transfer loads from heavy superstructures
(bridges, highways, embankments and high-rise buildings) to the subsoil safely without subsidence or
instability problems. In order to achieve an accurate and economical design in each of these projects, we must
have a good understanding of how deep foundations behave, which can be done through analytical methods,
numerical methods and experiments or experimental observations. Since a series of simplifications are usually
performed in numerical and analytical methods, the use of experimental methods is selected to achieve a more
desirable result. Experimental methods can be done by examining real models on site or performing
experiments on a small-scale model in the laboratory (physical modeling), which due to lower cost, simplicity
and reproducibility, usually physical modeling compared to Real on-site models are preferred. For physical
modeling, various devices such as Simple Chamber (1 g), Calibration Chamber (CC) and centrifuge device
can be used. The limitations and difficulties of these devices led to the recent introduction of a device called
the Frustum Confining Vessel (FCV) for the physical modeling of deep foundations. In this device, which is
in the form of an incomplete cone, by applying pressure to the bottom of the device, a linear stress gradient
can be created along the central axis to be consistent with what is actually happening. Despite the many
advantages that this device , it does not have a system for measuring the matric suction in unsaturated soils.
Therefore, in this article, by embedding precision instruments in the sample made of this device in Amirkabir
University of Technology (FCV-AUT) and implementing and loading of a pile in the soil inside it, It is possible
to study the behavior of piles in unsaturated soil and the effect of moisture and matric suction on the loaddisplacement curve. The pile used in this research is an open-end pile which was implemented by driving
method in Firoozkooh silty sand with gravimetric moisture contents of 0, 5, 10, 15 and 20% and in each case
a load-displacement curve was drawn. The floor pressure applied in this research is equal to 100 kPa, Which
was determined by placing several stress sensors at different soil depths and measuring the stress changes in
depth. The following results were obtained after experiments:
1- In the tested soil, up to 5% moisture content, no significant lateral friction is observed between the pile
and the soil, and the end resistance, which is the result of plugging the soil inside the pile, provides most
of the bearing capacity of the pile.
2- From 5 to 15% moisture content, the lateral friction between the pile and the soil is gradually mobilized
and while creating an initial jump in the curve of load-displacement, it increases the bearing capacity of the
pile exponentially. But from 15% moisture content onwards, the initial slope decreases again due to the
reduction of lateral friction.
3- Considering the final bearing capacity of the pile as the load corresponding to 10% of the displacement of
the pile head, the effect of matric suction on the changes of final bearing capacity was investigated. It was
found that By increasing the amount of matric suction, first the final bearing capacity of the pile increases
and after reaching a certain value, it decreases. The cause of this phenomenon is changes in wet soil
behavior in different areas of the soil-water characteristic curve (SWCC) and is affected by various factors
such as grain size, initial moisture content and soil density percentage.
