1دانشجوی دکترای تخصصی مکانیک خاک و پی؛ گروه مهندسی عمران؛ دانشگاه بین‌المللی امام خمینی
2دانشیار؛ گروه مهندسی عمران؛ دانشکده‌ی مهندسی؛ دانشگاه بوعلی سینا
3دانش‌آموخته کارشناسی‌ارشد عمران؛ دانشکده‌ی مهندسی؛ دانشگاه بوعلی سینا
چکیده
در این پژوهش در چارچوب روش تحلیل حدی، با بکارگیری یک مکانیسم گسیختگی سه‌بعدی با شکل اسپیرال لگاریتمی یکپارچه، پایداری جبهه‌ی حفاری تونل در محیط غیر اشباع مورد بررسی قرار گرفته است. برای استخراج معادلات سینماتیک حاکم از یک معیار گسیختگی خاک غیر اشباع استفاده شده است. پس از وارد‌‌سازی پارامترهای غیر اشباع در معادلات حاکم و اکسترمم‌سازی، یک مجموعه تحلیل‌های پارامتری انجام شده است. بر اساس نتایج حاصل، مکش بافتی تاثیر بسزایی در پایداری جبهه‌ی حفاری تونل دارد. با افزایش مکش بافتی، فشار حدی لازم برای تامین پایداری جبهه کاهش یافته که این موضوع می‌تواند در موارد کاربردی، مد نظر مهندسین قرار گیرد. در انتها با شبیه‌سازی عددی مسئله، نتایج حاصل از روش‌های تحلیل حدی و عددی مقایسه شده است. بر اساس نتایج بدست آمده روش تحلیل حدی، هر چند که روند گسیختگی برای هر دو روش شبیه است، فشار بیشتری را برای تامین پایداری جبهه‌ی حفاری پیش‌بینی می‌کند. بعلاوه اثرات مکش موجب تغییر در شکل گوه‌ی گسیختگی بخصوص در قسمت تاج می‌شود.
Assessment of Tunnel Face Stability in Unsaturated Media Based on Limit Analysis Method
چکیده [English]
In this research, in the framework of limit analysis method with consideration of a 3-D continuous failure mechanism, stability of tunnel excavation face in unsaturated media was studied. Governing kinematic equations were adapted for unsaturated conditions with introducing unsaturated form of Mohr Coulomb failure criteria. Afterward, a set of parametric analyses, in different form of suction distribution and tunnel geometry were performed. Based on obtained results, matric suction and its distribution considerably influence tunnel face stability. Therefore, increase in matric suction leads to decrease in limit pressure exerted on tunnel face. In the final section of paper, finite element numerical analyses in the same conditions of material and geometry were executed. The results indicate that limit analysis predict a limit pressure more than finite element method.
Introduction There are numerous studies in the literature concerning application of upper bond limit analysis for evaluation of tunnel face stability in soft ground. These works have used conventional soil mechanics theory. However, a large part of surface soils are in unsaturated condition. For many engineering problems especially in construction phase, the principle of unsaturated soil mechanics can be used. This can be more efficient for unloading problems such as Tunnelling or excavation. This approach describes the role of unsaturated parameters on tunnel face stability using limit analysis method.
Methodology and Approaches Governing kinematic equations were firstly adapted to the unsaturated condition. Then, parametric study was performed using direct calculations on kinematic equations. In the final step of paper PLAXIS3D was used for numerical analyses.
Results and Conclusions
Increase of matric suction leads to more dissipated energy in sliding surfaces of failure mechanism (decrease in limit pressure). Presence of matric suction in the analysis do not change the general form of failure mechanism. Limit pressures exerted on tunnel face obtained from limit analysis method are less than those obtained from finite element methods.
The continuous spiral logarithmic form of limit analysis are confirmed qualitatively by finite element method.
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