ارزيابي تغيير شكل گود هاي مهار شده در تحليل هاي خميري و تحكيمي

Evaluation of deflections in a braced excavation via plastic and consolidation analyses

در اين مقاله رفتار گودهاي مهار شده در خاك نرم چسبنده اصطكاكي با دو دسته تحليل مجزا از نوع خميري و تحكيمي با لحاظ نمودن زمان حفاري مرحله اي مورد ارزيابي قرار مي گيرد. به منظور بررسي اثر عامل زمان و فشار آب حفره اي فازهاي حفاري مرحله اي به كمك تحليل تحكيمي و تحليل خميري مدلسازي شده اند. در اين تحقيق علاوه بر عامل زمان به عوامل ديگري از قبيل طول بازه زماني حفاري، نوع تحليل به لحاظ شرايط زهكشي، مدل رفتاري براي خاك و اثر سطح آب زير زميني در بحراني ترين حالت يعني درست در سطح زمين و پشت ديواره ي گودبرداري توجه شده است. تحليل هاي اجزا محدود دو بعدي در نرم افزار PLAXIS2D اساس محاسبات عددي مطالعه ي حاضر را تشكيل مي دهند. مهاربندي گود از نوع ديوار حائل بتني و مهار شده با ميخكوبي انتخاب شده است. نتايج اين تحقيق نشان مي دهد كه مقادير تغيير مكان هاي افقي و قائم ديواره و كف گود در تحليل هاي تحكيمي با لحاظ كردن عامل زمان نسبت به تحليل هاي خميري، اغلب كمتر بوده و اختلاف آن ها در حد حداكثر 20% است. همچنين به نظر مي رسد كه تاثير زمان و حفاري مرحله اي تنها با انجام تحليل هاي تغيير شكلي عددي وابسته به زمان، همانند تحليل تحكيمي با مدل خزشي مي تواند ارزيابي شود و در تحليل عددي خميري حفاري مرحله اي بي معني است.

Behavior of braced excavation in a cohesivefrictional soil has been evaluated by present paper. Two groups distinct analyses based on plastic calculations and consolidation calculations were implemented by considering time intervals of staged excavation. In order to investigation of time effect and pore water pressure impact on the staged excavation phases, numerical modeling has been conducted by help of consolidation and plastic analyses. In major of former analysis and before present study numerical analyses of staged excavation procedure have selected without considering the effect of time and in form of plastic analyses, while pore water pressures were also ignored. In this, paper thereto the time effect other effects such as length of time interval, kind of analysis depending on drainage conditions, constitutive modeling for soil and location of ground water table were considered. Two dimensional finite element analyses in PLAXIS 2D software are the basis of the numerical calculations of present study. Excavation bracing selected as a kind of concrete facing wall and grouted soil nailing. The results of this research show that the values of excavation wall lateral displacement and soil heave in bottom of the excavation in consolidation analysis by considering time effect in comparison with plastic analysis often reduced approximately 20%. It seems that effect of time and staged excavation just with implementation of numerical deflection analysis depending on the time such as consolidation analysis with creep models can be evaluated and these conditions in plastic numerical analysis with staged excavation without creep (timedepended) models is meaningless. Soft soil creep model (i.e. SSC model) for considering time effect in plastic and consolidation analyses has been used by authors. The results of present paper show that neither consideration of analyses that consider time interval nor analysis such as consolidation analysis that considers time are not adequate, but soil constitutive model that defines the material behavior must be contains time and also in their mathematic equations structure parameters such as time, strain rate and stress rate that vary with time must be taken into account. Present paper analyses show that consolidation analysis by considering time effect obtain less wall deflections by comparison with plastic analyses. However, from present study outcomes can conclude that the plastic analyses also by considering constitutive model that contain time can take into account time effects in stressstrain calculations. On the other hand, responses of plastic analyses by comparison with consolidation analyses always are preservative and show more values. Therefore, in structural designing of bracing of an excavation, reliance on results obtained from plastic analysis is preservative and real values of timedependent deflections of wall and bottom of excavation via consolidation analyses are obtainable. This paper has recommended that both plastic and consolidation analyses for designing of braced cut were considered by engineers and optimum system between those according to the economically advantages and disadvantages be selected. Because, occasionally reliance on plastic preservative analyses lead to imposition of high values of design and construction costs on a certain project that is revealed by implementation of consolidation analyses that those are not necessary. At the end of the paper, verifications and comparisons are related to the topic of present study have been carried out by authors and the obtained results have been compared with together and then are investigated with the obtained results by present study.