بررسي اثر سيستم پيش نگهداري (فورپولينگ) بر كنترل نشست سطح زمين ناشي از حفاري تونل هاي كم عمق شهري تحت بارگذاري ترافيك ريلي

The Effect of Pre-Support System (Forepoling) on the Control of Ground Surface Subsidence caused by SEM/NATM in Shallow Urban Road Tunnels under Railway Traffic Loading

اين مطالعه بر اساس مدل‌سازي­هاي دوبعدي (2D) و سه‌بعدي (3D) مراحل مختلف حفاري تونل زيرگذر ضربعلي­زاده- مداح تحت بار ترافيك ريلي جهت ارزيابي تغيير شكل و نشست سطح زمين به كمك نرم‌افزار المان محدود پلكسيس (PLAXIS) انجام گرفته است. نتايج حاصل از مدل‌سازي عددي دوبعدي معرف شرايط پايدار تونل به‌واسطه ظرفيت قابل قبول سيستم­هاي نگهداري (اصلي و موقت) طراحي‌شده در بخش‌هاي مختلف تونل (ديواره­هاي اصلي، مياني و كف بند) در برابر بارهاي تركيبي مختلف است. همچنين بر اساس آناليز حساسيت انجام‌شده در مدل‌سازي دوبعدي، مقاومت چسبندگي و مدول الاستيسيته از پارامترهاي المان معادل سيستم فورپولينگ استفاده‌شده در اطراف تونل به‌عنوان سيستم پيش نگهداري، به ترتيب بيشترين تأثير در كنترل نشست سطح زمين دارند. علي‌رغم وضعيت پايدار تونل، مقدار نشست سطحي به‌دست‌آمده از مدل‌سازي عددي دوبعدي (44 ميلي‌متر) بزرگ‌تر از حداكثر مقدار نشست مجاز تونل (43 ميلي‌متر) بوده است و نيز معرف تغيير شكل نسبتاً زياد سطح زمين در مقايسه با داده­هاي ثبت‌شده توسط ابزار دقيق (30 ميلي‌متر) در حين حفاري و اجراي تونل دارد. به‌منظور بررسي دقيق‌تر اثر فورپولينگ بر كنترل نشست سطحي، مدل‌سازي سه‌بعدي مراحل حفاري با لحاظ نمودن پروفيل تغيير شكل طولي (Longitudinal Deformation Profile) (LDP) سينه­كار تونل انجام گرفت. نتايج حاصل معرف مجاز بودن حداكثر مقدار نشست سطحي به‌دست‌آمده از مدل‌سازي سه‌بعدي (32 ميلي‌متر) بوده و همچنين از تطابق بسيار خوبي با داده­هاي ابزار دقيق (30 ميلي‌متر) برخوردار است. با توجه به اينكه در مدل‌سازي سه‌بعدي مقدار ترخيص يا رهاسازي تنش در مراحل مختلف حفاري بر اساس پروفيل تغيير شكل طولي سينه­كار در نظر گرفته مي­شود، ازاين‌رو مقادير ترخيص تنش لحاظ شده در مدل‌سازي سه‌بعدي نسبت به مدل‌سازي دوبعدي كه بر اساس روابط تجربي لحاظ مي­شوند، به شرايط واقعي زمين در زمان اجراي تونل نزديك‌تر و دقيق‌تر مي­ باشند.

Summary In recent years, mountain tunneling method such as SEM/NATM has been used for tunnel construction even in urban areas because of its cost effectiveness. When this method is used in urban areas compared to mountainous areas, it is important to impose some constrained conditions, such as restrictions of surface settlement and ground water drawn down, because of existing constructions such as buildings, pile foundations and underground pipes around the tunnel face. It is said that the increase of the use of mountain tunneling method in the urban areas is due to the development of auxiliary methods such as face bolting and forepoling. Because of plain-strain condition in tunnel excavation along the tunnel axis direction, some two-dimensional (2D) models have been applied to evaluate the ground surface settlement generated by different phases of excavation. The deformation modes of tunneling face are different. The excavation face deformation is a three-dimensional (3D) problem. It would be more complicated considering forepoling implementation and face injection effects on deformations. The longitudinal displacement profile of the tunnel crown has been generated by 3D models and the typical proposed stress release relations in this research work have beem compared and evaluated. Introduction A method for reducing-induced ground movements in open-faced tunneling is to use tunnel support methods such as forepoling umbrella system. Forepoles are often used for supporting the ground ahead of the tunnel face during excavation. They also provide temporary support to unlined portion of tunnels. Forepoles are usually installed along the tunnel periphery in the longitudinal direction to form a supportive arch ahead of the tunnel face. This paper investigates the effect of forepoles on the stability of the tunnel face and ground surface settlement during a tunnel excavation in Tehran alluvium. Methodology and Approaches This study was conducted based on 2D and 3D numerical models of Zarbalizadeh-Maddah underpass tunnel for controlling the face stability and deformation of the surrounding ground using PLAXIS finite element software. Results and Conclusions The results of the 2D numerical modeling carried out in this research work represent the stable conditions of the tunnel due to the acceptable capacity of the support systems (primary and temporary) designed in different sections of the tunnel (e.g. side walls, middle walls, and invert) against different combinational loads. In spite of the stable condition of the tunnel, the surface subsidence obtained from the 2D numerical modeling (43 mm) represents a relatively high value of deformation of the ground surface compared to the instrumentation data (30 mm) recorded during the tunnel excavation. In order to more accurately investigate the effect of forepoling on the control of the surface subsidence, 3D modeling of the excavation stages was performed considering the longitudinal deformation profile (LDP) of the tunnel face. The results indicate a lower value of surface subsidence (32 mm) compared to the 2D modeling (43 mm). The 3D modeling results are also in very good agreement with the instrumentation data of the surface subsidence (30 mm).