بررسی عددی میزان تاثیرپذیری نشست زمین ناشی از حفر تونل خط دو متروی تبریز از تغییرات ضریب فشار جانبی زمین (k0)

نشست زمین از تبعات اجتناب‌ناپذیر حفر تونل‌های کم‌عمق (اعم از مکانیزه و سنتی) در زمین‌های نرم مانند مصالح خاکی است. این پدیده خود معلول به هم خوردن رژیم تنش‌های طبیعی زمین است که در اثر برداشتن مصالح در طی حفر تونل اتفاق می‌افتد. پیش‌بینی مقدار نشست زمین و محدوده گسترش آن ازنظر آسیب‌های احتمالی که می‌تواند به ساختمان‌های سطحی و سازه‌ها و تاسیسات زیرسطحی وارد کند، از اهمیت بالایی برخوردار بوده و با توجه به خطرات ناشی از آن می‌بایست با دقت هر چه تمام‌تر موردبررسی قرار گیرد. عوامل مختلفی می‌توانند بر وقوع این پدیده و مقدار آن موثر باشند که ازجمله می‌توان به نوع خاک موجود در محل حفاری تونل و خصوصیات ژئوتکنیک آن، قطر و عمق تونل، سازه‌های بالای محیط حفاری، سطح آب‌های زیرزمینی و... اشاره نمود. پارامتر k0 (ضریب فشار جانبی خاک) ازجمله خصوصیات ژئوتکنیکی مهم خاک بوده و نقش اساسی در طراحی تونل به‌ویژه پوشش بتنی آن دارد. در این تحقیق نشست زمین در اثر حفر تونل مکانیزه خط دو متروی تبریز موردمطالعه قرار گرفته و تاثیر پارامتر k0 بر آن با استفاده از روش تفاضل محدود و مدل‌سازی با نرم‌افزار flac 2d با مقادیر مختلف پارامتر k0 (0.25، 0.5، 0.75 و 1) بررسی شده است. مدل عددی ساخته‌شده و نتایج حاصل از آن با استفاده از نشست‌های واقعی که در حین حفاری تونل اندازه‌گیری می‌شوند، مورد صحت سنجی قرار گرفتند. نتایج حاصله نشان‌دهنده این است که با افزایش مقدار k0، میزان نشست زمین کاهش پیدا می‌کند و مقدار این کاهش از یک تابع‌نمایی تبعیت می‌کند.

numerical study of the impressionability of ground settlement due to excavation of tabriz metro tunnel, line 2, from the coefficient of lateral earth pressure (k0)

summaryin this research, ground settlement due to mechanized excavation of the tabriz metro tunnel, line 2, has been studied and the effect of the k0 parameter, coefficient of lateral earth pressure, has been discussed using finite difference method and modeling with flac 2d software considering different values of k0 parameter (0.25, 0.5, 0.75 and 1). the results of numerical model were verified according to real settlements measured simultaneously with tunnel excavation. the results show that in general, ground settlements due to tunneling decreases exponentially with increasing of the k0 value. introductionground settlement is one of the inevitable consequences of excavation of shallow tunnels (both mechanized and natm) in soft materials such as soils. this phenomenon has taken place due to the disruption of the initial stress distribution of the ground, which occurs because of the removal of materials during the tunnel excavation. predicting the amount of the settlement and its extension is of great importance in terms of possible damage to surface structures and subsurface infrastructures and facilities. according to the risk of this phenomenon, it should be carefully evaluated. the amount and extension of settlements are widely influenced by some factors that the geological and the geotechnical characteristics of soil are among the most significant of them. in addition, the tunnel geometry and its depth, the excavation method, the workmanship and management quality are the other parameters affecting the ground settlements caused by the excavation of shallow tunnels. the coefficient of lateral earth pressure (k0) is one of the most important geotechnical parameters of the soil mass. this parameter is defined as the ratio of the horizontal in-situ stress over the vertical in-situ stress, (σˊh /σˊv) and its effect on ground settlement due to tunneling was analyzed by several authors. estimation of the initial stresses with an accurate value of k0 is one of the most important issues in geotechnical engineering. k0 is greatly influenced by geological factors such as the soil type, the groundwater configuration, the overburden thickness (tunnel depth), etc. according to researches, the initial stress distribution in the ground is mostly unknown and quite a number of factors such as tectonic movements, thermic, creep or weathering can influence it. methodology and approachesseveral methods have been suggested to study and evaluate the amount of the settlement occurred due to tunneling such as: semi-empirical, analytical, numerical and etc. numerical methods are more paid attention because of extend utilizing of the geotechnical characteristics of host materials. in this study, a numerical parametric study on the effect of k0 on the ground settlements due to tunneling is presented. for this purpose, at first, real settlements obtained from studied section of the tabriz metro tunnel, line 2, are used to validate numerical models. then the effect of k0 on ground settlement due to the excavation of the tunnels is investigated using numerical method done by flac 2d.results and conclusionsthe final findings of this study are shown in fig. 2. the results show that the amount of ground settlement decreases generally with an increasing value of k0, and the changes follow an exponential function.according to fig. 2, the relation between these two parameters follows an exponential function which can be expressed using the following equation: where a and b are coefficients that are dependent mostly on the soil type, geometrical characteristics of the tunnel (depth and diameter). for this study, the amount of a and b parameters which are index of settlement reducing with increasing of k0, were determined respectively as 41.69 and 1.39 via numerical modeling. it was predicted that the changes of k0 causes a change in the magnitude and direction