ارزیابی پایدارسازی گود به‌روش ساخت از بالا در کنترل تغییر شکل دیواره‌های گود بر اساس بررسی‌های عددی و میدانی

جابه‌جایی زمین ناشی از حفاری در محدودۀ شهری، همواره با چالش‌های جدی از دیدگاه ژئوتکنیکی و سازه‌ای مواجه بوده است. در این زمینه آسیب دیدگی ساختمان‌ها و تاسیسات مجاور گود به‌دلیل تجاوز مقادیر تغییر شکل زمین از محدوده مجاز، خسارات جبران ناپذیری را به‌همراه دارد. بنابراین پیش‌بینی حرکت زمین مجاور گود در طراحی‌ها اهمیت ویژه‌ای دارد و طراحی سیستم مهاری گود، کنترل تغییرشکل جانبی دیوارۀ گود و نشست سطح زمین امری اجتناب‌ناپذیر است. این پژوهش، به بررسی پایدارسازی گود از دیدگاه تغییرشکل با استفاده از سازۀ اصلی به‌روش ساخت از بالا و بر اساس پروژه‌ای واقعی می‌پردازد. برای این منظور، مراحل اجرا در روش ساخت از بالا به‌صورت مقطعی و با استفاده از دوربین توتال استیشن مورد پایش قرار گرفته و داده‌های حاصل از ابزارسنجی گود، بر اساس روش‌های آماری تحلیل شده است. در این رابطه مدل‌سازی مراحل مختلف پایدارسازی گود با استفاده از نرم‌افزار المان محدود آباکوس[1] انجام شده و به‌منظور صحت سنجی و پیش‌بینی روند تغییرشکل‌ها، کالیبراسیون مدل عددی بر اساس نتایج حاصل از پایش‌های میدانی صورت پذیرفته است. در ادامه پارامترهای مختلف سیستم مهاری به‌منظور بررسی عملکردی، متغیر در نظر گرفته شده و در نهایت با استفاده از 160 مدل عددی، امکان تخمین سختی مورد نیاز سیستم مهاری با توجه به تغییرشکل مجاز، در 4 ساختگاه مختلف و در قالب روابط نمایی ارائه می‌شود.

Evaluation of Excavation Stabilization by the Top-down Approach in the Control of Excavation Wall Deformation based on Numerical and Field Studies

IntroductionExcavation in urban areas occasionally is accompanied by the improper performance of the support system for even small deformations. In this regard, deformation control design based on forcebased approaches provides a more realistic reprehensive of excavation performance. Topdown deep excavation techniques are among the modern excavation stabilization methods in urban areas. In this method, unlike the conventional methods, it is possible to perform the excavation and construction operations simultaneously. The present study aims to investigate excavation stabilization using the main structure through the topdown approach. For this purpose, field and numerical evaluations of the stabilized project were conducted based on the topdown approach in the downtown of Qom city, Iran. This research reports the information obtained through monitoring and modeling using the finite element ABAQUS software, predicting the occurred deformations until the end of excavation operations using the calibrated model, and offering an initial estimation of the required stiffness for the support system with respect to the lateral deformations in four sites proposed, according to the studies of Line A Qom Subway.Project specificationsBased on the geological studies of Line A Qom Subway Tunnel, the geological layers are classified into four soil classes. Qc1 consists of gravely sand with fine content of 5 to 20%; Qc2 is silty and clayey sand with fine content of 35 to 60%; Qf1 is clayey silt with fine content of 60%; and Qf2 is a silty clay layer with fine content above 60%. Line A of Qom subway passes the study area of the present study, which is located in Ammar e Yaser Street (Station A6). Based on the geotechnical studies of the project site, the site in the levels near the ground consists of Qc2 but in the lower elevations, it is composed of Qc1 and Qf2.Salam Trade Complex, located in the downtown of Qom city, has 6 underground stories and 6 aboveground stories. It is limited to the main street in the south and to urban decay in the three other directions. The final excavation depth, length, and width is 21, 36, and 3252 m, respectively. The project structure consists of a steel moment frame with a retaining wall in the negative elevations and metal deck frame for ceiling construction. In this project, excavation wall deformation was monitored in three important sections (A, B, and C). Due to the vicinity to urban decay, a total station TS02 was used for monitoring these sections. According to the field surveys, the maximum horizontal deformation of the walls in sections A, B, and C is 24.10, 42.16, and 47.21 mm, respectively, which were measured in the 0, 1.5, and 0 m elevations.Monitoring process and numerical simulationTo calibrate the prepared model, a sensitivity analysis was performed on geotechnical parameters including modulus of elasticity (E), internal friction angle ( phi;), and cohesion (C) of the layers by simulating 60 numerical models. Based on the sensitivity analysis results, an increase in internal friction angle and elasticity modulus for layer 1 (i.e., phi;1 and E1) and elasticity modulus of layer 3 (E3) results in a decrease in lateral deformation. Finally, using the sensitivity analysis results and after several trials and errors, the numerical models for sections B and C were calibrated when reaching the depths of 8 and 11 m, respectively. Using these models, then, it is possible to predict deformations up to the end of the project.To determine the required stiffness for the excavation support system, regarding the acceptable deformation of the adjacent soil mass, 160 numerical models were built and their results were analyzed. Based on the results of Brason and Zapata (2012), relative stiffens (R) were used to develop a relationship between the maximum lateral deformation of the wall and the required stiffness of the support system. R is a dimensionless parameter that represents the stiffness of a solid support system; the greater this value is, the more flexible the system would be. In this study, caisson pile length, excavation width, and buried depth of the wall were used for determining the R.R = (1)Figure 2 presents the maximum occurred deformation in terms of depth versus the relative stiffness for sites QC and QF. Figure 2. Maximum deformation in terms of depth versus the relative stiffness for sites QC and QFConclusionAccording to the monitory data, the maximum lateral deformation in sections B and C until the end of the project was 42.16 and 47.2 mm, respectively. Moreover, the deformation of the other points inside the excavation was 30 mm.Considering the occurrence of maximum lateral deformations in the higher elevations in the monitored sections, it is inferred that excavation support at the ground level plays a key role in this approach. Hence, the lack of completing the structural frames and slabs for facilitating the excavation operation can lead to an increase in deformation levels.Based on the prepared graphs, the topdown approach in sites QC2 and QF2, compared to sites QF1 and QC1, provides a more desirable performance for deformation control.