Journal of Engineering Geology

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Introduction
Excavation 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 force-based approaches provides a more realistic reprehensive of excavation performance. Top-down 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 top-down approach. For this purpose, field and numerical evaluations of the stabilized project were conducted based on the top-down 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 specifications
Based on the geological studies of Line A Qom Subway Tunnel, the geological layers are classified into four soil classes. Qc-1 consists of gravely sand with fine content of 5 to 20%; Qc-2 is silty and clayey sand with fine content of 35 to 60%; Qf-1 is clayey silt with fine content of 60%; and Qf-2 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 Qc-2 but in the lower elevations, it is composed of Qc-1 and Qf-2.
Salam Trade Complex, located in the downtown of Qom city, has 6 underground stories and 6 above-ground 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 32-52 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 simulation
To calibrate the prepared model, a sensitivity analysis was performed on geotechnical parameters including modulus of elasticity (E), internal friction angle (φ), 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., φ1 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 QF
Conclusion

1. According 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.
2. 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.
3. Based on the prepared graphs, the top-down approach in sites QC-2 and QF-2, compared to sites QF-1 and QC-1, provides a more desirable performance for deformation control.

Keywords:  Top-down excavation, Deformation assessment, Total Station TS02, FEM, Line A Qom Subway Tunnel

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