Geotechnical Engineering Online Library

In recent years, innovative underground construction techniques have been extensively utilized for many purposes in Iran. Using construction methods such as CAPS (concrete arc pre-supporting system) in the case of nearby special structures is regarded as a valuable technique. CAPS technique applied in this work is a supporting system implemented in urban areas, where the excavation-induced distribution of the soil settlement is considerably lower compared to other sequential excavation (SEM) methods. Our case study is Q7 station, which is an intersection station in Tehran metro line 7 located near Tohid Twin Tunnel and Gardoon Tower. Based on investigations carried out and presented in this paper, CAPS demonstrates an excellent performance and serviceability for structures located within congested urban areas. Q7 station was modeled using FLAC3D code. To ensure the accuracy of our model, monitoring data were compared with the numerical results. By performing sensitivity analysis on the shear parameters of the rehabilitated soil (c, φ) and the distance between beam elements (λ factor), we observed that increasing the shear parameters of the soil mass decreases the vertical displacement of the ground. The optimum value for the λ factor was estimated in this work based on the Rankin criteria for Gardoon Tower (a 20-story building) and Tohid Twin Tunnel.

An underground reinforced concrete tank was constructed for a project in the southwest region of India. The tank was 90 m x 35 m in plan and 7.3 m deep resting on partly filled-up and partly native soil. During the peak monsoon, a sudden uplifting of the base slab by about 300 mm and subsequent failure of the foundation raft and a partition wall was observed. Laboratory testing was executed and hydrogeological survey was carried out using ground penetrating radar, seismic refraction and infiltrometer testing, and an analytical study was carried out to identify the root cause of the tank uplifting. Based on this study, it was observed that the uplifting and structural failure was essentially due to the peculiar land terrain and soil properties and the development of excess hydraulic head below the bottom of the tank. After considering different options, the rectification measures were carried out by provision of dewatering wells along the tank periphery to release the excess hydrostatic pressure and stabilize the foundation raft. The structural repair of the top of the foundation raft and partition wall was carried out to strengthen the reinforced concrete members. The rectification measures worked well to increase the structural stability of the tank and to prevent build-up of excess hydrostatic pressure preventing uplift and subsequent damage in the future.