L2 STATION, LINE 2 METRO OF ISFAHAN, ISFAHAN, IRAN (L2S-L2MI)
FACTORS DESCRIPTION SERVICES Construction and Supervision, Engineering and Design NAME OF PROJECT L2 station, line 2 metro of Isfahan EMPLOYER / CLIENT Isfahan Urban Railway Organization (IURO) CONTACT INFORMATION OF THE EMPLOYER Website address: https://ghatarshahri.isfahan.ir/ Email: info.metro@isfahan.ir Phone No: (0098) 31 34358410 CONTRACT NUMBER 125/96 TYPE OF CONTRACT Design and Build (DB) CONTRACT DURATION Ongoing CONSTRUCTION METHOD NATM, Rib-Pile, Top-down, Braced secant piles with diagonal struts DATE OF START July 2017 DATE OF FINISH Ongoing DURATION OF THE PROJECT 6 years and 6 months (ongoing) KEY PROJECT CHARACTERISTICS · Geometry design: a three-story station configuration (25.55 m height) with three-arched roof is proposed, consisting of a ticket hall (35 m width), a middle story (22.5 m width), and a platform story (22.5 m width) · Excavation method = NATM, Top-down · Line alignment = -30.5 m · Overburden = 8 m · Type of retaining wall for ticket hall = Rib-pile, frame · Type of retaining wall for technical story = Secant pile, strut · Type of retaining wall for platform story = Secant pile, strut · Dimensions of ticket hall = 40.42 m width with 6 m depth · Dimensions of technical story = 23.6 m width with 5.18 m depth · Dimensions of platform story = 23.6 m width with 11.35 m depth · Width of platforms = Each of them 7.4 m, together 14.8 m · Effective width of station = 20.92 m · Number of rib-piles = 110 · Number of secant piles = 220 · Number of struts = 100 · Height of secant pile = 17 m · Main lithology and geological unit = Fine-grained with the middle layer of coarse-grained CHALLENGES AND DIFFICULTIES · The station is situated in one of the cultural heritage areas in Iran, where excavation on the surface is not permitted due to cultural heritage regulations. Therefore, all excavation and construction processes must be conducted underground. · The Zayandeh rood river traverses the central region of Esfahan, causing the groundwater level to be significantly high, and leading to the platform and middle stories to be completely submerged. · The station is built through clayey soil materials, with a middle layer consisting of coarse-grained materials that have high value of hydrodynamic factors such as transmissivity and coefficient of permeability. As a result, the pumping and dewatering operations are unable to decrease the underground water level as proven by a 24-hour pumping test which only resulted in a low reduction in the water level. · L2 station is a link tunnel with metro line 1, which is subject to floor level limitations requiring access to the desired depth on line 1. · The station serves as a link between line 1 and line 2 of the Esfahan metro. This is why the width of the station has been designed to be larger than the usual measurements, with a maximum width of 41 m and a minimum width of 24 m. There are two main reasons for this decision. First, there is a high demand for transferring passengers between these two lines due to a dense population. Second, side structures need to be integrated into the main structure, requiring additional space. · Braced secant piles and diaphragm walls from the ground surface is considered the most effective approach to mitigate groundwater issues. However, the station is situated in one of the cultural heritage areas in Iran, where excavation on the surface is not permitted due to cultural heritage regulations. · Although the station has an overburden of 30 m, the retaining wall of the station has an overburden of 8 m. DESCRIPTION OF SERVICES · The collection of data and conducting of foundational studies encompass various components: · The meticulous examination and identification of general physical characteristics and regulations · Investigation and study of building materials and construction methods · Scrutiny of the building services system and evaluation of necessary equipment · Architectural planning · The formulation of schematic designs and preparation of detailed reports. · Phase 2 studies encompass a comprehensive examination of architectural, structural, electrical, and mechanical systems. SOLUTION TO OVERCOME OBSTACLES · During the excavation process, it is advised to avoid using water pumping and dewatering techniques since they are not efficient ways for this specific project. Instead, a watertight wall (braced secant piles) should be constructed above the level of the groundwater to prevent water infiltration. · A three-slab station configuration with three-arched roof (25.55 m height) is proposed, consisting of a ticket hall (35 m width), a middle slab (22.5 m width), and a platform (22.5 m height). The ticket hall (top) slab houses the side structures and equipment such as RS and LPS, while the middle slab serves as a ventilation space. · The consideration of a three-story station design was prompted by the necessity for an interchange station with metro line 1, which is subject to floor level limitations requiring access to the desired depth on line 1. · In order to address the limitations regarding the performance of braced secant piles under the ground (in case of required equipment), a creative method was employed, involving the construction of two equipment galleries (above the underground water surface) to provide sufficient space for these piles. The crown of the tunnel is strategically positioned above the area where Braced secant piles need to be executed, ensuring adequate height and space. · During the operational stage, the braced secant piles were positioned 1.5 meters lower than the design measurement, resulting in a gap between the secant pile and the foundation of the ticket hall. This gap is filled with a pile cap to effectively connect these two structures. · The middle slab is then excavated using the new Austrian tunnelling method (NATM). It is necessary to perform strut to further enhance the stability and support of the top slab of the ticket hall. · Controlling and checking the process of excavation and construction. ACHIEVEMENTS · Introducing an innovative geometry design for construction this station (a three-slab station configuration with three-arched roof is proposed, consisting of a ticket hall (35 m width), a middle story (22.5 m width), and a platform story (22.5 m width)). · Formulating distinctive design following numerous brainstorming sessions as an innovative approach to construct the subway station in the central core of the historical area of Esfahan (fifty-thousand-year-old civilization and the historical capital of Iran) and underground water level. · The successful execution of an excavation and construction project for a L2 station with a larger cross-section than typical stations, necessitating the application of advanced techniques and distinctive designs, stands as a remarkable achievement. · Successfully performing three types of excavation methods including rib-pile (top slab), NATM, and top-down. · Employing a creative method, involving the construction of two equipment galleries (above the underground water surface) to provide sufficient space for Braced secant piles cause the excavation on the surface is not permitted due to cultural heritage regulations. · The construction methodology of the station harmoniously aligns with the architectural aspects, ultimately resulting in a mutually enhancing and complementary integration. · The station construction methodology has contributed to the creation of a unique architectural approach, wherein the architectural elements seamlessly align with the execution technique, thereby ensuring a harmonious integration of design and implementation. · The operation of underpass proceeded without causing any disruption. · Successfully evaluated the difficulties, challenges, and potential risks, ensuring its safety and mitigating potential damages. · Successfully controlled and minimized the risk of ground settlement, ensuring stability and safety during the construction process. · The approved settlement amount during the design phase closely matched the actual conditions experienced in the field · The results achieved from numerical modelling satisfactorily matched the operational stage. · The successful implementation of this innovative project within a historic context and beneath underground water, and with consideration for permissible settlements, was achieved through the utilization of a combination of various drilling methods, the use of braced secant piles, struts, and the top-down technique.
July 2017
