In the era of rapid urban rail transit development, the subway has evolved from a mere public transportation carrier into the "artery" of urban development, driving the vigorous rise of transit-oriented development (TOD) models. As the scale of urban underground space development continues to expand and the environment grows increasingly complex, deep foundation pit projects adjacent to operating subways face unprecedented challenges—how to achieve efficient underground space expansion while ensuring the safety of subway structures? This has become a key issue in urban underground engineering construction in the new era.As a technical team with years of deep expertise in geotechnical engineering, the Geological Survey Institute has leveraged rich engineering practice experience and sustained technological innovation to successfully master the "key technologies for design and construction of deep foundation pit subway protection in complex environments," forming a comprehensive technical system covering the entire cycle of design, construction, and monitoring, and providing a "geological survey solution" for the safe development and utilization of complex underground space in megacities.Engineering Challenge: The Difficult Problem of Underground Space Expansion Adjacent to Subways in Soft Soil AreasRepresented by the Kuntai Mingyuan, Kuntai Apartment, and Kuntai Center projects, this is the first underground complex project in China's soft soil areas to be expanded on both sides of an operating subway station and shield tunnel at a speed of 60 km/h. The project risk level is rated as Extra-Special, and its complexity is comparable to an "encyclopedia of underground engineering." The project required construction under stringent conditions: trains passing through the operating subway line at high speed without stopping, and four entrances simultaneously embedded into the expansion plots, while also addressing the challenges of depth differences of 11.8 m and 17.1 m on both sides of the subway. Furthermore, in Tianjin's soft soil characterized by marine-continental interactive sedimentary strata, the project had to solve the complex water system problems of multiple confined aquifers at the pit bottom and the adjacent North Canal. Most critically, the subway structure deformation indicators were controlled within 12 mm vertical and 6 mm horizontal, far exceeding industry standards. Achieving risk control through dynamic balance, the project wrote a story of extreme breakthrough in underground space development. Key Technical Breakthroughs: Multi-dimensional Safeguarding of Subway SafetyFacing extreme challenges, the geological survey technical team, guided by the principle of "safety as the framework, innovation as the blade" and centered on the core goal of "deformation control," developed five key technologies and built a full-chain technical system covering "design-construction-monitoring." Adoption of Innovative Large Compartmentalized Excavation DesignThe technical team broke through traditional models and boldly adopted the design principles of "large compartments, small excavations, distance from subway, early closure." After multiple rounds of optimization, the initial 12-compartment scheme was streamlined to 6, saving approximately 20 million yuan in support costs and significantly shortening the construction period of the soft soil deep foundation pit adjacent to the operating subway, providing a standardized paradigm of "compartmentalized design" for similar projects. Symmetric Support Design on Both Sides of the Subway and Integrated Support-Bridge DesignWall-strut support was adopted, combining opposing struts with large-area truss supports, with reinforced plates installed on the subway-adjacent side to meet the requirements of support deformation, soil extraction trestles, and local loading. Meanwhile, horizontal supports were symmetrically arranged on both the left and right sides of the foundation pit to ensure overall horizontal stiffness. At key support nodes, measures such as diagonal bracing, corner bracing, internal buttress diaphragm walls,slightly expansive concreteand axial force servo systems were employed to strengthen weak nodes. Real-time Dynamic Dewatering Coupling Control Measures for Subway DeformationThrough on-demand, quantitative dynamic dewatering, combined with subway deformation conditions, the phreatic water level difference on both sides of the subway was precisely controlled to reduce horizontal deformation of the subway. The hydraulic head of the first confined aquifer at the pit bottom was actively lowered, using compression settlement of the pit bottom soil to offset the station heave deformation caused by large-area excavation. When long-term dewatering inside the pit caused the hydraulic head of the confined water outside to decrease, resulting in increased differential settlement between the inside and outside of the tunnel section foundation pit, recharge wells on the outside were promptly activated to effectively reduce differential settlement of the tunnel. Dynamic Loading on Station Roof Sections to Control Station HeaveOn the basis of meeting the load control requirements for the station and tunnel areas,load plateswere installed above the station and commercial structure roof slabs. Through dynamic stacking of soil, steel, and other loads, the heave deformation of the subway station and tunnel was precisely controlled. Integrated Platform Based on Intelligent Monitoring and Numerical Analysis for Dynamic Feedback Control MeasuresNumerical models were established to determine design, monitoring, and dewatering schemes; simulation inversion was carried out based on comprehensive construction conditions to derive deformation data. The intelligent monitoring and analysis system automatically analyzed and issued warnings, promptly adjusting construction measures to ensure effective control of subway deformation. Promotion and Application of Key Technologies: Supporting Safe Construction of Multiple Rail-Related ProjectsKuntai Project: Refined Design Ensures SafetyThe foundation pit support design projects for Kuntai Mingyuan, Kuntai Apartment, and Kuntai Center, completed from 2019 to 2022, are located on Xinzhuang Street, Hebei District, Tianjin. In this project, the research results of the "Study on Compartmentalized Scheme and Construction Method for Deep and Large Foundation Pits" were adopted. The approach of "combining numerical simulation analysis with design" was applied in the support design, together with refined design measures such as the "large compartment, small excavation" foundation pit design concept and the "deep far, shallow near, interval skip-compartment" foundation pit design and construction method, effectively ensuring the deformation safety of the foundation pit and subway. Through these technical measures, construction costs, material costs, and measure costs were reduced by approximately 63 million yuan in total. The project won the First Prize of the 2024 Tianjin Excellent Survey and Design Achievement Award. Nanlou Project: Controlling Subway Deformation Based on Actual ConditionsThe Zongyi Plaza Project (Nanlou Project), completed from 2021 to 2022, is located on the southwest side of the intersection of Aiguo Road and Dagu South Road, Hexi District, Tianjin. Based on actual site conditions, the project applied the "Safety Technology Control Research" results from the "key technologies for design and construction of deep foundation pit subway protection in complex environments adjacent to subways." Through a series of measures in support design and construction, subway deformation was effectively controlled, ensuring the deformation safety of both the foundation pit and the subway while reducing engineering costs. The successful application of these technical results in this project provides a reliable practical example for subsequent similar projects.Nanmenwai Project: Strict Control Considering Deformation PatternsThe urban renewal foundation pit support design project for the northern area of the Nanmenwai Street commercial district, ongoing since 2022, is located at the intersection of Nanmenwai Street and Nanma Road, Heping District, Tianjin, and is divided into three plots A, B, and C, with a total land area of approximately 46,000 m². The project is adjacent to the operating Metro Line 2 and the under-construction Gulou Station of Metro Line 7. In the support design, the technical team of the Geological Survey Institute fully considered the influence of these results on the deformation patterns of foundation pits adjacent to subways and adopted a series of innovative process measures to strictly control subway deformation, ensuring the safety of both the foundation pit and the subway. This not only ensured the smooth progress of the project but also reduced construction costs, material costs, and measure costs, enhancing the reserve of geotechnical design technical experience for similar projects in the future. Technical Achievements: Win-Win in Economic and Social BenefitsThrough the summary and enhancement of the above multiple deep foundation pit subway protection design projects in complex environments, fruitful technical achievements have been obtained. In terms of patent applications, more than 20 invention patents, utility model patents, and software copyrights have been obtained, including the "Design and Construction Scheme Optimization Method and System for Heave-Settlement Control of Subway Double-Side Expansion," the "Excavation Settlement Prediction Method and System for Subway Double-Side Expansion," and the "Anti-Heave Dynamic Loading Analysis Method and System for Underground Structures." In collaboration with China Construction Eighth Engineering Division and Beijing Jiaotong University, leveraging the research advantages of universities, three special topic reports were produced, including the "Research Report on Design and Construction Risk Assessment and Control Technology for Large Subway Expansion Projects," and five papers were published in core journals, including "Analysis of the Impact of Compartmentalization of Large Foundation Pit Groups Adjacent to Subways on Operating Subway Structures." The successful application of these technical achievements has yielded significant economic and social benefits. Not only was subway operation safety ensured, but the smooth implementation of the expansion projects also effectively improved the utilization efficiency of underground space in urban core areas, winning unanimous praise from owners and all sectors of society. The Kuntai Mingyuan, Kuntai Apartment, and Kuntai Center projects, as the Extra-Special risk deep foundation pit projects with the greatest construction difficulty in Tianjin, attracted authoritative experts from universities, design institutes, and construction enterprises in the industry to visit and exchange multiple times, and mainstream media reported on the projects repeatedly, greatly enhancing our institute's technical influence in the geotechnical engineering industry, providing strong technical support for subsequent market development and undertaking similar projects, and offering valuable reference for similar projects.From "solving difficult problems" to "leading standards," from "individual projects" to "industry empowerment," the breakthrough of the "key technologies for design and construction of deep foundation pit subway protection in complex environments" is a vivid microcosm of our bureau's 65 years of innovative development. In the future, we will continue to use technological innovation as the engine and serving the city as our mission, writing new glorious chapters for the high-quality development of the geological and mineral industry and for building safer and better underground spaces in our cities!
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