Special Issue on Urban Underground Space Development Technologies

In well-established metropolitan areas, the underground space development for transportation and commercial usage is in high demand to meet both the sustainable population and economic growth. As the largest city in China, the underground space in Shanghai has been rapidly developed in past decades, including the basements of skyscrapers, underground streets and highways, subway tunnels and stations, underground transmission and substation, water supply and drainage pipe systems, etc. In 2013, the total area of underground space in use has reached up to $\mathrm{57,000,000}{\mathrm{m}}^2$ in Shanghai, most of which are constructed as deep excavations. The largest excavation area of a single project has reached up to $\mathrm{350,000}{\mathrm{m}}^2$ (Shanghai Hongqiao International Airport Transport Hub), whereas the maximum depth of deep excavation has been larger than 34 m (The Shanghai 500 kV World Expo Underground Transmission and Substation). The basement excavation for the Shanghai Tower–the tallest building in Shanghai—is shown in Fig. 1, and the excavation area and depth of which are $\mathrm{34,960}{\mathrm{m}}^2$ and 31.2 m, respectively. The tunnels of subways and underground highways are usually constructed by the shield tunneling method. At the end of 2013, the total length of the operating underground subway lines in Shanghai is more than 300 km. Moreover, many huge highway tunnels are constructed to cross the big rivers, in which the largest one is Shanghai Yangtze River Tunnel, with a diameter of 15.43 m (Fig. 2). The pipe jacking method is usually used in the water supply and drainage pipe systems. The largest steel pipe jacking for water supply pipe systems is about 3,600 mm in diameter, and the largest concrete pipe jacking for sewage pipe systems is about 4,640 mm in outer diameter (Fig. 3).

The construction of new underground space would impact the existing structures and pipelines. Sometimes the new deep excavations have to be very close to adjacent structures [Fig. 4(a)] and even cross the operating subway or high-speed railway [Fig. 4(b)]. If there are several deep excavations close to each other (Fig. 5), their environmental effect would be coupled, inducing the magnified consequence. New tunneling projects would also cross some existing buildings, tunnels, pipelines, and even airport (Fig. 6). Considering the fast development of underground space scale, the complex underground environment compounded with influences to existing surrounding structures need to be addressed while developing the large underground excavation technologies. Therefore, the design and construction methods for underground structures in Shanghai have made great progress in practices, and the performance of underground construction has also been extensively investigated.

This special issue of “Urban Underground Space Development Technologies” in the Journal of Aerospace Engineering aims to present new developments on large earth excavation technologies and to promote sustainable development on earth, in space, and on other planetary bodies. This unique special issue on underground space also diversifies the topics presented in the journal. In this special issue, the recent progress in underground development in Shanghai are presented, ranging from the novel design and construction methods for deep excavation and large tunnels, to convenient methods for ground response prediction, to special behavior of soil-water coupling, and to the new back-analysis method for parameters in numerical prediction, to theoretical study postbuckling behavior of buried geodesically-stiffened pipelines under combined external pressure and axial compression. In particular, the unique challenges and solutions in the special underground space development projects are elaborated.

In “Design and Performance of Large Excavations for Shanghai Hongqiao International Airport Transport Hub Using Combined Retaining Structures,” Wang et al. presented a combined retaining system for the largest excavation project in Shanghai, whose area reached up to $\mathrm{350,000}{\mathrm{m}}^2$. In this retaining system, the slope excavation, gravity retaining wall, and diaphragm wall were adopted according to the excavation depth in different locations. The performance of the combined retaining system and surrounding geoenvironment during and after the construction were monitored and analyzed based on an extensive instrumentation program, which verified the design work.

In “Novel Excavation and Construction Method of an Underground Highway Tunnel above Operating Metro Tunnels,” Chen et al. presented a divided alternate excavation method (DAEM) for an underground highway. To reduce the existing tunnel deformation, the overlying excavation of the highway was divided into small independent pits and constructed at intervals. Application of the proposed construction method revealed that the deformation of the underlying metro tunnels was controlled effectively.

In “Environmental Effect and Control of Large Diameter EPB Shield Tunneling below an Operating Airport,” Huang et al. introduced a case of an earth pressure balanced (EPB) shield tunnel with a diameter of 14.27 m crossing below an operating airport in a soft soil region. The environmental effect of an EPB shield tunneling project and its relationship with the construction parameters was investigated by field tests. The control criteria of EPB shield tunneling under the airport is then recommended and applied during the tunnel construction. The application demonstrated the effectiveness of the proposed control methods.

In “Practical Investigation into Two Types of Analyses in Predicting Ground Displacements Attributable to Dewatering and Excavation,” Tan et al. presented and compared two types of numerical analysis methods to predict the ground displacement caused by dewatering in a deep excavation. These two methods, i.e., transient analysis (TA) and steady state analysis (SSA), were carried out for the large-scale excavation of Shanghai Hongqiao International Airport Transport Hub. Comparisons between the calculated and measured results indicated that TA is more accurate and appropriate for detailed analysis of construction processes, and SSA requires less calculation and is suitable for preliminary evaluation.

In “Displacement Performance and Simple Prediction for Deep Excavations Supported by Contiguous Bored Pile Walls in Soft Clay,” Zhang et al. analyzed an excavation case supported by contiguous bored pile walls (CPWs). Based on the field measurements and statistical values available in the literature, some empirical expressions and parameters were presented to predict the wall deflection, ground movements, and building settlements. A simple method was then developed to predict the displacement performance of the deep excavations with CPWs, providing an approach to assess the deep excavations and leading to an optimized design and construction.

In “Stratified Settlement Characteristics of the Soil Strata in Shanghai Attributable to Dewatering,” Zhu et al. introduced a special deformation phenomenon of soil strata during the dewatering of confined aquifers. With an excavation case of an underground metro station, the dewatering-induced stratified settlement behavior was investigated by field monitoring. The monitoring results indicated that the clay layer swelled during dewatering in the underlying soil layer, which is referred to as the dewater-caused soil expansion. Such special phenomenon has not been reported in the existing literature, and this case study provides a basic insight into the phenomenon.

In “Back-Analysis and Parameter Identification for Deep Excavation Based on Pareto Multiobjective Optimization,” Huang et al. proposed a back-analysis method of deep excavation by the Pareto multiobjective optimization. A multiobjective optimization algorithm multialgorithm genetically adaptive multiobjective method (AMALGAM) was then implemented to identify the soil parameters based on the multiple types of field observations, and it was applied to a well-instrumented deep excavation. The Pareto front in the biobjective space exhibited a rectangular shape, which implied that the simultaneous minimization of both objectives could be achieved. The back-analyzed soil parameters of the compromise solution from the biobjective back-analysis could reasonably simulate both the wall deflection and ground surface settlement.

In “Postbuckling of Buried Geodesically Stiffened Pipelines under Combined External Pressure and Axial Compression,” Li and Qiao presented the post-buckling analysis of buried geodesically-stiffened pipe-lines of finite length under combined loading of external pressure and axial compression, simulating the underground pipelines embedded in soil and with axial compression construction load. The pipe–soil interactions are modeled as a Pasternak elastic foundation, and the nonlinear prebuckling deformation, initial geometric imperfection of the liner, and stiffener effect were considered to investigate the buckling and postbuckling behavior of stiffened cylindrical pipes. The theoretical study could facilitate design analysis and optimization of stiffened pipes, and it can be used to develop remedial schemes for underground pipes against instability during service and during the pipe-jacking process.