## 1 Introduction  
The stress redistribution induced by underground excavations causes soil movements around the excavation face in the earth mass, which is also translated to ground settlement. These soil movements can cause severe damage to adjacent buildings and underground facilities [1; 2]. Control of the soil movements due to excavations is of utter importance in subway projects, because they are often planned in the populated areas of cities that are already well developed. Several methods for surface settlement prediction due to underground excavations are currently available including those proposed by [PERSON], [PERSON] and [PERSON] [3; 4; 5]. Underground excavations in urban area have some characteristic features such as large spans, ultra-shallow buried depth, complex environments, and heavy overburdens due to street traffic. Hence, excavation-induced instability and settlement has become a major concern in the design and construction of the new underground excavations [6]. The pipe roofing method evolved from pipe umbrella method adopted in shallow ground cover and poor ground conditions during underground construction. As for this method, steel pipes are jacked to the design location by pipe jacking machines to act as the initial support system. These adjacent pipes are connected together and form a supporting structure, filled with grout or concrete in the inside and outside of the steel pipes before tunnel excavation. The pipe roofing method was firstly to be used to construct an underpass railway in 1971 and a total of six such projects were completed in the next decade [7]. The Antwerp subway station in Belgium was an early engineering feature, constructed by applying pipe roofing method in Europe in 1979 [8; 9]. It was first used in America in 1994 for a large shallow covered tunnel [10]. Subsequently, this method is applied to construct highways in Japan, and on the basis of the pipe roofing method, Endless Self-Advancing (ESA) and Front Jacking (FJ) were proposed [11; 12]. Meanwhile, the methods were widely applied to certain engineering projects in Singapore and Malaysia [7; 13]. A typical engineering project, 923 stations of the Seoul subway line 9 in Korea was constructed by using the pipe roofing method in the complex environments [14; 15]. Based on some engineering examples of previous pipe roofing method in China, it was used to build an underpass in Hong Kong in 1984, and the Taipei Songshan airport underpass was constructed using the pipe roofing method in 1989. The pipe roofing method was employed to construct the vehicle tunnel, which passes the Taipei International Airport [16]. The Roof-Box Jacking Method (RBJ) was used for the underpass project of Shanghai Beihong street in 2004. This support structure formed a rectangular section and was made up of 80 steel pipes with a diameter of 0.97 m and longitudinal length of 125 m [17]. By an arc-shaped supporting system, Xinle Yizhi station of Shenyang subway line 2 was constructed by the new roof tubular of method with 19 steel pipes with diameter of approximately 2 m in 2011 [18]. The Gongbei tunnel, as a part of Hongkong-Zhuhai-Macao Bridge project, was constructed a combination technology of curved pipe jacking roof for ground support and ground freezing for waterproof. This pipe roofing structure is composed of 36 steel pipe strings of 1.620 m diameter with a 355-358 mm gap between adjacent strings [19; 20]. In term of studies on the working mechanism of the pipe-roofing support systems, these have shown that the maximum bending moment of the steel grid is often much larger than that of the pipe-roofing system in the longitudinal direction [21]. Therefore, to ensure the stability of the pipe-roofing system during excavation, an intensive temporary supporting structure is often required. The tedious and expensive set up of the steel supporting structure is a big disadvantage of the traditional pipe-roofing method. And as for forming an arch structure, the space utilization is lower than the rectangular section.