## 1 Introduction
As a result, the column pile method is one most commonly used. The drawbacks of this method are longer construction period and higher cost than that for the two methods mentioned above. A diaphragm wall is another method which is commonly used in deep excavations nowadays, with merits of low vibration, low noise, high rigidity and relatively small wall deformation. The thickness and depth of the walls are adjustable for different conditions. The wall can also  
Figure 4: Collapse of an excavation in Foshan, China. Figure 5: Collapse of an excavation in Nanning, China. be used as a permanent structure with good sealing capability. However, large items of mechanical equipment are required with long construction period and great cost. In addition to retaining walls, strutting systems are other main components of the strutting system in a braced excavation method. There are three types of struts according to the material a strut is made of, including wood struts, reinforced concrete (RC) struts and steel struts. The wood struts can only be employed in excavations that are narrow and not deep, for their low axial stiffness. The RC struts are appropriate to excavations of various shapes with high axial stiffness. However, it takes some time for the strength of the RC strut to work and preload cannot easily act on it. Furthermore, dismantling it is difficult and costly. As for steel struts, they are widely employed for strutting systems because they are easy to install and dismantle with short construction periods and low cost, and they are easy to preload. However, they are not appropriate for excavations of great topographical undulation or of great width. Moreover, the joints of the struts are complex and highly susceptible to construction deficiency, and collapse may occur when the joints don't work (see Figures 1-3), and this issue has been discussed above. From the discussion above, the commonly used retaining structures have their own merits and drawbacks. Because of that, new forms of retaining structure need to be designed for use in deep excavations, which can make best use of the advantages and avoid the disadvantages of the common structures. In recent years, several researchers have carried out research aimed at addressing this issue. [PERSON] and [PERSON], for example, introduced an innovative prestressed support system [8; 9] in which earth pressure is resisted using wales, wires, and H-beam support. The system provides a large amount of room for excavation activity and makes construction easy. Cross/buttress walls have also been used to construct a new form of retaining system to protect excavations in urban areas in Taiwan [10; 11; 12; 13; 14; 15]. [PERSON] [10; 11] evaluated the effectiveness of cross and buttress walls using monitoring data taken from case histories and by carrying out finite-element (FE) studies. The results thus obtained indicate that buttress walls are effective in reducing wall deflection and ground settlement. [PERSON] [12] also studied the performance of buttress walls using FE models and found that their use can reduce the deflection of diaphragm walls moderately. [PERSON] used an FE method to study the effect of the shape of the buttress wall on excavation-induced movement and revealed the mechanism by which the shape controls the deformation [13; 14; 15]. The aforementioned research also suggests that innovative strut designs are commonly used in engineering practice to increase stiffness, reduce the amount of space occupied, and increase the reliability of the excavation process. In this study, we introduce a new form of retaining structure for use in deep excavations. We refer to it as the 'herringbone retaining system' (HRS). The system uses batter piles to form a system of retaining struts. In this work, we investigate the effectiveness of the support system via numerical calculation and also explore the mechanisms responsible for its action. The design scheme employed in the new system is first presented.