# Study on Share Rate of Support Structure for Super-Large Span Twin Tunnels with Small Interval
## 1 Introduction  
Recently, with the proceeding of urbanization, the demand from traffic is surging. As a result, the need for improving the capacity of the transportation system, specifically the cross section of tunnels in cities, is getting larger to contain four or more lanes [1]. The tunnels with large-span cross section, however, are usually oblate with a small rise-span ratio, which is challenging to design them in a safe and reliable way. At the same time, on the limitation of land resources and route optimization, twin tunnels with small intervals are becoming more usual at the portals, whose mechanical behaviour is much more complicated than ordinary tunnels.  
There are many studies on large-span tunnels and ordinary twin tunnels, respectively. In terms of large-span tunnels, [PERSON] [2] evaluated the different installation timing of initial ground support for large-span tunnel in hard rock. [PERSON] [3] and others [4; 5; 6; 7] studied the excavation (drill and blast) and construction of large-span tunnels and mechanical characteristics of primary support. [PERSON] [8] used hydro-mechanics to research the influence of groundwater on the surrounding rock of large-span tunnels.  
As for the ordinary, [PERSON] [9] quantifies the interaction effect and preliminarily estimates the main support requirements of twin tunnels based on the stress state of a single tunnel, and provided nomographs for preliminary assessments of the optimum spacing between twin tunnels. [PERSON] and others [10; 11] also studied the influence of shape and interval on twin tunnels and sandwiched rock. [PERSON] [12] researched themechanics performance of a twin arch tunnel under different rock covers. [PERSON] and others [13; 14; 15] found a method for selection of optimum distance between tunnel tunnels under different conditions. [PERSON] and others [15] studied the vibration of twin tunnels and found that certain factors (such as the soil and the interval) can affect it. [PERSON] and others [16; 17; 18; 19; 20] researched and monitored the twin tunnels under different conditions. [PERSON] and others [21; 22; 23; 24; 25; 26; 27; 28] have studied the interaction between the law of rock and soil and structural characteristics of constructions. Besides, [PERSON] and others [29; 30; 31] studied the surrounding rock pressure of deep twin tunnels. And [PERSON] and others [32; 33; 34] researched the rock pressure of shallow-buried twin tunnels.  
Nonetheless, for large-span twin tunnels there are few studies regarding it. [PERSON]. [35] analysed the mechanical mechanism of highway tunnels with large span and small spacing during the construction and found that large-span twin tunnels were hard to construct, depending on the excavation area, the thickness, and the attributes of the sandwiched rock and the terrain.  
The engineering of super large-span twin tunnels, which can contain eight lanes or more, is not usual at home and abroad. Although the total load of surrounding rock is almost explicit nowadays, its distribution law of surrounding rock pressure as well as the share rate of every part of the retaining structure has not yet been considered clearly. However, designs of primary support and secondary lining are respective, so finding out each part's specific share rate of load is logical for design and construction of actual engineering project. Therefore, in this paper, based on the theory of stratigraphic sliding failure mode and the outcome of monitoring and measurement in the Daling tunnel (twin tunnels, four lanes in each hole), the distribution law of surrounding rock pressure and the share rates of advance support, primary support, and secondary lining are studied. Eventually, by comparing and analyzing the theory and practical results, the correctness of the research method is ensured, which can provide reference for the design and optimization of the retaining structure of super large-span twin tunnels, especially 8-lane (or more) twin tunnels in cities.