## 1 Introduction  
Double-arched tunnel is a kind of special parallel tunnel structure which is quite different from the separated tunnel and the small spacing tunnel, and there is a central division wall which supports both the left and the right tunnel arches. At present, the double-arched tunnel has been successfully and widely applied in railway, highway and subway engineering, especially in those areas under complicated geological and topographic conditions when there are difficulties in the application of the separated tunnel [1; 2; 3]. Owing to the large span of the double-arched tunnel and its complex structure as well as the frequent stress conversion between surrounding rocks and tunnel linings, the construction of the double-arched tunnel is quite challenging with great difficulties and high risks. Engineering accidents, such as failures and collapses of surrounding rocks are prone to occur in the construction of the double-arched tunnel, which, therefore, has received wide attention from scholars and engineers in the field of tunnel engineering [4; 5; 6; 7]. Among many tunnelling construction methods, the new Austrian tunnelling method (NATM) [8] is the most popular method used in the construction of the double-arched tunnel, which has proved to be a very economical and flexible mode of construction [9]. The NATM is a technique through which the drilling and blasting method is adopted to excavate a tunnel in rock, and a shotcrete liner and rock bolts are used as a main support system [10; 11]. The most important of all, based on the concept of \"as-built or during-excavation classification system\", the NATM requires dynamic observations of the geological conditions of excavation faces and the performance of underground supporting structures such as the linings [12]. Rock mass classification systems most commonly RMR [13] and Q system [14] are utilized to translate these observed data to provide an estimate of the required excavation methods and support system components during tunneling [15]. As we can see, field monitoring is the core of the new Austrian tunnelling method and plays an irreplaceable role in judging the stability of surrounding rocks and evaluating the rationality and safety of supporting structures [16; 17; 18]. Current researches regarding the monitoring strategies for NATM tunnels tend to estimate forces and stresses in the linings by using the back analysis from the monitored displacements based on the proposed thin shell-theory [19; 20; 21; 22; 23]. However, there are still some notable exceptions by using direct stress measurement on condition of proper installation of the monitoring equipment. For example, [PERSON] measured radial earth pressure by means of pressure cells to check the compatibility of soil fracture grouting and the NATM technique [22]. Based on the direct monitoring strategy, many scholars have made plenty of researches with respect to the field monitoring of the tunnel construction by NATM. However, these monitoring objects mainly concentrate in the single-arched tunnel, researches referring to the systematic field monitoring aiming at the double-arched tunnel are relatively few [24; 25]. In this paper, the Gangkou double-arched tunnel in the Jinhua-Wenzhou expressway (Zhejiang Province, China) provided the engineering background and monitoring of the deformations and internal forces of both surrounding rocks and supporting structures was carried out. The field monitoring revealed the internal displacements of surrounding rocks, the contact pressures between surrounding rocks and primary linings, the axial forces in rock bolts and the internal forces in steel arches and secondary linings.