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Abstract For the purpose of saving construction cost，Wenchuan portal section of Zipingpu tunnel is so designed that twin tunnels in close proximity connected with Minjiang River bridge. On this portal section the spacing between right and left lines is small and the wall rock between them is weak. For the safety of tunnel construction，some monitoring tasks have been performed in Zipingpu tunnel，including the ground settlement，surrounding rocks pressure and internal force in lining，etc. Based on the measured data，it is shown that：(1) the influence of the later excavated tunnel on antecedence tunnel and the wall rock is existent in the construction of twin-bored tunnels，but the degree of influence is different. The surrounding rock is weaker，the extent of the influence is bigger. (2) The stress of the juncture section of the neighbor steps is relatively centralized，and the initial support is the most weak. The site monitoring is advised to adopted in the construction. (3) Because the wall rock is the most weak link in the construction of the small-distance tunnel，it is advised to take measures(such as the rational method of construction and site monitoring，etc.) to ensure the safety in the construction of Zipingpu tunnel.
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Received: 24 April 2009
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