| The small interval tunnel is getting more and more welcome in transportation industry with its unique advantages. This paper is based on parallel tunnels on Huang-Qu-Nan highway projects, has done some relatively systematic research on supporting time, supporting stiffness and supporting thickness of second lining through the tests on the site and computer numerical simulation method. the main achievements obtain as follows:1. According to 2D analysis for supporting parameters of second lining, about 40th day for this tunnel engineering is the best supporting time, Supporting stiffness has a little effect on second lining structure, E= 30GPa is taken as reasonable supporting stiffness as common value in engineering design and construction; Tiny data changes of mechanics state in second lining structure with different supporting thickness indicates 48cm is optimal supporting thickness.2. The results of 2D analysis demonstrate improving mechanical characteristics of tunnel lining structure by altering supporting time primarily, changing supporting thickness then and adjusting supporting stiffness lastly. To ensure the safety of the structure, when design combined lining in weak rock, changing supporting stiffness obtain little effect, should alter other parameters (such as primary support parameters) to make primary support undertake most of surrounding rock pressure, the second lining structure as security reserved structure, undertakes special load with initial lining.3. Based on simulating analysis of the fourth chapter and the fifth chapter, cross-section YK56+880 in weak surrounding rock range in Jiaowu parallel tunnel acquire sound support parameters of second lining as follow:E= 30GPa is regarded as reasonable supporting stiffness, 48cm is optimal supporting thickness,43rd day is rational supporting time of antecedence tunnel for second lining,+6m is the best construction layout between antecedence tunnel and later tunnel.
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