Abstract: Based on mechanical analysis of construction progress of double-arched tunnel under unsymmetrical pressure, the two kinds of partial pressures effects are put forward, which influence together the loaded state of double-arched tunnel. One of them is result from shallow depth ground partial pressure, named ‘terrain partial pressure’, the other is caused by construction order of the two bore of double-arched tunnel, named ‘construction progress partial pressure’. The result demonstrates that the direction of oblique load in ‘terrain partial pressure’ is definite and unchangeable, i.e. the direction of oblique load is from the inside of hill to the outside of hill-slope; the direction of oblique load caused by ‘construction progress’ depends on the early and late construction order of the two bore of double-arched tunnel; the study shows that the direction of oblique load is from the early excavated bore side to the late bore side. As a result, the procedure of the early excavation of external bore closed to the hill-slope is considered as the optimized construction procedure for double-arched tunnel under unsymmetrical pressure, since the oblique load caused by construction progress can be applied to balance out partial load in ‘terrain partial pressure’. The above conclusion is made use of the modification design and construction of double-arched tunnels under unsymmetrical pressure of Jinhua to Lishui highway in Zhe jiang province.

Key words: double-arched tunnel, center-pillar, unsymmetrical pressure, optimized construction procedure