7.2　Open tunnels

7.2.1　The construction of the open tunnels should not be carried out in rainy seasons or very cold seasons.

7.2.2　Before the construction of the open tunnel，the side slopes and front slopes shall be reinforced in advance as required in the design.Then，excavation and supporting shall be performed layer by layer.For the side slopes and front slopes，protections such as rockbolting，shotcreting，skeleton and vegetation shall be carried out layer by layer.

7.2.3　The side walls of the open tunnel shall be located on stable ground，and the width and depth of the foundations shall comply with the design requirements.When the side walls have soft or heterogeneous foundations，treatment measures shall be taken.When digging or trough cutting is performed，the supporting shall be strengthened and supporting shall be provided immediately after excavation.

7.2.4　If there is groundwater involved during the construction of the open tunnel foundation，the groundwater shall be diverted.

7.2.5　Prior to concrete casting for the open tunnel structure，the centerline，the elevation，and the dimension of the outer contour of the formwork shall be rechecked.The lining of the open tunnel shall not invade into the designed contour.The concrete for the side walls and arch of the open tunnel should be cast using formwork trolleys so as to achieve an integral structure.The steel arches for the open tunnel shall not be removed until the strength of the concrete for the arch ring has reached 100％of the design concrete strength. w2F/ILTktMymVXRXSAybvUNfJfadfhPFr6ZspjTohPcJ0cwT+7LBCWNZ9JDbuYIc