Stage method of tunnel construction
This method is used for construction of tunnels with cross-section over 120-130 m2 and height over 10 m mainly in hard rock mass. This method differs to continuous face method from the point that tunnel section is divided in two parts and each part is headed differently in different height and time.
There are two variants of stage method: upper-stage and lower-stage methods:
- Upper-stage method. At first lower part of profile is headed and then the heading of upper part is started to perform. This variant has a range of technological disadvantages and is rarely used.
- Lower-stage method. This variant is widely used in tunneling, especially for construction of tunnels with vast section and height. At first the upper part of profile is headed and then the heading of lower part is started to perform.
In tunnel construction by means of lower-stage method first of all the upper part of cross-section is headed with the help of continuous face method. The height of upper part is to be more than 4 metres according to the location conditions and heading equipment height. Then concreting (rock bolt support, shotcreting) are performed according to the project and tunnel construction conditions.
After heading and formwork of tunnel upper part the lower part is started to head with berms along the sides of excavation; the width of berms depends on crown pressure. Then sidepieces of lower part are headed and concreted as on a chess-board.