Based on the characteristics of NATM and the construction process of composite lining, this paper presents a thorough study on the mechanical responses and computational method of the lining. The concepts on bearing ring around an excavation circumference and high resistance point to enhance the bearing capacity of the ring are put forward to stabilize the surrounding rock Assuming that the behavior of the material of surrounding rock submits to Kelvin-Voigt model, the behavior of composite lining is elastic, and the inner forces of the lining are caused by deformation pressure and gravity load, a method to calculate the inner forces of composite lining is established. An example is given to illustrate the calculation procedure, the results of an example show that the method raised by this paper can simulate the construction procedure very well.
has been gradually widely used in construction of mining gallery, communication tunnel, hydraulic tunnel and other underground excavation all over the world since 1960's. The key point of the technique is that the surrounding rock of an excavation or tunnel is considered as having self-bearing capacity, which means that, basically, the surrounding rock has the ability to keep the stability by itself after reinforced. Usually, a thin, flexible shotcrete coating in contact with the surrounding rock of an excavation is firstly shotted against the surface shortly after excavated, rock bolts are installed simultaneously when necessary. If numbers of big displacement were happened in the surrounding rock or fissures occurred in the former shotcrete layer, then more shotcrete were added to cover the former layer by shooting again, steel bar net was placed along the surface of the former layer before shooting when needed. Finally, an inner lining is poured by using concrete or shotcrete again after the deformation of the surrounding rock tends to be unchanged. The support system formed in NATM is usually called composite lining. The outer layer directly against the rock is generally shotcrete or combined shotcrete and rock bolts, the middle is shotcrete, including steel bar net inside when necessary, the inner is monolithic concrete or shotcrete. Based on a study on the stressing procedure and bearing mechanism of composite lining related to the construction process, this paper is to establish a more rational method for the design and calculation of this kind of lining.
1.1 Ultimate bearing capacity of surrounding rock The ultimate bearing capacity of reinforced surrounding rock at any section is the basic index to check the stability of an excavation. The main effects of the support system are:
to change the distribution of the ground stress and displacement to help the surrounding rock tend to be stable:
to provide high resistance point by rock bolts or shotcrete so that the ground material could refrain from failure.
The latter is more effective to increase the ground self-bearing capacity and the designer can reflect the effect by taking a smaller value of safety factor.
