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2 Design Load 1
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2.1 Types of Loading 1
2.2 Lateral Earth Pressure 2
2.2.1 Earth Pressure at Rest 2
2.2.2 Rankine’s Earth Pressure Theory 3
2.2.3 Coulomb’s Earth Pressure Theory 5
2.3 Rock Mass Pressure 6
2.3.1 Loosening Masses Theory 8
2.3.2 Rational methods 13
2.4 The “Q” System 15
2.5 Pressures of Ground-Structure Interaction 16
2.6 Deadweight of Structure and other loads 17
Problems 18
Reference and Bibliography 36036
2 Design Load
The loading mechanism of an underground structure is different from that of a surface structure. For underground structures, the most important loading comes from the host ground itself. In competent host ground, the ground loading on the underground structure is quite insignificant and may be equal to zero whereas in incompetent ground, it may be quite significant. The host ground pressures on the underground structure is quite complex（R.S.Sinha,1989）. It is dependent on several factors such as the relative stiffness of the structure and the host ground, the elapsed time between the excavation and installation of support, the characteristics of the host ground, the in situ pressures, the size of the opening, the location of water table, and the adopted methods of construction.
2.1 Types of Loading
The loading of an underground structure is rather arbitrarily pided by the civil engineer into three categories, dead loading, dynamic loading and live loading.
Dead loading . A constant load on an underground structure (e.g. a tunnel) due to the weight of the supported structure itself, such as the deadweight of a structure, pressure of rock or soil and groundwater pressure, etc.
Dynamic loading. The short-term load placed on an underground structure during installation, and it changes in the direction or degree of force during operation, such as the pressure produced by explosion wave or earthquake wave.
Live loading. A moving, variable weight added to the dead load or intrinsic weight of an underground structure or vehicle, such as the floor load (people or equipment), crane load, rockfall load and other temporary loads.
In addition, design loads of an underground structure also include the internal forces resulting from concrete shrinkage, temperature fluctuation and different settlement.
With respect to a specific design of an underground structure, the aforementioned loads must be determined first and need to consider their combination. When the host media acts as a load-participating member, average design stresses become meaningless. The concentration of stresses becomes predominant in the design. This happens because the majority of the host medium material does not yield before failure and, as such, the determination of peak stresses becomes very important. An exception to this will be an underground structure in rock having low cover where the structural stability is more controlled by the geological discontinuities and where the movement of the host medium as a block is a more prominent factor than the peak stresses generated due to the excavation for the underground structure.
2.2 Lateral Earth Pressure
This section deals with the earth pressure against lateral supports such as retaining walls or the bracing in open cuts, with the resistance of the earth against lateral displacement, with the bearing capacity of footings, and with the stability of slopes. Proper design and construction of these structures require a through knowledge of the lateral forces that act between the underground structures and the soil masses being retained, and these lateral forces are caused by lateral earth pressure.
2.2.1 Earth Pressure at Rest
Let us consider the mass of soil shown in Figure 2.1. The mass is bounded by a frictionless wall that extends to an infinite depth. A soil element located at a depth is subjected to effective vertical and horizontal pressures, respectively. For this case, since the soil is dry, we have