荷载设计英文文献和中文翻译(5)
Loosening load is a generic term and indicates the load that comes on the support structure immediately after the ground is excavated. In some cases, the final load coming on the support structure may ultimately exceed the loosening load with time due to the existence of “genuine ground pressure”. The genuine ground pressure may be less than or equal to or be several times the in situ ground pressure that existed before the excavation.
2.3.1 Loosening Masses Theory
●Vertical pressure of shallow structures
Figure 2.5 shows a shallow structure with vertical pressure. In such condition, the vertical pressure equals to the total weight of the overlying rock column. We have
Figure 2.5 Vertical pressure of a shallow structure
（2.17）
Where = vertical pressure
= unit weight of rock mass
= thickness of overlying rock mass
It can be observed, the vertical pressure determined by equation (2.17) refers to the critical condition. In reality, there exists friction when the sliding block downward.
Let rectangle block ABCD be considered, then the total pressure acted on the structure can be expressed as
（2.17）
Where G = total weight of ABCD
F = Friction along surface AB or CD
And
（2.18）
Hence （2.19）
Based on the soil mechanics, the horizontal pressure of AB or CD is assumed triangle and the maximum value is at point A or D., then
（2.20）
Hence, the total horizontal pressure of AB or CD is equal to
（2.21）
And the friction of AB or CD is equal to
（2.22）
Substitution of Eqs. (2.19) and (2.22) into Eq. (2.17) gives
（2.23）
Or
（2.24）
Eq. (2.24) is the expression of surrounding pressure considering the effect of friction and is variable with the depth of underground structures. To determine the critical value of for maximum , we have
(2.25)
After solving Eq. (2.25), when the relationship of is substituted into Eq. (2.23), we have
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