The soil pressure on underground structures
Prior to production and to the construction of underground structures soil in an unlimited array is in a state of natural or geostatic equilibrium under the action of its own weight. In this case, the depth Z in the soil (Fig. 1), the following compressive stresses (pressure):
where is the average bulk weight of overlying soils; – the coefficient of lateral earth pressure under conditions of natural occurrence.
The open or underground excavation changes the equilibrium conditions of the soil and leads to the emergence of deformation and redistribution of stresses.
Earth pressure on underground structures is different than the pressure in the intact rock mass at the same depth, as:
first, the structure has a different stiffness than the soil,
secondly, the ground movement has time to happen in the time period between excavation and erection of structures, and finally,
thirdly, between structure and arrays are gaps that allow some movement of soil.
The pressure exerted by soil on a structure depends on the depth and hardness; moisture and degree of compaction on the construction and especially close to him and from the method of construction of buildings it is necessary to distinguish three osnovnega the erection of structures;
– the construction of the mound (Fig. 2, a), constructed or laid directly on the earth’s surface, or very small compared with the width of the excavation deepening with subsequent backfilling; so are normally constructed culverts under the road embankment;
– structure in the recess or the trench (Fig 2 b), when it is erected or laid in open development, with small compared to the width and depth is limited more or less solid walls; the space next to the building and above it is filled with soil; usually laid water supply pipelines, sewers, drains, etc ;
– the building, erected by the closed method (Fig 2), in which the soil is not disturbed from the surface, this method is used for the construction of tunnels and trenchless laying of pipelines.
Earth pressure on an underground structure is not constant but varies due to changes in temperature and humidity conditions and creep of the soil, In most cases, the pressure on the building increases gradually over time, reaching the highest values after a certain period of time, sometimes followed by a decrease.
To determine the pressure on underground structures from the ground last consider either as elastic or as a granular medium.
The first calculation model is applicable in cases where the stress in the soil after the production is much less destructive. If in the most stressed areas of the soil occurs damaging, the approximate calculation model of granular medium in a state of limit equilibrium.
The pressure on an underground structure located below the free groundwater, defined as the joint pressure of the soil in suspension and water. For the calculation of the optimal level of the underground water.
The pressure on an underground structure of unstable saturated soils (sand) was adopted by the hydrostatic law.
In the calculation of underground construction on the first limit condition of the load multiplied by the load factors of n taken from table:
The values of load factors, smaller units are cases when this load decreases the calculated total impact.
In the calculation of underground structures with consideration of additional or special combination of values of the calculated live loads are taken with coefficients of 0.9 and 0.8, respectively.