Collapsible loess has low compressibility and high strength under natural humidity, but when it is soaked by water, the strength of the soil decreases significantly. Under the combined action of additional pressure or the additional pressure and the saturated dead weight pressure of the soil, the collapsible loess has the property of sudden subsidence. Engineering practice shows that when the foundation of industrial and civil buildings (hereinafter referred to as buildings) is not treated or not sufficiently treated, during the use of buildings, due to various reasons of water leakage or groundwater level rise often cause subsidence accidents. Therefore, in the construction of collapsible loess area, it is necessary to take treatment measures to the foundation of buildings to improve the physical and mechanical properties of soil, reduce or eliminate the collapsible deformation caused by accidental flooding of collapsible yellow soil gene, and ensure the safety and normal use of buildings
The deformation of collapsible loess foundation includes compression deformation and collapsible deformation. The compressive deformation is caused by the load of the building under the natural humidity of the foundation soil, and gradually decreases with the increase of time, and tends to be stable about one year after the completion of the building. The annual rainfall in collapsible loess area is rare (approx300mm-500mm), the evaporation is much larger than the annual rainfall, which belongs to the arid and semi-arid climate region, and the natural humidity of collapsible loess is generally in 10％～22%Within, its saturation is mostly in40％~60％Or less. When the basement pressure is not greater than the bearing capacity characteristic value of the foundation soil, the compressive deformation value is very small. It usually does not exceed the allowable deformation value of the superstructure and does not have harmful effects on the building. Therefore, from the perspective of compression deformation, except for the newly piled loess with high compressibility and low bearing capacity and the high humidity loess that need to be treated for the foundation, the loess with low compressibility and high bearing capacity can not be treated for the foundation.
Collapsible deformation is the additional deformation caused by local water immersion of the foundation when the compressive deformation of the foundation is not stable or stable, the load of the building is not changed)It often occurs suddenly and unevenly, especially in the early stage when the foundation is soaked by water. It can often occur within a day or night 15cm～25cm As a result, it is difficult for the superstructure of the building to adapt to and resist the large quantity, fast rate and uneven foundation deformation, so it is more destructive to the building. The purposes of the treatment of collapsible loess foundation are as follows: first, to eliminate the total amount of collapsible loess foundation, so that the treated foundation becomes non-collapsible loess foundation, or to use deep foundation and pile foundation to penetrate all the collapsible soil layer, so that the upper load is transferred to the non-collapsible soil (or rock) layer through the foundation or pile foundation, so as to prevent the foundation from being collapsible; The other is to eliminate the partial subsidence of the foundation, reduce the total subsidence of the treated foundation, and control the residual subsidence of the untreated lower soil layer not greater than the design provisions.
In view of the importance of class A building, the possibility of foundation wet by water and the use of the uneven settlement of strict restrictions and other buildings are different, and the number of Class A building is small, large scale of investment, high project cost, once the problem, in politics or economy will cause serious impact and loss. Therefore, Class A buildings are not allowed to have any destructive deformation, nor is it allowed to affect the use because of deformation, so the foundation treatment is strict, requiring the elimination of all the foundation subsidence. Class B and C buildings cover a wide range of areas. Foundation treatment too strict, construction investment increased significantly, does not conform to our country existing technical and economic level, so only require to eliminate the collapsibility foundation part of the quantity, and then according to the degree of foundation treatment or residual wet fall quantity, the size of the corresponding waterproof measures and structural measures, in order to make up for the inadequacy of foundation treatment, prevent building harmful deformation.