Fig.2. Broken piles
However, in the capsized buildings and broken pile see in Fig 2, soil around the foundation that exposure to rollover can be judged visually more loosely, indicating higher porosity of the soil, the soil was soft , that the construction process probably not on the basis of effective backfill compaction process, soil soaked in the rain from the plastic state to plastic flow state, thus reducing the lateral restraint of upper pile and foundation, excavation unreasonably on the south of the building and high accumulation of soil cause high pressure on the north and lead to the upper part of the pile with the lateral displacement of soil deformation or differential foundation settlement. When the initial formation appeared, the whole eccentric gravity load of the upper structure will further exacerbate this eccentric process, which is called second-order effect of gravity, when such lateral overturning moment more than the rest of the pile, then the bending load capacity will cause pile instability, leading to overturning the whole building . That is why the collapse will happen in the main project basically completed (maximum vertical load at this time) raining ( at this time the soil into plastic flow state , losing the support of the upper part of the pile ) side foundation pit ( this an important reason for the plastic flow state soil will blasting out of foundation when the soil foundation pit will produce an initial lateral direction )
2.3The south side of the basement excavation and lethal effects of precipitation on Building 7
After the completion of 12 high-rise residential building in the Lotus riverside, construction side had just started construction of Building 7 on the south side of the outdoor basement, basement which has no horizontal support with excavation depth 4.6m excavation sidewalls take shotcrete slope, the north side away from the pit of Building 7 facades is about 7 ~ 8 m. Construction side began earth excavation foundation and basement floor cushion sidewall slope construction, in order to ensure the normal construction, it is necessary to take measures to reduce the ground water level, so that the soil remains dry when basement excavation and concrete construction. Since the soil is sandy soil , precipitation well point should be set within the surrounding foundation pit, and the basement excavation plane has larger size can be supplemented sump pit in the middle of precipitation in the region to ensure that the depth of the middle area of precipitation.
Since the origin design has no basement, foundation depth of building7 is about 2 m, and precipitation of south basement excavation depth should be 5 m or more , taking into account outdoor underground water table is generally about -1.0 m, then the 7th groundwater level difference between the lower part of the downstairs portion and the foundation pit is about 4 m .When excavation began pumping the well point , the water level of foundation pit decreased, the 7th downstairs and around foundation will have different pressure resulting groundwater level . In the water pressure difference will flow into the foundation pit under the hydrodynamic pressure
