Research On The Control Measure For Differential Settlement Of Composite Foundation In Karst Region And Its Effects On Force Conditions Of Raft Foundation

The geological structure is complicated in Karst region, however, a large number of high-rise buildings spring up in such region as China’s urbanization development. In some specialcases, one half of the building might stands on the shallow bedrock while the other half issupported by composite foundations with rigid piles sticking out ten to twenty meters deep intothe soil. The conventional design of composite foundation can hardly be carried out in suchcircumstances since the uneven geological structure is likely to lead to uneven settlement whichwill result in a sharp increase in the inner force of the raft. Therefore, the research on unevensettlement control of buildings constructed in such regions is of crucial importance to structuraldesign.As observed in most building engineering projects, the settlement of a30-to-40-storey RCstructures with a foundation pressure of approximately0.4to0.6Mpa is about20mm. With theapplication of a certain kind of material whose modulus is0.5Mpa, a simple and economicalapproach is given.First, this paper summarizes the basic theories of the soil-structure interaction analysisand the large deformation finite element analysis. With respect to the soil-structure interactionanalysis, two nonlinear static analysis methods are introduced: the nonlinear static analysis forthe overall structure and the nonlinear static analysis for the staged construction. As for thelarge deformation finite element analysis, the material’s constitutive model as well as the meshgeneration method and quasi-static analysis method are described. All the theories mentionedabove provide a theoretical basis and a research tool for the research on the control measure fordifferential settlement of composite foundation in Karst region.Furthermore, short-term axial compression tests with board specimens of differentmaterials were conducted. After analyzing the stress-strain curves of the test results, long-term axial compression tests with XPS (Extruded Polystyrene) and EPS (Expanded Polystyrene)board specimens were conducted. XPS is selected for the study case according to test results ofmaterials mentioned. Then, the large deformation finite element analysis for axial compression tests of XPS boards is done by ABAQUS. And according to the Mohr-Coulomb theory inPlastic Mechanics, the theoretical short-term tri-linear constitutive model of XPS is derived.Considering the effect of the loading rate, the XPS constitutive model is modified for the creepunder the sustained load, and combining parameter fitting for the creep of the test results, asimplified long-term tri-linear constitutive model of XPS for practical use is obtained.At last, according to the long-term tri-linear constitutive model based on the actual loadingrate, staged construction analysis is realized by SAP2000to study the effects of the controlmeasure on the force condition and the deformation as well as the overall lean of the raft. Bythis way, the application effectiveness of the control measure for differential settlement areevaluated. For the proposed project, another measure currently available was put forward forthe operability of the current construction conditions.