Study On Improvement Mechanism And Computation Theory Of Vacuum Combined Surcharge Preloading

Vacuum preloading and Vacuum combined Surcharge preloading are effective methods to improve soft foundation. Along with our country's construction upsurge, these two methods were used more widely. There are some developments of improvement principle, construction techniques and design theory, but there are more questions to solve such as the depth of improvement, the change of underground water, difference between vacuum preloading and surcharge preloading, effect of vacuum preloading to the environment, computation method, and so on. This status limited the more application and development of these two methods. More and more researchers' attention in the world focuses on these questions.Based on the former researchers' work, this thesis researched the questions above via field experiment, laboratory test and theoretical analysis. The main work of this thesis are:(l)Studied the change of underground water and depth of improvement under the vacuum combined surcharge preloading through field experiment.(2)Improved the laboratory test apparatus, and made four kinds of laboratory experiment, those are vacuum preloading, surcharge preloading, vacuum combined surcharge preloading and each direction equal loading. The result shows that the deformation and improvement of soil of vacuum preloading is more closely to each direction equal loading, and the improvement degree of soil is better than surcharge.(3)According to the principle of equality of consolidation degree, an equivalent method of converting vertical drain to layered soil was proposed, and we can use plane strain FEM to analyze ground with vertical drain via translating it to layered soil foundation.(4)Wrote the 3-D Biot consolidation program, improved the Borja's creep model and integrated in the program.(5) Considering the character of PVD in the foundation improved by vacuum combined surcharge preloading, this thesis proposed a new 3-D drain element to replace the element in general FEM grid, we can reduce the calculating work and satisfied the precision of of 3-D FEM analysis the same time. The 3-D drain element was compared with analytical method and general FEM, and validated.(6) Based on the work above and deformation character of highway's foundation improved by vacuum combined surcharge preloading, a practical engineering was computed via 3-D Biot consolidation FEM and compared with plane strain FEM, it showed that the result of 3-D Biot consolidation FEM analysis is more precise and worth researching more detail and extending.(7) Axisymetrical analytical solutions of vacuum preloading was deduced to compute pore water pressure and degree of consolidation, the effect of vacuum loading speed, well resistance and smear were considered.(8) For the sake of overcoming the shortage of the former work, axisymetrical analytical solutions of vacuum combined surcharge preloading were deduced, the effect of surcharge loading speed was considered and vacuum was regard as instantaneous load, so we can take account of effect of surcharge loading time, well resistance and smear, it is more closed to practical engineering.(9)Simplified calculate method of settlement under negative pressure was researched. Integrated with axisymetrical analytical solution, we can design the foundation improved by vacuum preloading and vacuum combined surcharge preloading.