Research On Swelling-shrinkage Law Of Expansive Soil And Pile-soil Interaction

Based on the research projects supported by the National Natural Science Fund (No.50378097) and Education Department of Hunan (No.07A017), the swelling and shrinkage rules of Nanning expansive soils, load transfer laws of single pile in expansive soils, settlements, stress analysis and pile-soil interaction etc are studied in this thesis by means of combining the research with experiments in-doors, model tests and theoretical analysis. The research included in this thesis has been accomplished with the following achievements:(1) A series of experiments about Nanning expansive soils contained side limit swelling test without load, swelling test with load, shrinking test and drying-wetting cycle test are carried out. Some mathematical relationships of expansive soils such as swelling strain vs. initial water content, swelling strain vs. initial density, swelling strain vs. vertical pressure are developed in this thesis. The tests show that there is a rectilinear relationship between swelling strain and initial density. Swelling strain is inverse proportion with initial water content and semi-log vertical pressure. At last, three-element regression equation about swelling strain of Nanning expansive soils is developed. The time-related characteristics of swelling-shrinkage deformation are studied. With regression analysis and curve fitting, an exponential function used to fit the relationship between swelling-shrinkage deformation and time is present.(2) The coupling relationship between vertical swelling force and initial water content, dry density and vertical pressure is studied based reformed swelling force measurement experiments. And the analytical expressions of swelling force considering initial water content or dry density are presented. Based of those and function principle, experimental investigation and theoretical derivation have been done from the view of the energy.(3) In order to study the frictional characteristics at the interface of expansive soil-structure, a lot of direct-shear friction tests on expansive soil-concrete interface has done by means of improved direct shear apparatus, in which water content and dry density of the expansive soils are various. The test results show that the vertical loads, water content of the soils and its dry density, respectively, are approximately linear with the ultimate shear stress. The ultimate shear stress would increase as the vertical load increases, water content decreases or dry density increases, respectively. As for residue shear stress, it also has the approximately linear relationship with the vertical load, water content of the soil; namely, residue shear stress increases with the increase of vertical loads or decrease of water content. But the dry density has little effect on the change of residue shear stress, which is different from its effect on ultimate shear stress.(4) A model test for a pile in expansive soil is carried out to study and analyze load transferring characteristics, including the behavior of side friction and pile end resistance together with distribution of both. The test shows a clear turning point occurs in a P-S curve; the axial force above the middle of the pile decreases dramatically and the increment of the axial force is linear with the increment of the applied load. Load transferring behavior mainly relies on the side friction resistance. Both the side friction and the pile-end resistance increase nearly linearly with the increase of the applied loads at the pile -top. As for the bearing ratio in pile side friction and pile-end resistance, an extremum value may be found.(5) The working characteristics of pile in soaking and shrinking expansive ground are studied based on groups of model tests. The ultimate displacement of pile is decided by balance equation about positive friction, negative friction and top load. Negative friction resistance and affected depth increase gradually with the process of shrink and the neutrality point also moves downwards. The neutrality point stops at a certain depth a few times later; Based on a long-term test observation to a pile embedded in expansive soil ground, a series of pile-soil interaction rules are obtained under the simulative rainfall and sunshine conditions. Tests show that, when foundation is swelling, tensile stress is distributed along the whole pile without top load. There is axial force peak at the middle-lower part of the pile, and the axial force peak increases with time. Neutrality point of the axial force shifts down in a certain degree with time. The load sharing ratio between side friction resistance and tip resistance shows a law of three stages that firstly decrease, secondly increase, and then tends to a constant gradually; when foundation is shrinking, the axial force distribution is complex and the axial force at the middle or lower section decreases with time. The development mechanism of side friction at different section is different. The load sharing ratio between side friction resistance and tip resistance decreases with time.Experimental study gives theoretical basis to pile design and engineering application.