Study On Bearing Capacity Of Sandy Ground By Centrifugal Model Test

With the development of centrifugal model tests, the centrifuge has become a valid method the deal with the geotechnical engineer problems.Though centrifugal model test, The stability, failing and deformation process of structure in geotechnical engineer, civil engineer, hydraulic projection, offshore engineer, highway bridge construction project and mining can be studied in the same stess level as the prototype. The safety and feasibility of the struture can be examined and the data can be used by other method. But there are some problems in centrifugal model tests such as the particle size effect, and these problems will influence the veracity of the test results and make it difficulte be used in practical engineer. So the rule and the extent of the influence of the problems on the test result should be researched at length, then the reasonable model can be designed and the veracity of the test results can be improved. The centrifugal model test can be better used to solve the geotechnical engineer problems.In this study, the characteristics of particle size effects in bearing capacity centrifugal model tests are studied at first, and the scale effect of footings and the influence of footing shape and depth burial on bearing capacity are also studied by centrifugal model tests under the conditions that particle size effects can be neglected.For the study on the particle size effect, according to modeling of prototype, it is deduced that as long as the footings used in centrifugal and gravitational tests are different, particle size effects always exists in which the bearing capacity obtained in centrifugal tests is always larger than that obtained in gravitational tests. And with the increasing of the ratio of footing width to mean particle diameter, the results of centrifugal tests are gradually close to the gravitational tests. When the ratio is large enough, the difference between the centrifugal and gravitational tests is so small that it could be ignored. So modeling of models can be used to study the particle size effect. But at present the rang of the ratio between footing width and mean particle diameter is little and the difference among the load-settlement curves is oberved by eyes. In the tests, the rang of the ratio is large , the maximum is 5 times of the minimum and the upper limit is 291. Morever, based on the test results, an index is introduced to quantitatively measure the influence of particle size effects on the test results. And the influence decreases with the increasing of the ratio of footing width to mean particle diameter.For the study on the scale effect of footings, A series of loading tests of circle footings on loose and dense sandy ground are conducted in the centrifuge under the condition that the particle size can be negelcted. It is shown that there is scale effect in the bearing capacity test in which the bearing capacity factor decreases with an increasing in the footing width. The extent of the scale effect gradually increases with the increase of ground density. The scale effect becomes less significant with an increase in the footing width and the scale effect is no longer exists when the footing width equal to the B0 value. The previous reaseachers only qualitatively described the phenomenon and there is no quantitatively based explanation for the variation of scale effect. In this study, the test results are compared with the results obtained from the literature and the empirical equations are proposed to present the relationship among the dependency of the bearing capacity factor on the confining stress level, the B0 value and the density of sand to examine quantitatively how the extent of scale effect continuously changes with the footing with.Through the research on the scale effect of footings, for the medium dense and loose sandy ground, the scale effects can be neglected in the range of plate size suggested by Specification for foundation design of construction ground (GB50007-2002), but for the dense sandy ground, the scale effects will still influence on the results of plate loading test in the same rang. According to the centrifugal test results, the corrected formulas of scale effect and plate size are introduced and the results of plate loading test are modified.In order to study the influence of footing shape and depth burial on bearing capacity, the centrifugal model tests with different footing shapes and depth burial are carried out under the condition that the particle size effect can be neglected. It is shown that for the same depth burial, the bearing capacity increases in the order of strip footing, circle footing, square footing and rectangle footing. For the dense sandy ground, the mode of failure gradually changes from general failure to local shear failure with the depth burial increasing and the extent of ultimate bearing capacity increasing is 16.53%~56.72%; for the loose sandy ground, the depth burial is little influence on the mode of failure, but the extent of ultimate bearing capacity increasing is greater than that of loose sandy ground, up to 66.7%~117%. And either the results from the model tests or the calculation by using three-dimensional footing bearing capacity experienced formulas is so different that the calculation of bearing capacity of the three-dimensional footing should be further studied.