Preliminary Analysis of Poisson’s Ratio of Shallow Stratum in Tianjin

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    Poisson’s ratio was characterized for shallow stratum in Tianjin from more than ten borehole logs collected during the period of the“10th five-year-plan”.Associated borehole histograms and laboratory test documents were also compiled for discussion of influencing factors such as the soil moisture,wet density,and porosity ratio. Based on the borehole logging data,we developed a scatter graph of Poisson’s ratio versus depth and fit a curve to the data to represent shear wave velocity and compression wave velocity. The graph shows that the Poisson’s ratio changes by three stages as a function of depth.In the first stage,in the range of 0 to 5 m,it increases rapidly from 0.420 near the surface to 0.495 at a depth of 5 m. Fluctuation in Poisson’s ratio may result from stiff artificial soil cover of different thicknesses that formed for thousands of years of natural processes and human activities near earth surface. In the second stage,in the range of 5 to 20 m,it tends to be stable and the value mostly remains at the level of 0.495,where the stratum mainly consists of soft,saturated,and plastic mucky stratum. In the last stage,in the range of 20 to 200 m,it slowly decreases but shows considerable discreteness and some kind of linear feature is revealed by the fitting correlation coefficient.This decrease in the last stage may be the result of changes in soil lithology,soil sedimentary environment,and soil provenance at greater depths.Many cycles of transgression and regression occurred in Tianjin during the Quaternary period,so the shallow stratum mainly consists of continental facies,marine facies,and marine continental mixed facies.These varying depositional settings caused great changes of lithology,and sedimentary environment in Tianjin. Moreover,the migrations of the Yellow River and Haihe River also influenced the sediment mineral compositions greatly:In summary,the sedimentary environment and evolution mechanism strongly affected Poisson’s ratio of the shallow stratum.Factors influencing Poisson’s ratio in the sediment were examining by selecting two typical boreholes and developing a Poisson’s ratio curve and associated histogram.Additional data including curves of soil moisture,porosity ratio,and wet density were all displayed in one figure together.It shows that the Poisson’s ratio is positively correlated with soil moisture and porosity ratio and negatively correlated with soil wet density.The form of Poisson’s curve is strikingly similar with that of soil moisture and porosity ratio curves and is opposite to that of a wet density curve.By comparing soil moisture,wet density or solid mineral components,it is easy to see that soil moisture has greater effect than wet density.In other words,soil moisture is may be the key parameter influencing the soil’s Poisson ratio.Further,comparing the curve of Poisson’s ratio and related borehole histogram, we can see that the changes of soil lithology at greater depths indicate a change of Poisson’s ratio. However,it is difficult to obtain Poisson’s ratio accurately based on soil lithology because the characteristics of stratum are very complicated and strongly regional.Because of the great number of factors contributing to Poisson’s ratio,further examination of the factors is necessary especially for application in a different geologic setting.

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  • Online: May 21,2014
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