Study On Creep Characteristics Of Dianchi Peaty Soil And Its Application In Long-term Settlement Of Tunnel

Dianchi peaty soil,as a special soft soil,has the properties of high organic matter content,high sensitivity,and significant creep characteristics.T he soil layer often appears the problems of low bearing capacity and large settlement,which has brought serious constraints to the urban construction of Kunming.With the wave of Kunming rail transit construction,this special soil has brought great difficulties to the construction and operation of shield tunnel.At present,the creep mechanism of dianchi peaty soil and its influence on the long-term settlement of tunnel still lack the systematic research.Therefore,it is of great theoretical and practical significance to study the long-term settlement of shield tunnel considering the creep characteristics of peat y soil.In this paper,the creep characteristics of dianchi peaty soil are studied based on the laboratory test.The long-term settlement caused by the disturbance of shield construction in peaty soil is calculated combining with theoretical analysis.The research contents are as follows:1.The basic physical properties of peatyy soil have been determined,and a large number of relevant experimental results have been collected for statistical analysis.The physical properties have been investigated deeply considering the soil depth and sampling sites.The statistical results show that for the peatyy soil at the same site and different depth,although the basic physical properties improve with the increase of soil depth,the peaty soil still belongs to the high compressibility soil in general.For these at the different site and same depth,there is a great discreteness for the physical properties.Therefore,the physical properties of dianchi peaty soil must be determined through tests.2.The consolidation creep test has been carried out to study the secondary consolidation characteristics of dianchi peaty soil from the four aspects of soil depth,disturbing degree,consolidation pressure,and loading modes.The results show:?1?The consolidation yield pressures?^'are similar with different depths due to the structure failure of the disturbed soil.And they are less than the pre-consolidation pressure P_c of the undisturbed soil.The loading mode has little effect on the value of P_c and?^'.?2?With the increase of the consolidation pressure P,the consolidation coefficients C_v show a decreasing trend for the undisturbed soil and disturbed one.And the changes of C_v is especially significant in the process of P from 100 k Pa to 200 k Pa.?3?The time t₉₀ when the consolidation degree reaches 90%and the primary consolidation time t_c have been obtained for the Dianchi peaty soil.The results show:the larger the value of t₉₀ t_c,the more obvious the inverse"S"shape of the e-lgt curve,and the higher the proportion of the secondary consolidation deformation accounts the total deformation.The t₉₀ and t_c obtained by the separate loading method are significantly smaller than that got by the graded loading method,and the soil compression rate is faster.?4?The second consolidation coefficient C_?and the compression index C_c have been compared,which shows that the undisturbed soil with a depth of 34³4.5 m exhibits lower secondary consolidation characteristics.For the undisturbed soil and disturbed soil with the depths of 17¹7.5m and 27²7.5m,and the disturbed soil with 34³4.5m deep,the value of C_?/C_c are basically consistent with the existing results,and ranges from 0.05 to 0.07.3.The direct shear creep tests has been carried out,and the creep characteristics of dianchi peatysoil have been discussed under the action of different vertical stress and shear stress.The results show:?1?the vertical stress and shear stress have significant impact on the creep characteristics of peaty soil.When the vertical stress is fixed,the larger the shear stress,the greater the total creep deformation.The shear stress has an obvious promoting effect on the nonlinear rheological characteristics of peaty soil.?2?Three kinds of direct shear creep empirical models for power function,logarithm function and exponential decay function have been established.And based on the exponential decay model,the generalized Kelvin model with five components has been proposed for describing the rheological properties of peaty soil.4.Based on the existing results,the rheological properties of peaty soil are described by five-component model.Coupling Terzaghi-Rendulic theory of two-dimensional consolidation,the governing equations for dissipation of excess pore water pressure has been established when the lining is impermeable.The vertical strain of soil has been integrated to get the calculation formula for long-term settlement of tunnel.The variation law of excess pore water pressure dissipating and the long-term settlement of tunnel in peaty soil has been analyzed combining with an example.The results show that in the initial stage the dissipating speed of excess pore water pressure is faster than that in soft clay in Shanghai,and then it becomes slow and steady.The lasting time of long-term settlement is longer and the settlement amount is larger in the peaty soil.The tunnel settlement tends to be stable in the 900 days,and the cumulative settlement is almost up to 150mm.In addition,the rheological properties of peaty soil in Kunming is significant,if the excess pore water pressure is dissipated by 90%as completion time of primary consolidation settlement,the secondary consolidation settlement accounts for about 36%of the total settlement of tunnel,and it is an important part of the long-term settlement of tunnel.