Effects Of Dry-Wet Cycle On Silt Mechanical Properties Of Guchengzhai And Response Characteristics Of The Wall

There are many earthen archaeological sites in the central plains,which are of high historical and cultural value.Most of the earthen sites in the central plains were built near water or surrounded by farmland,which was significantly different from the soil sites in the northwest arid region.In addition to the environment,the protection of soil sites is significantly affected by the soil properties.Most of the earthen sites in the central plains are built of silt.Due to the special gradation characteristics of silt,the stability of its granular framework structure is poor,capillary water plays a significant role and the water sensitivity is strong.The repeated action of capillary water dry and wet cycles such as rainfall(or irrigation)ponding or the rise and fall of groundwater level,leads to the deterioration of the mechanical properties of the shallow soil at the root of the soil site for the direct contact between the soil site and the earth,which is easy to induce root cutting and erosion,thus leading to the change of the soil site morphology and even instability.Therefore,it is necessary to systematically study the dry-wet cycle effect and its microscopic mechanism of the mechanical properties of silt at the site,which is of great significance to enrich the theory of soil site protection,the design and construction of preventive protection and restoration engineering of earthen sites.In order to study the development mechanism of root sapping at earthen sites in central China,this paper firstly designed the test equipment and method in line with the environment of soil sites in central China,and systematically studied the influence of sample preparation conditions,cyclic drying and wetting parameters on the shear mechanical properties of silt at the sites.Secondly,the internal mechanism was explored by means of mercury injection test,scanning electron microscope test and other microscopic tests,and then the correlation between mechanical property degradation and root sapping disease evolution of soil sites was discussed.Finally,an improved Duncan-Chang model for site silt with consideration of nonlinear strength criterion and dry and wet cycle times was established,and its rationality was verified by engineering examples by using the secondary development of ABAQUS software and Fortran programming language.The mechanism of root sapping in central China was discussed.The main research contents and results are as follows:(1)Static compaction and dynamic compaction have certain effect on the mechanical properties of silt,which are closely related to the shear test method.The shear strength and cohesion c values of the static compacted samples were higher than those of the dynamic compacted samples for direct shear test,while the results of the triaxial shear test were opposite,but both showed that the strength and cohesion reached the peak when the optimal moisture content was reached.Therefore,the selection of strength parameters should be combined with the site construction method and possible failure forms when repairing silt site.(2)The dry and wet cycle test was conducted for 15 months with a self-made stratified sampling test device.The results of the triaxial drainage shear test showed that the strain softening characteristics of the soil samples with the first dry and wet cycle were significantly enhanced compared with the initial soil samples.However,as the number of dry and wet cycles continued to increase,the strain softening properties of the soil samples decreased as a whole.At the same time,cohesion c and deformation modulus E_i also show a change rule of obvious increase and gradually decrease after one cycle.Combined with the results of mercury injection test(MIP)and nuclear magnetic resonance test(NMR),it is believed that the dry-wet cycling effect of the mechanical properties for silt at the site is mainly controlled by the uniform shrinkage and local failure of the clay"three-dimensional grid"in the silt.(3)The shear strength of silt samples is greatly affected by the range of dry and wet circulation,and its variation law is closely related to the consolidation confining pressure and the stable value of moisture absorption moisture content.During the range of confining pressure 10-40kPa,compared with the initial soil sample,with the increase of the range of dry and wet circulation,the shear strength first increases slightly and then decreases significantly,while it decreases monotonously when the confining pressure is 100-400kPa.(4)The silt at the site has a certain nonlinear characteristic of strength.Therefore,the mechanical parameters under the low stress level after the stabilization of the dry and wet cycle was suggested for the relevant calculation in the shallow failure restoration project of the soil site.The generalized power function strength criterion proposed by Baker can better describe the nonlinear strength characteristics of the site soil.As the number of dry and wet cycles increases,the parameter A of the strength criterion first increases,then gradually decreases and tends to be stable,while the n value is less affected by the effect of dry and wet cycles.(5)The Duncan-Chang model based on the nonlinear strength criterion describes the stress-strain relationship of the silt at the site and obtains the model parameters of the silt under each dry and wet cycle.The correlation function between the model parameters and the number of dry and wet cycles is established.The Duncan-Chang model is used to describe the dry-wet cycle effect and nonlinear strength criterion of silt in the central plains.(6)The secondary development was carried out with ABAQUS,and the improved Duncan-Chang model was numerically transformed by Fortran programming language.The rationality of the numerical model was verified by the indoor unit test with the numerical model.Based on the numerical response of dry and wet circulation to the typical section of Guchengzhai,the mechanism of root sapping in central China was discussed.