Study On Water Migration And Collapse Deformation In Field Immersion Test At Xianyang Loess Tableland

With the rapid development of China’s economy,many national key projects have been built in loess areas.The calculation and control of collapsible deformation of loess foundation is a key scientific and technical problem to be solved urgently in these major projects.In this paper,the self-weight collapsible loess foundation in Beidu Town,Xianyang is taken as the research object.The in-situ test,indoor test,model construction and finite element analysis are used to explore the water field migration law and self-weight collapsible deformation law during the immersion process of the test pit in the loess site.The main work and research results are as follows:（1）Through the field test pit immersion test,the water migration of the test site and the change of the water injection volume with time and the evolution of the groundwater level were obtained.The test results show that the infiltration rate of water above 1m is faster,and the lower the infiltration is,the slower the water infiltration rate is due to the weakening of suction relative to gravity.The daily water injection of the 22 m test pit was large in the first 12 days,but the overall trend was decreasing and gradually stabilized over time.The daily water injection of the 4 m test pit had no obvious regularity throughout the water injection cycle.（2）Through the field test pit immersion test,the self-weight collapsible deformation law of the test site was obtained.The test results show that the collapsible deformation of the 22 m test pit shows four stages of’stable-large-small-stable’during the soaking period and the water stop period.The settlement is inversely proportional to the distance from the center of the test pit,and the maximum settlement of the center point is 139 mm.The development of cracks shows the law of’fast-slow-tend to close’;the diameter of the test pit has a significant effect on the collapsible deformation.The test pit with a diameter of 4m has no collapsible deformation.The collapsible deformation of the test pit with a diameter of 22m mainly occurs in the Q₃ loess layer in the range of 10m,and the Q₃paleosoil layer and Q₂ loess and paleosoil interbedded collapsible deformation are very small.（3）Four in-situ double-ring tests with different inner diameters were designed.Combined with the indoor variable head test,the vertical and horizontal saturated permeability coefficients of undisturbed Q₃loess were tested.The water-holding curve of undisturbed loess was tested by direct tension meter test method,GDS unsaturated triaxial apparatus axis translation method and saturated salt solution gas phase method.The VG parameters of water holding curve fitting combined with COMSOL software were used to simulate and invert the double-ring infiltration test.The results show that the vertical saturated permeability coefficient obtained by the double-ring infiltration test decreases with the increase of the inner diameter size.The final wet-dry interface of the in-situ double-ring test can better invert the optimal in-situ vertical and horizontal saturated permeability coefficients.The permeability coefficient can reliably simulate the water infiltration process of large test pits.（4）The nuclear magnetic resonance test of soil column water infiltration process under water accumulation and rainfall was carried out,and the macropore information of soil samples with different areas was tested by image processing software.The results show that there is a preferential flow phenomenon in the undisturbed loess under the condition of water infiltration.According to the proportion of macropores obtained by image analysis,the two-domain infiltration model reflecting the mechanism can be used to reliably simulate the water infiltration in the field test pit immersion test.（5）The reliability of the standard method and the chord modulus method to calculate the self-weight collapsible deformation was explored.Based on the improved unsaturated loess collapsible deformation constitutive model,the reliability of the model was tested by laboratory tests,and the model was used to analyze the water infiltration and collapsible deformation development process of the field immersion test pit.The results show that the collapsibility obtained by the standard method is quite different from the measured value,and the correction coefficientβ₀ is 0.23.The chord modulus is not applicable to the test site,and the calculated deep soil still has large collapsible deformation.The improved constitutive model of collapsible deformation of unsaturated loess reflects the phenomenon of maximum and terminal collapsible pressure of loess.The finite element water-force coupling calculation program introduced into this model can effectively analyze the development law of water infiltration and collapsible deformation in the test pit immersion test,and to a certain extent,it reflects the size effect of test pit immersion.