Advanced Materials Research Vols. 261-263

Abstract: This paper introduces the hazards of urban land subsidence which caused by excessive extraction of groundwater, the construction of high-rise buildings and excavation, then analyses their mechanisms. It is considered that the essence of land subsidence is due to the soil compaction caused by effective stress augment and the soil loss caused by excavation. Through further analysis of current works, we suggest that regional differences in geological conditions, building loads, repeated action of groundwater withdrawal and recharge, and meso-mechanism of soil particles should be considered when establishing land subsidence model.

Abstract: The consolidation coefficient is the important parameters for soft soil foundation to predict its consolidation settlement. At present, one-way compression test is mainly used to study consolidation process and consolidation coefficient of soft soil. For saturated soft soil in marsh, through triaxial shear test, the dissipation of pore water pressure and consolidation process can be observed during the course of soil sample drainage. This study got test result bellow through above triaxial test. Firstly, pore water pressure has hysteresis effect during the consolidation process, and the reason for this is analyzed. Secondly, consolidation degree in different consolidation time can be got according to the change of discharged water volume. Thirdly, according to Terzaghi consolidation theory, consolidation coefficients can be got under different consolidation pressures. Consolidation coefficient first increases with the increasing of consolidation degree, when consolidation degree is between 40% and 80%, saturated soft soil has larger consolidation coefficient, consolidation coefficient of saturated soft soil in marsh is between 2Ч10^-4 cm² / s and 10Ч10^-4cm² / s.

Abstract: The method of evaluating the effect of geogrid reinforcement mainly is the finite element method at present. The interface between geogrid and soil is set to contacting element normally. The tangential stiffness expression has five parameters totally. The interaction characteristics of geosynthetics and filled soil are related to the soil type and water content. We designed the experimental research program, using direct shear friction test, study the five parameters of the interface between geogrid and soil varied with water content and soil type. The experimental results show that: Whether weathered sands or silty clay, the value of the parameters (k₁,n,c,φ) increase with the increase of water content, when reaching the peak point, then the value of the parameters (k₁,n,c,φ) will decreases with the increase of water content. The water content corresponding to the peak is near the optimum water content. The value of the parameters (R_f) decreases with the increase of water content, when reaching a minimum, the value of the parameters (R_f) will increase with the increase of water content. The water content corresponding to the minimum also is near the optimum water content. The research results can provide reliable basis for the selection of evaluation parameters.

Abstract: Through the undisturbed samples and reconstituted samples for compression test, analyzed and got the characteristics of compression deformation and the influence law of structure properties on soft clay. The s-t curves deformation of reconstituted soil in small loads is the largest, as the load increases, tends gradually to horizontal line, but undisturbed soil deformation is the largest under the condition of a bigger load. Undisturbed soil due to structural exists, its s-lgt curves are divided into three categories according to load level. The e-lgp curve of soft clay is bounded by structure yield stress very clearly divided into two sections, but e-lgp curve of reconstituted soil is a straight line. Because structure of the reconstituted soil has been destroyed, its law of compression characteristics is comparative markedly. But due to the existence of undisturbed soil structural properties, near the structural yield stress, the compression parameters appear mutations. Research results show that influence of soft clay structural properties on its compression characteristics is not allowed to ignore.

Abstract: The coefficient of permeability of unsaturated soil is an important parameter which is used to analyze moisture migration. For complexity and diversity of unsaturated soil behaviors, it is difficult to perform test of unsaturated soil because of many factors. With GDS system, the permeability characteristics of unsaturated soil are tested and studied in this paper which makes it possible to directly measure the coefficient of permeability at various combinations of net normal stresses and matric suction values. The research shows the coefficient of permeability of unsaturated soil is the function of suction and cell pressure.

Abstract: Xi’an ground fracture, caused by the extraction of groundwater and the movement of fault under soil strata, is a geo-hazard. The movement of ground fracture originates the uneven settlement of upward block and downward block. In Xi’an ground fracture region, the segmented lining structure was adopted in subway tunnel to pass through the ground fracture, so as to adapt for the uneven settlement. Three-dimensional elastic-plastic finite difference method was applied to simulate the initial lining structure, second segmented lining structure, surrounding soils and ground fracture. The horizontal and vertical displacement of segmented lining structure, surrounding soils pressure and internal force of segmented lining structure in subway tunnel were analyzed by the calculation results. The knowledge on mechanical behavior of segmented lining structure passing through an active ground fracture and surrounding soils was shown as following. The relative vertical displacement between segmented lining structure sects beside the ground fracture increases remarkably with the movement of ground fracture, and the segmented lining structure located in upward displaceent block near ground fracture originates notable rotary. Tension or compression deformation occured in the deformation joint between adjacent segmented lining structures near the ground fracture.There was a significant change in the contact pressure of the first sect of lining structure in the upward displace block. Under the same uniform settlement at the bottom of upward diaplacement block, the relativly vertical displacemtn on the surfaceof ground fracture strata without tunnel equals 50cm, but the relativly vertical displacement between adjacent segmented lining structure at ground fracture is 18.2cm on the design level of arch top of lining strcutre. the maximum tensile stress of segmented lining structure is 2.02MPa, the maximum compressive stress of segmented lining is 3.49MPa. In conclusion, segmented lining structure can adapts to the uneven settlement caused by the movement of ground fracture. Though maximum tensile and compressive stress of sengmented lining structure passing through the active ground fracture is bigger than the general lining structure located in soils strata without the ground fracture, the segmented lining structure constructed by the steel fibre concrete can bear with the maximum tensile stress.

Abstract: In this paper, the construction technique of a new soil anchor-the cement-soil mixing anchor, which is applicable in the area of soft clay, is introduced. During the construction of the anchor, the drill pipe and blade is served as a reinforced body and formed an anchorage body with the cement-soil body. The material and making, technics parameters, mechanical equipment and construction requirement of the cement-soil mixing anchor are presented firstly. Then, the uplift tests of the cement-soil mixing anchor are carried out, and the ultimate pullout force which is 1.6 times greater than that of the soil nailing in the same condition is obtained. At last, an application of the cement-soil mixing anchor in a foundation pit is given. The test results shown that the application of the cement-soil mixing anchor can reach the expected reinforcement effect. Therefore, this framework can then be used by design engineers to more confidently estimate the pullout capacity of cement-soil mixing anchors under tension loading.

Abstract: In the process of tunnel construction, because the rock stress redistribute, the vault and the two groups will generate displacement constantly. This paper adopts the genetic algorithm to optimize the weight and threshold of BP neural network, taking the tunnel depth, rock types and part measured values of displacement as input parameters to construct a neural network time series prediction model of tunnel surrounding rock displacement. The method proposed in the paper has been applied in the Ma Tou Tang tunnel construction successfully, and the results show that the model can predict the displacement of the surrounding rock quickly and accurately.

Abstract: Rockburst happened during underground excavation caused by stress re-distribution of surrounding rocks. Stress difference was critical factor to determine whether rockburst happened or not. The stress difference-strength ratio method of rockburst classification was put forward. Rockburst intensities of the same size and different cave shapes were calculated without considering the influence of excavation unloading and considering the influence of excavation unloading by finite element method. Results showed that the grade of rockburst predicted by the stress difference-strength ratio method was lower than by stress-strength ratio method. Rockburst grade was lower one grade for not considering the excavation unloading.

Abstract: In order to analyze the effect of subsoil liquefaction on the seismic response of pile foundation, typical computation model in liquable ground was set up. Based on different constitutive models of the foundation soil before and after liquefaction and considering the nonlinear relation of pile-soil interaction, the dynamic finite difference analysis of group piles was carried out by using FLAC3D. The dynamic response characteristics of pile foundation considering liquefaction during earthquake are studied. Results show that the peak value of pore pressure ratio is different in different depth of the soil around pile, and the arriving time of the peak value of pore pressure ratio is later than that of the peak value of earthquake acceleration. At any time during the earthquake whether before or after the liquefaction the top moment of pile are larger than that of the pile shaft when the top pile is solidly embedded.