순환골재와 쇄석을 이용한 연직배수재의 실내모형실험

In this study, a laboratory model test on utilization of recy cled aggregates and crushed stone as vertical drains to use alternative material of sand in soft ground is performed. The v ertical and horizontal coefficient of permeability of the recyc led aggregates and crushed stone showed largely 1.2～4.0 times and 3 .0～3.3 times greater than sand, respectively. Therefore, it showed enough to be an alternative material to the sand which had been being used as the vertical and horizontal drainage material before. The variations of pore water pressure with time showed constantly regardless of the load in all vertical drainage materials. When water level drops suddenly, the pore w ater pressure of the recycled aggregate and crushed aggregate is reduced to nearly zero. Therefore, it was applicab le to the field because discharge capacity was similarity to th at of sand. The settlement in crushed aggregates and recycled aggregate decreases gradually with the load increase. When water level drops suddenly, earth pressure in all drains materials was evaluated the equivalent drainage capacity similarity to sand because it show approaching the nearly zero. Key words: Recycled aggregates, crushed stone, drain material, pore water pressure, settlement, earth pressure

The variation of pore water pressure in earthen dams plays an important role in maintaining its stability. The pore water pressure within the dam are altered by the external loading conditions like rapid drawdown of reservoir water, earthquake loading and raise of water table caused by infiltration of rainfall. The seepage through an earthen dam involves saturated and unsaturated flows but to avoid complexity in solving the non-linear partial differential equations, the flow in unsaturated zone is neglected and seepage analysis is carried by constructing the flow net in which the pore water pressures beyond the free surface is taken as zero. In actual conditions negative pore water pressure develops beyond the free surface due to the capillarity which leads development to the matrix suction of the soil. In this paper a comparative study on distribution of pore pressure in a zoned earthen dam under steady state and transient conditions had been carried out considering unsaturated-saturated seepage theory. To solve the non-linear partial differential equations, finite element method has been adopted in the present study. The earthen dam has been modeled in different stages. At each stage a new parameter was added and parametric analysis was carried out. The results indicate that negative pore water pressure developed at the downstream side and the pore pressures at the mid-levels of the core are high. This specifies that, soils with low permeability have higher pore pressure. The pore pressures appeared to be higher in upstream side during rapid drawdown compared to steady state.

Experimental study of one-dimensional nonlinear consolidation of an aquitard under multistage loading

Stresses and pore water pressure induced by machine foundation on saturated sand

Rainfall-induced landslides are a significant hazard in many areas of loess-covered terrain in Northwest China. To investigate the response of a loess landslide to rainfall, a series of artificial rainfall experiments were conducted on a natural loess slope, located in the Bailong River Basin, in southern Gansu Province. The slope was instrumented to measure surface runoff, pore water pressure, soil water content, earth pressure, displacement, and rainfall. The hydrological response was also characterized by time-lapse electrical resistivity tomography. The results show that most of the rainfall infiltrated into the loess landslide, and that the pore water pressure and water content responded rapidly to simulated rainfall events. This indicates that rainfall infiltration on the loess landslide was significantly affected by preferential flow through fissures and macropores. Different patterns of pore water pressure and water content variations were determined by the antecedent soil moisture conditions, and by the balance between water recharge and drainage in the corresponding sections. We observed three stages of changing pore water pressure and displacement within the loess landslide during the artificial rainfall events: Increases in pore water pressure initiated movement on the slope, acceleration in movement resulting in a rapid decrease in pore water pressure, and attainment of a steady state. We infer that a negative pore water pressure feedback process may have occurred in response to shear-induced dilation of material as the slope movement accelerated. The process of shear dilatant strengthening may explain the phenomenon of semi-continuous movement of the loess landslide. Shear dilatant strengthening, caused by intermittent or continuous rainfall over long periods, can occur without triggering rapid slope failure.

On the soft grounds, effects of consolidation on performance of the system of piled rafts as well as superstructures cannot be overlooked due to nonlinear interaction characteristics. Conventional methods of engineering design and numerical procedures of two-dimensional interaction analysis cannot realistically simulate loading and deformation mechanism of the interaction system and cannot completely predict variations of deformations and internal forces of both structure and piled raft foundation. Therefore in order to well understand long-term behavior of the interaction system, time-dependent effect of soil deformations on performance of the interaction system of piled rafts and foundations as well as superstructure is evaluated in this paper. The nonlinear deformation and strength behavior of foundation soils are taken into account by using the elasto-plastic constitutive model based on Mohr-Coulomb’s yield criterion while the consolidation effect of subsoil under loading is incorporated by numerically solving the coupling Biot’s equations of consolidation. Numerical analyses are conducted for the structure-raft-foundation interaction system by using the finite element methods. Based on numerical analyses for a given interaction system, both the features of variation in time of internal forces, settlements and the characteristics of compatibility mechanism of interaction system are examined. It is shown that both nonlinear behavior of soft soils and consolidation characteristics of foundation remarkably affect the time-dependent performance of the interaction system. Such dependency is intimately related to variation of excess pore water pressure of subsoil.

This paper presents an elasto-plastic model for determination of the ground response curve of a circular underwater tunnel excavated in elastic-strain softening rock mass compatible with a nonlinear Hoek-Brown yield criterion. The finite difference method (FDM) was used to propose a new solution to calculate pore water pressure, stress, and strain distributions on periphery of circular tunnels in axisymmetric and plain strain conditions. In the proposed solution, a modified non-radial flow pattern, for the hydraulic analysis, is utilized. To evaluate the effect of gravitational loads and variations of pore water pressure, the equations concerning different directions around the tunnel (crown, wall, and floor) are derived. Regarding the strain-softening behavior of the rock mass, the stepwise method is executed for the plastic zone in which parameters of strength, dilatancy, stresses, strains, and deformation are different from their elasto-plastic boundary values as compared to the tunnel boundary values. Besides, the analytical equations are developed for the elastic zone. The accuracy and application of the proposed method is demonstrated by a number of examples. The results present the effects of seepage body forces, gravitational loads and dilatancy angle on ground response curve appropriately.

Analysis of the hydraulic interaction between clay buffer and host rock in a large scale test

A new approach is proposed for evaluating the fieid performance of vertical drains. This method involves comparing the minimum discharge capacity qw(min) of a vertical drain installation required for negligible well resistance with the discharge capacity mobilized in the field, qw(mob). A procedure has been developed for the estimation of qw(min), which has a typical value in the range Of 2 to 100 m3/yr, depending on the horizontal permeability of the soil and the maximum drainage length Of the vertical drain. A computer program, ILLICON, was developed for settlement analysis of embankments on soft ground and was used to back-calculate qw(min) from field observations of settlement and pore water pressure. The drain performance at four vertical drain installation schemes was evaluated using the proposed method. It was found that the magnitude of the compressive strain undergone by soil during preloading is a critical factor to the proper functioning of vertical drains.

An investigation of pore water pressure and consolidation phenomenon in the unfrozen zone during soil freezing

Isotropic consolidation test and consolidated-undrained triaxial test were first undertaken to obtain the parameters of the modified cam-clay (MCC) model and the behavior of natural clayey soil. Then, for the first time, numerical simulation of the two tests was performed by three-dimensional finite element method (FEM) using ABAQUS program. The consolidated-drained triaxial test was also simulated by FEM and compared with theoretical results of MCC model. Especially, the behaviors of MCC model during unloading and reloading were analyzed in detail by FEM. The analysis and comparison indicate that the MCC model is able to accurately describe many features of the mechanical behavior of the soil in isotropic consolidation test and consolidated-drained triaxial test. And the MCC model can well describe the variation of excess pore water pressure with the development of axial strain in consolidated-undrained triaxial test, but its ability to predict the relationship between axial strain and shear stress is relatively poor. The comparison also shows that FEM solutions of the MCC model are basically identical to the theoretical ones. In addition, Mandel-Cryer effect unable to be discovered by the conventional triaxial test in laboratories was disclosed by FEM. The analysis of unloading-reloading by FEM demonstrates that the MCC model disobeys the law of energy conservation under the cyclic loading condition if the elastic shear modulus is linearly pressure-dependent.

Excavation-induced unloading effects and dewatering-induced groundwater seepage inevitably result in basal soil reverse consolidation in deep excavation. However, this reverse consolidation process is rarely considered because most previous analytical methods were developed based on total stress analyses. This study proposes a one-dimensional reverse consolidation model for basal soil of deep excavation. Based on the consolidation theory proposed by Terzaghi, governing equations of soil reverse consolidation caused by excavation and dewatering were separately established. The continuous drainage boundary was introduced to describe the construction processes. The reverse consolidation responses of basal soil were obtained by superimposing analytical solutions of the excess pore-water pressures that resulted from excavation and dewatering. The proposed model was verified by existing solutions and a well-documented excavation case history. Moreover, the reverse consolidation characteristics of basal soil were investigated by parametric analyses. Results indicate that the excavation-induced variations in pore-water pressure decreased with increasing excavation depth. The final pore-water pressure and effective stress were predominantly affected by excavation duration rather than interval distribution. In addition, a smaller coefficient of consolidation led to lower pore-water pressure and greater effective stress at a given excavation depth.

Post-failure process and kinematic behavior of two landslides: Case study and material point analyses

Glaciers and ice streams can move by deforming underlying water‐saturated sediments, and the nonlinear mechanics of these materials are often invoked as the main reason for initiation, persistence, and shutdown of fast‐flowing ice streams. Existing models have failed to fully explain the internal mechanical processes driving transitions from stability to slip. We performed computational experiments that show how rearrangements of load‐bearing force chains within the granular sediments drive the mechanical transitions. Cyclic variations in pore water pressure give rise to rate‐dependent creeping motion at stress levels below the point of failure, while disruption of the force chain network induces fast rate‐independent flow above it. This finding contrasts previous descriptions of subglacial sediment mechanics, which either assume rate dependence regardless of mechanical state or unconditional stability before the sediment yield point. Our new micromechanical computational approach is capable of reproducing important transitions between these two end‐member models and can explain multimodal velocity patterns observed in glaciers, landslides, and slow‐moving tremor zones.

In this paper, tests on silty soil obtained from the Yellow River Delta under waves were conducted in a wave flume (size: 15(L) ×1.0(H) ×0.5(W), unit: m) to investigate characteristics of its consolidation behaviors. In addition, soil penetration strength was measured with a mini-penetrometer, and variations of pore water pressure were analyzed to study the mechanisms. Results indicated that the sliding interface between the oscillatory layer and the underlying layer would move downwards under another round of wave actions after a period of time&s consolidation under self-weight in static water, while the magnitude of the oscillation would be much smaller than that of the last time. Analytical results of pore water pressure variations clearly illustrated that the liquefaction tended to be initiated in the surface layer and spreading fast into lower layers. The penetration strength data revealed that there was a layer of extremely high strength in the sliding zone, which might be the embryo of the hard crust in the tidal flat of the Yellow River Estuary. All in all, the strength of such silt soil increased unequally along the depth continuously under wave loadings, and the consolidation speed was much higher than that in the static water.