Charles Wang Wai Ng

A Numerical Investigation of the Stability of Unsaturated Soil Slopes Subjected to Transient Seepage

Abstract Slope instability in unsaturated residual soils and loose fills has attracted increasing attention in recent years around the world such as Brazil, South Africa, Japan and in the Far East. Rain-induced failures are the most common ones. Rainfall leads to the development of perched water table, rising the main groundwater level and soil erosion (due to concentrated water flow), resulting in an increase in pore water pressure or a reduction in soil matric suction. This, in turn, results in a decrease in shear strength on the potential failure surface to a point where equilibrium can no longer be sustained in the slope and then failures occur. However, the present understanding of the influence of transient seepage in unsaturated soils, due to water infiltration under various boundary and ground conditions, and hydrogeological regimes on slope stability is still relatively poor compared with other elements of geomechanics. To investigate the influence of various rainfall events and initial ground conditions on transient seepage and hence slope stability, a parametric study has been conducted using the finite element method. A typical steep unsaturated cut slope in Hong Kong has been adopted for the parametric study. Variables considered in the parametric study include isotropic and anisotropic soil permeability, initial water table at upslope boundary, rainfall intensity and duration. Pore water pressures or suctions predicted during the transient seepage analyses are then used as input ground water conditions for subsequent limit equilibrium analyses of the stability of the slope. Factor of safety is calculated using Bishop’s simplified method, with modified Mohr–Coulomb failure criterion to allow for shear strength variation due to the presence of matric suction. Infiltration due to rain water causes a reduction of matrix suction, but an increase in moisture content and soil permeability in unsaturated soils. Perched water table is developed above the main water table. The factor of safety is not only governed by the intensity of rainfall, initial ground water table and the anisotropic permeability ratio, but it also depends on antecedent rainfall duration. A critical rainfall duration can be identified, at which the factor of safety is the lowest.

Advanced unsaturated soil mechanics and engineering

Part 1: Physical and flow charcteristics of unsaturated soil 1 Basic physics, phases and stress state variables 2 Measurement and control of suction: methods and applications 3 Flow laws, seepage and state-dependent soil-water characteristics Part 2: Collapse, swelling, strength and stiffness of unsaturated soils 4 Collapse and swelling caused by wetting 5 Measurement of shear strength 6 Measurement of soil stiffness Part 3: State-dependent elasto-plastic modelling of unsaturated soils 7 A state-dependent elasto-plastic critical state-based constitutive model Part 4: Field trials and numerical studies in slope engineering of unsaturated soil 8 Instrumentation and performance: A case study in slope engineering 9 Engineering applications for slope stability Appendix A: Definitions Appendix B: Notations

Influence of rainfall intensity and duration on slope stability in unsaturated soils

Abstract Rain-induced landslides are major geotechnical hazards. The influence of infiltration under various rainfall, ground conditions on slope stability is still poorly understood. In this paper, a finite element parametric study was carried out to investigate the influence of different rainfall events and ground conditions on transient pore water distributions in unsaturated soils. A steep, unsaturated colluvium hillside in Hong Kong was chosen: the initial water table, rainfall intensity and rainfall duration were variables. Pore water pressures predicted during the transient seepage analyses were used as input groundwater conditions for subsequent limit equilibrium analyses of the stability of the slope. Shear strength variation due to the presence of matrix suction was also taken into account. It was found that the factor of safety not only depended on the intensity of rainfall and the initial groundwater table, but also on rainfall duration. A critical rainfall duration was identified, when the factor of safety was the lowest.

Effects of wetting–drying and stress ratio on anisotropic stiffness of an unsaturated soil at very small strains

The very small strain shear modulus of soil, G0, is affected by many factors including soil properties, current stress state, stress history, and matric suction. Very little research has been conducted on anisotropic shear moduli of unsaturated soils. In this study, the effects of wetting–drying and stress ratio on anisotropic shear stiffness of an unsaturated completely decomposed tuff (CDT) at very small strains have been investigated using a modified triaxial testing system equipped with three pairs of bender elements. During drying and wetting tests, the measured very small strain shear moduli increased in a nonlinear fashion, but at a reduced rate as the matric suction increased. Similar to the stress-dependent soil-water characteristic curves (SDSWCCs), there was hysteresis between the drying and wetting curves showing the variations in shear moduli with matric suction. Variation in suction on the specimens under isotropic conditions produced changes in stiffness anisotropy (expressed as G0(hh)/G0(h...

Weathering indices for rhyolitic tuff and granite in Hong Kong

A series of chemical analyses and dry density tests of rhyolitic tuff and granitic samples have been carried out. The test results have been interpreted using a newly developed theoretical weathering model for calculating a mobility index of major elements in rock and soil. It is found that this index is useful for describing changes of rock and soil during weathering and classifying the degrees of weathering. Based on the theoretical model, a new parameter called the volume index, which combines chemical and physical data, is suggested. This index appears to be a good indicator for classifying the degree of weathering in Hong Kong. A quantitative classification scheme is suggested.

Centrifuge and Numerical Modeling of Axial Load Effects on Piles in Consolidating Ground

This paper reports the results of four centrifuge model tests that were undertaken to investigate behavior of floating piles subjected to negative skin friction (NSF) and to study effects of axial load on the load-transfer mechanism along single floating piles and shielded center piles inside a group of sacrificing piles. In addition, three-dimensional numerical analyses of the centrifuge model tests were carried out with elasto-plastic slip considered at the pile–soil interface. Prior to applying load, the measured neutral plane position of the single floating piles was located at approximately the three-quarter depth level of the embedded pile length. The neutral plane elevation shifts lower down the pile shaft as the distance of pile tip above the bearing stratum decreases. Under the application of axial load, the dragload generated by excessive soil settlement decreases and is eventually eliminated. The amount of axial load for complete NSF elimination does not seem to be significantly affected by the...

Weathering mechanisms and indices of the igneous rocks of Hong Kong

Weathering mechanisms of volcanic and granitic rocks in Hong Kong have been studied by chemical analysis, optical microscopy on thin sections and magnetic susceptibility measurements. The mobility of elements of calcium, sodium, iron, potassium, magnesium, silicon, aluminium and constitutional water is investigated in detail using the Mobility Index. Chemical weathering processes in Hong Kong are dominated by decomposition of feldspars and biotite, leaching of alkali and alkaline earth metals, and enrichment of ferric oxide under prolonged subtropical climate conditions. Data for some granitic residual soils from five neighbouring regions in Southern China are also gathered and examined. It is found that the residual soils in Hong Kong are abnormal and immature with respect to its latitude. By examining various quantitative chemical indices and comparing them with the current six-grade material classification scheme, WPI, LOI and Imob are found to be good indicators of the degree of weathering for both volcanic and granitic rocks in Hong Kong. Regarding the optical microscopy, the micropetrographic index (Ip) is only suitable for granitic rock but not for volcanic rock due to the presence of fine-grained minerals and small voids. For the magnetic susceptibility measurements, consistent results with weathering grades for volcanic rocks are obtained by applying a moving average technique. However, due to the low content of Al2O3 and Fe2O3 in granitic rock, SOC and the magnetic susceptibility index, which are appropriate for quantifying the degree of weathering of volcanic rock, are not quite suitable for the granitic rock.

A three-dimensional parametric study of the use of soil nails for stabilising tunnel faces

Abstract Applying an effective nailing system at a tunnel heading, not only improves the stability of the tunnel heading and limits deformation at the tunnel face, but it also reduces volume loss during excavation and hence reduces ground surface settlement. The effectiveness of a soil nail system is affected by many factors such as the diameter and stiffness of the nails. In this paper, a systematic parametric study was conducted to study the axial rigidity of a nail, E n A n , for improving the stability of tunnel headings and reducing ground movements in stiff clay. The parametric study involved a series of three-dimensional elasto-plastic coupled-consolidation finite element analyses. The stability of the tunnel face is improved with increasing E n A n . For a given nail density applied at the tunnel face, an optimum axial rigidity of the nail ( E n A n ) opt can be identified. The efficiency of the nailing system diminishes when ( E n A n ) opt is reached. The use of a soil nailing system reduces the magnitude of stress relief at the tunnel heading during excavation. Thus, this reduction of stress relief minimises the amount of soil yielding and excess pore water pressure generated in the soil around the tunnel heading.

Dimensional analysis of natural debris flows

Debris flow is generally composed of a wide range of solid particles and viscous pore fluid . I t flows at a high traveling velocity down a slope channel. Interactions between solid and fluid phases affected by multiple parameters govern the rheological properties of debris flows. A dimensional analysis for a systematic study of the governing parameters is presented. Multiple dimensionless numbers with clear physical meanings are critically reviewed. The applications of field data for studying natural debris flows are demonstrated. Specific values of dimensionless numbers for classifying flowing regimes of natural debris flows on the large scales are obtained. Compared to previous physical model tests on small scales, this study shows that the contact friction between particles dominates in natural debris flows. In addition, the solid inertial stress due to particle collisions and the pore fluid viscous shear stress play key roles in governing the dynamic properties of debris flows. The channel width as a confinement to the flows can affect the solids discharge per unit width significantly. A dimensionless number related to pore fluid pressure dissipation is also found for distinguishing surge flows and continuous flows in field satisfactorily.

Physical and numerical modeling of an inclined three-layer (silt/gravelly sand/clay) capillary barrier cover system under extreme rainfall

As an extension of the two-layer capillary barrier, a three-layer capillary barrier landfill cover system is proposed for minimizing rainfall infiltration in humid climates. This system consists of a compacted clay layer lying beneath a conventional cover with capillary barrier effects (CCBE), which is in turn composed of a silt layer sitting on top of a gravelly sand layer. To explore the effectiveness of the new system in minimizing rainfall infiltration, a flume model (3.0 m × 1.0 m × 1.1 m) was designed and set up in this study. This physical model was heavily instrumented to monitor pore water pressure, volumetric water content, surface runoff, infiltration and lateral drainage of each layer, and percolation of the cover system. The cover system was subjected to extreme rainfall followed by evaporation. The experiment was also back-analyzed using a piece of finite element software called CODE_BRIGHT to simulate transient water flows in the test. Based on the results obtained from various instruments, it was found that breakthrough of the two upper layers occurred for a 4-h rainfall event having a 100-year return period. Due to the presence of the newly introduced clay layer, the percolation of the three-layer capillary barrier cover system was insignificant because the clay layer enabled lateral diversion in the gravelly sand layer above. In other words, the gravelly sand layer changed from being a capillary barrier in a convention CCBE cover to being a lateral diversion passage after the breakthrough of the two upper layers. Experimental and back-analysis results confirm that no infiltrated water seeped through the proposed three-layer barrier system. The proposed system thus represents a promising alternative landfill cover system for use in humid climates.