Increase In Pore Water Pressure Research Articles

EFFECT OF LIQUEFACTION ON LOCAL SCOUR AROUND A CIRCULAR BRIDGE PIER UNDER SUDDEN WATER PRESSURE DROP

This paper presents experimental results carried out in a laboratory channel to investigate the effect of bed liquefaction on local scour development around circular bridge pier under sudden water pressure variation. Many hydraulic structures collapse due to scouring under sudden attack of flood flows or storm waves. During such a natural conditions, the water level rises abnormally above its initial level and then decreases to its original level. This phenomenon creates abrupt change of water pressure variationnear the structures and cause to failure due to scouring. Considering such a real occurrence in the hydraulic engineering field, this paper deals with the distribution of pore water pressure, effective stresses, velocity distribution and local scour around a circular bridge pier. The results show that the sediment bedis liquefied by an increase of excess pore water pressure under sudden water pressure variation and that the equilibrium local scour increases considerably than that of clear-water steady flow at the bridge pier.

Elastic viscoplastic consolidation modelling and interpretation of pore-water pressure responses in clay underneath Tarsiut Island

It has been reported that the excess pore-water pressure in clay underneath Tarsiut caisson retained island increased for many months following completion of construction (with vertical load unchanged). It was thought that this increase might be caused by the creep behaviour of the soft clay. However, the phenomenon of excess pore-water pressure increase under constant loading has not been simulated successfully at the Tarsiut Island site. In this paper, a newly developed elastic viscoplastic (EVP) model is implemented in a finite element (FE) program for consolidation analysis. This FE program is used to model the consolidation behaviour of the clay under Tarsiut Island loading. The phenomenon of pore-water pressure increase with time following completion of construction of the island has been successfully simulated using the FE program with the new EVP model. The mechanism of the pore-water pressure increase is explained. It is found that the creep compression nature of the clay is the main internal factor causing the increase. However, the volumetric strain (change) produced due to the dissipation of excess pore-water pressure is the external factor which combines with the internal factor to cause decreasing effective stress and increasing pore-water pressure.Key words: elastic, viscoplastic, finite element, pore-water pressure, consolidation, creep, clay.

Quasi-steady state: a real behaviour? Discussion

Discussions 191 In discussing the quasi-steady state behaviour of sand under undrained condition, a distinction should be made on whether the tests are conducted under load-controlled or deformation-controlled loading conditions. If it is loadcontrolled as used in the authors’ experiments, instability in the form of a runaway deformation will occur once the deviator stress exceeds its peak value as shown in Fig. 4b of the discussed paper. In Fig. 4b, the axial strain increased from 2% to about 15% in less than 2 s. This process is accompanied by a sudden increase in pore-water pressure (also shown in Fig. 4b). By any engineering standard, the soil has failed in a catastrophic way. This instability behaviour is un likely to be affected by whether the end platen is lubricated or not as the axial strain at which instability occurs is nor mally small, e.g., 2% in the test shown in Fig. 4b. After the occurrence of instability, the deviator stress tends to increase with further deformation, as shown in Figs. 4a and 4c. This behaviour is defined as quasi-steady state. The Discusser wonders whether it is still meaningful to study this quasi-steady behaviour as an element behaviour, as severe non-homogenous deformation in the forms of distortion, bulging, etc. normally will occur after the axial strain has suddenly increased to a large strain. In addition to Fig. 4b, another example is shown in Fig. 5 of the discussed paper. The axial strain has increased to 27% in less than 2 s in this test. A more visualized example was given by Peters (1991) in which the development of non-homogeneous deformation during instability is shown by a series of photographs. A similar pattern of non-homogenous deformation was observed by Castro (1969), Chu (1991), and Lee (1997) after instability had occurred. Considerable bulging was also observed by the authors for their tests on medium loose sand (Dr c = 31.4%) as shown in Fig. 11 of the discussed paper. As considerable non-homogenous deformation has occurred

Stress–strain behaviour of a loosely compacted volcanic-derived soil and its significance to rainfall-induced fill slope failures

The stress–strain behaviour of a soil is crucial to the recognition of the mechanism of slope failure. Triaxial tests, composed of isotropically consolidated drained (ICD) and undrained compression (ICU) tests and anisotropically consolidated constant deviatoric stress path (CQD) tests, were carried out with the aim of investigating the stress–strain behaviour of loosely compacted volcanic-derived soils. The fact that the critical states are the same for ICU, ICD and CQD tests may show that the critical state is independent of the above three stress paths. The critical state line, as defined in critical state soil mechanics, is obtained from the e–log p′ and q– p′ plots based on the results of the above tests. The initial state of the consolidated specimens at initiation of failure may be classified as dilative or contractive in the light of the locations of the soil state relative to the critical state line. For contractional soils, the increased pore water pressure generated by rainfall infiltration leads to a contractive failure in a drained manner, giving rise to high excess pore water pressure. The excess pore water pressure caused by contraction cannot be dissipated instantly, resulting in a decrease in the shear resistance of the soil. The failure process is rapid. The failed soil mass is prone to flow after failure under the action of gravity due to its high moisture content and inflow of surface runoff and rainwater. For dilational soils, the increased pore water pressure resulting from infiltration leads to dilation, which reduces pore water pressure and thus increases the shear resistance of the soil. However, continued rainfall infiltration may be able to equilibrate the reduction in pore water pressure caused by dilation and, therefore, the dilation or displacement can continue. In Hong Kong, volcanic-derived soil is characterized by high permeability. Both the high permeability of volcanic-derived soil and a shallow failure surface make it possible for the reduction in pore pressure to equilibrate relatively quickly. Therefore, the failure is also rapid, at least for poorly compacted fill slopes.

Time-Dependent Stress-Strain Behavior of Soft Hong Kong Marine Deposits

Abstract To investigate the time-dependent stress-strain behavior of Hong Kong marine deposits (HKMD), three types of triaxial tests have been conducted under both compression and extension states. The first is conventional undrained shear tests at different strain rates ranging from 0.0025 to 0.25%/min. The second type is stress relaxation tests. The last type is undrained creep tests. The results are reported in this paper. One order increment of logarithmic strain rate causes about 5 to 9% increase of undrained shear strength. In stress relaxation tests, the pore-water pressure increase is about 10% of the consolidation pressure under extension states. A power law equation is presented to calculate the decay of the deviator stress in relaxation. In multi-stage triaxial creep tests, the log(strain rate)-log(t) relationship of the lower stress level is linear. The slopes of these lines are dependent on current deviator stress as well as stress history.

Analysis of rainstorm-induced slide-debris flows on natural terrain of Lantau Island, Hong Kong

Lantau Island, the largest outlying island of the territory of Hong Kong, experienced a severe rainstorm on 4–5 November 1993, which induced >800 slope failures on natural terrain there. Detailed field investigations were carried out to study the failure modes, in relation with various influencing factors. It was found that the occurrence of slide-debris flows has a close relationship with bedrock geology, slope gradient, vegetation cover and micro landform. The failure modes of slide-debris flows may be classified into translational slides and rotational slides, and the former are predominant. Analysis of the hydrological response of colluvial slopes during the rainstorm indicated that the majority of the failures were caused by the development of a perched water table in the thin surface layer of colluvium of volcanic origin due to infiltration during the heavy rain. Undisturbed soil samples from south Lantau have been subjected to anisotropically consolidated undrained compression tests at comparatively low stress levels. Constant deviatoric stress path tests (CQD) simulating the stress path in the field at in situ stress levels have been performed to investigate soil behavior. The CQD test results indicate that the material of slopes at undisturbed state is brought to dilation because of the increase in pore water pressure caused by infiltration of rain water. For a translational slide, the displacement, resulting from dilation, may destroy cohesion along the failure surface and locally within the interior of the slide. The surplus water during the intense rainstorm was able to equilibrate the reduction in pore pressure caused by dilation, and the dilation and displacement may be further increased. The strain-softening after significant strains triggered debris flow mobilization. However, for a rotational slide, the increase in pore water pressure caused by surplus water infiltration during the intense rainstorm could not equilibrate the reduction in pore pressure caused by dilation, much or even all of the sliding block could not mobilize into a debris flow.

EFFECT OF PILE LENGTH ON SEISMIC RESPONSE OF CAISSON TYPE QUAY WALL RESTING ON PILE FOUNDATION

Dynamic behavior of a caisson type quay wall resting on pile foundation against the 1995 Hyogoken Nanbu Earthquake was numerically evaluated by a liquefaction analysis based on effective stress model. The effect of pile length on seismic response of caisson type quay wall against earthquake motion was investigated through some numerical examples. The displacement response of the caisson quay wall with pile foundation was smaller than that of the caisson without pile foundation. When the pile length becomes longer, the displacement response of caisson type quay wall becomes smaller. As the deformation was mainly induced by the increase of pore water pressure in the soil behind the caisson type quay wall, it is indispensable to reduce the pore water pressure in case of applying longer pile to the caisson type quay wall.

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.

Seismic Behavior of a Caisson Type Quay Wall Resting on Pile Foundation with Expanded Base

Dynamic behavior of a caisson type quay wall resting on pile foundation with exp anded base was numerically evaluated by a liquefaction analysis based on effective stress model during 1995 Hyogo-ken Nanbu Earthquake. The effective stress model used for the liquef action analysis during earthquake motion was a strain space multiple mechanism model. The deformation during the earthquake ground motion induced due to pore water press ure in the soil behind the caisson quay wall. Though the displacement response of the caisson q uay wall with pile foundation was smaller than that of the caisson without pile foundation, the deformation was mainly induced by the increase of pore water pressure.

Deformation Characteristics of a Saturated Cohesive Soil Subjected to Increase in Pore Pressure

Landslides resulting from rain and melting snow are generally known to be caused by an increase in the ground water level within the slopes. In these cases, an element on a slip surface before failure is subjected to the increase in pore water pressure, and deforms under a drained condition. It is very important, therefore, to estimate the drained deformation characteristics of clay during this increase in pore pressure. A drained test of cohesive soils, however, requires a long-term period to complete, and the drained deformation of cohesive soils during the decrease in mean effective stress has never been made clear. In this paper, deformation characteristics of a saturated cohesive soil subjected to the increase in pore water pressure were investigated and were compared with its undrained deformation characteristics. State points (p', q) at which a given value of equivalent shear modulus Ge(= Δη / Δεs, where εs is shear strain and η is stress ratio, q / p') is the same locate in the same very narrow zone for both of the two tests stated above. Any zone for an other Ge value locates in parallel with the other zones. Therefore, the Ge-value at a given stress state is independent of stress paths. Stress points showing the same εv-value forms a contour line in the pore pressure increase tests, and the line can be uniquely defined for a given εv-value. The distribution pattern of these lines is similar to the effective stress paths obtained from the consolidated undrained tests on overconsolidated specimens with different OCR-values. As a result, drained deformation characteristics in the pore pressure increase tests may be estimated with little error from the results of rapid undrained shear tests. In the final, to investigate the deformation behavior under general stress conditions, the effects of direction of principal stress and stress induced anisotropy on the above deformation characteristics were examined using the triaxial extension and torsional shear test results, and using the results on anisotropically consolidated specimens.

Porewater pressure increases in soil and rock from underground chemical and nuclear explosions

A review and analysis of chemical and nuclear explosive-induced porewater pressure increases and induced rise in groundwater table elevations (groundwater mounding) is presented. Our analysis indicates that residual pore pressure increases and groundwater mounding can be induced by underground chemical and nuclear explosions to scaled distances of 879 m/(kt) 1 3 . This relationship is linear over seven orders of magnitude of explosive energy ranging from a 0.01 kg chemical explosion to a 100 kt nuclear explosion and is valid for a wide variety of saturated geological profiles. Underground chemical explosions, and probably underground nuclear explosions have the potential to induce liquefaction of water-saturated soils to scaled distances of about 260 m/(kt) 1 3 .

Pore-water pressures in soft to firm clay during driving of piles into underlying dense sand

During driving of steel piles through soft, sensitive clay into very dense sand and gravel, pore-water pressure responses were monitored. As a result of the large length of the piles and also because of the high sensitivity of the soft clays, the piles were driven in two stages. During the initial stage of driving in the soft clay, only very small pore-water pressure increases were recorded together with very low pile driving resistances; however, during the second stage of driving, high pore-water pressure increases were observed in the clay as soon as the piles penetrated into the underlying very dense sand and gravel. It was concluded that the clay deposit was loaded from below, as the piles were driven into very dense sand. The total stress changes and the resulting pore-water pressure changes in the clay were analyzed, assuming that the pile driving load was equivalent to a flexible load acting on the surface of an elastic half-space, which represents the soft clay deposit. This interpretation of the pore-water pressure increases is important for the assessment of the bearing capacity of engineering structures affected by piles driven through soft soils into very dense deposits. The potential for high pore-water pressure increases in the clay during undrained loading as well as for volume increases in the dense sand due to pile driving can be predicted from piezocone test data. Key words: pile driving, pore-water pressure, piezocone testing, soft sensitive clays, dense sand deposits.

Performance of Caisson Type Quay Walls at Kobe Port

ABSTRACT Many caisson type quay walls were damaged in Kobe Port during the 1995 Hyogoken-Nambu earthquake. Seaward displacements of the caisson walls were about 5 m maximum and 3 m average. Although the caisson walls also tilted and settled, they did not collapse or overturn and deformed quite uniformly maintaining almost straight face lines of the walls along the port front alignment (pier head). The high seismic resistant quay walls of a caisson type, which were specifically designed for unloading emergency supplies, performed very well and resisted the strong earthquake motion. Geotechnical investigations including in-situ soil freezing sampling and shake table tests were performed in order to understand the mechanism of deformation in the caisson walls. The displacements of the caisson obtained from the shake table tests agreed very well with those measured in Kobe Port after the earthquake. The results of these investigations suggested that the excess pore water pressure increase in the foundation soil underneath the caissons and in the backfill soil significantly increased the deformation of the caisson walls.

A partial explanation of the dependency of hydraulic conductivity on positive pore water pressure in peat soils

AbstractPart of the relationship between positive pore water pressures and hydraulic conductivity in peat soils may be explained by accumulations of methane bubbles. We show how compression and expansion of gas bubbles with changes in pore water pressure could cause changes in hydraulic conductivity and thus help to explain some observations of dependency of hydraulic conductivity in peats on pore water pressure. Consideration is also given to the effect on hydraulic conductivities of methane gas going into solution with increase in pore water pressure.

Coupling GIS with Physical Models to Assess Deep-Seated Landslide Hazards

Deep-seated landslides pose a dilemma for land managers who must utilize certain natural resources while protecting others from adverse environmental impacts. The physical factors controlling deep-seated mass movement are often poorly constrained, and the consequences of a particular land use may therefore be unknown. Empirical techniques work well for mapping areas affected by deep-seated landsliding, but may be incapable of predicting the impacts of land use on specific landslides. Numerical models of the physical processes of mass movement offer an alternative method for determining landslide potential and provide additional information for evaluating hazards where empirical evidence is insufficient or ambiguous. Modeling efforts can utilize topographic and geotechnical data mapped at regional scales (typically 1:24,000) if the physical controls on landsliding are resolved. A strategy for coupling the data processing capabilities of geographic information systems (GIS) software with the predictive power of a physical model is demonstrated with a small basin in northwestern Washington state, managed for timber production, where deep-seated landsliding has degraded an important fisheries resource. Historic data were insufficient to clearly identify the primary factors controlling landslide activity. Modelling results, however, indicate that certain deep-seated landslides react primarily to increases in pore-water pressure, while others react primarily to stream-channel incision. These results guide further field efforts and provide a rational basis for land-use decisions.

Approaches to the study of hillslope development due to mass movement

Slope-angle histograms have traditionally provided a data base for the evaluation of changing angles over geological time. Ideas relating to hillslope development due to mass movement have considered a lowering in regolith shear resistance due to weathering, producing slope-angle decline. Decreasing values for angles of internal friction, along with increasing pore water pressures, have been suggested to explain slope-angle decline through time. These ideas have considered simple changes in undifferentiated regolith. This article considers the role of progressive pedogenesis in determining the changing stability of slopes. For this it is necessary to evaluate the changes which occur within individual horizons to produce an increasingly differentiated soil cover. Angles of internal friction alter at different rates and in different ways depending on whether the horizon is losing or gaining weathered material through translocation. Furthermore, the increasing internal differentiation of the soil cover has complex effects on its hydrological response. Instead of the two scenarios previously envisaged, one involving the water table below the slip surface and the other involving the water table at the ground surface, slope stability needs to be evaluated in the light of continually changing negative or positive pore water pressures. Each storm produces a different response, and this response alters with soil development, complicating the assessment of failure timing and depth. The study of evolving soil profiles is of fundamental significance to a range of geomorphological processes, requiring closer evaluation in the future.

Effective stress and water pressure in saturated clays during heating–cooling cycles

Experiments with heating and cooling cycles in undrained constant total stress conditions in triaxial apparatus are presented. Heating induces a large pore-water pressure increase, which eventually leads to a large irreversible strain and possible mechanical failure. Subsequent cooling produces a drop in water pressure. In one test the drop during cooling was more than two times higher than the previous increase during heating, reaching values of up to 2.30 MPa. An analysis of these findings in terms of a thermoplastic model is presented. The interpretation of these tests relies heavily on the kind of stress-partitioning hypothesis that is used. It was found that the described phenomena can be quantitatively dealt with using the classical effective stress principle, if the shear strength and consolidation are described in terms of temperature-dependent plastic yield limit. Key words : temperature, plastic strains, effective stress, pore pressure.

On Subglacial Stone Pavements in Till

Four pavements from central and western Canada display uneven pavement surfaces, abundant tilted and inverted stones, reversed lee ends, fine crossing striae in various directions, weak striae around curves, worn facets distributed evenly on rounded stones, and till fabric girdles; collectively inconsistent with the popular concept of stones firmly held in till being abraded by overriding ice. Based on these data, new models are offered that involve stone rotation in viscously deforming till to produce deformation pavements. In the specific model, increased pore water pressure under moving ice locally mobilizes previously formed lodgment and/or erosional pavements into deformation pavements. In the more general model stones released from the sole of a wet-based glacier rotate, interact, and settle into a viscous suspension zone in deforming till according to velocity, density, and viscosity gradients. Stone axes and striae settle into various orientations following dewatering and till stiffening. Conceptually, pavement types include lodgment, deformation, meltout, and erosional formed by end-member processes in a continuum schematically depicted in a pavement prism. Pavements of this study are hybrids, and probably so are many others. An abundance of deformation-dominated pavements in an area has important implications as a record for rapid glacier flow that influences the stability of ice sheets.

A model for dilatation, densification, and static liquefaction of loose sands

Utilizing basic balance laws of saturated granular media, a theoretical model for describing the phenomena of dilatation and densification of dry sand, as well as the increase of pore water pressure of saturated sand during shear loading, is developed. An expression for the free-energy function of loose sands is postulated, and the problem of static shearing of a sand sample is analyzed. Predictions of the model for pore water pressure are compared with those obtained experimentally for dry pluviated and moist vibrated samples of fine Ottawa sand and solid-glass beads and discussed.

Field Measurements on Seepage Water Drainage Effect of Geotextiles (Spun-Bonded Non-Woven Fablics)

Drainage effect of geotextiles (spun-bonded non-woven fabrics) was examined, based on field measurements on the embankment in which geotextiles were used as horizontal drainage layers.The observations on the pore water pressure changes inside the embankment during and after rain show that increased pore water pressures due to seepage water after rain smoothly decrease with time by the drainage effect of geotextiles. Therefore the effectiveness of geotextiles as drainage layers was confirmed from the filed measurements.