Pore Water Pressure Research Papers (original) (raw)

2002, Earth Surface Processes and Landforms

Riparian vegetation strips are widely used by river managers to increase streambank stability, among other purposes. However, though the effects of vegetation on bank stability are widely discussed they are rarely quantified, and generally underemphasize the importance of hydrologic processes, some of which may be detrimental. This paper presents results from an experiment in which the hydrologic and mechanical effects of four riparian tree species and two erosion-control grasses were quantified in relation to bank stability. Geotechnical and pore-water pressure data from streambank plots under three riparian covers (mature trees, clump grasses and bare/cropped turf grass) were used to drive the ARS bank stability model, and the resulting factor of safety (F s ) was broken down into its constituent parts to assess the contribution (beneficial or detrimental) of individual hydrologic and mechanical effects (soil moisture modification, root reinforcement and surcharge). Tree roots were found to increase soil strength by 2-8 kPa depending on species, while grass roots contributed 6-18 kPa. Slope stability analysis based on data collected during bank failures in spring 2000 (following a very dry antecedent period) shows that the mechanical effects of the tree cover increased F s by 32 per cent, while the hydrologic effects increased F s by 71 per cent. For grasses the figures were 70 per cent for mechanical effects and a reduction of F s by 10 per cent for the hydrologic effects. However, analysis based on bank failures in spring 2001 (following a wetter than average antecedent period) showed the mechanical effects of the tree cover to increase F s by 46 per cent, while hydrologic effects added 29 per cent. For grasses the figures were 49 per cent and 15 per cent respectively. During several periods in spring 2001 the hydrologic effects of the tree cover reduced bank stability, though this was always offset by the stabilizing mechanical effects. The results demonstrate the importance of hydrologic processes in controlling streambank stability, and highlight the need to select riparian vegetation based on hydrologic as well as mechanical and ecological criteria. Published in

2000, Geomorphology

Gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion. The interaction of these forces control streambank mechanics. Hydraulic forces exerted by flowing water on in situ bank-toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower-bank profiles. Seepage forces exerted on in situ bank material by groundwater, downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength. Data from a study site on Goodwin Creek, MS, USA clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore-water pressures. Negative pore-water pressures (matric suction) have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear. A stable bank can be transformed into an unstable bank during periods of prolonged rainfall through:increase in soil bulk unit (specific) weight,decrease or complete loss of matric suction, and, therefore, apparent cohesion,generation of positive pore-water pressures, and, therefore, reduction or loss of frictional strength,entrainment of in situ and failed material at the bank toe, andloss of confining pressure during recession of stormflow hydrographs.

2005

The intensity of the Asian summer-monsoon circulation varies over decadal to millennial time scales and is reflected in changes in surface processes, terrestrial environments, and marine sediment records. However, the mechanisms of long-lived (2-5 k.y.) intensified monsoon phases, the related changes in precipitation distribution, and their effect on landscape evolution and sedimentation rates are not yet well understood. The arid high-elevation sectors of the orogen correspond to a climatically sensitive zone that currently receives rain only during abnormal (i.e., strengthened) monsoon seasons. Analogous to present-day rainfall anomalies, enhanced precipitation during an intensified monsoon phase is expected to have penetrated far into these geomorphic threshold regions where hillslopes are close to the angle of failure. We associate landslide triggering during intensified monsoon phases with enhanced precipitation, discharge, and sediment flux leading to an increase in pore-water pressure, lateral scouring of rivers, and oversteepening of hillslopes, eventually resulting in failure of slopes and exceptionally large mass movements. Here we use lacustrine deposits related to spatially and temporally clustered large landslides (Ͼ0.5 km 3) in the Sutlej Valley region of the northwest Himalaya to calculate sedimentation rates and to infer rainfall patterns during late Pleistocene (29-24 ka) and Holocene (10-4 ka) intensified monsoon phases. Compared to present-day sediment-flux measurements, a fivefold increase in sediment-transport rates recorded by sediments in landslide-dammed lakes characterized these episodes of high climatic variability. These changes thus emphasize the pronounced imprint of millennial-scale climate change on surface processes and landscape evolution.

2001, Global and Planetary Change

The assertion that pure conductive heat transfer always dominates in cold climates is at odds with decades of research in soil physics which clearly demonstrate that non-conductive heat transfer by water and water vapor are significant, and frequently are for specific periods the dominant modes of heat transfer near the ground surface. The thermal regime at the surface represents the effective boundary condition for deeper thermal regimes. Also, surface soils are going to respond more quickly to any climatic fluctuations; this is important to us because most facets of our lives are tied to earth's surface. To Ž . accurately determine the surface thermal regime for example, the detection of climate change , it is important to consider all potential forms of heat transfer. Gradients that have the potential to alter the thermal regime besides temperature include pore water pressure, gravitational, density, vapor pressure and chemical. The importance of several non-conductive heat transport mechanisms near the ground surface is examined.

2011, Acta Mechanica Sinica

This paper describes the results of an experimental study on the undrained shear behaviour of loose sand collected from the location close to the epicenter of the recent Chlef (Algeria) Earthquake (October 10, 1980). The study focuses on the effects of the mode of the soil deposition on the liquefaction resistance of the Chlef sand. For this purpose, the results of undrained monotonic triaxial compression tests performed on samples with initial density of 0.29 under initial confining pressures ranged from 50 kPa to 200 kPa are presented. The specimens were prepared by two depositional methods namely dry funnel pluviation and wet deposition. It was found that there was a marked difference in the undrained behaviour of sand in terms of maximal deviatoric stress, peak strength, residual strength and excess pore water pressure, even though the density and stress conditions were identical. The conclusion was that the soil fabric was responsible for this result. The results indicated also that at low confining pressures, the specimens reconstituted by the wet deposition method exhibited complete static liquefaction (zero effective confining pressure and zero stress difference).

2001, Geotechnical and Geological Engineering

A case study is presented in order to identify the effect of antecedent rainfall on slope stability for Singapore. A storm in February 1995 (during which 95 mm of rain fell in 2 1 2 h) caused more than twenty shallow landslides on the Nanyang Technological University Campus. Details of the location, size and morphology of the landslides are presented. The antecedent rainfall during the ¢ve days preceding the event was signi¢cant in causing these landslides since other rainfall events of similar magnitude (but with less antecedent rainfall) did not cause landslides. To further understand the effect of antecedent rainfall, numerical modelling of one of the slope failures is presented. The changes in pore-water pressure due to different rainfall patterns were simulated and these were used to calculate the changes in factor of safety of the slope. The results demonstrate that antecedent rainfall does play an important role in slope stability.

2008, Earth-Science Reviews

Calciclastic submarine fans are rare in the stratigraphic record and no bona fide present-day analogue has been described to date. Possibly because of that, and although calciclastic submarine fans have long intrigued deep-water carbonate sedimentologists, they have largely been overlooked by the academic and industrial communities. To fill this gap we have compiled and critically reviewed the existing sedimentological literature on calciclastic submarine fans, thus offering an updated view of this type of carbonate slope sedimentary system.

2007, Earth Surface Processes and Landforms

Limited information exists on one of the mechanisms governing sediment input to streams: streambank erosion by ground water seepage. The objective of this research was to demonstrate the importance of streambank composition and stratigraphy in controlling seepage flow and to quantify correlation of seepage flow/erosion with precipitation, stream stage and soil pore water pressure. The streambank site was located in Northern Mississippi in the Goodwin Creek watershed. Soil samples from layers on the streambank face suggested less than an order of magnitude difference in vertical hydraulic conductivity (K s ) with depth, but differences between lateral K s of a concretion layer and the vertical K s of the underlying layers contributed to the propensity for lateral flow. Goodwin Creek seeps were not similar to other seeps reported in the literature, in that eroded sediment originated from layers underneath the primary seepage layer. Subsurface flow and sediment load, quantified using 50 cm wide collection pans, were dependent on the type of seep: intermittent low-flow (LF) seeps (flow rates typically less than 0·05 L min − − − − −1 ), persistent high-flow (HF) seeps (average flow rate of 0·39 L min − − − − −1 ) and buried seeps, which eroded unconsolidated bank material from previous bank failures. The timing of LF seeps correlated to river stage and precipitation. The HF seeps at Goodwin Creek began after rainfall events resulted in the adjacent streambank reaching near saturation (i.e. soil pore water pressures greater than − − − − −5 kPa). Seep discharge from HF seeps reached a maximum of 1·0 L min − − − − −1 and sediment concentrations commonly approached 100 g L − − − − −1 . Buried seeps were intermittent but exhibited the most significant erosion rates (738 g min − − − − −1 ) and sediment concentrations (989 g L − − − − −1

2009, Quaternary Science …

2003, International Journal for Numerical and Analytical Methods in Geomechanics

This paper presents a complete finite-element treatment for unsaturated soil problems. A new formulation of general constitutive equations for unsaturated soils is first presented. In the incremental stress-strain equations, the suction or the pore water pressure is treated as a strain variable instead of a stress variable. The global governing equations are derived in terms of displacement and pore water pressure. The discretized governing equations are then solved using an adaptive time-stepping scheme which automatically adjusts the time-step size so that the integration error in the displacements and pore pressures lies close to a specified tolerance. The non-linearity caused by suction-dependent plastic yielding, suction-dependent degree of saturation , and saturation-dependent permeability is treated in a similar way to the elastoplasticity. An explicit stress integration scheme is used to solve the constitutive stress-strain equations at the Gauss point level. The elastoplastic stiffness matrix in the Euler solution is evaluated using the suction as well as the stresses and hardening parameters at the start of the subincrement, while the elastoplastic matrix in the modified Euler solution is evaluated using the suction at the end of the subincrement. In addition, when applying subincrementation, the same rate is applied to all strain components including the suction.

2004, Earth Surface Processes and Landforms

Pore water pressures (positive and negative) were monitored for four years (1996–1999) using a series of tensiometer-piezometers at increasing depths in a riverbank of the Sieve River, Tuscany (central Italy), with the overall objective of investigating pore pressure changes in response to ﬂow events and their effects on bank stability.The saturated/unsaturated ﬂow was modelled using a ﬁnite element seepage analysis, for the main ﬂow events occurring during the four-year monitoring period. Modelling results were validated by comparing measured with computed pore water pressure values for a series of representative events. Riverbank stability analysis was conducted by applying the limit equilibrium method (Morgenstern-Price), using pore water pressure distributions obtained by the seepage analysis.The simulation of the 14 December 1996 event, during which a bank failure occurred, is reported in detail to illustrate the relations between the water table and river stage during the various phases of the hydrograph and their effects on bank stability. The simulation, according to monitored data, shows that the failure occurred three hours after the peak stage, during the inversion of ﬂow (from the bank towards the river). A relatively limited development of positive pore pressures, reducing the effective stress and annulling the shear strength term due to the matric suction, and the sudden loss of the conﬁning pressure of the river during the initial drawdown were responsible for triggering the mass failure.Results deriving from the seepage and stability analysis of nine selected ﬂow events were then used to investigate the role of the ﬂow event characteristics (in terms of peak stages and hydrograph characteristics) and of changes in bank geometry. Besides the peak river stage, which mainly controls the occurrence of conditions of instability, an important role is played by the hydrograph characteristics, in particular by the presence of one or more minor peaks in the river stage preceding the main one. Copyright © 2004 John Wiley & Sons, Ltd.

2007, Journal of Terramechanics

An application of smoothed particle hydrodynamics (SPH) to simulation of soil-water interaction is presented. In this calculation, water is modeled as a viscous fluid with week compressibility and soil is modeled as an elastic-perfectly plastic material. The Mohr-Coulomb failure criterion is applied to describe the stress states of soil in the plastic flow regime. Dry soil is modeled by one-phase flow while saturated soil is modeled by separate water and soil phases. Interaction between soil and water is taken into account by means of pore water pressure and seepage force. Simulation tests of soil excavation by a water jet are calculated as a challenging example to verify the broad applicability of the SPH method. The excavations are carried out in two different soil models, one is dry soil and the other is fully saturated soil. Numerical results obtained in this paper have shown that the gross discontinuities of soil failure can be simulated without any difficulties. This supports the feasibility and attractiveness of this a new approach in geomechanics applications. Advantages of the method are robustness, conceptual simplicity and relative ease of incorporating new physics.

2004, Journal of Geophysical Research

2010, Landslides

A gigantic rapid landslide claiming over 1,000 fatalities was triggered by rainfalls and a small nearby earthquake in the Leyte Island, Philippines in 2006. The disaster presented the necessity of a new modeling technology for disaster risk preparedness which simulates initiation and motion. This paper presents a new computer simulation integrating the initiation process triggered by rainfalls and/or earthquakes and the development process to a rapid motion due to strength reduction and the entrainment of deposits in the runout path. This simulation model LS-RAPID was developed from the geotechnical model for the motion of landslides (Sassa 1988) and its improved simulation model ) and new knowledge obtained from a new dynamic loading ring shear apparatus ). The examination of performance of each process in a simple imaginary slope addressed that the simulation model well simulated the process of progressive failure, and development to a rapid landslide. The initiation process was compared to conventional limit equilibrium stability analyses by changing pore pressure ratio. The simulation model started to move in a smaller pore pressure ratio than the limit equilibrium stability analyses because of progressive failure. However, when a larger shear deformation is set as the threshold for the start of strength reduction, the onset of landslide motion by the simulation agrees with the cases where the factor of safety estimated by the limit equilibrium stability analyses equals to a unity. The field investigation and the undrained dynamic loading ring shear tests on the 2006 Leyte landslide suggested that this landslide was triggered by the combined effect of pore water pressure due to rains and a very small earthquake. The application of this simulation model could well reproduce the initiation and the rapid long runout motion of the Leyte landslide.

2007, Journal of Geophysical Research

1] The erosion of sediment from riverbanks affects a range of physical and ecological issues. Bank retreat often involves combinations of fluvial erosion and mass wasting, and in recent years, bank retreat models have been developed that combine hydraulic erosion and limit equilibrium stability models. In related work, finite element seepage analyses have also been used to account for the influence of pore water pressure in controlling the onset of mass wasting. This paper builds on these previous studies by developing a simulation modeling approach in which the hydraulic erosion, finite element seepage, and limit equilibrium stability models are, for the first time, fully coupled. Application of the model is demonstrated by undertaking simulations of a single flow event at a single study site for scenarios where (1) there is no fluvial erosion and the bank geometry profile remains constant throughout, (2) there is no fluvial erosion but the bank profile is deformed by simulated mass wasting, and (3) the bank profile is allowed to freely deform in response to both simulated fluvial erosion and mass wasting. The results are limited in scope to the specific conditions encountered at the study site, but they nevertheless demonstrate the significant role that fluvial erosion plays in steepening the bank profile or creating overhangs, thereby triggering mass wasting. However, feedbacks between the various processes also lead to unexpected outcomes. Specifically, fluvial erosion also affects bank stability indirectly, as deformation of the bank profile alters the hydraulic gradients driving infiltration into the bank, thereby modulating the evolution of the pore water pressure field. Consequently, the frequency, magnitude, and mode of bank erosion events in the fully coupled scenario differ from the two scenarios in which not all the relevant bank process interactions are included.

1999, Journal of Geotechnical and Geoenvironmental Engineering

Low plasticity silts and silty clays occur extensively in the Central United States, India and China. For evaluating their liquefaction potential during an earthquake, no accepted guidelines are available based on their density, void ratio, plasticity index, standard penetration values, or any other simple soil property. Their liquefaction behavior is not properly understood at present and is often confused with that of sand-silt mixtures.

2004, Soil and Tillage Research

''Close-to-nature forest stands'' are one central key in the project ''Future oriented Forest Management'' financially supported by the German Ministry for Science and Research (BMBF). The determination of ecological as well as economical consequences of mechanized harvesting procedures during the transformation from pure spruce stands to close-to-nature mixed forest stands is one part of the ''Southern Black Forest research cooperation''. Mechanical operations of several typical forest harvesting vehicles were analysed to examine the actual soil stresses and displacements in soil profiles and to reveal the changes in soil physical properties of the forest soils. Soil compaction stresses were determined by Stress State Transducer (SST) and displacement transducer system (DTS) at two depths: 20 and 40 cm. Complete harvesting and trunk logging processes accomplished during brief 9-min operations were observed at time resolutions of 20 readings per second. Maximum vertical stresses for all experiments always exceeded 200 kPa and at soil depths of 20 cm for some vehicles and sequences of harvesting operations approached !500 kPa. To evaluate the impacts of soil stresses on soil structure, internal soil strengths were determined by measuring precompression stresses. Precompression stress values of forest soils at the field sites ranged from 20 to 50 kPa at soil depths of 20 cm depth and from 25 to 60 kPa at soil depths of 40 cm, at a pore water pressure of À60 hPa. Data obtained for these measured soil stresses and their natural bearing capacities proved that sustainable wheeling is impossible, irrespective of the vehicle type and the working process. Re-occurring top and subsoil compaction, increases in precompression stress values in the various soil horizons, deep rut depths, vertical and horizontal soil displacements associated with shearing stresses, all affected the mechanical strengths of forest soils. In order to sustain naturally ''unwheeled'' soil areas with minimal compaction, it is recommended that smaller machines, having less mass, be used to complete forest harvesting in order to prevent or at least to maintain currently minimal-compacted forest soils. Additionally, if larger machines are required, permanent wheel and skid tracks must be established with the goal of their maximum usefulness for future forest operations. A first step towards accomplishing these permanent pathways requires comprehensive planning with the Federal State Baden-Württemberg. The new guideline for final opening with skid tracks (Landesforstverwaltung Baden-Württemberg, 2003) proposes a permanent skid track system with a width of 20-40 m.

2010, Geomorphology

In the Argentera massif (French Southern Alps), large active landslides develop along strike of an active corridor of dextral strike-slip faults revealed by shallow ongoing seismicity. Glacially polished bedrock outcrops are offset by right-lateral strike-slip faults. Gravitational structures appear to be spatially connected to these active faults. Dating using the in situ-produced 10Be cosmogenic nuclide performed on glacial, tectonic and gravity surfaces. The late glacial–interglacial Holocene transition is constrained by 10Be ages between 12 and 15 ka obtained on glacially polished surfaces. The main tectonic activity closely post-dates the main deglaciation event and is constrained by 10Be ages of 11 and 7–8 ka obtained on fault scarps. Three successive periods of landsliding are recognized, at 11–12, 7–9 and 2.5–5.5 ka. These Holocene ages were obtained on right-lateral strike-slip fault scarps indicating that recent Alpine tectonics are expressed by transcurrent movements. The discussed close age relationship between deglaciation and a tectonic pulse may suggest that post-glacial rebound and enhanced pore water pressure do influence seismogenic tectonic activity. Gravitational destabilizations at 11–12 and 7–9 ka are coincidental with the main tectonic activity, and suggest tectonic shaking as a landslide trigger. The third gravitational destabilization at 2.5–5.5 ka could be attributed either to slope weakness resulting from multiple low-magnitude earthquake events, as currently revealed by the seismic activity or to climatic causes during the wetter optimum climatic period. These early and middle Holocene ages coincide with a phase of large landslide throughout the Alps scale which suggests that these large gravitational mass movements could be related to combined effects of intense tectonic activity and transitions form cold and dry period to warm and wetter phase.

2004, Applied clay science

The fracture network in the excavation-disturbed zone (EDZ) of an argillaceous formation, the Opalinus Clay in the Mont Terri Rock Laboratory in Switzerland, is characterised by applying structural and hydrogeological techniques. Unloading fractures are studied by mapping side walls of newly excavated niches and by analysing resin-injected overcores taken from the EDZ. The result is a structural data set of fracture orientations, frequencies, and extent of the EDZ. Pneumatic and hydrogeological tests are carried out in short boreholes crossing the EDZ to derive hydraulic parameters such as permeability and transmissivity distributions of the fracture network. Hydraulic transmissivities are orders of magnitude higher when compared to those of undisturbed rock, and are in the order between 1E À 8 and 1E À 7 m 2 /s. Regular repetition of these hydrotests resulted in decreasing transmissivities in the range of two orders of magnitude over a time period of about 2 years. These observations indicate a hydraulic self-sealing of the initially highly transmissive fracture network with ongoing saturation of the EDZ. The dynamic evolution of EDZ fractures can be derived by interpreting pore water pressure measurements during gallery excavation. Finally, all structural and hydrogeological information is synthesised in a conceptual model of the EDZ in the Mont Terri Rock Laboratory.

2006, Journal of Geotechnical and Geoenvironmental Engineering

A vertical drain radial consolidation equation based on a parabolic reduction in permeability toward the drain is presented. The proposed equation, based on Hansbo's equal strain theory, is compared with settlement data from a laboratory... more

2003, Soil and Tillage Research

The discussion about the effect of repeated short time wheeling on long-term changes in soil structure and pore functioning reveals a great uncertainty. On the one hand it is told that soil structure elements are rigid and do not undergo intense changes in pore functions as a consequence of the short loading interval during each single wheeling. On the other hand, the complete deterioration of the structure elements and pore functions is assumed to occur, which also results in changes of the shrinkage pattern, soil strength including even strength regain. Consequently, the effect of wheeling on soil deformation and stress/strain distribution was investigated in a soil bin which contained Hiwassee clay at the NSDL, Auburn. If the soil is very strong due to aggregation, plow pan formation or dryness, soil stress applied by repeated wheeling results in an increased primarily vertical soil particle displacement in the Hiwassee clay soil while during repeated wheeling (up to 10×) a more pronounced displacement linked with a more intense movement of particles can be proofed. With increasing number of wheeling events, new platy or again coherent structure elements are formed, which create a very different pore system. The more intense is soil wheeling, the smaller is the saturated hydraulic conductivity and the higher is the unsaturated one at a given pore water pressure value. Such changes are the more pronounced the more completed is the rearrangement of the still existing aggregates into new units like plates. Due to shear because of the three-dimensional soil displacement even under dry conditions such aggregates can be redisturbed and a coherent but very compacted soil horizon can be formed. Under those conditions the values of bulk density are even higher than the Proctor density.

2007, Canadian Geotechnical Journal

A full-scale field study was conducted to investigate the effects of rainfall infiltration on a natural grassed expansive soil slope in China. A 16 m wide × 28 m long area was selected for instrumentation. The instrumentation included jet-filled tensiometers, moisture probes, a tipping bucket rain gauge, and a vee-notch flow meter. One artificial rainfall event amounting to about 370

2009, International Journal of Earth Sciences

Subglacial deformation is crucial to reconstructing glacier dynamics. Sediments associated with the Late Ordovician ice sheet in the Djado Basin, Niger, exhibit detailed structures of the subglacial shear zone. Three main types of subglacial shear zones (SSZ) are discriminated. The lowermost SSZ, developed on sandstones, displays Riedel macrostructures and cataclastic microstructures. These resulted from brittle deformation associated with strong glacier/bed coupling and low porewater pressure. Where they developed on a clay-rich bed, the overlying SSZ display S-C to S-C 0 fabrics, sheath folds, and dewatering structures. These features indicate high ductile shear strain and water overpressure. On finegrained sand beds, the SSZ exhibit homogenized sand units with sand stringers, interpreted as fluidized sliding beds. The succession of subglacial deformation processes depends on fluid-pressure behavior in relation to subglacial sediment permeability. Fluid overpressure allows subglacial sediment shear strength and ice/bed coupling to be lowered, leading to ice streaming.

1997, Canadian Geotechnical Journal

A suction probe was developed to directly measure matric suction greater than 100 kPa. The suction probe contains a small volume of water, a nonthreaded, high-range pressure transducer, and a 15 bar (1 bar = 100 kPa) ceramic disk. Cyclic prepressurization up to 12 000 kPa was used to dissolve potential cavitation nuclei in the water in the suction probe to increase the sustainable tension of the water. Using a pressure-plate cell, the suction probe has been verified to be accurate and has a rapid response for pore-water pressures as low as -500 kPa. Measurements performed on various types of soils indicate that the suction probe is able to measure matric suction up to 1250 kPa with satisfactory accuracy. The filter-paper method and the thermal conductivity sensor generally gave lower values of matric suction than the suction probe, whereas the null-pressure plate gave higher values of matric suction than the suction probe. The suction probe appeared to be best suited for measuring suction in wet clayey-type soils. Some difficulties were encountered in measuring suction in soils with a low degree of saturation.

1999, Journal of Geotechnical and Geoenvironmental Engineering

The limit-equilibrium method is commonly used for slope stability analysis. However, it is well known that the solution obtained from the limit-equilibrium method is not rigorous, because neither static nor kinematic admissibility conditions are satisfied. Limit analysis takes advantage of the lower-and upper-bound theorems of plasticity to provide relatively simple but rigorous bounds on the true solution. In this paper, threenoded linear triangular finite elements are used to construct both statically admissible stress fields for lowerbound analysis and kinematically admissible velocity fields for upper-bound analysis. By assuming linear variation of nodal and elemental variables, the determination of the best lower-and upper-bound solution may be set up as a linear programming problem with constraints based on the satisfaction of static and kinematic admissibility. The effects of pore-water pressure are considered and incorporated into the finite-element formulations so that effective stress analysis of saturated slopes may be done. Results obtained from limit analysis of simple slopes with different ground-water patterns are compared with those obtained from the limit-equilibrium method.

2011, Geomorphology

Landslides are a serious threat to life and property throughout the world. The causes of landslides are various since multiple dynamic processes are involved in driving slope failures. One of these causes is prolonged rainfall, which affects slope stability in different ways. Water infiltrating in a hillslope may cause a rise of the piezometric surface, which, in turn, involves an increase of the pore water pressure and a decrease of the soil shear resistance. For this reason, knowledge of spatio-temporal dynamics of soil water content, infiltration processes and groundwater dynamics, is of considerable importance in the understanding and prediction of landslides dynamics. In this paper a spatially distributed and physically based approach is presented, which embeds a slope failure method in a hydrological model. The hydrological model here used is the tRIBS model (Triangulated Irregular Network Real-Time Integrated Basin Simulator) that allows simulation of most of spatial-temporal hydrologic processes (infiltration, evapotranspiration, groundwater dynamics and soil moisture conditions) that can influence landsliding. Slope stability is assessed by implementing the infinite slope model in tRIBS. The model, based on geotechnical and geomorphological characteristics, classifies each computational cell as unconditionally stable or conditionally stable. Soil moisture conditions resulting from precipitation can trigger landslides at conditionally stable locations. When a landslide occurs, the model also computes the amount of detached soil and its possible path downslope. Model performance has been initially tested on a small catchment with very steep slopes, located in the northern part of Sicily (Italy), after a sensitivity analysis concerning some model parameters.

2005, Geomorphology

Landslides on black marl slopes of the French Alps are, in most cases, complex catastrophic failures in which the initial structural slides transform into slow-moving earthflows. Under specific hydrological conditions, these earthflows can transform into debris flows. Due to their sediment volume and their high mobility, debris flow induced by landslides are far much dangerous than these resulting from continuous erosive processes. A fundamental point to correctly delineate the area exposed to debris flows on the alluvial fans is therefore to understand why and how some earthflows transform into debris flow while most of them stabilize.

2007, Natural Hazards

Landslides are triggered by earthquakes, volcanoes, floods, and heavy continuous rainfall. For most types of slope failure, soil moisture plays a critical role because increased pore water pressure reduces the soil strength and increases stress. However, in-situ soil moisture profiles are rarely measured. To establish the soil moisture and landslide relationship, a qualitative comparison among soil moisture derived from AMSR-E, precipitation from TRMM and major landslide events was conducted. This study shows that it is possible to estimate antecedent soil moisture conditions using AMSR-E and TRMM satellite data in landslide prone areas. AMSR-E data show distinct annual patterns of soil moisture that reflect observed rainfall patterns from TRMM. Results also show enhanced AMSR-E soil moisture and TRMM rainfall prior to major landslide events in landslide prone regions of California, U.S.; Leyte, Philippines; and Dhading, Nepal.

1999, Earth Surface Processes and Landforms

To investigate the role of pore water pressures in the stability of a streambank, a series of tensiometers and piezometers was installed in a bank of the Sieve River, Tuscany, Italy. Fluvial entrainment at the bank toe was monitored by repeated crossprofiling, erosion pins and marked pebbles. Fluctuations in matric suction measured at the tensiometers reflected the overall response of pore water pressures to rainfall, evapotranspiration, rising and drawdown of the river stage, and variations in water table. An expression was derived for the safety factor with respect to mass movement of the upper bank, incorporating the failure criterion for unsaturated soils and the normal Mohr±Coulomb criterion for saturated conditions. Variations in matric suction have important effects on the stability of the streambank. During low-flow periods, the shear strength term due to the matric suction allows the bank to remain stable at a steep angle. However, during rainfall and flow events, reduction in matric suction and increase in unit weight of the material from vertical and lateral infiltration may be sufficient to trigger a mass failure, without development of significant positive pore water pressures. During the rising limb of highflow events, the factor of safety increases as a consequence of the stabilizing confining pressure of the water in the river, despite a reduction in matric suction. During drawdown in the river, when the suction values are still low and the confining pressure in the river decreases to zero, the factor of safety falls to lower values than those experienced prior to the runoff event. Measurements of fluvial entrainment reveal that, although the processes, mechanisms and the frequency of retreat of basal and upper bank zones differ significantly, the amount of retreat at the bank toe due to fluvial erosion is comparable to that of the upper portion of the bank due to mass failure.

1999, Sedimentary Geology

The South Pyrenean Foreland Basin contains numerous units of Eocene carbonate megabreccias intercalated with siliciclastic turbidites and derived by resedimentation of shallow-marine carbonate platforms. Previous studies were limited mainly to the foreland eastern part, known as the Jaca Basin. The present study from the Pamplona Basin, a western part of the foreland trough, sheds new light on the origin and regional significance of these South Pyrenean Eocene carbonate megabreccias (SPECMs). The number of the SPECM units in the foreland basin is higher than previously recognized and their age is somewhat older than originally assumed. The SPECM units appear to occur as time-stratigraphic clusters, which can be correlated with the relative sea-level lowstands and linked with phases of tectonic activity. The megabreccias were derived from a carbonate-platform system hosted by the foreland basin's southern (passive) margin. The episodic instability and mass wasting were triggered by phases of structural steepening (forebulge uplift) accompanied by high-magnitude earthquakes, with the former causing platform emergence, increased load stresses and excess pore-water pressure in the carbonate ramp. The SPECM deposits were emplaced by cohesive debris flows evolving into high-density turbidite currents. An ideal SPECM unit consists of (1) an immature, homogeneous debrite in the proximal part; (2) a differentiated, bipartite debrite and turbidite in the medial part; and (3) an incomplete, base-missing debrite overlain by turbidite, or a turbidite alone, in the distal part. The debrite component volumetrically predominates in the SPECM units, and the original terms 'megaturbidite' and 'seismoturbidite' thus seem to be inappropriate for these deposits.

2001, Hydrological Sciences Journal

A one-dimensional numerical model for dam failure due to flow overtopping is developed. The MacCormack explicit finite difference scheme is used to solve the one-dimensional equations of continuity and momentum for unsteady varied flow over steep bed slopes. In the computation of erosion process, sediment transport equations are considered and the modified Smart formula developed for steep bed slope is selected. The sliding stability of the overtopped dam is checked by modified ordinary method of slices. The model has been successfully calibrated and verified using laboratory experimental data. By comparing with the experimental results, it was found that the model accuracy depends largely on the sediment transport formula and pore water pressure coefficient. The model was found to predict actual breach outflow of the Buffalo Creek Dam reasonably well and closer than other existing numerical models.

1996, Coastal Engineering

2011, Cold Regions Science and Technology

Rapid mass movements involving large proportions of ice and snow can travel significantly further downslope than pure rock avalanches and may transform into debris-flows as the ice melts and as water from the stream network or water-saturated debris is incorporated. Currently, ice is thought to have three distinctive effects: 1) reduction of the friction within the moving mass itself, 2) increase of pore pressure as the ice melts and consequent reduction of the shear resistance of the flowing material, and 3) reduction of boundary friction where the failing mass travels on a glacier. However, measurement-based evidence to support these hypotheses is largely missing. In this study, laboratory experiments on the first two mechanisms were carried out in two partially-filled large rotating drums, one in Vienna (Austria) and a second in Berkeley (USA). Varying proportions of cold gravel and gravel-sized ice were mixed and added to the rotating drum running at constant rotational velocity until all ice had melted. Flow behavior was recorded with flow depth, normal force, shear force, pore-water pressure, and temperature sensors. The bulk friction coefficient was found to decrease linearly with increasing ice content by ~20% in the early phase of the experiments, before significant portions of the ice transformed into water. For ice contents larger than 40% by volume, the transformation from a dry granular flow to debris-flow-like movement or hyperconcentrated flow was observed when pore-water pressures rose and approached the normal forces along the flow profile. Pore-water pressure from melting ice developed within several minutes after the start of the experiments and, as it increased, progressively reduced the friction coefficient. The results emphasize that the presence of ice in granular moving material can significantly reduce the friction coefficient of both dry and partially-saturated debris. Due to size effects and the absence of other factors reducing friction (e.g. surfaces with low friction and rock comminution), the absolute measured friction coefficients from the laboratory experiments were larger than those found from natural events. However, the relative changes in friction coefficients depending on the ice and water content may also be considered in realscale hazard assessments of rapid mass movements in high mountain environments.

2007, Canadian Geotechnical Journal

During the placement of fine-grained cemented mine backfill, the high placement rates and low permeability often result in undrained self-weight loading conditions, when assessed in the conventional manner. However, hydration of the cement in the backfill results in a net volume reduction-the volume of the hydrated cement is less than the combined volume of the cement and water prior to hydration. Though the volume change is small, it occurs in conjunction with the increasing stiffness of the cementing soil matrix, and the result in certain circumstances can be a significant reduction in pore-water pressure as hydration proceeds. In this paper, the implications of this phenomenon in the area of cemented mine backfill are explored. An analytical model is developed to quantify this behaviour under undrained boundary conditions. This model illustrates that the pore-water pressure change is dependent on the amount of volume change associated with the cement hydration, the incremental stiffness change of the soil, and the porosity of the material. Experimental techniques for estimating key characteristics associated with this mechanism are presented. Testing undertaken on two different cement-minefill combinations indicated that the rate of hydration and volumes of water consumed during hydration were unique for each cement-tailings combination, regardless of mix proportions.

1997, Engineering Geology

Tests on specimens of reconstituted illitic clay have examined the influence of temperature on the mechanical behaviour of clay soils. The program involved consolidation to effective confining pressures up to 1.5 MPa, heating to lOO"C, and tests on normally consolidated and overconsolidated specimens with OCR=2. The tests included isotropic consolidation, undrained triaxial compression with pore water pressure measurement, drained tests along controlled stress paths to investigate yielding behaviour, and undrained tests which involved heating and measurement of the resulting induced pore water pressures. The large strain strength envelope is independent of temperature. However, peak undrained strengths increase with temperature because smaller pore water pressures are generated during shearing. An important contribution from the study is a series of results for the yielding of illitic clay at three different temperatures. For the first time, there is clear evidence of yield loci decreasing in size with increasing temperature. An associated flow rule can be assumed without serious error. The results contribute to the confirmation of a thermal elastic-plastic soil model developed by the authors from cam clay following the addition of a small number of extra assumptions. Depending on the initial stress state, heating under undrained conditions may produce shear failure. #Q 1997 Elsevier Science B.V.

2004, Water Resources Research

upslope watershed area, and soil physical and engineering properties are other factors that affect generation and subsequent dissipation of piezometric response during rainstorm events [

2004, Marine Geology

The late Quaternary sediments of the Adriatic Sea show clear evidence for fluid flow and shallow subsurface sediment mobilization. Amplitude blanking in high-resolution seismic data and limited sediment core information document that free gas is diffused at very shallow stratigraphic levels; pockmarks and blow-out pipes indicate expulsion of overpressured fluid; possible polygonal fault systems denote areas of sediment contraction and dewatering. Core data and VHR seismic profiles suggest that very high accumulation rates of homogeneous and fine-grained deposits result in fluid accumulation (high pore water pressure) within low-permeability sediments. The fluid flow processes that are active under such regime are still poorly understood. Enhanced accumulation rates during particular intervals can potentially result in trapping of fluids leading to increased pore pressure and decreased shear strength. Seismicity is high in the study area, and recurrent earthquakes or tsunamis provide an effective mechanism leading to the cyclic loading of sediment, favoring mobilization of gas-charged sediments.

2007, Journal of Glaciology

To avoid some of the limitations of studying soft-bed processes through boreholes, a prism of simulated till (1.8 m Â Â 1.6 m Â 0.45 m) with extensive instrumentation was constructed in a trough blasted in the rock bed of Engabreen, a temperate glacier in Norway. Tunnels there provide access to the bed beneath 213 m of ice. Pore-water pressure was regulated in the prism by pumping water to it. During experiments lasting 7-12 days, the glacier regelated downward into the prism to depths of 50-80 mm, accreting ice-infiltrated till at rates predicted by theory. During periods of sustained high porewater pressure (70-100% of overburden), ice commonly slipped over the prism, due to a water layer at the prism surface. Deformation of the prism was activated when this layer thinned to a sub-millimeter thickness. Shear strain in the till was pervasive and decreased with depth. A model of slip by ploughing of ice-infiltrated till across the prism surface accounts for the slip that occurred when effective pressure was sufficiently low or high. Slip at low effective pressures resulted from water-layer thickening that increased non-linearly with decreasing effective pressure. If sufficiently widespread, such slip over soft glacier beds, which involves no viscous deformation resistance, may instigate abrupt increases in glacier velocity.

A large landslide (40×10 6 m 3 ) was reactivated on the left bank of Canelles reservoir, Spain. The instability was made evident after a considerable reduction of the reservoir level. The drawdown took place during the summer of 2006 after several years of high water levels. The drawdown velocity reached values between 0.5 and 1.2m/day (registered at low elevations). The paper reports the geological and geotechnical investigations performed to define the movement. The geometry of the slip surface was established from the detailed analysis of the continuous cores recovered in deep borings and from limited information provided by inclinometers. Deep piezometric records provided also valuable information on the pore water pressure in the vicinity of the failure surface. These data allowed validating a flow-deformation coupled calculation model, which takes into account the changes in water level that occurred 4 years previous to the failure as well as the average rainfall. The analysis indicates that the most likely reason for the instability is the rapid drawdown that took place during the summer of 2006. The potential sudden acceleration of the slide is also analysed in the paper introducing coupled thermal hydraulic and mechanical effects that may develop at the basal shearing surface of the sliding mass. The results indicate that the slide velocity may reach values around 16m/s when displacement reaches 250m.

1999, Journal of Quaternary Science

The stratigraphy of a trench excavated through a solifluction lobe lying at an altitude of 860 m a.s.l. on the eastern flank of the Okstindan mountains is described. Sedimentological evidence suggests that the movement was probably dominated by a flow process, with silty sands episodically bursting-out through a thinly vegetated lobe front in the spring and early summer thaw phases, when pore-water pressures were likely to be increased. A continuous buried soil extends for some 14 m. Fourteen new radiocarbon age estimates from thin-slice samples of this buried soil and organic fractions derived from laboratory pre-treatment procedures are discussed. These data indicate that the solifluction probably commenced in the mid-Holocene and continued throughout the Neoglacial. The slope instability may be correlated tentatively with the record of glacial variations, shifts in tree lines and archaeological evidence, supporting a link with regional climatic deterioration. Figure 1 Location of the 860 m a.s.l. (EL-1 ) solifluction lobe study site, on the east facing flank of the Okstindan massif, Nordland County, north Norway. Note the position of the Okstinddalen solifluction site at 710 m.

2006, Journal of Structural Geology

Layers of fine-grained muscovite were hot-pressed then sheared between alumina sliders to shear strains up to 2, at temperatures between 300 and 700 C, confining pressures of 206 MPa and various pore water pressures. High pore water pressures helped suppress dehydroxylation of the mica and permitted testing at temperatures higher than previously used. Shear strain rates between 10 À3 s À1 and 10 À7 s À1 were accessed using constant shear strain rate and stress relaxation testing. Except for strain rates <10 À5 s À1 at 700 C, deformation was strain rate and temperatureinsensitive, but effective normal stress-sensitive with a friction coefficient at yield of 0.3, rising with strain to 0.5. Steady-state sliding was not attained. From the mechanical data and microstructural study, deformation was inferred to have occurred by a mixture of brittle/frictional and crystal plastic processes. At 700 C and low strain rates the shear strength falls rapidly with a linear-viscous characteristic, in a way not previously reported. This is tentatively attributed to rate-control by viscous glide of basal dislocations. Extrapolating these results to geological strain rates, we expect mica-rich fault zones will exhibit frictional behaviour with a low friction coefficient between 0.25 and 0.5, giving way at mid-crustal conditions to a rapid strength drop as viscous creep supervenes. Thus mid-to lower-crustal, mica dominated faults of any orientation, and terrains of schistose, metapelitic rocks in the cores of orogeneic belts, are expected to be very weak, supporting shear stresses in the range 1e10 MPa. The frictional behaviour of mica-rich faults in upper-crustal regions is, however, expected to be too strong to account alone for the proposed weakness of some major fault zones.

The degree of consolidation is usually used as one of the criteria for assessing the effectiveness of soil improvement work using the fill surcharge or vacuum preloading method. It is also often used as a design specification in a soil improvement contract. Degree of consolidation is normally calculated using settlement data. However, as the effect of vacuum preloading is controlled largely by pore water pressure changes, it is necessary to analyze the pore water pressure variations and to assess the degree of consolidation using pore water pressures. In this paper, the problems involved in the estimation of degree of consolidation using settlement data are discussed. A method to estimate the average degree of consolidation using pore water pressure data is suggested. Two case studies are presented to examine the characteristics of the pore water pressure variation of soil under vacuum loading. The degree of consolidation achieved in each of the two cases is assessed using pore water pressure data and compared with that estimated using settlement data. Factors affecting the degree of consolidation assessment are discussed.

2007, Physics and Chemistry of the Earth, Parts A/B/C

Indurated clays are being considered in a number of counties as a host rock for disposal of radioactive waste. This paper describes laboratory experiments carried out at the GRS geotechnical laboratory in Braunschweig on core samples taken from the Callovo-Oxfordian argillite at Bure in France and the Opalinus clay at Mont Terri in Switzerland. The experiments focused on investigating responses of the clay rocks to thermo-hydro-mechanical (THM) loading similar to that expected in repositories. Very complex coupled THM phenomena were observed in the tests, such as significant swelling and shrinking induced by wetting and drying, stress reaction to hydration and dehydration, pore-water pressure increase, thermal expansion and contraction, thermal effects on deformation and strength, and self-sealing. All the laboratory observations suggest that adsorbed pore-water is a key factor dominating the THM properties and processes in indurated clays.

2000, Journal of Contaminant Hydrology

The application of a DC electric field down gradient to the contaminated zone in the subsurface can create an electrokinetic barrier. The electrical potential gradient causes the movement of ions which in turn imposes a viscous drag on the pore water. In clayey soils, this viscous drag can generate a high enough pore water pressure capable of counteracting the ground water gradient. This phenomenon can be used effectively as a barrier to prevent contaminant migration. A finite element model was developed to simulate the contaminant migration in soil under coupled hydraulic, electrical and chemical gradients. The model is also capable of predicting the associated changes in the soil like the pore water pressure, pH and voltage gradient as a function of time and distance from the electrode. This model was validated using the w experimental data presented by Yeung Yeung, A.T., 1990. Electro-kinetic barrier to contaminant x transport through compacted clay. PhD Thesis, University of California, Berkeley, 260 pp. . The results indicate very good agreement between the experimental and simulated results. The model predications show that when the anode is placed down gradient of the cathode, the cation migration could be completely arrested. However, the anions were found to move faster in the direction of the ground water flow and reduce the effectiveness of the barrier. This could be avoided by carefully designing the placement of the electrodes. If the contaminant of interest is either a cation or an anion, a simple double row of anode and cathode electrode arrangement could serve as an effective barrier. On the other hand, if the contaminant includes both anions and cations, then a triple-row configuration of electrodes need to be implemented. The model ) Corresponding author. 0169-7722r00r$ -see front matter q 2000 Elsevier Science B.V. All rights reserved.

2008

The Disturbed State Concept (DSC) provides a general approach for constitutive modeling of deforming materials. Here, we briefly explain the DSC and present the results of laboratory tests on two regionally significant North American tills, along with the results of a numerical simulation to predict the behavior of one of the tills in an idealized physical system. Laboratory shear tests showed that plastic strain starts almost from the beginning of loading, and that failure and resulting motion begin at a critical disturbance, when about 85% of the mass has reached the fully adjusted or critical state. Specimens of both tills exhibited distributed strain, deforming into barrel shapes without visible shear planes. DSC parameters obtained from shear and creep tests were validated by comparing model predictions against test data used to find the parameters, as well as against data from independent tests. The DSC parameters from one of the tills were applied in a finite-element simulation to predict gravity-induced motion for a 5000-m long, 100-m thick slab of ice coupled to an underlying 1.5-m thick layer of till set on a 41 incline, with pore-water pressure in the till at 90% of the load. The simulation predicted that in the middle segment of the till layer (i.e., from x ¼ 2000 to 3000 m) the induced (computed) shear stress, strain, and disturbance increase gradually with the applied shear stress. Induced shear stress peaks at 60kPa.Thecriticaldisturbance,atwhichfailureoccurs,isobservedafterthepeakshearstress,ataninducedshearstressof60 kPa. The critical disturbance, at which failure occurs, is observed after the peak shear stress, at an induced shear stress of 60kPa.Thecriticaldisturbance,atwhichfailureoccurs,isobservedafterthepeakshearstress,ataninducedshearstressof23 kPa and shear strain of 0.75inthetill.Calculatedhorizontaldisplacementovertheheightoftheentiretillsectionattheappliedshearstressof65kPais0.75 in the till. Calculated horizontal displacement over the height of the entire till section at the applied shear stress of 65 kPa is 0.75inthetill.Calculatedhorizontaldisplacementovertheheightoftheentiretillsectionattheappliedshearstressof65kPais4.5 m. We note that the numerical prediction of critical disturbance, when the displacement shows a sharp change in rate, compares very well with the occurrence of critical disturbance observed in the laboratory triaxial tests, when a sharp change in the rate of strain occurs. This implies that the failure and concomitant initiation of motion occur near the residual state, at large strains. In contrast to the Mohr-Coulomb model, which predicts failure and motion at very small (elastic) strain, the DSC thus predicts failure and initiation of motion after the till has undergone considerable (plastic) strain. These results suggest that subglacial till may be able to sustain stress in the vicinity of 20 kPa even after the motion begins. They also demonstrate the potential of the DSC to model not only local behavior, including potential ''sticky spot'' mechanisms, but also global behavior for soft-bedded ice.

2007, Bulletin of the Seismological Society of America

Excess pore-water pressure and liquefaction at the Wildlife Liquefaction Array in 1987 were caused by deformation associated with both high-frequency strong ground motion and 5.5-second-period Love waves. The Love waves produced large (ϳ1.5%) cyclic shear strains well after the stronger high-frequency ground motion abated. These cyclic strains generated approximately from 13 to 35% of the excess pore-water pressure in the liquefied layer and caused excess pore-water pressures ultimately to reach effective overburden stress. The deformation associated with the Love waves explains the "postearthquake" increase of pore-water pressure that was recorded at the array. This explanation suggests that conventional methods for predicting liquefaction based on peak ground acceleration are incomplete and may need to consider cyclic strains associated with long-period surface waves. A postearthquake survey of an inclinometer casing indicated permanent shear strain associated with lateral spreading primarily occurred in the upper part of the liquefied layer. Comparison of cone penetration test soundings conducted after the earthquake with pre-earthquake soundings suggests sleeve friction increased. Natural lateral variability of the liquefied layer obscured changes in tip resistance despite a ϳ1% reduction in volume. The large oscillatory motion associated with surface waves explains ground oscillation that has been reported at some liquefaction sites during earthquakes.

2009

Among the various laboratory studies to investigate the Thermo-Hydro-Mechanical (THM) behaviour of Boom clay, relatively few were devoted to the time dependent behaviour, limiting any relevant analysis of the long-term behaviour of the disposal facility. The present work aims at investigating the time-dependent behaviour of Boom clay under both thermal and mechanical loading. High-pressure triaxial tests at controlled temperatures were

2010, Marine and Petroleum Geology

Late Quaternary shallow biogenic gas reservoirs have been discovered and exploited in the Qiantang River (QR) estuary area, eastern China. The fall of global sea level during the Last Glacial Maximum resulted in the formation of the QR incised valley. From bottom to top, the incised valley successions can be grouped into four sedimentary facies: river channel facies, floodplain-estuarine facies, estuarineshallow marine facies, and estuarine sand bar facies.