Positive Pore Water Pressure Research Articles

Dynamic response of seabed in wind power of deep-shallow sea induced by internal solitary waves

With the development of offshore wind power projects worldwide, the stability of deep and shallow sea wind power sites has become a crucial concern. Internal solitary waves (ISWs) are nonlinear and large amplitude waves in the stratified ocean. Their strong vertical and horizontal motion and vorticity and turbulence caused by fragmentation have an important impact on the marine environment, seabed sediments, and marine engineering. This study focuses on the interaction between ISWs and the seabed of wind power sites in deep and shallow seas. Specifically, we investigate the interaction between ISWs and monopile foundations or floating fan plate anchor structures in the seabed. Through the simulation of CFD software and Comsol software, a law of seabed pore pressure variation during the effect of ISWs on the structure can be well demonstrated. Our results demonstrate that ISWs induce the accumulation of excess pore water pressure around the structures. The monopile foundation experiences negative excess pore water pressure at its bottom and the plate anchor at its top. The pore water pressure at the bottom of the monopile foundation initially decreases and then increases. In contrast, the pore water pressure at the right side of the monopile foundation increases, decreases, and then increases. This phenomenon promotes the settlement of the structure. At the bottom of the plate anchor, there is a sharp increase in positive excess pore water pressure by ISW. The pore water pressure above the plate anchor structure initially decreases and then increases, while below the plate anchor, it increases and then decreases. This phenomenon leads to the movement of the anchorage structure and instability of the surrounding soil. The influence of ISWs on the seabed bottom intensifies with the obstruction of the structure, resulting in significant positive pressure at the bottom. Between the two anchoring structures, there is increased seepage of pore water, making the seabed more prone to instability. Through the study of this article, we know that ISW has different effects on the force of monopile foundation and plate anchor structure, which provides a favorable basis for its design and maintenance.

Seismically progressive motion mechanism of earth slopes considering shear band porewater pressure feedback under earthquake excitations

The complicated seismic response of earth slopes and the transition mechanism from progressive to catastrophic motion of landslides have long been recognized as one of the most challenging research topics in geotechnical earthquake engineering. In this paper, this fundamental conundrum will be tentatively unveiled from a new perspective combining the seismic slope displacement analysis with (mechanical) dynamic porewater pressure feedback of the shear band soils against the earthquake excitation. We present both the conceptual scheme and the calculation framework to numerically study the dynamic changes of porewater pressure, shear strength parameters, and hydraulic properties of the shear band soils. Their significant influences to the change of earthquake-induced landslide movement modes are systematically investigated. Different dilation angles are chosen to characterize the shear dilation and shear contraction processes in the shear band soil, while the dynamic porewater pressure feedback in the shear band soil during the continuous seismic displacement is investigated. The results show that the seismic slope displacement and failure strongly depend on the mechanical feedback of the shear band soil. When the dilation angle is positive, the porewater pressure feedback in the shear band during the slope movement is negative, and the effective resistance stress increases, which is not susceptible to catastrophic motion. On the other hand, when the dilation angle is negative, the positive porewater pressure feedback may lead to catastrophic failure. At last, the effects of shear softening/hardening combined with dilation angle evolution on the ultimate slope movement mode are also put forward.

Numerical Modelling for effect of rainfall on unsaturated soil slopes

Expansive soils are predominant in many parts of the world. Expansive soil treated as problematic soils, as economic and design a safe slope on expansive soils is major challenge for civil engineers. Expansive soils were compacted under unsaturated condition, in various construction projects. Major failures reported in many countries around the world are due to swell shrinkage characteristics causing unstable slopes. Fills and compacted soils have an inherent suction when they are first compacted. The suction is the most important factor which affects the shear strength of unsaturated soil. variation of suction in unsaturated soil is an important factor considered for solving issues like slope stability and bearing capacity failure in unsaturated soil. Rainwater infiltration are major triggering factor for failureof expansive soil slopes. The strength of unsaturated soil changes with water evaporation and infiltration. Due to infiltration of rainwater into the soil a significant change in the pore water takes place. Hence, this fluctuation in pore water pressure responsible for the distinct mechanical behavior of unsaturated soil under different Geotechnical problems.A numerical modeling was done with Plaxis LE software, considering parameters of unsaturated soil. As the changes in pore-water pressure and moisture content are a function of the flow of moisture through the soil-atmosphere boundary.The effect of soil type, rainfall intensity, location of ground water table, permeability functionsareconsidered to reproduce the soil-atmosphere interaction. Obtain results illustrates the combine effect of suction and increase of saturation due to rainfall on slopes. It was observed that rainfall increase the induced matric suction and increase the positive pore water pressure finally results in slope failures.

Water level fluctuations drive bank instability in a hypertidal estuary

Abstract. Hypertidal estuaries are very dynamic environments characterized by high tidal ranges (> 6 m) that can experience rapid rates of bank retreat. Whilst a large body of work on the processes, rates, patterns, and factors driving bank erosion has been undertaken in fluvial environments, the process mechanics affecting the stability of the banks with respect to mass failure in hypertidal settings are not well-documented. In this study, the processes and trends leading to bank failure and consequent retreat in hypertidal estuaries are treated within the context of the Severn Estuary (UK) by employing a combination of numerical models and field-based observations. Our results highlight that the periodic fluctuations in water level associated with the hypertidal environment drive regular fluctuations in the hydrostatic pressure exerted on the incipient failure surfaces that range from a confinement pressure of 0 kPa (at low tide) to ∼ 100 kPa (at high tide). However, the relatively low transmissivity of the fine-grained banks (that are typical of estuarine environments) results in low seepage inflow/outflow velocities (∼ 3 × 10−10 m s−1), such that variations in positive pore water pressures within the saturated bank are smaller, ranging between about 10 kPa (at low tide) and ∼ 43 kPa (at high tides). This imbalance in the resisting (hydrostatic confinement) versus driving (positive pore water pressures) forces thereby drives a frequent oscillation of bank stability between stable (at high tide) and unstable states (at low tide). This transition between stability and instability is found not only on a semidiurnal basis but also within a longer time frame. In the spring-to-neap transitional period, banks experience the coexistence of high degrees of saturation due to the high spring tides and decreasing confinement pressures favoured by the still moderately high channel water levels. This transitional period creates conditions when failures are more likely to occur.

An Integrated Bayesian Risk Model for Coastal Flow Slides Using 3-D Hydrodynamic Transport and Monte Carlo Simulation

The literature suggests two forms of flow slides: breaching and liquefaction. Both forms of failure have comparable ultimate circumstances, but the progression and sand movement mechanisms of breaching failure diverge from those of liquefaction. The first type, breaching, occurs in densely packed sand and is characterized by slow sand grain discharge throughout the dilation of the failing soil particles and negative excess pore pressures. The latter form, known as liquefaction, is the process by which a mass of soil abruptly begins to behave like a flowing liquid, and as a result, it can flow out across overly mild slopes. The process begins in compacted sand and is linked to positive surplus pore water pressures that are caused by the compaction of the sand. Despite the available literature on flow slide failures, our understanding of the mechanisms involved remains limited. Since flow slides often begin below the water surface, they can go undetected until the collapse reaches the bank above ground. The complexity of flow slides requires the use of cutting-edge technological instruments, diving equipment, advanced risk assessment, and a variety of noteworthy probabilistic and sensitivity analyses. Hence, we developed a new sensitivity index to identify the risk of breach failure and vulnerable coastal areas to this risk. In addition, we developed a sophisticated hybrid model that allows for all possibilities of flow slides in sync with random variables used in this new sensitivity index. In this new hybrid model, three distinctive models exist. The 3D Hydrodynamic Model addresses waves, wind, current, climate change, and sediment transport. The Monte Carlo Simulation is responsible for sensitivity analysis, and the Bayesian Network focuses on joint probabilities of coastal flow slide parameters of this new index that incorporates all environmental parameters, including climate change. With the assistance of these three models, researchers aim to: (a) expand the application scope by presenting a method on coastal flow slides; (b) consider different particle diameters corresponding to critical angle slope failure; (c) analyze variables that can play a pivotal role in the flow slides; and (d) present a methodology for coupling coastal flow slide projections with reliable outcomes. The hybrid model incorporates random variables of retrogressive breach failures, and the new risk index considers their ranges to control the simulation. The use of such a hybrid model and risk index offers a robust and computationally efficient approach to evaluating coastal flow slides.

Pore pressure response during tube sampling in gold tailings

Capturing the in-situ state of mine tailings is important in determining its behaviour under loading for evaluating the risk of failure of tailings dams. Obtaining a high-quality undisturbed sample through tube sampling is thus of great value to the responsible engineer. However, the sampling process itself disturbs the soil and changes its state. Since soil behaviour is driven by effective stress, one method to evaluate the disturbance induced during tube sampling is to monitor the soil’s positive and negative pore water pressure response during the process. A laboratory study was conducted where tailings samples were prepared at a controlled void ratio and subsequently saturated with water. Tube samplers of different diameters were driven into the material and extracted by means of an electric actuator during which internal sample pore pressures were monitored using tensiometers. The material experienced significant instantaneous contractive and dilative tendencies during insertion. Instantaneous negative responses during extraction of the tube samplers were also measured. It was found that the largest tube sampler (100mm diameter) performed best in the study.

Some Approaches to the Prediction of Permeability Parameters in a Finite Element Program for Early Warning

Recently, landslides often occurred in natural soil slopes in the tropical region, which correlate with the rainy season. Rainfall infiltration leads to groundwater level fluctuations. The increased positive pore-water pressures due to rainfall influence have affected the properties and behavior of the unsaturated soil slope. In this research, the Finite Element Method of SEEP/W and SLOPE/W analyzes the factor safety of the slope affected by pore water pressure change due to rainfall. The Soil Water Characteristic Curve (SWCC) and Hydraulic Conductivity function were obtained from sieve analysis and Atterberg's limit. In addition, unsaturated soil properties from the UNSODA code are estimated based on grain-size distribution using the SWRC program. The study area is in Khanom District, southern Thailand. The results show that the soil slope at the site became unstable on November 18, 2021, with F.S. = 1.0, which agrees well with the date of the disaster. In conclusion, the slope stability analysis without the parameters from the unsaturated soil hydraulic database (UNSODA) leads to the F.S. value being higher than the actual value, and the alarm estimation would be inaccurate. Doi: 10.28991/CEJ-2022-08-12-014 Full Text: PDF

A Simple Method for Predicting Rainfall-Induced Shallow Landslides

A method of practical interest is presented in this study for predicting shallow landslide triggering due to expected rainfall scenarios. The proposed approach is based on some closed-form expressions derived using physically-based models and requires a limited number of material parameters obtained from conventional tests. Two typical triggering mechanisms are considered: in the first case, a landslide is caused by a reduction in suction due to rain infiltration in unsaturated soils; in the second one, a slope failure occurs due to the formation of a water table with consequent generation of positive pore water pressures within the slope. A simple criterion is also provided to establish which triggering mechanism occurs. For each considered mechanism, a rainfall threshold curve is derived, which is formally similar to several empirical intensity-duration relationships proposed in the literature for predicting the occurrence of shallow landslides. However, unlike these empirical relationships, the present threshold curves explicitly depend on basic parameters such as slope geometry, soil properties, and pore water pressure existing at the depth of the potential slip surface before the rainfall. The proposed approach is also applied to well-documented case studies to assess its predictive capacity.

A Thermodynamic Formulation of Water Potential in Soil

AbstractIn the mid‐20th century, harnessing of thermodynamics in describing water movement in soil, viz the concept of water potential, marks the emergence of modern soil physics, unsaturated soil mechanics, and vadose zone hydrology. Yet to date, a seamless linkage between thermodynamics and water potential is still missing, leading to several long‐lasting dilemmas regarding soil properties, for example, abnormal soil water density, peculiar film water viscosity and relative permittivity, pore water pressure, and water freezing temperature depression. Here, a thermodynamic framework is established by synthesizing recent advancements in soil‐water interaction. The classical thermodynamic concepts are revisited, highlighting the difference between macroscopic systems commonly treated in conventional theories and the intermolecular scale system subject to external fields. Soil water is conceived as an intermolecular scale open thermodynamic system subject to external fields of gravity, osmosis, and adsorption. The formulated thermodynamic framework is verified by reducing to the conventional definition of matric potential and a recently proposed unitary definition. The accuracy of the framework is further justified in terms of mechanical equilibrium criteria. The framework predicts the existence of spatially varied pore water pressure in soil pores and can serve as the theoretical basis for reconciling the physical origin of abnormal soil behavior such as water density, film water viscosity, relative permittivity, and negative or positive pore water pressure.

An assessment of the mechanical performance of a novel sand bentonite-glass fiber composite for the avoidance of catastrophic landfill failure

• Octahedral shear stress increased higher at the fiber aspect ratio than at the fiber dosage. • At peak and residual stress, bearing capacity factors improved significantly. • Negative excess pore waster pressure increased with an aspect ratio of 40. • Energy absorption capacity was the function of fiber dosage and fiber aspect ratio. Compacted sand bentonite (CSB) blend is one of the finest materials for landfill construction. CSB mixes with a strain-softening tendency may fail abruptly, increasing soil permeability. Water movement over the shear zone can induce flow slides, triggering a catastrophic collapse. Waste glass fiber was added to the CSB mixture (90:10) in various fractions (0.5 %, 1, and 1.5 %) and aspect ratios (l/d = 40, 80, and 120) to prevent the soil structure from collapsing. A full-scale consolidated undrained (CU) test was conducted to assess (i.e., c-phi-analysis) the load-carrying capabilities of waste fiber soil composite. The octahedral shear stress was increased significantly for both stress level (i.e., peak and residual stress). Positive pore water pressure (PWP) increased progressively after initial yield, while negative PWP reduced as the aspect ratio increased from 80 to 120. Fiber dosage and fiber aspect ratio improved bearing capacity factors (N c ,N q ,N ϒ ) and undrained bearing capacity. The EAC (Energy absorption capacity) was determined using a numerical integration approach through MATLAB (version14), and the use of waste fiber resulted in a considerable improvement. Several variables contributed to the principal stress and soil modulus. Accordingly, they were framed with all the variables and built a model that matched the experimental results well.

Analysis of Maximum-Rainfall-Infiltration-Induced Slope Stability Using the Transient Rainfall Infiltration and Grid-based Regional Slope-stability Model in Cililin, West Java, Indonesia

DOI:10.17014/ijog.9.2.263-278Landslide is one type of geological disasters that frequently occurs during the rainy season. Rainfall infiltration can cause soil saturation that increases the positive pore water pressure, disturbing the slope stability. Therefore, knowledge of future landslide-triggering rainfall is required for mitigation efforts and reducing the risk of landslide hazards. This paper presents slope-stability modeling in the Cililin area using the well-established infinite slope model called the transient rainfall infiltration and grid-based regional slope-stability (TRIGRS). The modeling used the rainfall data obtained from the statistical analysis of the maximum daily rainfall by using the Gumbel distribution. The present study applied six scenarios in the modeling. Scenario I is the initial condition without rainfall, showing the slope stability influenced by topography, slope, and soil characteristics. TRIGRS modeling involves rainfall infiltration in scenarios II, III, IV, V, and VI. The maximum rainfalls used in the modeling are 66, 76, 101, 120, and 132 mm/d, showing that rainfall infiltration affected the slope stability. The result indicates that rainfall triggered an increase and expansion of the area distributions critical to the slope stability.

Unsaturated Hydraulic Conductivity Estimation—A Case Study Modelling the Soil-Atmospheric Boundary Interaction

Pore water pressure changes due to soil-atmospheric boundary interaction can significantly influence soil behaviour and can negatively affect the safety and stability of geotechnical structures. For example, prolonged rainfall events can lead to increased pore water pressure and lower strength; repeated cycles of pore water pressure changes can lead to degradation of strength. These effects are likely to become more severe in the future due to climate change in many parts of the world. To analyse the behaviour of soil subjected to atmospheric boundary interactions, several parameters are needed, and hydraulic conductivity is one of the more important and is difficult to determine. Hydraulic conductivity deduced from laboratory tests are often different from those from the field tests, sometimes by orders of magnitude. The problem becomes even more complicated when the soil state is unsaturated, where the hydraulic conductivity varies with the soil’s state of saturation. In this paper, a relatively simple alternative approach is presented for the estimation of the hydraulic conductivity of unsaturated soils. The method involved a systematic re-analysis of observed pore water pressure response in the field. Using a finite element software, the soil-atmospheric boundary interaction and related saturated/unsaturated seepage of an instrumented slope have been analysed, and results are compared with field measurements. The numerical model could capture the development of suction, positive pore water pressure and changes in water content with reasonable accuracy and demonstrated the usefulness of the hydraulic conductivity estimation method discussed in this paper.

Field Collection of Geotechnical Measurements for Remote or Low-Cost Datalogging Requirements

Abstract Reliable, low-cost datalogging alternatives promote transfer of knowledge and technology to the wider geotechnical and geoscientist community. Alternative systems can ease increased data resolution on large projects, operate in remote locations with restricted site access, or allow developing countries access to reliable and cost-effective datalogging solutions. A low-cost prototype datalogger was developed and tested in the laboratory with the use of open-source materials. Open-source example code is provided at the permanent links included in this paper. The materials for the prototype were 20 % the cost of commercial datalogging units with similar capabilities. With labor, these custom-built units were 35–45 % the cost of a purchased datalogger. Measurements from commercial units and the prototype datalogger were compared to determine the prototype’s accuracy. The datalogger was deployed in place of commercially available dataloggers at three sites across western Canada in the past two years. Laboratory and field testing of the low-cost datalogger has shown the prototype to be easily adaptable to various sensor types. The study experimented with negative pore water pressure (matric suction), volumetric water content, and temperatures from SDI-12 sensors as well as positive pore water pressure and temperature from vibrating wire piezometers. Telemetry modules have been attached to remote dataloggers, transmitting occasional data points, and periodically verifying system operation. Assembly, installation, and monitoring with the low-cost datalogging system over the past two years has demonstrated their durability in field applications. The implementation of a low-cost, open-source geotechnical datalogging system can be a challenge in some locations and requires the consideration of limitations, which are addressed.

Recycled waste tire fiber as a sustainable reinforcement in compacted sand–bentonite mixture for landfill application

A layer of compacted sand-bentonite (CSB) mixture is employed as a liner at the landfill. The formation of desiccation-induced shrinkage cracks can increase the leachate migration into groundwater. The usefulness of the liner system may be endangered if the liner material's strength is insufficient. To address this issue, the CSB barrier can be modified by incorporating with waste tire fiber. This study aimed to assess the undrained shear strength, consolidation, and desiccation-induced crack of mixtures of CSB and reinforced CSB. An analytical model was derived to assess the behaviour of tire fiber composite soil from the experimental result. Positive excess pore water pressure was reduced significantly by the inclusion of tire fiber in the consolidated undrained triaxial test results. By the addition of 15% tire fiber, the effective frictional angle was raised from 21.0° to 24.2°, while the effective cohesion component was increased from 15.0 kPa to 24.2 kPa at the same fiber content. The crack density factor and crack intensity factor of the mixture without tire fiber were much higher than those of the CSB with fiber. By the addition of tire fiber, time for 90% consolidation was reduced, while the coefficient of consolidation increased. Although the permeability increased marginally, but it was within the limiting value. An analytical model adequately captured the stress-strain and pore water pressure responses of all proportion of tire fiber, and the yield surface of the composite was expanded maximally with only 10% tire fiber.

Effect of root orientation on the strength characteristics of loess in drained and undrained triaxial tests

Ecological engineering measures are gaining popularity in the control of mountain hazards, particularly in shallow landslides, in which plant roots improve the shear strength of shallow soil. To measure such improvement, a series of drained and undrained triaxial tests were performed to investigate the role of Cunninghamia R. Br roots in loess soil mechanical behavior collected from Longchi Forest Center, southwest China. The specimens were prepared by distributing roots in four (4) distinct spatial orientations types, i.e., Horizontal, Vertical, Horizontal-Vertical (grid), and Randomly mixed in the soil. All specimens were first fully saturated and subsequently sheared in compression tests under drained and undrained conditions. The test results are evaluated in terms of monotonic stress-strain behavior, volume change, development of excess pore water pressure, and critical state behavior. The results showed that the addition of roots increased the soil shear strength, provides additional resistance to volumetric deformations and increased the positive excess pore water pressure as compared to plain loess soil. The results also revealed that the mechanical response of the soil-root composite depends on the type of root orientation. The shear strength improved highest with horizontal-vertical (grid) root orientation in both drained and undrained conditions among four root orientations. In contrast, the horizontal root orientation showed the least shear strength improvement. Additionally, for the same root orientation type, the shear strength improvement was more pronounced in undrained conditions compared to drained conditions. The plain and soil-root composite shear strength parameters showed that the addition of roots increased the cohesion with little effect on the mobilized angle of friction in both drained and undrained conditions. The findings of this research will provide a strong baseline for developing a predictive constitutive model for soil–root composite.

Characterization of dynamic response of asphalt pavement in dry and saturated conditions using the full-scale accelerated loading test

Asphalt pavement exhibits different dynamic responses to vehicle loading in saturated conditions and in dry conditions. It is essential to directly conduct field full-scale tests to systematically investigate comprehensive influence of water on dynamic response of asphalt pavement. However, real vehicles which were often applied in field tests might result in a certain deviation in dynamic responses measured from each field loading test due to the inevitable influence of the driver’s subjective operation and the tread pattern on vehicle wheel. This study adopted the full-scale accelerated loading test system to comprehensively characterize dynamic response of asphalt pavement in both dry and saturated conditions based on the strict control of loading vehicle’s path. The results indicated that dynamic responses showed relatively larger magnitudes in saturated condition than those in dry condition. Strain and stress respectively showed slowly increasing and decreasing trends with the increase of vehicle load and speed correspondingly. Pore water pressure was apparently sensitive to vehicle speed compared with vehicle load. Pore water pressure generated by the front axle wheels showed obviously larger magnitudes than those produced by the rear axle wheels due to the change in thickness of surface runoff. The transverse distribution of pore water pressure obtained from both field tests and numerical simulation exhibited consistent variation trends. Decline degree of pore water pressure magnitudes was obtained at 86.1% for the two adjacent testing points with a distance of 100 mm under the vehicle wheel, which proved the great influence of the tread pattern of vehicle wheel on the transverse distribution of dynamic pore water pressure. A prediction model of positive pore water pressure that considered vehicle load and speed was proposed.

Field model experiments and numerical analysis of rainfall-induced shallow loess landslides

Shallow loess landslide induced by rainfall is a widespread form of slope failure on the Loess Plateau of China. The hydrological response, evolutionary mechanism and failure processes of unsaturated loess slopes under precipitation in a field scale are currently lacking. Three groups of full-scale field experiments with different rainfall intensities were conducted in Jingyang County, Northwest China. Instrumentations including soil moisture sensors, tensiometers and earth pressure cells were installed in the slope. This paper presents the results from three types of sensors to reveal the surface infiltration process and deformation characteristics of unsaturated loess slopes. In addition, SEEP/W and SLOPE/W were selected to quantify the effects of soil suction strength on the stability of the homogeneous loess slope. The results show that the maximum infiltration depth of homogeneous loess slope after two days of continuous heavy rainfall is about 0.6 m. The infiltration depth and seepage velocity on the slope crest and slope toe are faster than the middle of the slope; The decrease amplitude and decrease speed of matric suction increase with the increase of rainfall intensity. Moreover, it is found that the rainfall intensity only affect the development of slope deformation and failure in terms of the time scale, but has little influence on the failure mode; Tensile cracks at the slope shoulder develope more rapidly due to the creep deformation of slope, the deformation and failure at the slope shoulder are the key factors controlling the occurrence of loess landslides. In addition, the proportion of suction strength in shear strength is limited, and the finiteness of the suction strength determines that its attenuation will not determine the stability of homogeneous loess slope without preferential infiltration channels. However, the increase of positive pore water pressure at the potential slide surface may be the key factor to determine the slope failure.

Post-cyclic mechanical behaviors of laterite clay with different cyclic confining pressures and degrees of reconsolidation

The post-cyclic mechanical behaviors of subgrade soil are usually assessed by conventional cyclic triaxial tests, however, the stress field induced by traffic loads consist of not only a cyclic deviator stress, but cyclic confining pressure and shear stress. Meanwhile, the pore water is allowed to be discharged under traffic loads. Furthermore, the traffic load-induced excess pore water pressure can be partially dissipated after load-cycling. Thus, different reconsolidation processes should be considered. Recognising this, cyclic loading with variable confining pressure was applied to remoulded laterite soils under partially drained conditions, and reconsolidation was allowed. The development of effective stress paths, excess pore water pressure, and undrained strength of post-cyclic shear process were analysed. Results show that the development of effective stress paths is similar and all stress paths cross the critical state line. Besides which, the remoulded laterite clay always has a positive excess pore water pressure and tend to dilate. The undrained strength will be increased under different conditions such that the greater the cyclic confining pressure, cyclic deviator stress, number of cycles, and degree of reconsolidation, the larger post-cyclic undrained strength. Moreover, the post-cyclic undrained strengths between cyclic loading with and without cyclic confining pressure were compared: the undrained strength ratio of cyclic confining pressure to that under constant confining pressure increases linearly with increasing slope of the stress path. Nevertheless, the post-cyclic undrained strength after reconsolidation can be obtained by the corresponding undrained strength without reconsolidation: the ratios of post-cyclic shear strength with and without reconsolidation increase with increasing degree of reconsolidation. At a certain slope of stress path, the undrained strength increases linearly with degree of reconsolidation.

Post-Cyclic Mechanical Behaviors of Undisturbed Soft Clay with Different Degrees of Reconsolidation

Soft soil is often subjected to cyclic loading such as that imposed during storms, under traffic, or in an earthquake. Furthermore, the cyclic-loading-induced excess pore water pressure can be partially dissipated after cyclic loading. Thus, different reconsolidation processes should be considered. A series of static and dynamic triaxial tests were conducted on undisturbed soft soil to determine the post-cyclic mechanical behavior thereof, such as the variation of undrained shear strength, the development of excess pore water pressure, and the evolution of effective stress path. The effects of consolidated confining pressure, cyclic stress ratio, and degree of reconsolidation were analyzed. Results show that the trend of all stress–strain curves is similar under different conditions. The effect of the degree of reconsolidation is such that, with increasing the degree of reconsolidation, the shear strength is enhanced. Meanwhile, compared with undrained shear strength without cyclic loading, the shear strength after cyclic loading with full reconsolidation is increased. These factors also have a significant effect on the undrained shear strength: the greater both the confining pressure and cyclic stress ratio are, the higher the undrained shear strength. A positive excess pore water pressure is always observed during post-cyclic shearing process, irrespective of different factors. The S-shaped effective stress paths under different test conditions are observed and cross the critical state line. The microstructures of undisturbed soil and post-cyclic specimens with different degrees of reconsolidation were quantitatively investigated. Besides that, the degree of influence of different factors on the post-cyclic undrained strength was analyzed. Based on the test results, the undrained shear strength with cyclic load-history was well predicted by existing models.

Long-term dynamic bearing capacity of shallow foundations on a contractive cohesive soil

The calculation of the long-term dynamic bearing capacity arises from the need to consider the generation of maximum pore-water pressure developed from a cyclic load. Under suitable conditions, a long-term equilibrium situation would be reached, when pore-water pressures stabilized. However, excess pore-water pressure generation can lead to cyclic softening. Consequently, it is necessary to define both the cohesion and the internal friction angle to calculate the dynamic bearing capacity of a foundation in the long term, being necessary to incorporate the influence of the self-weight of soil and therefore the width of the foundation. The present work is based on an analysis of the results of cyclic simple shear tests on soil samples from the port of El Prat in Barcelona. From these experimental data, a pore-water pressure generation formulation was obtained that was implemented in FLAC2D finite difference software. A methodology was developed for the calculation of the maximum cyclic load that a footing can resist before the occurrence of the cyclic softening. The type of soil studied is a contractive cohesive soil, which generates positive pore-water pressures. As a numerical result, design charts have been developed for long-term dynamic bearing capacity calculation and the charts were validated with the application of a real case study.