Pore Pressure Generation Research Articles

Influence of Fault Architecture on Induced Earthquake Sequence Evolution Revealed by High‐Resolution Focal Mechanism Solutions

AbstractThe increasing seismicity and improved seismic observation network in recent years provide an opportunity to explore factors that influence the triggering processes, spatiotemporal evolution, and maximum magnitude of induced sequences. We map the fault architecture and stress state of four induced sequences in Oklahoma to determine their influence on the seismicity. We systematically relocate the earthquakes and compute hundreds of focal mechanisms of small to medium events (1.0 < M < 5.1) using various techniques, including machine learning, for the Guthrie, Woodward, Cushing, and Fairview sequences. The detailed fault geometry and spatiotemporal evolution of seismicity and stress states reveal different dominant driving forces for each sequence. In Cushing and Fairview (largest event ≥M5.0), the main fault structures are near‐vertical narrow strike‐slip faults, with most of the small earthquake fault planes optimally oriented. The two sequences exhibit discontinuous temporal migration but strong earthquake self‐driven rupture growth. In Guthrie and Woodward (largest event <M5.0), the two sequences show more complex diffuse fault structures with varying dipping angles along depth. The inverted focal mechanisms show a mix of strike‐slip faulting and normal faulting in both sequences, and the normal faulting events are less optimally oriented than strike‐slip events. The two sequences are dominated by continuous diffusive migration in time driven by pore pressure propagation. The above results suggest that fault architecture and stress state influence sequence evolution, major driving forces, and possibly maximum magnitude.

Evidence of the climate control of strong seismicity in the Italian Apennines through groundwater recharge

This work explores the possibility that the destructive earthquakes occurring along the Apennine Chain in Italy are systematically triggered by groundwater recharge. The focus is on multi-year transitions toward phases of wet climate rather than on short-term heavy rainfall occurring in a few days or seasonally. The analysis takes into consideration the earthquakes with a moment magnitude of Mw ≥5.8 that have occurred since 1901. Their time distribution is compared with the fluctuations of the self-calibrated Palmer Drought Severity Index (scPDSI), an indicator of soil moisture, here assumed to be a proxy for groundwater recharge. It is found that the scPDSI evolved through six main oscillations lasting from 11 to 25 years, and that, with one exception, the strongest earthquake in each phase is placed within 2 years from the maximum of soil moisture. Based on a statistical test for pairs of point processes, such a coincidence indicates a significant synchrony between the two phenomena. In particular, the two strongest earthquakes of the study period (the 1915 Marsica earthquake and the 1980 Irpinia–Basilicata earthquake, with moment magnitudes of Mw 7.1 and 6.8, respectively) occurred exactly in the year of two of the largest peaks of scPDSI. The connection between wet climate conditions and the occurrence of strong earthquakes is further investigated by comparing the time distribution of Mw ≥6.1 historical earthquakes that have occurred since 1200 AD with the evolution of the Great Aletsch Glacier in Switzerland, representative of water accumulation at the continental level. Even in this case, the earthquakes clustered during time periods of increased precipitation and lower water evaporation, corresponding to the extreme phases of the Little Ice Age. The agreement of the results at different time scales and using different climate indexes leads to postulate a significant and systematic role of groundwater recharge in the triggering of large earthquakes along the Apennines. It is also suggested that the earthquakes might be triggered by pore-pressure propagation, where the necessary hydraulic continuity is made possible by the intersection of the shallow karst structures with the seismogenic faults.

Application of a coupled hydro‐mechanical interface model in simulating uplifting problems

AbstractThis paper presents the detailed formulation of a coupled hydro‐mechanical structure‐soil interface and demonstrates its application in simulating uplifting problems. This interface features real‐time prediction of the pore pressure generation and structure‐soil separation, and thus rate dependency and ‘breakaway’ can be modeled without user intervention. Constitutive relations of this interface were derived by considering the coupling between soil skeleton and fluid along the interface. A complete finite element formulation and numerical implementation of the interface is provided based on an eight‐node element. The performance of this interface is demonstrated by simulating lifting a surface footing at varying rates (spanning across undrained, partially drained and drained conditions), compared with existing theoretical solutions, numerical results and experimental data. The good agreement achieved indicates that this interface is capable of modelling uplift at varying rates, which is an extremely challenging topic in offshore engineering. Sensitivity studies were conducted to investigate the parameters affecting uplifting behaviour. A unified backbone curve was established correspondingly, which is shown to be different from existing studies in compression, due to the difference in the mechanism between the two cases.

Cyclic porewater pressure generation in intact silty soils

The results of cyclic strain-controlled, constant volume direct simple shear (CDSS) tests and field shaking tests have been evaluated for intact, natural, low-plastic silts from six different fine-grained soils with 54%–100% fines content, 47%–83% silt content, and plasticity indices (PI) ranging from nonplastic to 16. These tests constitute a subset of a larger archive of CDSS tests performed on silt deposits from the Pacific Northwest, British Columbia, and Alaska collected and analyzed by the co-authors. The cyclic data are presented in this paper for two objectives: (a) to characterize cyclically-induced excess pore pressure generation in intermediate soils with various soil index properties and stress histories, and (b) to provide calibrated Vucetic and Dobry model parameters for simulating excess pore pressure generation in the silt soils based on the data and trends presented in the first objective. The CDSS test results showed that excess pore pressure ratios decrease with PI over the narrow range of PI evaluated and decrease with overconsolidation ratio. The cyclic threshold shear strain amplitude for pore pressure generation extracted from field shaking tests on silts were within the range proposed in the literature, confirming that the cyclic threshold shear strain amplitude is a fundamental soil property. Calibrated Vucetic and Dobry model parameters for these intermediate, fine-grained silts were significantly different than those reported for sands in the literature and were heavily influenced by the overconsolidation ratio. The calibrated parameters obtained in this study can be used as a benchmark in selecting model parameters for silts.

Radial consolidation of prefabricated vertical drain-reinforced soft clays under cyclic loading

Previous studies have revealed that prefabricated vertical drains (PVDs) are effective in stabilizing the soft subgrade soils under traffic-induced cyclic loads. A consolidation model of soft clays under cyclic loading incorporating both radial and vertical drainages, and nonlinear variation of compressibility and permeability is presented in this paper. The prediction of the model matches well with the result from a large-scale cyclic triaxial test conducted on a soil sample installed with a single PVD. Parametric analysis has also been carried out to explore the impacts of cyclic degradation parameter, cyclic stress ratio, nonlinear compressibility and permeability, cyclic loading frequency, drainage condition, and number of loading cycles on the behaviors of the PVD-reinforced soil. Results indicate that radial drainage helps to decelerate the accumulation of excess pore pressures under cyclic loading due to two factors: increased rate of excess pore pressure dissipation and decreased rate of internal excess pore pressure generation. In addition, radial drainage also facilitates the rapid dissipation of excess pore pressures during rest period, leading to a denser soil which is stronger for resisting the subsequent set of cyclic loads. The findings of this study demonstrate the usefulness of PVDs in reinforcing soft subgrades under cyclic loading conditions, e.g., road pavement, railway embankment, etc.

Liquefaction responses of fibre reinforced sand in shaking table tests with a laminated shear stack

Mixing unidimensional tension-resistant elements into cohesionless soils, which is well known as fibre-reinforcement, is effective at strengthening the soils as well as increasing their liquefaction resistance in triaxial tests. In this study shaking table tests were performed on unreinforced and fibre reinforced sand models, contained in a laminated shear stack, to gather evidence as to whether or not the fibre reinforcement technology might reduce liquefaction susceptibility in a loading condition that has the semblance of an earthquake. The test results show that the excess pore pressures generated, and the amount by which the equivalent shear modulus is reduced, are significant in unreinforced and reinforced models when exposed to continuous sinoidal biaxial shaking simultaneously in horizontal and vertical directions. Fibre-reinforcement delays and reduces the build-up of excess pore pressures, slightly. Fibre-reinforcement also decreases slightly the attenuation of the acceleration amplitudes as well as the degradation of stiffness resulting from the generation of excess pore pressures. A constitutive model based on the rule of mixtures, which captures the load sharing between fibres, sand skeleton and pore water in triaxial loading conditions, and accounts for the anisotropy of the fibre orientation distribution, was modified and then used to study the shaking table deformation pattern. The stress the fibres impose on the sand skeleton in the vertical direction was determined and a new pore pressure ratio was used to assess whether or not liquefaction occurred. This enabled a mechanistic understanding of how the fibres alter the sand skeleton stresses and suppress liquefaction development. Liquefaction eventually occurred, or was close to occurring, in the reinforced model at different depths even though the fibres added stresses to the sand skeleton. The earth pressures prevailing in these 1 g shaking table tests were not sufficiently large for optimal interaction between fibres and sand skeleton. The fibres were not sufficiently tensioned as the confining (clamping) stresses provided by the neighbouring sand particles were not large enough. This means that the benefits of this reinforcement technology may be minor in practical situations where the fibre reinforced soil is at shallow depths.

Insight into excess pore pressure generation leading to liquefaction of sand with stress history under saturated and unsaturated conditions

Assessments of the liquefaction resistance of clean sand still involve considerable uncertainties, which are a current research topic in the field of soil liquefaction. The factors considered and discussed in this study include the loading history, degree of saturation, and partial drainage. The effects of each of these factors on pore pressure generation and liquefaction resistance have been studied for decades in the laboratory, and empirical relationships have been derived. In this paper, an attempt is made to explain these effects using the unique index of volumetric strain. A pore pressure generation model is developed which is similar to that of Martin et al. (1975), but based on stress-controlled triaxial tests. The model is verified through comparisons of its results with those of laboratory tests. It is confirmed that the plastic volumetric strain that has accumulated in sand, either by drained or undrained cyclic loading, dominates the increase in the liquefaction resistance of the sand. However, the plastic volumetric strain caused by overconsolidation is less effective in reducing the volumetric strain potential for subsequent cyclic shearing, thus enhancing its resistance to liquefaction. The model provides a better understanding of the physical processes leading to the liquefaction of saturated and unsaturated sand with and without stress history.

On frictional heating and pore pressure generation:Implications for coseismic landslides hypermobility

Pore pressure plays a critical role in the hypermobility mechanism of landslides. In coseismic landslides, earthquake can further influence the variation of pore pressure. Among these factors, the dominant factor to explain the hypermobility mechanism of such coseismic landslides remains unknown. In this paper, considering the seismic loading and thermo-hydro mechanism, a novel FEM-FDM model is proposed to simulate the temperature increase and excessive pore pressure buildup from the initiation of the earthquake to the end of the landslide. The entire runout process of the landslide was simulated by the DAN model. Through the nonlinear finite element method, seismic records from the nearby seismic stations were analyzed to calculate the seismic excess pore pressure. The rising temperature and excess pore pressure in the shear zone were simulated through the seismic-thermo-hydro model. Based on the seismic-thermal-hydro model, the simulated results of the landslide indicated a rapid excess pore pressure increase within the shear zone. While seismic load may weaken the slope materials and induce the landslide, thermal pressurization mechanism can generate much larger excess pore pressure than seismic load does, leading to much lower effective normal stress and shear force resistance in the shear zone during the landslide motion.

Time-Dependent Behavior of Embankment Resting on Soft clay Reinforced with Encased Stone Columns

In this study, the time-dependent behavior of embankment resting on soft clay reinforced with encased stone column (ESC) is examined numerically. A three-dimensional slice model of an embankment resting on reinforced soft clay was developed to examine the influence of the stiffness of encasement material, length of encasement, and floating ESC on the time-dependent response considering various design parameters like generation and dissipation of excess pore pressure, settlement of composite ground, column deformation, and effective stress experienced by columns and surrounding soils. This investigation reveals that the performance of embankment resting on soft clay reinforced with ESC is better than resting on unreinforced soft clay and soft clay reinforced with granular columns without encasement. Apart from strength enhancement, consolidation characteristics of composite ground were also significantly improved by encasing granular columns with geosynthetic material. Moreover, with the increase in the stiffness of encasement, length of ESC, and length of encasement a significant reduction in the generation of excess pore pressure and lateral deformation were noticed. Further, load transfer mechanism, failure pattern, and temporal variation of stress concentration ratio depend on the encasement stiffness, column length, and encasement length.

Insight into the Behavior of a Caisson Anchor under Cyclic Loading in Calcareous Silt

This paper provides insight into the behavior of a stiffened caisson anchor under inclined cyclic loading in calcareous silt. A series of tests was conducted in a beam centrifuge. A monotonic test was first performed, quantifying the pure monotonic capacity, and then four cyclic loading tests varying the mean load, amplitude, and number of cycles. Cyclic soil characterization T-bar tests and caisson tests were linked. Undrained cyclic T-bar tests led to generate excess pore pressure, resulting in degradation of soil strength and stiffness. For partially drained cyclic caisson tests, the excess pore pressure generated during initial undrained monotonic loading experienced partial dissipation. Healing due to consolidation outweighed the damage due to initial pore pressure generation. Postcyclic monotonic capacity was found to be up to 35% higher compared with the pure monotonic capacity unless the anchor failed during cyclic loading. Measured rotation indicated the evolution of anchor failure mechanism. Caisson capacity under inclined loading was presented as a failure envelope, with the effect of cyclic loading accentuated. The contribution of the soil–chain interaction on the caisson capacity was minimal. No trenching was apparent on the soil surface, and no gap was formed around the anchor in the considered centrifuge testing conditions.

Liquefaction potential of reinforced sand with plastic wastes

Granular soil liquefaction, due to developed pore water pressure during undrained cyclic shear of saturated soils, is regarded as a common phenomenon under earthquakes loading. This phenomenon results in the huge damage to infrastructures. Various reinforcement materials have been successfully implemented with particular attention to use waste materials to satisfy design specifications and also reducing the adverse environmental effects. This paper investigates the possibility of using waste plastic fibers as a reinforcement material to mitigate liquefaction potential and pore pressure generation of reinforced sand under cyclic loading. For this purpose, 42 stress-controlled cyclic triaxial tests were conducted on Babolsar sand, reinforced by polyethylene terephthalate (PET) and polypropylene (PP) fibers with fiber contents of 0.25%, 0.5% and 1%, under confining pressures of 50, 100 and 200 kPa, and with cyclic stress ratios (CSR) of 0.2 and 0.35. Results revealed that the addition of these waste plastic fibers could significantly increase the liquefaction resistance of Babolsar sand, and also, with an increase in the waste content, the number of cycles leading to liquefaction increased. Adding wastes decreased the pore water pressure generation, and this effect was more pronounced with an increase in the waste content.

Model-scale tests to examine water pressures acting on potentially buoyant underground structures in clay strata

Throughout the service life, underground structures are subjected to transient and sustained hydrostatic pressures. The reservoir impoundment results in an increase in water level, as well as hydraulic gradient, which can endanger the uplift performance of infrastructure. In uplift design, a reduction factor is often suggested for buoyant force acting on underground structures in clays due to the time lag effect. However, the mechanism of pore pressure generation in clays is not fully understood. This investigation presents a novel U-shaped test chamber to assess the pore pressure generation with time in the horizontal branch subjected to an increase in reservoir level in the left vertical branch. A mathematical model is developed to explain the time lag effect of pore pressure generation. The test program also involves the evaluation of uplift pressure acting on foundation model in the right vertical branch due to adjacent reservoir impoundment. It is found that the time lag effect of pore pressure generation in clays can be observed irrespective of hydraulic gradient, but a higher hydraulic gradient can lead to a faster response in pore pressure sensors. A reduction factor of 0.84–0.87 should be considered to reduce the conservatism of uplift design.

Numerical modeling of contaminant advection impact on hydrodynamic diffusion in a deformable medium

The self-weight of solid waste or machine-rolled compaction can induce or trigger contaminant migration in the landfill. Although the consolidation-induced hydraulic gradient driving solution transport has been extensively investigated, little attention has been paid to ion migration caused by its concentration gradient variation. It is necessary to more precisely predict the multi-stage contaminant transports in deforming porous material. Based on the modified Cam-clay model, the proposed fluid-solid coupled model can simulate the elastoplastic deformation behavior of layered kaolinite and KBr solution transport/sorption, and its modeling results were validated by published laboratory data. The solid-fluid interactions were analyzed by comparing various transport manners of K + and Br − from excess pore pressure generation to dissipation. Results reveal that the consolidation process can accelerate KBr solute advection from the contaminated layer into the uncontaminated layer, and then affects the subsequent diffusion, mechanical dispersion and sorption for K + and Br − . The simulations also indicate that consolidation-induced solute transport is time-dependent, and therefore the ion diffusion and mechanical dispersion should receive more attention.

Effect of hydraulic conductivity and impeded drainage on the liquefaction potential of gravelly soils

Investigating the role of sand and fines content and in situ drainage conditions in governing the hydraulic conductivity of gravelly deposits is highly important to characterize the liquefaction potential of gravelly soil. In this study, a variation of hydraulic conductivity with sand content has been empirically obtained based on the existing gravel liquefaction case histories. It is found that the hydraulic conductivity of a soil matrix having more than about 20%–30% sand content by mass is low enough to cause liquefaction without the presence of any impervious confining layer. In addition, a numerical study has been performed using the commercial software FEQDrain to study pore pressure generation in gravelly soil at a variety of relative densities and hydraulic conductivities with and without an impermeable cap layer when subjected to a variety of earthquake loadings. For both unconfined and confined condition, excess pore pressure ratios consistently increase with a decrease in hydraulic conductivity ( k) and relative density ( Dr). Excess pore pressure ratio is correlated with hydraulic conductivity, soil compressibility, and cyclic stress ratio (CSR). For the confined condition, pore pressure in the gravel layer is primarily governed by the overlying cap layer and even a sandy cap layer instead of highly impervious clay layer can cause liquefaction.

A viscoplastic recoverable sensitivity model for fine-grained soils

Full-flow penetrometer testing in fine-grained soils indicates that undrained shear strength decreases due to remoulding induced generation of excess pore pressures and increases due to strain rate effects and dissipation of excess pore pressures. This paper presents a viscoplastic strain-softening–hardening constitutive model that captures this duality of behaviour via non-local regularisation, strain rate dependency, and consolidation induced recovery of sensitivity. The model is based on Structured Modified Cam Clay and Perzyna’s overstress framework. Validation of the implementation is first demonstrated through simulation of constant rate of strain triaxial and oedometer compression tests performed on different clays. The model is then applied in large deformation finite element analyses of monotonic and variable rate T-bar penetration to demonstrate successful numerical implementation and its ability to simulate strain rate effects. Finally, the model is applied in the simulation of undrained cycles of penetration of a T-bar penetrometer in kaolin clay and a carbonate silt and compared with data from geotechnical centrifuge experiments. The results show that the model without sensitivity recovery significantly underestimates the consolidated-undrained resistance in both soils, suggesting that recoverable sensitivity is an aspect of behaviour that ought to be considered in the development of constitutive models for soft sensitive soils.

Rock Deformation Estimated by Groundwater-Level Monitoring: A Case Study at the Xianshuihe Fault, China

Rock deformations induced by active faults is an important topic in earthquake studies. Such deformations are usually measured with crossfault measurements (CFM), which are time-consuming and labor-intensive. In this study, rock deformations induced by the famous Xianshuihe fault in Xialatuo of China were estimated by groundwater-level monitoring (GLM) and CFM for the period of January 1, 2016 to December 31, 2018. The pattern of the variations in areal strain estimated with GLM matches that from CFM well. The estimated strain by the GLM and CFM both changed from positive to negative with time, indicating that the fault plane switched from tensile to compressive. This indicates that the rate of rock deformation had slowed down during this period, which is consistent with the long-term creep rates obtained by CFM at the site, implying that the fault may have gradually entered the next relock state. The estimated strain changes using the GLM method lag slightly behind those of CFM, which is probably due to the diffusive effects of pore pressure propagation that is caused by the rock deformation under the crustal stress. This study demonstrates that GLM is a more convenient and efficient addition to traditional geophysical techniques and raises the possibility for the characterization of continuous rock deformations. The method may be used to obtain the changing regional strain field with a network of monitoring wells.

Laboratory investigation of geogrout inclusion: the influence of the substitution ratio

Laboratory investigations using balloon tests were performed on normally consolidated kaolin samples to investigate their behaviour during and after the balloon expansion. The tests were carried out to simulate a soft soil improvement technique called CPR grouting (acronym for deep radial consolidation). The results allowed estimations of the parameters used in the design method, such as the consolidation potential, and evaluating the generation of pore pressures, injection pressures, post improvement deformation and optimised expanded volume. It was observed that the consolidation potential was extremely dependent on the substitution ratio. The tests satisfactorily reproduced the behaviour of the technique and represent a new device for geotechnical engineers, especially for those concerned with the improvement of soft soils.

Perceiving moisture damage of asphalt mixes containing RAP using survival analysis based on Kaplan-Meier estimator and Cox proportional hazards model

Moisture induced stress tester (MIST) has been recently developed which aims to simulate stripping due to repeated pore pressure generation. In the current work, asphalt mixes containing recycled asphalt pavement (RAP) (in proportion of 0, 10, 20, 30 and 40% by weight) were subjected to varying stress cycles of MIST (0, 1000, 2000, 3500, 5000, 10,000 cycles) and indirect tensile (IDT) strength test was conducted on such mixes. AC30 was used for control mix whereas AC10 was adopted for RAP mixes. The results of IDT strength tests assisted in computing the failure times and such times were analysed using survival analysis based on Kaplan Meier (KM) estimator and Cox Proportional Hazards (CPH) model. The results indicated that survival analysis can be effectively utilised to analyse the IDT strength test results. It was observed that increasing stress cycles of MIST increase the time to failure whereas RAP is expected to reduce the time to failure. Moreover, KM estimator indicated that RAP reduces the survival of asphalt mixes. Hazard ratio (HR^) computed using CPH model shows a decrease with the increase in RAP proportion. Both MIST tensile strength ratio (M−TSR) and HR^ revealed that RAP leads to an improvement in moisture damage resistivity of asphalt mixes. Hence, HR^ has the potential to be an effective indicator of moisture damage susceptibility of asphalt mixes. Also, “Good” and negative correlation was observed to exist between M−TSR and HR^. The current study is expected to further enhance our understanding regarding moisture induced damage of asphalt mixes and assist in effectively screening good mixes from poor ones.

Fully coupled thermal-hydro-mechanical model of pore pressure propagation around borehole with dynamic mudcake growth

Pore pressure is a significant parameter in drilling engineering. A better understanding of near-wellbore pore pressure propagation by taking into account the thermal-hydro-mechanical coupling process and dynamic mudcake growth is beneficial for wellbore stability analysis, formation testing while drilling and well logging while drilling. To reveal the pore pressure evolution mechanism, a new fully coupled thermal-hydro-mechanical model of pore pressure propagation around a borehole with dynamic mudcake growth is proposed. A comparison with the conventional approach suggests that neglecting the dynamic mudcake effect or the coupling effect can overestimate the pore pressure in the vicinity of borehole. The variation process of pore pressure can be divided into four stages, and the coupling effect weakens as time progresses. Compared with the Young's modulus of the rock, formation porosity, and temperature difference, the near-wellbore pore pressure is more sensitive to the overbalanced pressure, in situ stresses, formation permeability, and the Poisson's ratio of the rock. The near-wellbore pore pressure disturbance in formations with a low permeability and high in situ stress is more significant; therefore, to consider the effects of thermo-hydro-mechanical coupling and dynamic mudcake growth in deep and ultra-deep well drilling is required.

Static liquefaction mechanisms in loose sand fill slopes

To investigate potential triggering and failure mechanisms of loose fill slopes, two centrifuge model tests were conducted on loose sand with a contractive tendency andhigh liquefaction potential. The behaviour ofloose sand fill slopes subjected to rising groundwater or rainfall was simulated using a geotechnical centrifuge. Moreover, a three-dimensional back-analysis and parametric study were conducted to investigate the role of soil nails in preventing the triggering of static liquefaction in loose fill slopes. Contrary to the rainfall test where only excessive settlements are observed, arapid, and brittle fluidised flowslide in the slope can be identified whensubjected to rising groundwater. The fluidised flowslide is triggered by a localised drained surface failure in the form of a gully at the crest of the slope. The initial failure at the crest induces the generation of significant excess pore pressures within the slope, causing a significant reduction in effective stress andloss of shear strength due to static liquefaction. For an identical slope, numerical simulations indicate that a fluidised flowslide can be prevented by soil nailing. Soil nailing can prevent the peak shear strength of the soil from being mobilised and limits the excessive strains required to trigger static liquefaction.