Showing papers on "Pore water pressure published in 2019"

Effect of aggregate size and inclusion of polypropylene and steel fibers on explosive spalling and pore pressure in ultra-high-performance concrete (UHPC) at elevated temperature

Abstract: This paper investigates the individual and combined effects of polypropylene (PP) fibers, steel fibers, and aggregate size on spalling behavior and pore pressure build-up of ultra-high-performance concrete (UHPC) exposed to elevated temperature. Simultaneous measurements of pore pressure and temperature were conducted at different depths in UHPC specimens under one-sided heating with a heating rate of 2 °C/min. Compressive, tensile, and permeability tests were performed to analyze spalling behavior. Addition of PP fibers fully prevented spalling and they are much more effective in increasing permeability than steel fibers and larger aggregates. The combined use of PP and steel fibers, and PP fibers and larger aggregates showed strong synergistic effect on increasing permeability. The higher the permeability, the lower was the maximum pore pressure measured in the samples. Two plateaus were observed from the temperature history due to vaporization of liquid water (between 115 and 125 °C inside the specimens) and release of water vapor (starting from 180 °C), respectively. The second plateau was identified as the functional temperature of PP fibers. Maximum pore pressures in spalled specimens were much lower than their tensile strengths, which could imply the contribution of hydraulic pressure in the region of moisture clog on spalling.

Water-Weakening Effects on the Mechanical Behavior of Different Rock Types: Phenomena and Mechanisms

Abstract: The presence of water strongly affects rock properties and would be related to a series of geological disasters. To understand water saturation effects on the mechanical behavior of different rock types and interpret the underlying mechanisms of differences in water sensitivity, three kinds of rocks, namely sandstone, granite and marble, were selected for tests. Uniaxial compression experiments were conducted on specimens under oven-dried and water-saturated conditions. Acoustic emission (AE) techniques were also applied to monitor and record AE signals during tests. Experimental results reveal that water weakens the mechanical parameters of the three tested rocks, such as uniaxial compressive strength (UCS), elastic modulus and critical strain. The sandstone undergoes the greatest weakening with the addition of pore water, the mechanical properties of the granite exhibit relatively minor reductions, while the marble is the least affected by water saturation. The water-weakening degree of rock properties depends on the porosity as well as the mineralogy, especially the proportion of quartz and swelling clays. Moreover, after water saturation, the failure pattern of the sandstone and the granite tends to transform into the shear-dominant mode from the tensile one in dry state, probably due to frictional reduction. However, the water presence does not change the failure mode of the marble.

Hydrothermal alteration of andesitic lava domes can lead to explosive volcanic behaviour

Abstract: Dome-forming volcanoes are among the most hazardous volcanoes on Earth. Magmatic outgassing can be hindered if the permeability of a lava dome is reduced, promoting pore pressure augmentation and explosive behaviour. Laboratory data show that acid-sulphate alteration, common to volcanoes worldwide, can reduce the permeability on the sample lengthscale by up to four orders of magnitude and is the result of pore- and microfracture-filling mineral precipitation. Calculations using these data demonstrate that intense alteration can reduce the equivalent permeability of a dome by two orders of magnitude, which we show using numerical modelling to be sufficient to increase pore pressure. The fragmentation criterion shows that the predicted pore pressure increase is capable of fragmenting the majority of dome-forming materials, thus promoting explosive volcanism. It is crucial that hydrothermal alteration, which develops over months to years, is monitored at dome-forming volcanoes and is incorporated into real-time hazard assessments.

Soil Sorptive Potential: Concept, Theory, and Verification

Abstract: Despite the widely accepted notion that water potential is the fundamental variable for describing soil–water interactions in soil under unsaturated conditions, it is unable to describe sev...

Induced seismicity in geologic carbon storage

Abstract: . Geologic carbon storage, as well as other geo-energy applications, such as geothermal energy, seasonal natural gas storage and subsurface energy storage imply fluid injection and/or extraction that causes changes in rock stress field and may induce (micro)seismicity. If felt, seismicity has a negative effect on public perception and may jeopardize wellbore stability and damage infrastructure. Thus, induced earthquakes should be minimized to successfully deploy geo-energies. However, numerous processes may trigger induced seismicity, which contribute to making it complex and translates into a limited forecast ability of current predictive models. We review the triggering mechanisms of induced seismicity. Specifically, we analyze (1) the impact of pore pressure evolution and the effect that properties of the injected fluid have on fracture and/or fault stability; (2) non-isothermal effects caused by the fact that the injected fluid usually reaches the injection formation at a lower temperature than that of the rock, inducing rock contraction, thermal stress reduction and stress redistribution around the cooled region; (3) local stress changes induced when low-permeability faults cross the injection formation, which may reduce their stability and eventually cause fault reactivation; (4) stress transfer caused by seismic or aseismic slip; and (5) geochemical effects, which may be especially relevant in carbonate-containing formations. We also review characterization techniques developed by the authors to reduce the uncertainty in rock properties and subsurface heterogeneity both for the screening of injection sites and for the operation of projects. Based on the review, we propose a methodology based on proper site characterization, monitoring and pressure management to minimize induced seismicity.

Coseismic Groundwater Drawdown Along Crustal Ruptures During the 2016 Mw 7.0 Kumamoto Earthquake

Abstract: Author(s): Hosono, T; Yamada, C; Shibata, T; Tawara, Y; Wang, CY; Manga, M; Rahman, ATMS; Shimada, J | Abstract: Groundwater-level changes after earthquakes provide insight into changes in hydrogeological properties such as permeability and pore pressure. Quantifying such changes, both their location and magnitude, is usually hindered by limited data. Using extensive high-resolution water-level monitoring records, we provide direct evidence of significant groundwater drawdown (4.74-m maximum) over a 160-km2 area along crustal ruptures after the Mw 7.0, 2016, Kumamoto earthquake. Approximately 106 m3 of water disappeared within 35 min after the main shock. The loss of water was not caused by static-strain driven pore-pressure decrease nor by releasing of water through structural pathways, but most likely by water transfer downwards through open cracks. Such changes may impact the security of water resources, the safety of underground waste repositories, and contaminant transport in seismically active areas.

Controlling Effects of Residual Deformation on Pore Pressure: A Loess Soil Case

Abstract: What the role of each phase medium plays and how their interactions do work should be essential problems to understand dynamic behaviours of soils. In order to disclose interactions between solid, water, and air phases of soils, we applied loess samples to analyse controlling effects of residual deformation on pore pressure based on three kinds of laboratory tests. We obtained the similarity and difference of mechanical behaviors of soil samples under different water contents and loading. Both process and cause of pore air/water pressures are independent of initial stress conditions or loadings. However, absolute values of pore water pressure depend on the confining pressure, whereas the pore air pressure is contrary. The uniformity of responding process and cause of pore pressure depend upon the interaction mechanism between solid particles and air/water media, but the different absolute values depend upon the permeability and compressibility of air/water.

Mechanisms of consistently disjunct soil water pools over (pore) space and time

Abstract: . The storage and release of water in soils is critical for sustaining plant transpiration and groundwater recharge. However, how much subsurface mixing of water occurs, and how much of the water is available for plants or otherwise percolates to streams and the groundwater is not yet understood. Based on stable isotope ( 2H and 18O ) data, some studies have found that water infiltrating into soils can bypass older pore water. However, the mechanisms leading to the separation of water routed to the streams and water held tightly in smaller pores are still unclear. Here, we address the current limitations of the understanding of subsurface mixing and their consequences regarding the application of stable isotopes in ecohydrological studies. We present an extensive data set, for which we sampled the isotopic composition of mobile and bulk soil water in parallel with groundwater at a fortnightly temporal resolution and stream water and rainfall at a much higher resolution in a Mediterranean long-term research catchment, in Vallcebre, Spain. The data reveal that the mobile and tightly bound water of a silty loam soil in a Scots pine forest do not mix well; however, they constitute two disjunct subsurface water pools with little exchange, despite intense rainfall events leading to high soil wetness. We show that the isotopic compartmentalization results from the rewetting of small soil pores by isotopically depleted winter/spring rain. Thus, stable isotopes, and, in turn, water residence times, do not only vary across soil depth, but also across soil pores. Our findings have important implications for stable isotope applications in ecohydrological studies assessing the water uptake by plants or the process realism of hydrological models, as the observed processes are currently rarely implemented in the simulation of water partitioning into evapotranspiration and recharge in the critical zone.

Reliability analysis of unsaturated soil slope stability under infiltration considering hydraulic and shear strength parameters

Abstract: In general, soil properties, including shear strength and hydraulic parameters, are characterised as a spatial variability. This paper aims to investigate the effect of spatial variability of the soil properties on slope stability during rainfall infiltration. The effective friction angle, saturated hydraulic conductivity, and soil water characteristic curve parameters of sand are simulated using random field theory. A seepage analysis is conducted using the random finite element method to obtain pore water pressure distribution. A stability analysis is performed to show the variation of safety factors and failure probability. The results show that the random field of the soil-water characteristic curve produces a significant variation of pore water pressure, while the random field of the effective friction angle is the most important parameter for probabilistic stability analysis.

Mechanisms of wetting-induced loess slope failures

Abstract: Frequent occurrence of landslides induced by rainfall or irrigation has seriously threatened the urban and rural development in the Loess Plateau, China. The increase in pore water pressure has been identified as a key factor for understanding wetting-induced loess slopes failures. However, experimental studies are limited regarding the increase in pore water pressure of the collapse loess from an initial negative value until failure occurs under constant total stress condition. An old landslide with progressive retreat development at an early stage of the construction of the Lvliang Airport was selected as a case study. Field surveys including exploration wells and boreholes revealed very fresh sliding shear planes and clearly visible cracks, suggesting the creeping movement of the old landslide. In case of heavy rain or long-term rainfall, this old landslide may be resurrected, threatening the stability of the airport site. To examine the mechanism of the failure induced by wetting for this unsaturated loess landslide, loess specimens were taken from the field, followed by performing a series of laboratory tests, including triaxial shear tests at constant matric suctions and wetting tests at constant deviator stresses. The test results revealed that the wetting-induced deformations of the loess included volume and shear deformations, reflecting compression and shearing behaviour induced by wetting. The failure behaviour of the loess along a wetting path was dependent on the stress level and the loss degree of matric suction as well as the hydro-mechanical path, and could be well described by the linear form of the Mohr-Coulomb strength theory. On this basis, the threshold value of the stress level was identified, which could be used to judge whether the wetting-induced failure of the loess occurs. The threshold value of matric suction at failure was also identified to analyse the loss degree of matric suction from stable conditions to failure. The mechanism of the failure of the soil due to wetting revealed from the present study could interpret the rainfall-induced landslide in unsaturated loess.

New Model for Pore Pressure Prediction While Drilling Using Artificial Neural Networks

Abstract: Pore pressure is one of the main formation conditions that affects the efficiency of drilling operations and impacts its cost. Accurate prediction of the pore pressure and the parameters controlling it will help reduce the drilling cost and avoid in some cases catastrophic accidents. Many empirical models reported in the literature were used to predict the pore pressure based on either drilling parameters or log data. Empirical models require trends such as normal or abnormal pressure to predict the pore pressure. Few researchers applied artificial intelligence (AI) techniques to predict the pore pressure using one or maximum two AI methods (which are black box). There is no developed empirical correlation for pore pressure prediction based on optimized AI techniques. The objective of this paper is to predict the pore pressure based on both drilling parameters and log data, namely weight on bit (WOB), rotary speed (RPM), rate of penetration (ROP), mud weight (MW), bulk density (RHOB), porosity ( $$\phi $$ ), and compressional time ( $$\Delta {t}$$ ). Real field data will be used to predict the pore pressure using artificial neural network (ANN). Finally, for the first time, a new empirical correlation for pore pressure prediction will be developed based on the optimized ANN model. The obtained results showed that it is very important to combine the drilling parameters and the logging data to predict the pore pressure with a high accuracy. A new empirical correlation was developed using the optimized ANN method that can estimate pore pressure with high accuracy (correlation coefficient of 0.998 and average absolute percentage error of 0.17%). Unlike the published empirical models, the new model requires no prior pressure trends (such as normal or abnormal pressures) to perform prediction.

Influence of initial state and intermediate principal stress on undrained behavior of soft clay during pure principal stress rotation

Abstract: It is important to be fully aware of the dynamic characteristics of saturated soft clays under complex loading conditions in practice. In this paper, a series of undrained tests for soft clay consolidated with different initial major principal stress direction ξ were conducted by a hollow cylinder apparatus (HCA). The clay samples were subjected to pure principal stress rotation as the magnitudes of the mean total stress p, intermediate principal stress coefficient b, and deviator stress q were all maintained constant. The influences of intermediate principal stress coefficient and initial major principal stress direction on the variation of strain components, generation of pore water pressure, cyclic degradation and non-coaxiality were investigated. The experimental observations indicated that the strain components of specimen were affected by both intermediate principal stress coefficient and initial major principal stress direction. The generation of the pore water pressure was significantly influenced by intermediate principal stress coefficient. However, the generation of pore water pressure was merely influenced by initial major principal stress direction when b = 0.5. It was also noted that the torsional stress–strain relationships were affected by the number of cycles, and the effect of intermediate principal stress coefficient and initial major principal stress direction on the torsional stress–strain loops were also significant. Stiffness degradation occur under pure principal stress rotation. Anisotropic behavior resulting from the process of inclined consolidation have considerable effects on the strain components and non-coaxial behavior of soft clay.

Estimation of Pore Pressure and Fracture Gradient in Volve Field, Norwegian North Sea

Abstract: Maintaining a stable borehole and optimizing drilling are still considered to be vital practice for the success of any hydrocarbon field development and planning. The present study deliberates a case study on the estimation of pore pressure and fracture gradient for the recently decommissioned Volve oil field at the North Sea. High resolution geophysical logs drilled through the reservoir formation of the studied field have been used to estimate the overburden, pore pressure, and fracture pressure. The well-known Eaton’s method and Matthews-Kelly’s tools were used for the estimation of pore pressure and fracture gradient, respectively. Estimated outputs were calibrated and validated with the available direct downhole measurements (formation pressure measurements, LOT/FIT). Further, shear failure gradient has been calculated using Mohr-Coulomb rock failure criterion to understand the wellbore stability issues in the studied field. Largely, the pore pressure in the reservoir formation is hydrostatic in nature, except the lower Cretaceous to upper Jurassic shales, which were found to be associated with mild overpressure regimes. This study is an attempt to assess the in-situ stress system of the Volve field if CO2 is injected for geological storage in near future.

Source discrimination of mine water inrush using multiple methods: a case study from the Beiyangzhuang Mine, Northern China

Abstract: The problem of distinguishing the source of water inrush in mines and tunnels has been addressed by studying the specific case of significant water inrush along the haulage roadway of the Beiyangzhuang Mine and applying three different methods to determine the source of the water inrush from a range of angles. The first of these methods was to determine the source by analyzing the dynamic response law of the groundwater in the water filling aquifers, including a Quaternary porous aquifer and a Cambrian–Ordovician karst aquifer. The second was to establish a linear equation for stratum burial depth and ground temperature to calculate water temperature. The source of water inrush is identified by comparing the calculated water temperature for the filling aquifer and the measured water temperature at the water inrush point. The third was to analyze the hydrochemical types of the water filling aquifers and water inrush point samples using a Piper diagram, followed by Fisher discriminant analysis to discriminate water inrush sources with eight hydrochemical components; the mixture ratio is roughly evaluated based on chloride mass balance. These three methods consistently showed that the primary source of water inrush is karst water. The hydrogeochemistry discrimination analysis further indicated that the mixing ratio of karst water to pore water was about 6.0, suggesting that this method is the powerful and more practical of the three methods tested. The results presented here provide significant guidance for the management of mine water inrush.