Pore Pressure Measurements Research Articles

Thermomechanical properties of sand–structure interface using temperature-controlled triaxial instrument

To study the effect of temperature on the mechanical properties of a sand–structure interface, a temperature-controlled interface shear test system based on a triaxial instrument was developed by placing a cylindrical structure inside a triaxial soil sample and connecting them to two individual thermostat water baths. Sand–aluminum interface shear tests under effective cell pressures of 50, 100, and 200 kPa and temperatures of 6, 19, 32, and 45 °C were carried out to evaluate the thermal effects on the interface strength under drained conditions. Constant vertical force–stiffness coupled with a thermal load applied to the aluminum top before the interface shearing was also considered. The results show that the new apparatus can serve a variety of thermomechanical load paths, including maintaining a constant rate of displacement and a constant vertical force–stiffness on the structure top prior to the shearing process. The apparatus also has advantages over other interface shear devices in controlling drainage conditions and measurement of volume change and excess pore pressure. A comparison with the previous literature on interface shear strength verifies the results in this study. Different thermomechanical load paths applied on the interface supplement the insignificant thermal effect on the sand–aluminum interface shear strength. However, it is the first time that the displacement of the aluminum is observed to be related to thermal variations and independent of the magnitude of the vertical force.

Significance of coarse clasts in viscous debris flows

Debris flows are flowing mixture of water and sediments (from clay to boulder-sized particles). The sediments control both occurrence and characteristics of debris flows, however, the relevant literature has concentrated mainly on fine debris. Considering their large fractions in debris flows, the significance of coarse clasts (≥0.005 mm) is investigated by comparing the crude debris (CD, the fractions of <10 mm and <5 mm of the Jiangjia Gully debris-flow deposit) with the reconstituted debris (RD, prepared by replacing the coarse clasts of the CD with the corresponding sizes of the Dongyuege debris-flow clasts following the gradation of the CD). Rheometrical tests, propagation tests, and pore-pressure measurements are made on the CD and RD samples in a systematic manner.Aggregation of data collected in 39 tested debris-water mixtures leads us to conclude that the coarse clasts can greatly affect the slurrying susceptibility of debris masses and bulk behavior of slurries. The debris-flow indices (Id) from the CD to RD slurries with maximum grain sizes of 10 and 5 mm decrease by 29% and 17%, respectively. The CD slurries possess higher yield strength and viscosity than their RD counterparts with the same solid volumetric concentrations (Cvs). The areas inundated by 10 CD slurries with maximum grain sizes of 10 and 5 mm are approximately 17%–39% of those of their RD counterparts. The excess pore pressures in the RD slurries dissipate more quickly than those in the CD slurries. The shape and surface texture of coarse clasts regulate slurrying of debris masses, as well as influence their liquefaction potential, which in turn has an impact on the mobility and persistence of subsequent slurries.The platy/flaky debris-producing mountain catchments generally experience lower-magnitude/higher-frequency, lower Cv debris flows of relatively less destructive power, and the debris flows dominated by equidimensional clasts are often more powerful and hazardous, despite their potentially lower frequency.

Failure Mechanism of a Granular Bed Induced by a Horizontal Water Jet Using Particle Image Velocimetry

Particle image velocimetry (PIV) was used to investigate the failure mechanism of a granular bed subjected to a water jet emerging laterally from a vertical, impermeable face. Experiments were performed in a modified seepage tank made of plexiglass with dimensions of 700 × 600 × 103 mm. The bed of particles was subjected to a lateral seepage flow at a controlled rate, and the motion of the particles was recorded using a digital camera. Measurements of flow and pore pressure were also made. Several stages of failure were observed during the repeated experiments: 1) outwards movement of the particles (horizontally and vertically upwards), 2) cavity formation, 3) cavity enlargement, and 4) the emergence of a fluidised zone at the bed surface. Trends relating to the flow rate of fluid flow to the onset of failure were also noted. The PIV analyses revealed the movement of sand grains and boundaries between regions in which sand grains either did or did not move. An analytical model based on the Ergun equation is presented to predict the onset of failure. This study provides valuable insights into the failure mechanism of granular materials triggered by lateral seepage flow.

Shallow penetrometer tests: theoretical and experimental modelling of the rotation stage

Shallow penetrometers are a new type of device that measures the properties of surficial offshore sediments via multi-phase tests involving penetration, dissipation, and rotation stages. In fine-grained soils such as silts and clays, these testing stages yield properties relevant to subsea pipeline and shallow foundation design; namely, undrained strength, consolidation, and interface friction. This paper describes the fundamentals of the rotation stage, including models required for data interpretation, encompassing both a total and an effective stress framework. Additionally, new relationships to evaluate the pore pressure scaling factor, which is a key interpretation parameter required to convert discrete measurements of pore pressure on the penetrometers to an average pore pressure over the contact area, are developed based on large-deformation finite element simulations. Results from an experimental campaign using kaolin clay samples are presented, illustrating the potential of the devices to rapidly and repeatably measure interface friction properties of fine-grained sediments offshore. The results compare well with comparative measures obtained from shear box tests conducted at similarly low effective stress levels. Recommendations regarding future in situ applications are given at end of the paper.

Ground response during offshore pile driving in a sandy slope

This paper presents an analysis of field measurements obtained during pile installation tests in a submerged sandy slope. The field test was performed in 2016 as part of the construction of a new sea lock in the Netherlands. Ground vibrations and excess pore pressures were to be limited in order to prevent damage to neighbouring locks and maintain slope stability. Geophones and piezometers were installed in the slope, at depth, and at various lateral distances from the pile axis. In the analysis, focus is placed on the installation of a tubular steel pile, which was subjected to both vibratory and impact driving. An assessment is made of some factors, such as driving equipment and pile tip penetration depth, affecting both the magnitude of ground vibration and its spatial and temporal characteristics. A similar analysis is conducted of the pore pressure measurements, with particular attention paid to the part of driving carried out in a homogeneous sand layer. Measured vibrations and excess pore pressures are compared to established attenuation relationships and published data, respectively. An approach is suggested in which the test results and interpretation may be used in assessing the potential implications of excess pore pressure development in the sand layer for slope stability.

Spatial variability of medium dense sand deposit

This paper presents a study of the spatial variability of the cone resistance in a medium dense sand deposit in Norway. Spatial variability studies have been done earlier for clay deposits, but there are few results available for sands. To bridge this gap and to compare the spatial variability of sand and clay, the in situ cone resistance measured with the piezocone at the Oysand benchmark site near Trondheim was studied. Corrected cone resistances qt derived from the measured cone resistance and pore pressure measurements were used to establish the autocorrelation structure of the Oysand deposit. The qt data were examined visually and conclusions were drawn on the data that should be analyzed. A depth interval of 7 to 12 m was selected for spatial variability further analysis. The scale of fluctuation was identified. Several autocorrelation functions were considered, and the single exponential function was found to be the one offering the best autocorrelation. The spatial variability in the vertical and horizontal directions was analyzed statistically, using three different approaches, the auto-correlation fitting, maximum likelihood estimation and simplified Vanmarcke method. The results indicate short autocorrelation distances of 3 m or less in the horizontal direction, suggesting a very variable sand at the Oysand site. These horizontal autocorrelation distances are much shorter than those obtained for clays. In the vertical direction the scale of fluctuation was less than one meter, as obtained for other soils.

Geopressure evaluation using integrated basin modelling, well-logging and reservoir data analysis in the northern part of the Badri oil field, Gulf of Suez, Egypt

The evaluation of pore and fracture pressures represents the gist of well planning and execution in drilling operation and exploration, where accurate pore and fracture pressures modelling enables the oil and gas companies to drill targets safely. The scope of this paper is to evaluate pore and fracture pressures in the northern part of the Badri Field, in the Gulf of Suez, Egypt to determine the convenient pore pressure fracture gradient model (PPFG) and to define the pore pressure regime in the subsurface within the Zeit, South Gharib, Belayim and Kareem formations from the top downwards.The pore pressure and fracture gradient model has been assessed using integrated data that include: burial history, well logs (sonic, resistivity and density), offset wells drilling problems, and reservoir pressure. The Amoco overburden equation and Eaton's sonic and resistivity methods were used to estimate the overburden stress and the pore pressure values respectively, while fracture pressure was calculated using the Eaton method. Results show that the maximum pore pressures is in the South Gharib Formation and the lowest pore pressure is within the reservoir sections. Six abnormal pressure zones were detected, these includes four overpressure zones and two sub-normal pressure zones, where the maximum estimated pore pressure was up to 10.5 pound per gallon (ppg) equivalent gradient, which was recorded in the South Gharib Formation. While the minimum measured pore pressure value was recorded at 4.7 (ppg) in the Kareem sandstone reservoir. The fracture gradient ranged between 11.5 and 13.1 (ppg) equivalent density for both the Belayim (Hammam Faraun Member) and Kareem sandstone reservoirs.The main conclusions of this study are, 1) the pore pressure profile is normal, sub-normal and over-pressurized in the study area, 2) the integration of basin modelling, well-logging, drilling problems and direct measurements allow us to evaluate the geopressures in the subsurface horizons, 3) the use of low mud density while drilling into the depleted reservoir zones is the best solution for fluid losses mitigation, 4) the modified surface, intermediate and production casing seats is adequate for the future planned wells profile. 5) providing insights on downhole pressure behavior across stratigraphy in the area of study, to achieve optimum drilling mud designing and adequate casing seats as well as safe and successful operational planning.

Pore pressure prediction in orthotropic medium based on rock physics modeling of shale gas

Pore pressure prediction is critical to reducing drilling costs and exploring hydrocarbons. Current seismic approaches for pore pressure prediction mainly initially establish the relation between vertical effective stress and physical parameters (velocity, resistivity or porosity), and then predict pore pressures with the known effective stress theory. The commonly used Eaton's equation requires the estimation of normal compaction trend (NCT), which cannot be measured but only be inferred. However, the conventional NCT is a curve which is able to describe the normal trend but is relatively rough, limiting the accuracy of pore pressure prediction. In this study, the shale rock physics modeling in orthotropic (OA) medium is designed to equate the NCT. The modeled and the measured P-wave velocity match well in normal pore pressure segments, but they differ in abnormal pore pressure segments. The modeled NCT is utilized to predict pore pressures, and the comparison with the measured pore pressures proves that the accuracy can be improved. Then P-wave velocity is inverted from azimuthal seismic data based on the reflectivity approximation of OA medium, and the pore pressure of the research area is predicted using the modeled P-wave velocity (the target play is a shale gas field in Sichuan basin of China). The prediction result is consistent with the characteristics of overpressure mechanism of hydrocarbon generation, indicating the effectiveness of rock physics modeling in pore pressure prediction.

Dynamic Consolidation Measurements in a Well Field Using Fiber Bragg Grating Sensors.

Currently available groundwater flow prediction tools and methods are limited by insufficient spatial resolution of subsurface data and the unknown local heterogeneity. In this field study, fiber Bragg grating (FBG) sensors were installed in an extraction well field to investigate its potential to measure groundwater flow velocity. Reference in-situ pore pressure and temperature measurements were used to identify possible sources of FBG responses. FBG strain sensors were able to detect soil consolidation caused by groundwater extraction from 250 m distance. The results show that FBG responses were influenced by interface friction between soil and FBG packaging. FBG packaging slipped in soil and the effect was more pronounced during higher groundwater flow around a nearby well. These FBG fibers could be applied for indirect flow monitoring that does not require any tracer and provide real-time and long-term data during regular operation of extraction wells.

On the role of pore pressure in dynamic instabilities of saturated model granular materials

Drained triaxial axisymmetric isotropic and subsequently shear compression tests are performed on fully water-saturated short cylindrical samples of nearly monodisperse glass beads, initially assembled in a loose state by moist tamping and undercompaction technique. Both fully drained isotropic and shear tests are affected by dynamic instabilities triggered under uncontrolled isotropic or deviatoric stress. These dynamic instabilities encompass isotropic collapses, compression stick-slips and especially the unexpected isotropic liquefaction. The most reasonable but imprecise scenario points to a possible dynamical collapse of the granular structure triggered by a spontaneous failure of the force chains, generating an instantaneous surge of excess pore pressure in a fully saturated system. This paper aims in exploring the key role of pore pressure outburst on some global macroscopic parameters of these dynamic events. It focuses on the unforeseen, short-lived and stabilised excess dynamic pore pressure and proposes an in-depth assessment of the validity and the reliability of pore pressure measurements. Irrespective of loading condition (i.e. isotropic or drained compression shear) the ephemeral stabilised excess dynamic pore pressure controls the evolution of the axial and volumetric strains in an essentially dynamic consolidation process. Some appearance conditions for these unexpected dynamic instabilities are discussed.

Design and Evaluation of a Deformable Sensor for Interstitial Pore Pressure Measurement in Concrete under Very High Stress Level

Previous studies have shown the strong influence of free water saturation ratio on the triaxial behavior of concrete at high confinement. This influence of the free water is usually attributed to a pore pressure effect. This article presents an experimental method aiming at measuring the pore pressure of free water into concrete samples under very high mean stress. Two types of deformable pressure sensors were designed and tested. The first one works in hydrostatic compression while the second one acts as a flexible membrane. The two sensors give comparable results and show that pore pressure may reach several hundred MPa in saturated concrete sample under a maximum 400 MPa hydrostatic compression. Such levels of pressure may explain the loss of shear strength and increase in volumetric stiffness observed on the macroscopic behavior of concrete due to the presence of free water.

Load-indentation testing to assess porewater pressure effects on linear drag and disc cutting of a saturated permeable brittle rock

In this study load-indentation tests with porewater pressure measurement were conducted to explore the unexpected differences owing to water saturation in the linear cutting of a permeable quartzose sandstone using drag and disc type cutters. This was done to evaluate the probable role of porewater pressure in rock fragmentation by drag and disc type cutters. The contrasting response to water saturation of these two types of cutters might be explained by the effects of their different fragmentation mechanisms, such as the relative size of the crushed zone that forms beneath the cutters. The relationship between cutting speed and rock permeability was expected to be a major factor influencing the effective pressure beneath a cutter in saturated rock. However, load-indentation tests with pore pressure measurement at the same speed showed that the pore pressure within the tested sandstone remained too low to affect the rock fracture process. Other possible mechanisms are discussed.

Pore Pressure Prediction Using Eaton and Neural Network Method in Carbonate Field “X” Based on Seismic Data

Abnormal pore pressure can cause some problems during the drilling process such as a blowout or sticking pipe while drilling. Pore pressure prediction may prevent the drilling hazard, especially in carbonate field that known as a complex reservoir. It is useful for mud weight determination to prevent blowout and sticking pipe while drilling. This study focuses on predicting pore pressure values and maps it through 3D seismic data. The field is carbonate reservoir which known as a heterogeneous formation with shale above the reservoir. Due to the difference of lithologies, the two different empirical parameter is used in each lithology for Eaton equation. The pore pressure prediction then correlates with the seismic attribute using a neural network method. The input parameter of the Eaton is sonic and density log. Then, the result of Eaton’s method is calibrated by leak-off test (LOT) and repeat formation test (RFT), hence the results are more accurate and verified. Then, the pore pressure is correlated to acoustic impedance, shear impedance, seismic frequency, and seismic amplitude to create a subsurface model by the neural network machine learning. The result shows that the pore pressure prediction of the model is verified by the measured pore pressure well-log data with good accuracy up to 90%. The combination method of Eaton and neural network was proven to be able to predict and map pore pressure distribution in a complex carbonate field.

Paleo-pressure evolution and its origin in the Pinghu slope belt of the Xihu Depression, East China Sea Basin

Abstract The Xihu Depression in the East China Sea Basin is one of the prime gas and light oil-producing regions of eastern China. Overpressure evolution and origin in the sandstone reservoirs of the Pinghu (E2p) Formation in the Pinghu slope belt of the Xihu Depression were investigated through a combination of fluid inclusion analyses and basin numerical modeling, combined with measured pore pressures (drill stem and wireline formation tests), well loggings, gas geochemistry, and casting thin section data. Four oil inclusion types, exhibiting different API gravities and hydrocarbon gas inclusions, were identified, using inclusion petrography, microscopic fluorescence, and Raman spectra. Homogenization and ice melting temperatures of coeval aqueous inclusions, determined using microthermometry, combined with burial-thermal histories determined by basin numerical modeling, suggested multiple episodes of oil and natural gas charge in the E2p reservoirs, from the Miocene to the present day. Pore pressure evolution history, reconstructed with fluid inclusion PVTx analysis, indicated that the Pinghu Formation experienced three phases: overpressure generation, followed by overpressure release, and then further overpressure generation. The first phase of weak overpressure was generated during burial in the middle–late Miocene. Currently, it is hard to definitely determine the origin of the first phase of overpressuring. Nevertheless, it can be speculated that the origin may be related to disequilibrium compaction, oil generation and tectonic compression based on burial, tectonic and hydrocarbon generation histories. Following this, a sudden uplift of formations led to overpressure release in the late Miocene, and then, the second phase of moderate overpressure generation occurred during burial in the late Pliocene and Quaternary. Based on well-log signatures, vitrinite reflectance data of mudstones and gas geochemistry, combining with seismic analysis, the second phase of overpressuring was interpreted to have been mainly caused by fluid expansion, which resulted from local gas generation by kerogen maturation and long-distance vertical and lateral pressure transfer.

Numerical and experimental analysis of the extraction mechanism of an anchor plate embedded in saturated sand

Floating offshore structures can be founded on submerged foundations such as anchor plates. Their extraction resistance is of major importance during and at the end of the lifetime cycle. During their lifetime cycle, the foundation is suspended to complex loading conditions due to waves, tidal currents and wind loads and its resistance has to be known. At the end of the lifetime cycle, the anchor plates have to be removed from the seabed. The extraction resistance of the anchors is thereby a lot higher than the sum of its self-weight and hydrostatic and earth pressure. When the anchor plate is moved, the volume underneath increases. This leads to a suction until inflowing pore water fills the newly created volume. In order to investigate this effect, an experimental study at model scale with a displacement-driven extraction is performed. Pore pressure measurements are carried out at various locations in the soil body and underneath the plate. The soil movement is tracked with the particle image velocimetry method (PIV). In the conducted experiments different extraction velocities are considered to investigate its effect on the extraction resistance. Numerical investigations with unresolved coupled CFD-DEM are carried out and validated against the experimental results.

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

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.

Field measurement of pore pressures and liquid-gas distribution using drilling and ERT in a high food waste content MSW landfill in Guangzhou, China

Systematic field measurements were carried out at the sliding area of the Xingfeng Landfill, which is a large high food waste content municipal solid waste (HFWC-MSW) landfill. Sixteen boreholes were drilled and pore pressure transducers, gas pressure pipes and inclinometer pipes were installed to measure pore water pressures, pore gas pressures and waste displacements, respectively. Two measuring lines of electrical resistivity tomography (ERT) were arranged to analyze the liquid-gas distribution in the waste mass. In addition, a stability analysis was conducted on the basis of the measured results. The main findings are as follows: (i) In the regions at the depths of 5–15 m in the waste mass, the saturation level of the waste was relatively high, and high gas pressures were not evident. However, in the regions at depths blow 15–20 m, the waste was unsaturated, and there were obvious local high gas pressures, (ii) The low-water-content and high-water-content zones obtained by ERT corresponded to the low-saturation and high-saturation zones inferred by the measurements of the water and gas pressures, respectively, indicating the consistency between the results of ERT and drilling, and (iii) By field measurement and stability analysis, the correlation between high gas pressures and sliding was confirmed, indicating high gas pressures may be the critical cause for the frequent waste sliding. In addition, the possible causes for the liquid-gas distribution and high gas pressures in the waste mass were discussed by considering the biodegradation characteristics of HFWC-MSW. This study provides the references for field measurement and stability control in similar landfills.

Systematic slope stability assessment through deformation field monitoring

Field monitoring is frequently carried out during excavations and other geotechnical activities and provides additional information during the execution of a construction project. The interpretation of field monitoring data is often obscured by measurement noise and disturbance, and a systematic approach to assess both the quality and implications of the field monitoring data is very helpful in geotechnical practice. The possibility to infer practical conclusions from the field monitoring data depends on the type of field measurements, especially in monitoring of the stability of slopes. Pore pressure measurements can serve as a direct measurement of utilized soil strength for a slope, while deformation measurements are significantly more ambiguous and complicates the interpretation. The assessment of slope stability through field monitoring of deformations requires inverse or back analysis of the soil properties, followed by a forward analysis of the resulting slope stability. Such an inverse or back analysis is frequently influenced by non-uniqueness of the material properties and the stability of the measurement data. Systematic approaches to inverse or back analysis have been demonstrated in the scientific literature, but the practical use of these methods is not entirely straight-forward. The current paper presents a case study of systematic slope stability assessment through field measurements of deformations with a review of the field monitoring programme, numerical simulations of deformations, and a simplified approach to back analysis of the soil parameters. The excavation of a slope in an urban environment including layers of organic clay covered with highly heterogeneous gravel fill is used as an example of geotechnical back analysis. The aim is to elucidate some of the challenges in geotechnical back analysis while providing some practical solutions for practice.

Halden research site: geotechnical characterization of a post glacial silt

This paper describes the geology and geotechnical engineering properties of the Halden silt; a 10–12 m thick deposit of fjord-marine, low plasticity clayey silt. Over the last six years, the test site has been well characterized by combining the results from a number of geophysical and in situ tools, including; electrical resistivity tomography, multi-channel analysis of surface wave surveys, cone penetration testing, dissipation testing, in situ pore pressure measurements, seismic flat dilatometer testing, field vane testing, self-boring pressure meter testing, screw plate load testing and hydraulic fracture testing. The results from these investigations assist the interpretation of layering and in situ soil properties. Soil sampling and advanced laboratory testing have provided data for interpretation of geological setting and depositional history, soil fabric, strength, stiffness and hydraulic properties. However, interpretation of the stress history, based on oedometer tests and clay-based correlations to the cone penetration test, are unreliable. They contradict the depositional history, which suggests that the soil units at the site are near normally consolidated, except for some surface weathering and desiccation. Further, undrained shear strength interpretations are complex as the in situ tests are potentially influenced by partial drainage, and conventional undrained triaxial tests do not provide a unique (peak) undrained shear strength. Despite certain interpretation challenges the paper presents important reference data to assist in the interpretation and assessment of similar silts, and provide some guidance on important geotechnical properties for projects where limited design parameters are available.

Measurement of stresses in soils with modernized sensors

This article is devoted to the modernization of handicrafts manufactured to measure the stress in the ground (messdose). Today, when designing buildings and structures, an important task is to determine the amount of expected precipitation. The calculated values are not always objective, especially in the conditions of Western Siberia, where weak organic-mineral soils are common. Based on the experience of the introduction of sensors measuring the voltage in the ground, the author identified a number of factors by which they most likely fail. According to approximate calculations, until the end of the average natural experiment, about 25 % of the messdose remain operable. The author proposed options for upgrading sensors to minimize the risks of their failure. The main causes of sensor failure: contact breakdown, depressurization, short circuit. They are eliminated by inserting fiberglass boards and two-stage sealing into the sensor body. The body of the sensors was made of titanium. Cyanoacrylate glue was used for sticking the strain gauges onto the body of the messdose. Sealing was performed with epoxy resin. As a signal cable used 4-core cable KSPV. Calibration of the sensors was carried out in an aerostatic tank. Upgraded sensors were tested during a laboratory experiment. The author tested a sample of weak organic soil in compression conditions. No total pressure sensor has failed. The convergence of the sensor readings with the load applied to the sample is very high. The measurement error did not exceed 10 %. The author also attempted to make handicraft sensors to measure pore pressures in the ground, but during the implementation they all failed.