Vertical Effective Stress Research Articles

Evaluation of a simplified soil constitutive model considering implied strength and pore-water pressure generation for one-dimensional (1D) seismic site response

Pore-water pressure (PWP) generation can lead to soil softening and liquefaction of sandy soils during earthquakes, with potential influence on site response and seismic design. The authors evaluated the generalized quadratic/hyperbolic (GQ/H) constitutive model, which captures small-strain stiffness, large-strain shear strength, and is coupled with a widely used cyclic strain–based PWP generation model (termed GQ/H+u). A suite of cyclic direct simple shear tests with a range of relative densities (∼30%–80%) and effective vertical stresses (∼25–200 kPa) and dynamic centrifuge tests with liquefiable sands were used to evaluate the ability of the GQ/H+u model to simulate cyclic soil behavior. Results indicate that GQ/H+u provides reasonable estimates of PWP increase during cyclic shear, with differences between measured and computed excess PWP ratios (ru) for both element and centrifuge tests generally smaller than 0.1. Computed spectral accelerations are comparable to centrifuge test measurements, with almost no bias at medium to long periods (T > 0.4 s) when the computed maximum shear strain (γmax) was smaller than the limit shear strain (γlimit). When computed ru > 0.8 and computed γmax > γlimit, spectral accelerations may be underestimated at both short and long periods as dilative behavior is not captured by GQ/H+u.

Overpressure Generation Mechanisms and Its Distribution in the Paleocene Shahejie Formation in the Linnan Sag, Huimin Depression, Eastern China

The Linnan Sag is one of the main oil-producing units in the Huimin Depression, Eastern China, and the pore pressure gradients obtained from drill stem tests (DSTs) range from 9.0 to 16.0 MPa/km. Uncertainty about the origin and distribution of abnormally high pressures in the Linnan Sag has led to different interpretations of hydrocarbon accumulation and resource assessments, and it interferes with safe drilling. In the Linnan Sag, mudstone compaction curves are substantially affected by several non-compaction factors, and the normal trend of the compaction curve is difficult to determine. The determination of the origin and distribution of overpressure in the Linnan Sag is a challenge. In this study, the factors that may affect mudstone compaction—such as the shale volume, higher calcareous, and organic matter content—were carefully examined and processed. The pressures in the mudstones were estimated by the corrected mudstone compaction curves, which were compiled from acoustic, density, and neutron logs, and calibrated using DST and mud weight data. The log response–vertical effective stress and acoustic velocity-density crossplots were used to identify the mechanisms that generate overpressure. The comprehensive compaction curve shows that the mudstones in the overpressured layer exhibit clear disequilibrium compaction characteristics. The logging response crossplots demonstrate that those overpressured points were consistent with the loading curve. The findings suggest that, the fundamental mechanism resulting in overpressures is the disequilibrium compaction of thick Paleocene mudstones. Hydrocarbon generation and vertical transfer of overpressure may be the main unloading mechanisms, which corresponds to the overpressure points that deviate from the loading curves. Since organic matter cracking may occur in formations at depths greater than 4000 m (Ro > 1.0%), the contribution of hydrocarbon generation to overpressuring is expected to be limited. The transfer of overpressure through opening faults is therefore considered to be the main cause of higher overpressure in local sandstones. The overpressures in the mudstones are characterized by a gradual decrease from the center to the margin in the Linnan Sag. The pressure in the isolated sand bodies are generally similar to that in the surrounding mudstones, whereas it can be lower or higher when the overpressure in the sand bodies are vertically transferred by faults to other pressure systems. The results of this analysis provide an indication of the magnitude, mechanism, and distribution of overpressure in the Linnan Sag. This insight can be used to guide further exploration of the Linnan Sag and similar geological basins.

On fluid-filled mixture model for suction pile foundation analysis

The purpose of this study was to investigate the installation water pressure of the offshore wind turbine suction bucket caisson foundation. The fluid-filled mixture theory was used in the study and the theoretical equations are derived from the basic theory of ocean fluid dynamics and flow in porous media and they were solved by finite difference methods. Benchmark verification was performed prior to further simulation cases. Numerical examples were made based on several penetration depths and different suction pressures. Different boundary conditions were tested and the most reasonable boundary conditions were proposed. Suction induced stresses in response to various suction pressures and penetration depths were investigated. The overall lateral resistance and possible piping potential were studied. The frictional stress and bearing capacity on the outside of the caisson are greater than those on the inside. Even the critical suction pressure is given and the penetration depth increases to 2.5 m, the suction induced stress at the outer corner of the caisson tip only increases to 70% of the static effective soil stress and does not reduce the effective vertical soil stress to zero, and no piping is occurred.

Overpressure and its positive effect in deep sandstone reservoir quality of Bozhong Depression, offshore Bohai Bay Basin, China

Bohai Bay Basin is a Meso-Cenozoic terrestrial sedimentary basin in eastern China. Its offshore regions, including Bozhong and Liaodongwan Depressions, are favourable exploration targets which provide near a half of the petroleum reserves in the basin. Eocene Shahejie (Es) Formation and Oligocene Dongying (Ed) Formations are two important exploration targets in Bozhong Depression, and overpressure is commonly seen in Es and Ed Formations in this area. Our research examined the distribution characteristics of overpressure in the formations and suggest the main mechanism of overpressure is compaction disequilibrium due to the rapid sedimentation rates (~500 m/Ma) of fine-grained sediments in this area. Also, oil and gas generation within the thick mudstones of the two formations has added the magnitude of overpressure. We investigated the reservoir quality especially primary porosity in Es and Ed formations, and their relationship with overpressure. The positive effect of overpressure on reservoir porosity preservation was validated through microscopic observations and vertical effective stress (VES) analysis. We established a quantitative model for evaluating the relationship of overpressure, pore structures, porosity, and VES. The result suggests the overpressure in the targeted formations were primarily originated from undercompaction. The overpressure kept VES from increasing and helped preserve the primary intergranular porosity. The porosity preserved by overpressure can be significantly higher than normally compacted porosity under the same condition of depth and temperature.

Compressibility of biocemented loose sands under constant rate of strain, loading, and pseudo K0-triaxial conditions

The compressibility behavior of loose sands treated with Microbially Induced Carbonate Precipitation (MICP) is presented in this paper. The paper discusses the strain rate effects and evolution of at-rest earth pressure coefficient and elastic shear modulus during K0-loading. The soil samples were prepared in a triaxial cell in which a biological solution containing the ureolytic bacteria Sporosarcina pasteurii was injected and held under a small back pressure. Cementation treatments were injected following an alternated top and bottom sequence. The constant rate of strain, constant rate of loading, and pseudo K0-triaxial tests were performed at different strain and stress rates. On-specimen internal instrumentation consisting of a submersible load cell, three Hall Effect transducers, and vertical Bender Elements were used to control radial strains during K0-loading and measure small-strain shear modulus changes. Based on shear wave velocity measurements, the MICP-treated sand was lightly cemented and displayed soil-like behavior. The experimental results demonstrated a significant reduction in soil compressibility after MICP treatment. The material response was remarkably similar for every tested strain rate. The very small values of axial strains measured for the biotreated samples in relation to untreated control specimens for vertical effective stress levels below 200 kPa is evidence of the suitability of this treatment and shows its potential for use in field applications at relatively shallow depths.

Quantification of bound water content, interstitial porosity and fracture porosity in the sediments entering the North Sumatra subduction zone from Cation Exchange Capacity and IODP Expedition 362 resistivity data

In this study, we investigate porosity evolution through the sedimentary input section of the North Sumatra Subduction zone by quantifying interstitial porosity, bound water content and fracture porosity based on IODP Expedition 362 data and post-cruise chemical analyses. During IODP Expedition 362, total porosity of the sedimentary section entering the North Sumatra subduction zone was measured. This total porosity is derived from the total water content of core samples thus including pore water and water bound to hydrous minerals like smectite. Clay mineral composition varies over the sedimentary section and is mainly kaolinite/illite in the Nicobar Fan units and smectite/illite in the prefan pelagic unit below. The prefan pelagic unit shows anomalously high total porosity values and is stratigraphically correlated to a high amplitude negative polarity (HANP) seismic reflector located landward. This HANP reflector has been previously interpreted as a porous fluid-rich layer where the d{\'e}collement may develop along parts of the margin as a consequence of pore pressure buildup. We estimate clay bound water content from Cation Exchange Capacity (CEC) which gives information about the smectite/illite composition and soluble chloride content data. Interstitial porosity corresponds to onboard total porosity corrected from clay bound water and is more relevant in terms of sediment compaction state and fluid flow properties. Interstitial porosity versus vertical effective stress curve shows no evidence of undercompaction and suggests that the input section 2 has been experiencing normal consolidation due to high sediment accumulation rate. The porosity anomaly observed in the prefan pelagic unit results from the local occurrence of water-bearing minerals like smectite rather than excess pore pressure, which might, however, buildup more landward in the basin. We also estimate fracture porosity using a resistivity model for shales used in previous works based on wireline resistivity log and show that fracture porosity yields 4-6% in damaged parts of the sedimentary section investigated.

Experimental Study on Influence Factors and Empirical Formula of K0 for Coarse Grained Soil

The earth pressure coefficient at rest K0 is a key mechanical parameter of soil, so studying K0 of coarse grained soil has important theoretical value and practical significance in geotechnical engineering works. To investigate the influence factors of K0 for coarse grained soil, a series of K0 compression tests for a sandy gravel and a rockfill were conducted by utilizing a newly-developed large-scale K0 device. The test data show that there is a negative linear correlation between K0 and initial relative density Dr0. The data also show that the increasing vertical effective stress σ′v, will decrease the K0 value, and the effect of low σ′v, on the K0 is more obvious. Large-scale consolidated drained (CD) triaxial tests for soils used for K0 compression test were carried out, and according to the data of CD triaxial tests and K0 compression tests the correlation between K0 and effective internal friction angle ϕ′ is established. Based on the result, an empirical equation is presented. This equation can be used to predict the K0 value of the coarse grained soil under a random consolidation condition and stress condition according to the value of ϕ′. Furthermore, this empirical formula is verified by K0 compression tested data.

Development of a Cutting Force Model for a Single PDC Cutter Based on the Rock Stress State

Polycrystalline diamond compact (PDC) bits are currently the most widely used bits in the petroleum industry. However, the rock-breaking mechanisms of PDC bits are not well understood, especially in deep hard-rock drilling. By modifying the Nishimatsu model, a single PDC cutter force model based on in situ stress is developed in this study. By coupling the mechanical properties of rock and the technological measures of rock breaking, the model can accurately solve the cutting force and assess the rock drillability when applied to a PDC bit. The analysis of research findings shows that the vertical stress initially has a greater impact on the cutting force despite a larger horizontal stress relative to the vertical stress. Moreover, the optimal cutting angle of a PDC cutter is between 15° and 20°. The applicable conditions of the model are obtained experimentally. The model can predict cutting forces in non-expansive rocks well with 2% error. Furthermore, the mechanical-specific energy under low confining pressure has been theoretically demonstrated to increase more quickly than that under high confining pressure. The proposed model can analyse the influence of effective horizontal and vertical stresses and rock strength on the cutting force, which can lead to a better understanding of the rock-breaking mechanisms of PDC drill bits. The results of this work can be used to study PDC bit performance and provide guidelines for the application and design of PDC bits for specific rocks.

Undrained Shear Strength and Monotonic Behavior of Different Nonplastic Silts: Sand-Like or Clay-Like?

The common assumption in geotechnical engineering research and practice has been that nonplastic silts behave sand-like, whereas, as silts become more plastic, they start to behave clay-like. To date, there are practically no studies focusing solely on various nonplastic silts and their monotonic response, perhaps because of the solid belief that nonplastic silts behave sand-like in all aspects. A series of constant volume direct simple shear tests were conducted on three different nonplastic silts. Because the specimens were reconstituted to have loose grain structures, volumetrically contractive tendency was observed for all the silts with relatively small effective stress friction angles. It was observed that Ko compression lines and critical state lines are parallel to each other for all three silts, which is considered to be a clay-like behavior in literature. It was shown that undrained shear strengths can be normalized with the effective vertical consolidation stress, which is also considered to be a clay-like behavior in literature. Normalized values are compared with the ones obtained for plastic silts compiled from previous studies. Normalized undrained shear strength values for plastic silts in literature were seen to be typically greater. Possible reasons such as density effect and strength anisotropy were discussed.

Shear Strength of Coal Combustion Product by Vane Shear

The objective of this study was to evaluate the shear strength of a coal combustion product (CCP) by using the vane shear test. A series of small-scale vane shear (SS-VS; diameter = 12.7 mm and height = 25.4 mm) and large-scale vane shear (LS-VS; diameter = 25 mm and height = 50 mm) laboratory tests were conducted. Undrained and drained strength envelopes were determined using consolidated undrained triaxial compression tests, and drained strength was checked via consolidated drained direct shear. In addition, effects of (1) rate of vane rotation, (2) time delay between vane insertion and beginning rotation, and (3) elapsed time under the final vertical effective stress before shear were evaluated via SS-VS. Vane shear strength results were represented in terms of peak shear strength and the initial horizontal effective stress acting on the vertical-oriented failure surface during rotational shear. Both SS-VS and LS-VS yielded shear strengths that plotted between the drained and undrained triaxial strength envelopes. Shear strength of CCP increased with increasing time under load, which was potentially attributed to diagenesis. Peak shear strength in SS-VS was reached after approximately 72 hours of elapsed time under constant vertical effective stress. Peak shear strength in SS-VS and LS-VS compared favorably to the drained strength measured via direct shear testing.

Shear Strength of Coal Combustion Product by Vane Shear

The objective of this study was to evaluate the shear strength of a coal combustion product (CCP) by using the vane shear test. A series of small-scale vane shear (SS-VS; diameter = 12.7 mm and height = 25.4 mm) and large-scale vane shear (LS-VS; diameter = 25 mm and height = 50 mm) laboratory tests were conducted. Undrained and drained strength envelopes were determined using consolidated undrained triaxial compression tests, and drained strength was checked via consolidated drained direct shear. In addition, effects of (1) rate of vane rotation, (2) time delay between vane insertion and beginning rotation, and (3) elapsed time under the final vertical effective stress before shear were evaluated via SS-VS. Vane shear strength results were represented in terms of peak shear strength and the initial horizontal effective stress acting on the vertical-oriented failure surface during rotational shear. Both SS-VS and LS-VS yielded shear strengths that plotted between the drained and undrained triaxial strength envelopes. Shear strength of CCP increased with increasing time under load, which was potentially attributed to diagenesis. Peak shear strength in SS-VS was reached after approximately 72 hours of elapsed time under constant vertical effective stress. Peak shear strength in SS-VS and LS-VS compared favorably to the drained strength measured via direct shear testing. © 2019 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved.

Generation and evolution of overpressure caused by hydrocarban generation in the Jurassic source rocks of the central Junggar Basin, northwestern China

The Jurassic black mudstone and coal beds in the central Junggar Basin, northwestern China, are the major source rocks for the basin with type II2 and type III (gas-prone) kerogens. Widespread overpressures are developed in the Jurassic stratigraphic interval. Sonic and resistivity logs display strong characteristic responses of overpressure in the mudstones, with anomalously high acoustic traveltimes and low resistivity compared with the normally pressured mudstones. The overpressured Jurassic sediment sequences appear to have undergone normal compaction because the mudstones exhibit no anomalously low bulk density. The overpressured mudstones deviate from the normally pressured mudstones in density–effective vertical stress space. The overpressure in the Jurassic source rocks is, therefore, not caused by disequilibrium compaction. The overpressured Jurassic sandstone reservoirs are predominantly oil and gas saturated or oil bearing. The well-log responses of the overpressured mudstones and seismic velocity characteristics indicate that the top depth of the overpressure zone ranges from 3800 to 4600 m (12,500 to 15,100 ft), corresponding to formation temperatures of approximately 94°C to 111°C (∼201°F to 232°F), with estimated vitrinite reflectance values of 0.6% to 0.75%. The Jurassic source rocks with overpressure are capable of generating hydrocarbons at present and are currently overpressured. All the evidence suggests that the overpressure in the Jurassic source rocks in the central Junggar Basin is caused by hydrocarbon (HC) generation. The overpressure evolution was modeled quantitatively in response to pressure changes caused by HC generation during basin evolution. The results indicate that multiple episodes of overpressure development and release occurred within the Jurassic source rocks, suggesting multiple episodes of HC expulsion. The timing and numbers of these episodes of HC expulsion were thus determined from the modeled overpressure evolution.

Seafloor Burial of Surrogate Unexploded Ordnance by Wave-Induced Sediment Instability

Burial of munitions and other unexploded ordnance (UXO) on the seafloor complicates the detection and remediation of associated threats in coastal areas. We observed up to 0.6-m burial of surrogate UXO placed near the 8-m isobath during a series of winter storm events from late January to early March 2015 in Duck, NC, USA. We used time series from a pressure sensor located roughly 1 m above the bed during the experiment to apply an existing analytical solution for dynamic poroelastic wave-sediment interaction. The analytical solution yielded the spatial and temporal distributions of wave-induced pore pressures and effective stresses, which were used to evaluate five different criteria for wave-induced sediment instability. Four of the criteria have been applied previously considering different deterministic or stochastic wave types, whereas the fifth is novel and may be regarded as a generalization of the Sleath condition for plug flow. Using the estimated maximum depths of sediment instability (or bed failure) as predictors for observed surrogate UXO burial depths, we concluded that criteria based on vertical effective stress and the vertical distribution of pore pressures were most accurate. We believe that our data and the accompanying analysis are unique in the literature, where field observations of wave pressures were combined with a dynamic wave-seafloor interaction model to make predictions of sediment failure depths that were validated against actual observations of surrogate UXO burial depths.

Large near-surface block rotations at normal faults of the Iceland rift: Evolution of tectonic caves and dilatancy

Abstract Surface ramps in normal fault zones of the Iceland plate boundary have been described in many studies, but their structure and evolution are not well understood. We show that surface ramps are manifestations of large tilted blocks (TBs) formed in dip relays of normal faults. Based on existing modeling studies, we propose three classes of TBs defined by kinematics and location of the hinge of the TB. TBs are considered a member of the family of fault relay structures that form near the surface, commonly, but not exclusively, in columnar basalts with orthotropic strength. We present high-resolution aerial vehicle–based observations of a representative set of normal faults in Iceland and compare these with geometric models we derived from modeling studies. We predict extensive tectonic cave (fluid conduit) systems under the TB, which interact with magma and groundwater flow. The general fault structure is dominated by large, subvertical open fractures reactivating cooling joints that are locally filled by basalt rubble. We propose the existence of a hybrid failure zone at larger depths before the effective vertical stress is sufficient to initiate shear fractures in intact basalt.

Analisis Potensi Likuifaksi Akibat Gempa (Studi Kasus : Reklamasi Pelabuhan Kontainer Belawan Fase-2)

One of the impacts caused by an earthquake is the strength loss of the soil due to cyclic stresses calledliquefaction. Liquifaction usually occurs in loose sand. The objective of this study is to determine thepotential of liquefaction in observed project. The project observed in this study is reclamation project.Analysis for potential liquefaction is done using the Simplified Procedure Method. Firstly, determinethe value of peak acceleration in bedrock in the study area based on the 2010 Indonesian EarthquakeHazard Map. Secondly, collect soil data based on CPT-u testing. Based on these data, the accelerationvalue (Amplification/Deamplification) can be found on each soil layer when being moved from thebedrock to the ground surface. By comparing the average shear stress caused by the acceleration ofthe earthquake to the effective vertical stress at each soil layer, we can obtain the value of CyclicStress Ratio. Lastly, calculate CRR (Cyclic Resistant Ratio) which is the value of soil resistance toliquefaction. Based on CRR value, it can be seen the corrected conus penetration value (qc1N). Thethree parameters are then connected in the NCEER 1996 chart to assess which soil layers arepotentially liquefied or not during an earthquake.

Influence of the Effective Vertical Stresses on Hydraulic Fracture Initiation Pressures in Shale and Engineered Geothermal Systems Explorations

Influence of the Effective Vertical Stresses on Hydraulic Fracture Initiation Pressures in Shale and Engineered Geothermal Systems Explorations

Determination of Friction Capacity of Driven Pile in Clay Using Gaussian Process Regression (GPR), and Minimax Probability Machine Regression (MPMR)

Friction capacity (fs) of driven pile in clay is key parameter for designing pile foundation. This study employs Gaussian Process Regression (GPR), and Minimax Probability Machine Regression (MPMR) for determination of fs of driven piles in clay. GPR is a Bayesian nonparametric regression model. MPMR is a probabilistic model. Pile length (L), pile diameter (D), effective vertical stress (σ’v), undrained shear strength (Su) have been used as input variables of GPR and MPMR. The output of the models is fs. The developed GPR, MPMR models have been compared with the Artificial Neural Network (ANN). GPR also gives the variance of predicted fs. The results prove that the developed GPR and MPMR are efficient models for prediction of fs of driven piles in clay.

Shear wave velocity-based evaluation of liquefaction resistance for calcareous sands of different origin

This paper presents the results of an experimental research carried out in laboratory aimed at establishing a relationship between cyclic liquefaction resistance (CRRfield) and corrected shear wave velocity (VS1) of carbonate sands. Two carbonate skeletal sands, namely Quiou and Dubai, were investigated together with a carbonate non-skeletal sand from Kenya. The programme included also tests on two silica sands prepared and tested in similar conditions for comparison purposes. The research was based on undrained cyclic simple shear tests and bender element tests in triaxial cell performed on reconstituted specimens of sand at different initial density and stress states. The results obtained highlight that data points for carbonate sands are all located to the right of the currently used cyclic liquefaction resistance vs. shear wave velocity curves suggested for silica sands, implying that the latter curves should be considered non-conservative when applied to carbonate crushable sands. An insight into the effect of the initial effective vertical stress on liquefaction susceptibility of carbonate sands was presented and a stress normalization procedure was also proposed.

The influence of a cyclic loading history on soil-geogrid interaction under pullout condition

The knowledge of soil-geosynthetic interface behaviour is a key point in the design of geosynthetic-reinforced soil structures. The pullout ultimate limit state can be reproduced conveniently by means of pullout tests performed with large-size laboratory apparatuses, which allow studying the interaction mechanisms that develop in the anchorage zone. During the service life of geosynthetic-reinforced soil structures, reinforcements may be subjected to long-term cyclic vehicular loads or short-term seismic loads in addition to dead loadings, such as the structure's self-weight and other sustained loads. In order to study the influence of a cyclic loading history (a sinusoidal function with fixed amplitude A, number of cycles N and frequency f) on the post-cyclic peak pullout resistance, the writers carried out a series of multi-stage pullout tests on a high density polyethylene extruded uniaxial geogrid embedded in a compacted granular soil for different vertical effective stress σ′v values. Moreover, the stability of the soil-geosynthetic interface from a point of view linked to the cyclic loading application has also been investigated. Test results showed that the design pullout resistance parameters are affected by the applied cyclic loading history for specific combined conditions (A, N and σ′v) and it should be taken into account for designing geosynthetic reinforced soil structures.

One-dimensional normal compression laws for resedimented mudrocks

Abstract Four different empirical models used to describe the one-dimensional normal compression behavior of mudrocks have been evaluated by investigating the behavior of 15 different resedimented mudrocks. These mudrocks originate from a wide variety of geologic origins and were tested in the laboratory over an effective stress range of 0.1–100 MPa. The normal compression of silt-rich, low liquid limit mudrocks is best described by assuming a log-linear relationship between vertical effective stress, σ′v and void ratio. For smectite-rich, higher liquid limit mudrocks their behavior is better described by assuming a log-linear relationship between σ′v and porosity. The assumption of a log-log relationship between (1 + void ratio) and σ′v is not the most appropriate for any particular mudrock type, but gives a reasonably good description of compression behavior for all mudrocks, and is therefore ideal for situations in which the general composition of a sediment is unknown. For all mudrocks, the assumption of an exponential relationship between porosity and σ′v gives a very poor fit to the experimental data. As mudrocks undergo compression to high effective stresses, high liquid limit mudrocks display a much greater compressibility and a corresponding larger loss in porosity compared to more silt-rich mudrocks. The porosities of all materials tend to converge into a much narrower range above about 10 MPa, where all mudrocks display a similar compression behavior and a constant value of compression index of 0.21 regardless of their composition. At low effective stresses though, their compression behavior is strongly controlled by composition.