Decrease In Effective Stress Research Articles

Analysis of simplified time of liquefaction triggering methods by laboratory tests, physical modelling and numerical analysis

The damage resulting from earthquakes can result from the combination of seismic excitation and/or due to a build-up of excess pore pressure in the soil (liquefaction). These two effects are related since the reduction of soil stiffness due to a decrease in effective stress, modifies the seismic response of the soil deposit. Therefore, the expected level and type of damage is dependent on the amount of seismic energy reaching the ground surface before liquefaction. The development and validation of simplified liquefaction assessment methods to provide reasonable estimates of the build-up of excess pore pressure is essential for improving estimates of the level of seismic demand (ground shaking and permanent ground deformation) that may be experienced by a building. This paper presents two methods, one based on equivalent cyclic stress loading, and another based on the cumulative strain energy, which are used to predict the evolution of the pore pressure build up throughout time. The centrifuge tests performed in ISMGEO (Italy) during the LIQUEFACT project (www.liquefact.eu) were used as a benchmark to evaluate the predictive performance of the methods. Additionally, a series of one dimensional soil column effective stress and total stress analyses and single soil element simulations were run. Available laboratory tests were used to calibrate the parameters of the simplified methods, as well as input parameters for the numerical simulations. The results showed that both simplified methods had considerable bias. A direct comparison of the effective stress analyses, a set of effective stress analyses with limited drainage, and the centrifuge results, highlighted that the centrifuge experiments exhibited significant pore water flow that was not captured in the simplified models. Comparisons between the irregular loading in the one dimensional analyses compared to the uniform loading in the element tests highlighted shortfalls in the conversion from irregular to equivalent uniform loading. Comparisons between stress demands from total stress, effective stress and the simplified methods illustrated the limitations of relying on the total stress acceleration to estimate demands on a soil in a liquefying deposit.

The State of Pore Fluid Pressure and 3‐D Megathrust Earthquake Dynamics

AbstractWe study the effects of pore fluid pressure (Pf) on the pre‐earthquake, near‐fault stress state, and 3‐D earthquake rupture dynamics through six scenarios utilizing a structural model based on the 2004 Mw 9.1 Sumatra‐Andaman earthquake. As pre‐earthquake Pf magnitude increases, effective normal stress and fault shear strength decrease. As a result, magnitude, slip, peak slip rate, stress drop, and rupture velocity of the scenario earthquakes decrease. Comparison of results with observations of the 2004 earthquake support that pre‐earthquake Pf averages near 97% of lithostatic pressure, leading to pre‐earthquake average shear and effective normal tractions of 4–5 and 22 MPa. The megathrust in these scenarios is weak, in terms of low mean shear traction at static failure and low dynamic friction coefficient during rupture. Apparent co‐seismic principal stress rotations and absolute post‐seismic stresses in these scenarios are consistent with the variety of observed aftershock focal mechanisms. In all scenarios, the mean apparent stress rotations are larger above than below the megathrust. Scenarios with larger Pf magnitudes exhibit lower mean apparent principal stress rotations. We further evaluate pre‐earthquake Pf depth distribution. If Pf follows a sublithostatic gradient, pre‐earthquake effective normal stress increases with depth. If Pf follows the lithostatic gradient exactly, then this normal stress is constant, shifting peak slip and peak slip rate updip. This renders constraints on near‐trench strength and constitutive behavior crucial for mitigating hazard. These scenarios provide opportunity for future calibration with site‐specific measurements to constrain dynamically plausible megathrust strength and Pf gradients.

Experimental investigation on the influence factors of primary production performance of tight oil

The effects of permeability, initial fluid pressure, effective stress, gas-oil ratio, pressure depletion rate, and oil viscosity on the primary production oil recovery were investigated in detail. The results show that the permeability of tight core is the most significant influence factor. When the permeability decreases from 3.0 × 10−3 µm2 to 0.0021 × 10−3 µm2, the oil and gas recoveries decrease from 45% and 90% to 10% and 20%. Furthermore, the initial fluid pressure also has great influence and the oil recovery linearly increases with the fluid pressure as well as the decrease of effective stress. When the initial fluid pressure increases from 6 MPa to 15 MPa, the oil and gas recoveries can be enhanced about 10% and 22%. Gas-oil ratio is another important parameter. When the gas-oil ratio is 26 m3⋅m−3, the oil recovery can reach approximately 30% compared to 2.7% at the gas-oil ratio of 0. The pressure depletion rate has a more obvious influence on the ultimate oil recovery for the cores with a low permeability, while with a low depletion rate after bubble point can enhance the oil recovery. The work can provide useful information for the choosing of key parameter and development method in tight oil production.

Experimental study on deformation and permeability enhancement of oil sand reservoir by hydraulic fracturing technique under true triaxial stress

AbstractHydraulic fracturing technology is often applied to form a permeable zone for geothermal resources near the borehole in tight reservoirs to improve the permeability and extraction efficiency. Steam or hot water is often injected to dissolve the heavy oil to increase its fluidity during the extraction. As the numerical simulation or two‐dimensional physical experiments have great limitations to simulate hydraulic fracturing of oil sand reservoirs, this study conducted a large‐scale three‐dimensional physical simulation experimental study of hydraulic fracturing with high temperature of oil sand reservoir under true triaxial stress state and monitored the water pressure change at various points inside the box during the hydraulic fracturing by laying pressure sensors. The experimental results showed that the hydraulic breakdown pressure of oil sands increases with the increase in σ2, which is related to the increase in σ2 limiting the fracture growth and the decrease in effective stress during fluid injection. To better describe the breakdown pressure variation, we developed a new oil sand breakdown pressure prediction model considering the true triaxial stress condition, which showed a good prediction effect. Due to the formation of fluid hysteresis zone during the fracture expansion, the fluid pressure recorded by the internal pressure sensor lagged behind the injection pressure. Hydraulic fracturing produced shear fractures inside the oil sands under low σ2 conditions and tensile fractures under high σ2 conditions, and the permeability was significantly increased after hydraulic fracturing, in which the shear fractures formed under low σ2 showed a better permeability enhancement effect than the tensile fractures formed under high σ2. The research and analysis in this study can provide reasonable suggestions and feasibility analysis for the design and construction of heavy oil extraction.

Particle-void fabric and effective stress reduction in cyclic liquefaction of granular soils

During undrained cyclic loading, granular soils undergo the reduction of effective stress in the constant volume condition. Revealing the evolution of particle-void fabric in accompany with effective stress reduction provides a significant insight into the fundamental mechanism of cyclic liquefaction. In this study, numerical tests were conducted in DEM simulations to explore the cyclic liquefaction of granular packings with different particle size distributions and void ratios. With the decrease of mean effective stress p' during cyclic liquefaction process, the decrease of particle-void descriptor Ed can be observed for all packings. From micromechanical perspective, large size voids are redistributed and local void distribution around particle becomes relative uniform. The change of particle-void fabric from consolidated state to initial liquefaction state is irrelevant to density of the packing. A power function is further adopted to describe the positive correlation between Ed and p'.

Analysis and numerical simulation of toppling deformation and failure characteristics of anti-dipping rock slope under impoundment

High dam engineering is characterized by high water head, steep slope and complex geological conditions. Excavation or water storage will destroy the original equilibrium state of the bank slope and induce the deformation of the bank slope. Based on the monitoring data, this paper analyzes the spatio-temporal evolution of slope deformation during the impoundment of a reservoir bank in China. On this basis, considering the influence mechanism of water pressure, temperature, rainfall, seepage, rock mass rheology and other factors on slope deformation in the process of water impoundment, a statistical regression model of slope deformation is established, and the regression analysis of the deformation process of typical measuring points is carried out, and the main influencing factors of slope deformation are analyzed. Based on regression analysis, the slope deformation and failure process under impoundment are simulated and the influence degree of effective stress change and parameter weakening of rock mass on bank slope deformation and failure in the process of seepage field change is studied to reveal the main inducement of slope deformation under impoundment through the fractured rock mass seepage-stress-rheology coupling finite element analysis method. The results show that the slope deformation is the rheologic deformation triggered by the decrease of effective stress at slope toe caused by the rise of water level and the softening of rock mass, and the toppling form is the dominant one in space.

Precise Monitoring of Pore Pressure at Boreholes Around Nankai Trough Toward Early Detecting Crustal Deformation

In our recent study, we detected the pore pressure change due to the slow slip event (SSE) in March 2020 at the two borehole stations (C0002 and C0010), where the other borehole (C0006) close to the Nankai Trough seems not because of instrumental drift for the reference pressure on the seafloor to remove non-crustal deformation such as tidal and oceanic fluctuations. To overcome this problem, we use the seafloor pressure gauges of cabled network Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) stations nearby boreholes instead of the reference by introducing time lag between them. We confirm that the time lag is explained from superposition of theoretical tide modes. By applying this method to the pore pressure during the SSE, we find pore pressure change at C0006 about 0.6 hPa. We also investigate the impact of seafloor pressure due to ocean fluctuation on the basis of ocean modeling, which suggests that the decrease of effective normal stress from the onset to the termination of the SSE is explained by Kuroshio meander and may promote updip slip migration, and that the increase of effective normal stress for the short-term ocean fluctuation may terminate the SSE as observed in the Hikurangi subduction zone.

Experimental Evaluation on the Conductivity of Branch Fracture with Low Sand Laying Concentration and Its Influencing Factors in Shale Oil Reservoirs

Abstract The productivity of shale oil reservoirs is mainly determined by the hydraulic fractured reservoir volume. The branch fractures with low sand laying concentration are the main channels connecting the shale matrix and the main fractures. Maintaining the branch fracture conductivity is significant to the production of shale oil. In this study, a series of shale branch fracture conductivity and soaking experiments were conducted using a core flooding device and a small reactor, and the influences of different factors on the fracture conductivity were evaluated. The results show that when the sand laying concentration in fractures is less than 3 kg/m2, the branch fractures present a significant stress sensitivity. Particularly when the sand laying concentration is less than 1 kg/m2, and the closure pressure is greater than 15 MPa, there will be a risk of proppant embedded and fracture closed. The antiswelling agent has an inhibitory effect on the shale swelling. When the concentration of the antiswelling agent in the gel-breaking fluid is 2%, the swelling factor of shale powder is only 1.84%. Comparatively, the 0.5% of the antiswelling agent has a poor effect which can cause the shale rock to crack when the effective stress decreases. It can cause the fracture conductivity to decline by 70-90% when the gel-breaking fluid flows back associated with the shale oil. The probability of core breaking and proppant embedding will increase. The frequent shut-in and rapid open-flow for production can accelerate the damage of fracture conductivity. It is necessary to optimize the fracturing fluid and the open flow scheme to prevent the rapid decline of production in shale oil reservoirs.

Evaluation of synthetic and recorded ground motions for soil liquefaction analyses

In performance-based earthquake engineering, simulated earthquake ground motions are often generated for time history analyses of structures and geotechnical systems. Liquefaction refers to dramatic loss of stiffness and strength of soils during earthquake shaking. As a complicated nonlinear process associated with accumulation of excessive pore pressure and decrease of effective stress due to soil-fluid interaction, whether synthetic motions can be appropriately used in liquefaction assessment still remains a grand challenge. In this study, various synthetic ground motions are generated using different simulation techniques, whose performances are compared with real recorded motions through nonlinear time history analyses of liquefiable grounds with different soil densities and ground inclinations. One-to-one comparison of results in this study clearly demonstrates the importance of preserving accumulative intensity measures of synthetic motions in liquefaction assessment. Strong linear relationships are also found in a log-log space between the maximum ground displacement and evolutionary Arias intensity during post-liquefaction stage for different liquefiable grounds, and the predictive relationships seem not to be affected by the types of synthetic ground motions.

FEM SIMULATION ASSESSMENT OF MACHINING TITANIUM GRADE 2 USING TEXTURED TOOL

This work focuses on the 3D numerical investigation of machinability criteria using textured tools for the dry machining of titanium grade 2. For the development of the 3D finite element model, DEFORM-3D (version 10.1) software was used. Dimple patterns were created nearer to the cutting edge region on the rake face of carbide inserts so as to predict their effect and investigate the machinability criteria. Comparison of dimple-textured tool was being made with a conventional tool based on the cutting forces ([Formula: see text], [Formula: see text] and [Formula: see text], cutting temperature, tool wear, effective stress and effective strain. The numerical study revealed that the dimple-patterned tool improved the machinability of titanium grade 2. The cutting forces (tangential cutting force, feed force and radial force) reduced while machining with a dimple-patterned tool as compared to the conventional nontextured tool. There was an enhancement of tool life and a decrease in effective stress and effective strain while dry turning grade-2 titanium alloy using the textured tool.

Research of the Behavior of Clay Materials with Double Porosity

This contribution presents results from a series of compression and undrained triaxial tests to study the mechanical behavior of dump clay from the north of Bohemia. The use of these materials as a foundation for construction can’t be achieved without the adoption of some precautions. This comes from embankment, formed by digging the ground (altered claystone), up to the level of coal mining which is in a sub horizontal stratigraphic layer. A potential static liquefaction behavior was observed in undrained tests for high confinement stress. A structural collapse was noticed with the results obtained in the triaxial test. This collapse is characterized by an unexpected large decrease in deviator and mean effective stress. The soils formed have strength properties that are potentially dangerous. These concepts can improve the use of these kinds of soils in geotechnical engineering work. It continues and expands the results obtained in previous research, especially the future problematic use of these materials as the foundation soil for line or building structures.

Wave velocities depending on shear strain, directionality, and excess pore water pressure from wildlife liquefaction array

Data from the Wildlife Liquefaction Array, which is located at southern California’s Imperial Valley in the USA, are analyzed. Variations in shear wave (Vs) and compression wave (Vp) velocities are identified during strong motions. Shear strains (γ) are computed from recorded acceleration time series between the sensors. All these variables are compared with the ground-surface accelerations and excess pore water pressure ratio (Ru) at different depths. Results show that Ru starts to accumulate when γ exceeds the threshold strain of 0.01–0.015%. Vs is negatively correlated to Ru and γ. Further investigation shows that Vs degradation depends on maximum horizontal shear strain with the different azimuthal angles at the same time window. By contrast, Vs reduction with Ru is caused by the decrease in effective stress; hence Ru reduces Vs regardless of the azimuthal angles. Vp degradation is limited compared with that of Vs under strong shaking, indicating that the constrain modulus is determined by the combination of soil skeleton and pore fluid.

Methodology to Characterize the Seismic Compression of Unsaturated Sands under Different Drainage Conditions

Abstract This study focuses on a new experimental setup and methodology to characterize the volumetric contraction of unsaturated sands during cyclic shearing under different drainage conditions. A new cyclic simple shear device was developed with suction-saturation control using a hanging column suitable for testing unsaturated granular soils with a maximum suction of 11 kPa. The pore water and pore air pressures are monitored during cyclic shearing using an embedded tensiometer and a gauge pressure transducer protected by a hydrophobic filter, respectively. In addition to describing the specimen preparation techniques and testing methodology, typical results for the seismic compression of nearly saturated, dry, and unsaturated sand specimens during cyclic shearing under different drainage conditions are compared. The differences in the response of these sand specimens were interpreted using the changes in mean effective stress and secant shear modulus during cyclic shearing. In drained conditions, the unsaturated sand specimen showed lower seismic compressions than the dry sand specimen and the nearly saturated sand specimen. In undrained conditions, a greater contraction was observed for the unsaturated conditions due to decreases in mean effective stress and secant shear modulus. The decrease in effective stress occurred due to a decrease in matric suction associated with different rates of increase in pore water and air pressure and an increase in degree of saturation with volumetric contraction.

Methodology for calculate bearing capacity of soft soils under cyclic loading

The bearing capacity in soft soils under cyclic loading has not been widely studied as the static bearing capacity. In soft saturated soils under cyclic load, the phenomenon of cyclic softening occurs, this consists of an increase in the generation of pore-water pressure due to the cyclic load and a decrease in effective soil stresses. The purpose of this work was to propose a methodology for calculating the bearing capacity in soft soils under cyclic loading. The research is based on the results of monotonic and cyclic simple shear tests carried out with samples from the PRAT port of Barcelona. From an analysis carried out on all the experimental data, an equation for the generation of pore-water pressure is proposed. This formulation was implemented in the FLAC2D finite difference software and is part of the methodology. The methodology of the proposed calculation process consists of 6 stages and verification under two criteria. This process is iterative in case of not meeting any of the two verification criteria. Under this methodology of the calculation process, charts are proposed to calculate the bearing capacity of soft soils under cyclic loading. This application takes into account the properties of the soil, the static bearing capacity (P he), and the effective load outside the foundation (q). Also, an analysis of the parameters that most influence the bearing capacity of soft soils under cyclic loading was carried out.

Cyclic Secant Shear Modulus and Pore Water Pressure Change in Sands at Small Cyclic Strains

AbstractWhen fully saturated sandy soil is subjected to cyclic straining in undrained conditions, pore water pressure increases and effective stress decreases. Therefore, it has been assumed that i...

NUMERICAL INVESTIGATION ON PILE BEHAVIOR DUE TO THE RISING GROUNDWATER EFFECT

Since the late 1990s, the changing of groundwater level in the aquifer beneath Bangkok has been realized because the enforcement of groundwater laws and improvement of water supply have been adopted. Then, the groundwater with piezometric drawdown has risen to almost hydrostatic equilibrium. In the principle of soil mechanics, when the porewater pressure in soil mass increases, the effective stress decreases. This can affect underground structures. Under the soft ground condition such as Bangkok, Thailand, pile foundations are commonly constructed to support structures. With decreased effective stresses, the loss of ultimate load-carrying capacity of existing piles can occur, and the structures can be damaged. This research investigates the effects of the groundwater rising on the pile foundation in the Bangkok subsoil. An advanced numerical model with the fully coupled flow deformation analysis, both groundwater flow and soil deformation behaviors with time-dependent conditions are modeled, is conducted by using PLAXIS 2D. The results found that the calculation type of fully coupled flow deformation can express the pile-soil movement behavior. The pile-soil heave and the pile load capacity be significantly reduced during the groundwater rising event. Despite the reduction of pile load capacity with groundwater changed not enough to failure (Factor of safety <1.0), the pile stability should be concerned, and the optimum groundwater level must be controlled.

Evaluation of various seismic response analysis methods for underground structures in saturated sand

Underground structures in liquefiable saturated sand are prone to earthquake damage. Various simplified pseudo-static seismic response analysis methods for underground structures have been recommended by current design codes and guidelines, but lack adequate systematic validation in liquefiable ground. In this study, the effectiveness of four categories of simplified pseudo-static methods in saturated sand are evaluated. These methods include a displacement-based method, a force-based method, a F(flexibility ratio)-R(racking ratio) method, and a detailed equivalent static method. High-fidelity dynamic analysis of circular and rectangular cross-section underground structures in non-liquefiable and liquefiable ground under different input ground motions are conducted, using a validated plasticity constitutive model for large post-liquefaction deformation of sand, to serve as a basis for the evaluation. Although the pseudo-static methods can generally provide adequate analysis results for underground structures in non-liquefiable ground, especially the displacement-based method, their performance in saturated sand are generally poor. The deviation of pseudo-static analysis results from reality stems from their inadequacies in considering the effects of dynamic soil-structure interaction in liquefiable ground. Temporal and spatial variability of soil-structure interaction in saturated sand, caused by the accumulation of excess pore pressure and subsequent decrease in soil effective stress and constraint on underground structures, are not considered in pseudo-static analysis methods. Modifications to the pseudo-static analysis methods and high-fidelity dynamic analysis methods are suggested to be adopted for the evaluation of the seismic response of underground structures in liquefiable ground.

Undrained Seismic Compression of Unsaturated Sand

Unsaturated soils in engineered geostructures like embankments or retaining walls may experience seismic compression during earthquakes due to particle rearrangement associated with large-strain cyclic shearing. Although previous studies have investigated volume changes during drained cyclic shearing of unsaturated soils, undrained cyclic shearing presents a more complex situation. During undrained cyclic shearing, changes in total volume, matric suction, degree of saturation, effective stress, shear modulus, and damping ratio may occur that have complex coupling effects that affect seismic compression. To better understand these coupling effects, this study performed a series of undrained cyclic simple shear tests on unsaturated sand specimens. Contractile volumetric strain after 200 cycles in these tests was found to vary nonlinearly with degree of saturation. The largest volumetric contraction occurred at a degree of saturation of 0.4 and coincided with the largest decrease in mean effective stress. The sand specimens followed a wetting-path scanning curve during shearing, with small changes in matric suction. The decrease in mean effective stress during cyclic shearing for all specimens followed the same linear relation with the accumulation of volumetric strain.

Seismic imaging of the double seismic zone in the subducting slab in Northern Chile

Double seismic zones are commonly observed in the subducting slabs in a global scale, serving as ideal examples for studying the seismogenetic mechanism of the intermediate-depth earthquakes. In this study, we relocate earthquakes and determine seismic velocity models using the double-difference seismic tomography method in the northern Chile subduction zone where a double seismic zone exists. The results suggest that the double seismic zone in northern Chile is located at about 50–140 ​km depth, with an interval of approximately 20 ​km between the two zones. The upper seismic zone is characterized by relatively low Vp (~7.8–8.0 ​km/s), low Vs (~4.4–4.5 ​km/s) and high Vp/Vs (~1.85) above the depth of ~90 ​km, while the region below ~90 ​km is distinguished by relatively high Vp (~8.2 ​km/s), high Vs (~4.8 ​km/s) and slightly high Vp/Vs (~1.75), which may be related to a series of dehydration reactions of hydrous minerals in the subducted oceanic crust. In comparison, the lower seismic zone is featured by the anomaly of low Vp/Vs (~1.7), although some local areas may consist of relatively high Vp/Vs values (~1.8), possibly due to the dehydration reaction of serpentine. Based on the Vp, Vs, Vp/Vs anomalies combined with previous petrological experiments and thermodynamic models, it can be derived that intermediate-depth earthquakes are mainly related to the dehydration of various hydrous minerals in the subducting slab. The dehydration process of hydrous minerals releases water into the subducting slab and subsequently leads to the increase of pore fluid pressure and the decrease of effective normal stress, thus causing the occurrence of brittle failure and intermediate-depth earthquakes in subduction zones. The imaging results of the northern Chile subduction zone further indicate that the existence of the double seismic zone is related to the dehydration process of different hydrous minerals.

Dynamic Wave‐Induced Settlement Behavior of a Caisson Breakwater Built on a Sandy Seabed

Monitored breakwater settlements taken from an actual breakwater structure over an extended period of time (more than five years) were analyzed. The analysis revealed that the waves clearly affect the settlement of the breakwater, especially during high wave conditions such as typhoons. Breakwater settlement is caused by a decrease of effective stress of seabed during partial liquefaction due to wave‐induced cyclic loads, which occurs due to an increase in excess pore pressure and the combination of oscillatory and residual pore water pressures. A new combined numerical model was suggested that allows the storm wave‐induced seabed settlement underneath the caisson breakwater to be examined qualitatively. The technique uses a combined wave model (2D‐NIT) and soil model (FLIP). The dynamic wave load calculated by the 2D‐NIT was used as the input data for the soil model. This soil model can simulate both oscillatory and residual pore water pressures at the same time. There is a different feature to other previous studies adopting similar techniques.