Water Pressure Changes Research Articles

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.

Study on radius of influence of groundwater circulation well in the field experiment

In recent years, groundwater circulation wells (GCW) have gradually become a research hotspot as an In-situ remediation technology. This technology reduces the construction of gas phase extraction wells and saves underground space compared to the traditional Air sparging (AS) remediation technology, the remediation process involves both volatile removal and microbial degradation. This study fully combines the background conditions of an organic contamination site and designs a positive circulation airflow lift circulation well. By analyzing the diffusion law of chloride tracer with aeration time and the changes of water level and water pressure of the monitoring well, it is known that the influence radius of the circulation well can be stabilized at 5.00 m when the aeration volume is 10 L/min. It not only has an essential demonstration significance for the development of groundwater circulation well remediation technology, but also provides reference process parameters for subsequent applications in type site remediation engineering applications.

Numerical analyses of embankment dams containing plastic concrete cut-off walls using finite difference method (A case study: The Karkheh Embankment Dam)

In this paper, numerical analyses have been performed on the Karkheh embankment dam with a clayey core and plastic concrete cut-off wall during construction, impounding, and permanent seepage stages. The dam has 127 meters height and is located in a high seismic hazard zone in Iran. Different stages of construction, water impounding, and steady state seepage were modelled and analyzed using the hyperbolic and Mohr-Coulomb models with the two dimensional finite difference method (FDM). So, nonlinear analyses were performed using FLAC 2D to investigate the settlements and the pore water pressure changes in different zones of the dam during above-mentioned stages and the results were compared to those of the other studies. The results show that at the end of the construction stage, the maximum settlement equal to 1.45m occurs inside the clay core at the height of 65m. Then, after impounding of the reservoir and steady state stage, the maximum magnitude of the horizontal deformations occurs in the downstream of the dam equal to 0.55m; however, these magnitudes reach to 0.17m at the crest of the dam. Moreover, it was shown that the maximum horizontal displacement of the plastic concrete cut-off wall has happened at the top of the wall in the clay core which is in a good agreement with the other studies’ result.

The Influence of Shell Permeability on Stability of Upstream Slope during Rapid Drawdown – Khassa Chai Earth Dam as a Case Study

Several factors affect the stability of earth dam during sudden drawdowns such as permeability and mechanical properties of soil, upstream side slope, drawdown ratio, and drawdown rate. This paper investigates the influence of shell permeability on earth dam upstream slope stability and its role in the change of pore water pressure at different locations of the embankment during the sudden drawdown, using different limit equilibrium methods. To accomplish the objective of this study, (Geo Studio 2012 Software) as one of the powerful geotechnical programs was used for the modeling and numerical analysis. The study shows that decreasing in the shell permeability resulted in the reduction of pore water pressure dissipation and variation of shell hydraulic conductivity plays a vital role in the overall stability of the upstream slope under rapid drawdown conditions.

Deterioration and Cavity of Surrounding Rocks at the Bottom of Tunnel Under the Combined Action of Heavy-Haul Load and Groundwater: An Experimental Study

In China, the first tunnel was built in accordance with the 30-ton heavy-haul railway standard. Based on the change in water and soil pressure obtained from long-term on-site monitoring, the cavity mechanism of the surrounding rock at the bottom of a heavy-haul railway tunnel under rich water conditions was explored in this study. The cavity characteristics and degradation depth of the three types of surrounding rock under different axial loads and hydrodynamic pressures were analyzed through laboratory tests. The structural defects at the bottom of the tunnel and local cracks in the surrounding rock were determined to provide a flow channel for groundwater. The dynamic load of heavy-haul trains causes groundwater to exert high hydrodynamic pressure on the fine cracks. The continuous erosion of the bottom surrounding rock leads to a gradual loss of surrounding rock particles, which would further exacerbate with time. The cohesive soil surrounding rock is noticeably affected by the combined action of heavy-haul load and groundwater in the three types of surrounding rock, and the surrounding rock cavity is characterized by overall hanging. In the simulation experiment, the particle loss of the surrounding rock reached 1,445 g, which is 24.2% higher than that of the pebble soil surrounding rock and 40.8% higher than that of sandy soil surrounding rock. The findings of this study could be helpful for developing methods for defect prediction and treatment of heavy-haul railway tunnels.

Pore water pressure responses of saturated sand and clay under undrained cyclic shearing

In this study, changes in the pore water pressure were observed for saturated specimens of a loose fined-grain sand (Nam O sand) and a soft silty clay (Hue clay) subjected to undrained cyclic shearing with different testing conditions. The cyclic shear tests were run for relatively wide range of shear strain amplitude (g = 0.05%-2%), different cycle numbers (n = 10, 50, 150 and 200) and various shear directions (uni-direction and two-direction with phase difference of q = 0o, 45o and 90o). It is indicated from the experimental results that under the same cyclic shearing condition, the pore water pressure accumulation in Hue clay is at a slower rate, suggesting a higher cyclic shear resistance of Hue clay than that of Nam O sand. Liquefaction is reached easily in nominally 50% relative density specimens of Nam O sand when g ³ 0.4%, meanwhile soft specimen of Hue clay is not liquefied regardless of the cyclic shearing conditions used in this study. The threshold number of cycles for the pore water pressure generation generally decreases with g meanwhile, the threshold cumulative shear strain for such a property mostly approaches 0.1%. In addition, by using this new strain path parameter, it becomes more advantageous when evaluating the pore water pressure accumulation in Nam O sand and Hue clay subjected to undrained uni-directional and two-directional cyclic shears.

Study on the Variation Rule and Characteristics of Pore Water Pressure in the Failure Process of Saturated Rock

With the development of tunnels and other engineering constructions into the deep strata, rock masses are more prone to dynamic damage such as rock bursts under in situ conditions and excavation disturbances. The pore water in the rock mass will produce pressure changes during this process. According to the relationship between the change of pore water pressure and the development of rock mass damage, the variation rule and precursor characteristics of pore water pressure in the process of rock mass failure can be found. In this paper, through mechanical analysis, the evolution law of pore water pressure during the failure process of saturated rock is obtained. The study found that, in the process of rock failure, the pore water pressure presents three stages of linear growth, transition, and decrease. The rise and fall of pore water pressure are closely related to rock damage and influence each other. Through the observation of pore water pressure during coal mining, it is found that the coseismic effect of pore water pressure is significant. It is proved that there is a close correlation between the evolution of the stress field in the surrounding area of the stope and the change of pore water pressure in the surrounding area under the effect of mining disturbance. During the engineering practice, dynamic monitoring can be carried out on the change of pore water pressure inside the rock mass according to the law, and the precursor information of rock mass instability and failure can be explored.

Data‐Driven Inference of the Mechanics of Slip Along Glacier Beds Using Physics‐Informed Neural Networks: Case Study on Rutford Ice Stream, Antarctica

AbstractReliable projections of sea‐level rise depend on accurate representations of how fast‐flowing glaciers slip along their beds. The mechanics of slip are often parameterized as a constitutive relation (or “sliding law”) whose proper form remains uncertain. Here, we present a novel deep learning‐based framework for learning the time evolution of drag at glacier beds from time‐dependent ice velocity and elevation observations. We use a feedforward neural network, informed by the governing equations of ice flow, to infer spatially and temporally varying basal drag and associated uncertainties from data. We test the framework on 1D and 2D ice flow simulation outputs and demonstrate the recovery of the underlying basal mechanics under various levels of observational and modeling uncertainties. We apply this framework to time‐dependent velocity data for Rutford Ice Stream, Antarctica, and present evidence that ocean‐tide‐driven changes in subglacial water pressure drive changes in ice flow over the tidal cycle.

Impacts of COVID-19 social distancing policies on water demand: A population dynamics perspective

Social distancing policies (SDPs) implemented in response to the COVID-19 pandemic have led to temporal and spatial shifts in water demand across cities. Water utilities need to understand these demand shifts to respond to potential operational and water-quality issues. Aided by a fixed-effects model of citywide water demand in Austin, Texas, we explore the impacts of various SDPs (e.g., time after the stay home-work safe order, reopening phases) using daily demand data gathered between 2013 and 2020. Our approach uses socio-technical determinants (e.g., climate, water conservation policy) with SDPs to model water demand, while accounting for spatial and temporal effects (e.g., geographic variations, weekday patterns). Results indicate shifts in behavior of residential and nonresidential demands that offset the change at the system scale, demonstrating a spatial redistribution of water demand after the stay home-work safe order. Our results show that some phases of Texas's reopening phases had statistically significant relationships to water demand. While this yielded only marginal net effects on overall demand, it underscores behavioral changes in demand at sub-system spatial scales. Our discussions shed light on SDPs' impacts on water demand. Equipped with our empirical findings, utilities can respond to potential vulnerabilities in their systems, such as water-quality problems that may be related to changes in water pressure in response to demand variations.

Soil structure effects on deformation, pore water pressure, and consequences for air permeability during compaction and subsequent shearing

Soils may react to the impact of wheeling with volume-changed deformation by compaction and volume-constant deformation by shearing. The different deformation mechanisms with their deteriorated effects on soil pore functions have been studied frequently, but less is known about the compaction/shearing-induced deformation on structured soils as compared with homogenized substrates. In order to document both the effect of aggregation on soil deformation behaviors as well as the sensitivity due to texture, compaction and shearing measurements were performed on soil samples from the A- horizon of a homogenized Luvisol (a silt loam with a bulk density of 1.37 g cm-3) and two structured Gleysols (clay loam soils with bulk densities of 1.34 g cm-3 and 1.11 g cm-3). The changes of vertical settlement (ΔH), air/water-filled pores (ε/θ), air permeability (ka), and pore water pressure (uw) during compaction and subsequent shearing were studied. Results showed that the soil deformation behaviors due to compaction and shearing depended highly on the level of applied normal stress, especially on structured soils. At a normal stress of 50 kPa, which was smaller than the precompression strength, the structured Gleysols had only minor compaction-induced deformation (ΔH: 0∼1 mm) and a contractive shear behavior. At a high normal stress of 200 kPa, which exceeded the precompression strength, both homogenized and structured soils displayed greater compaction-induced deformation (ΔH: 3.5∼6.0 mm) and a dilative shear behavior. Compared with static loading, cyclic loading resulted in further deformation and dilative shear behaviors in both structured and homogenized soils. In addition, the structured soils showed a smaller decrease in ε/θ and maintained 10 times higher ka value than homogenized soils. However, shearing reduced the inter-aggregate pore continuity and enhanced the relative functionality of the sheared intra-aggregate pores, as was proofed by the more pronounced changes of uw (Δuw) in the structured soils (from -93.0 hPa to +335.6 hPa) compared with that in the homogenized silt loam (from +2.1 hPa to +140.2 hPa). In conclusion, the well-structured clayey soils exhibited less deformation during compaction compared with the homogenized (tilled) soil with coherent structure and more silty texture. The dynamic stress application and shearing resulted in more intense weakening of soil structure because the accessibility of particle surfaces for mobilized water coincides with an enhanced stress dependent swelling and sliding due to the rapidly increased uw.

Demon Monsters or Misunderstood Casualties?

Demon Monsters or Misunderstood Casualties?

Evaluation of Negative Skin Friction on Bridge Abutment Piles

Piles are used to transfer loads of structures to deeper and stronger soil layers through skin friction and/or end bearing. Surcharge loads, site grading, or dewatering may induce downward movement of soil adjacent to piles installed in a compressible medium. This movement creates negative skin friction stresses acting downward at the pile-soil interface, which applies additional loads “drag forces” to the pile causing a maximum axial load in the pile shaft at the “neutral plane”. To evaluate the development of drag forces, a comprehensive field monitoring program was conducted over four years for three instrumented abutment H-piles as part of a three-span bridge project. The soil settlement and changes in pore water pressure in the soil adjacent to the piles due to the construction of an approach embankment were monitored using multiple-point extensometers and vibrating wire piezometers. The piles’ elastic settlement and strains were measured using single-point extensometers and vibrating wire strain gauges. The field measurements are presented and discussed in terms of responses time histories and load distribution along one pile shaft. In addition, the calculated forces from vibrating wire strain gauges are compared with the unified design method prediction considering the total stress method (α-method) for cohesive soils. The results show that the maximum drag force was developed after the complete dissipation of excess pore water pressure and that the location of neutral plane varied during the embankment construction stages. Employing the total stress method in the unified design method provided a reasonable prediction of the drag force and the neutral plane’s location.

Calculation Method to Determine the Zone of Underground Damage in Water Supply System Pipelines

The application of water conservation and leakage prevention at all stages of water delivery to the consumer can provide the rational and efficient use of water resources. The long length of heating and water supply networks, their high degree of wear and tear lead to the appearance of leaks. The paper describes research results on developing the calculating method for detecting underground damage zones in the water supply system pipeline. The basis for this study is the main difference between the regulated water intake for the needs of the water user and the uncontrolled loss of water from a damaged pipeline. The water user takes water from the water supply system in accordance with the designed parameters, and such a water intake will not cause unusual changes in the water pressure in the pipeline. The volume of water lost through pipeline damage will always be proportional to the sharp water pressure change in the pipeline. We developed the mathematical model to substantiate the detecting method of the pipeline damage zone. The model considers the operational characteristics of pipelines to the full extend: head and water flow, the degree of deterioration of water conduit, the geodetic position of water intake points. The mathematical model, confirmed earlier by dozens of independent experiments, made it possible to replace expensive hydraulic experiments and prove the possibility of using the developed method of detecting hidden damages of water conduits by extrapolating the measured piezometric heads. The proposed software system for calculating the underground pipeline damage zone in water supply systems makes it possible to determine the pipeline damage hidden zones based on solving the hydraulic problem for a linear pipeline and studying hydraulic processes. We can use the created optimization model as a proven analog of a physical hydraulic stand in hydraulic studies in the simulation mode.

Tracing of stresses and pore water pressure changes during a multistage modified relaxation test model on organic soil

Tracing of stresses and pore water pressure changes during a multistage modified relaxation test model on organic soil

A case study of vacuum tube-well dewatering technology for improving deep soft soil in Yangtze River floodplain

Deep soft soil of the Yangtze River floodplain in Nanjing has a special interlayer structure, which provides favorable conditions for the application of vacuum tube-well dewatering technology. This paper focuses on the design, construction and treatment effect of vacuum tube-well dewatering technology. The hydrological parameters and the hydraulic connections between the various layers of the site have been ascertained through two simple single-well multi-holes pumping tests. Moreover, the layout of vacuum tube wells and depressurization wells in a test site with 84 m length and 84 m width was designed based on the above parameters. Field tests of vacuum tube-well dewatering technology were conducted on the test site. And the changes in groundwater level, ground settlement and pore water pressure during the test was monitored. The CPTU test technology was used to quickly evaluate the engineering properties of the site before and after treatment. Finally, combined with the settlement monitoring data, the hyperbolic method was used to predict the final settlement. The results indicated that the consolidation efficiency of vacuum tube-well dewatering technology has obvious advantages. Implications of this study can provide a reference for the construction design of the site's ground improvement consolidation and other similar projects.

PERBANDINGAN HASIL UJI KAPASITAS DUKUNG TIANG PANCANG PADA TANAH LEMPUNG JENUH SKALA LABORATORIUM DENGAN PERHITUNGAN ANALISIS STATIS

Pile bearing capacity, is an important parameter in infrastructure design. The pile driving process, especially in saturated clay soils, results in an increase in pore pressure which affects the bearing strength capacity of the pile foundation immediately after erection. By knowing the com-parison of various ways to calculate the bearing capacity of a pile foundation, especially in clay soils, various influencing parameters can be studied and to be taken in determining the bearing ca-pacity of a pile foundation. The test is carried out by comparing the value of the pile bearing capacity of a laboratory scale pile model with the results of the calculation of the pile bearing capacity using the static analysis method based on the results of the test parameters of the soil. The clay soil con-ditions in the test are limited to saturated conditions. The reading of the change in pore water pres-sure is carried out using a pressure transducer during the pile driving process. Based on the pile loading test results in the laboratory, the pile bearing capacity results were 0.31 kN, while the bearing capacity results based on the static analysis method were 0.75 kN. The difference in pile bearing carrying capacity is caused by excess pore water pressure that appears around the ground during the driving process.

Active earth pressure of 3D earth retaining structure subjected to rainfall infiltration

Rainfall infiltration is a key factor that determines the active earth pressure on an earth-retaining structure. Previous investigations on the stability of earth retaining structures were mainly conducted under the assumption that the backfills was dry, saturated, or subjected to a steady unsaturated flow, ignoring the time-varying characteristics of the rainfall infiltration process. Based on the upper bound theorem of limit analysis, this study conducted a stability analysis of a three-dimensional (3D) earth retaining structure (ERS) subjected to various rainfall infiltration patterns. An analytical model that was, capable of calculating the pore water pressure distribution due to rainfall infiltration, was employed to calculate the time-dependent pore water pressure change of unsaturated soil. A horizontal slice method was employed to take into account the change of effective unit weight of soil. Afterward, an analytical expression of the active earth pressure coefficient is derived from the energy balance equation. The effects of the 3D characteristics of ERSs and rainfall patterns on the active earth pressure coefficient and the failure pattern of ERSs were investigated. It was found that compared with plain strain conditions, the 3D characteristics of ERSs lead to a lower value for the active earth pressure and better stability. The rainfall patterns not only directly determined the stability and active earth pressure of an ERS, but also played a dominant role in critical failure pattern of the ERS.

The Undrained Characteristics of Tengger Desert Sand from True Triaxial Testing

Aimed at the characteristics of aeolian sand under rapid construction conditions in desert geotechnical engineering, a series of the true triaxial undrained test were carried out on the GDS apparatus. The 3D deformation, failure, and other characteristics of the dense sand are obtained. Under the condition of same p c , the state transition point where the void water pressure changes from increasing to decreasing appears earlier and leads to enhanced dilatancy with the increase of b, which means the enhanced dilatancy of dense sand caused the increase in strength. The results of the same b shows that the void water pressure generally indicates a decrease at low confining pressure and an increase at high confining pressure, indicating that the aeolian sand shows dilatancy at low confining pressure and contraction at high confining pressure. The state transition point increases with the increase of p c , but all points tend to the same critical state line and state transition line. When b = 0, the critical state line is q = 1.57 p ′ , and the state transition line is q = 1.23 p ′ . When b = 1, the critical state line is q = 1.24 p ′ , and the state transition line is q = 1.04 p ′ . The results at same b obtained the unified critical state line and the state transition line. Therefore, the true triaxial test can obtain the unified relationship of void ratio, p c and b, which overcomes the fact that the existing test cannot consider the influence of b. The test results provide a basis data for the design, construction, and maintenance of geotechnical engineering in Tengger Desert.

Failure mechanism and stability analysis of bank slope deformation under the synergistic effect of heavy rainfall and blasting vibration

Underwater blasting technique is often adopted to clear away the obstructed reef and dredge-cut in the mountainous waterway dredging engineering. However, the vibration of the underwater blasting and excavation of the riverbed has a certain influence on the stability of bank slopes. Combined with the partial bank collapse phenomenon in the waterway regulation project between two dams in the Three Gorges reservoir area, the change of pore water pressure of unsaturated soil under heavy rainfall and the increase of permanent displacement of soil under blasting vibration in the reservoir area are discussed. This paper explains the weakening effect of rainfall infiltration and blasting vibration on the stability of the bank slope. By studying the law of slope failure deformation under the separate action and coupling action, it is found that rainfall infiltration promotes the increase of slope permanent displacement and the blasting vibration enlarges the infiltration area of the soil, which demonstrated the accelerated failure mechanism of the rainfall-blasting slope.

Causes analysis of shield tunnel cracks in deep stratum with high water pressure

In the process of shield tunneling, there is a risk of cracks and diseases caused by geological changes, construction process, design and other factors, and leads to the construction process and even engineering accidents. This paper analyzes the cracks on the top of the segment under the geological conditions of deep burial and high water pressure by analyzing the cracks of a traffic rail project in Shenzhen, and analyzes the reasons for the cracks under normal construction. Based on the conventional stress model of segment and the screening of surrounding geological and hydrological conditions, the model assumption of local stress of integral segment is made, and the modified idiom and beam method are adopted- The results show that under the geological condition of fractured water stratum, the sudden change of water pressure caused by rainstorm and the joint action of broken rock block and segment floating will lead to the reduction of segment top space and direct contact with rock stratum, which will exceed the normal pressure range of segment and lead to cracks in the top segment.