Negative Pore Water Pressure Research Articles

Study on Creep-consolidation Coupling Characteristics of Soil of Deep Foundation Pits in Soft Clay Regions

This paper starts from the mechanism of the coupled effect of consolidation and creep of soft soil, regarding the effect of time, adopting time hardening and Druker-Prager yield criterion coupled creep model, studies the variation law of the excess pore water pressure of soil with time after foundation pit excavation through numerical simulation. The results show that unloading of foundation pit excavation will produce negative excess pore water pressure in the pit bottom and surrounding soil, and the maximum pore water pressure (absolute value) will occur in the center of the foundation pit; after the the excavation completion of the foundation pit, the pore water pressure is continuously dissipated, and the pore pressure of the soil before the pit is dissipated faster, while the soil behind the pit is relatively slow. The excess pore water pressure after dissipation is basically the same as the ultra-static pore pressure when the excavation is completed; due to the impermeability of the underground continuous wall, the pore pressure boundary is very complicated, resulting in a sudden change of the excess pore water pressure behind the wall front wall.

Laboratory tests of electro-osmotic consolidation combined with vacuum preloading on kaolinite using electrokinetic geosynthetics

Laboratory tests were conducted on kaolinite to investigate the effectiveness of electro-osmotic consolidation combined with vacuum preloading using electrokinetic geosynthetics (EKG). The results showed that the combined method could remove more water and induce larger surface ground settlements compared with the traditional vacuum preloading or electro-osmotic consolidation. Vacuum preloading was quite effective during the first 4 h, though the electro-osmotic consolidation took a main role in dewatering process after 9 h. The combined method could also hinder the development of cracks, induce higher negative pore water pressure and hence increase the efficiency of electro-osmotic consolidation. The results showed that deep electro-osmotic consolidation technique combined with vacuum preloading could result in significant water removal efficiency along with shorter electrode length. Furthermore, both electro-osmotic consolidation and the combined method could consolidate the soil efficiently with low energy consumption.

Effect of water infiltration on the mechanical behaviour of unsaturated soil

Natural slopes and embankments are generally unsaturated in nature with negative pore water pressure adding to the shear strength and hence the stability of the slopes. During the event of rainfall, pore water pressure becomes less negative or even positive as a result the shear strength of soil decreases and failure occur. Therefore, the strength and deformation characteristics for unsaturated soil become important when analysing the stability of these types of slopes. In this study, a triaxial test apparatus was used to study the effect of water infiltration on the mechanical behaviour of unsaturated soil. The test results show that water infiltration decreased with increase in net confining stress and the stress paths were independent of the matric suction, the net confining stress and the shearing conditions for the present experimental study.

Assessing the Effect of Density and Water Level on the Degree of Compaction of Sand Using Dynamic Cone Penetration Test

Saudi Arabia has witnessed unparalleled development in all types of construction, especially in eastern Saudi Arabia where the major oil and petrochemical industries are located. The major projects are mostly located on sandy soil, and it is difficult to assess the physical properties of this soil using conventional methods. Consequently, laboratory and field studies were conducted to evaluate the density of sandy soil utilizing dynamic cone penetration test (DCPT). The laboratory study was conducted on sand specimens prepared with different relative densities (40%, 60% and 90%). Additionally, the effect of the level of water table on DCPT values was investigated. The density of soil at two field sites was assessed by using DCPT and nuclear gauge. Test results indicated that an increase in the dry density and shear strength was associated with a decrease in the dynamic cone penetration index. Further, the variation in the water table level had a significant impact on DCPT results due to the changes in the effective stress. The DCP resistance for fully drained sand increased significantly by as much as 170% compared with the fully submerged or dry sand, due to the development of negative pore water pressure. Further, the results of this investigation have shown that DCPT is an effective and reliable tool to assess the degree of compaction of sandy soils. Reliable correlations between density, relative density and degree of compaction of sandy soils have also been developed.

A Tensile Strength Apparatus with the Facility to Monitor Negative Pore-Water Pressure

Abstract This article presents a new testing method for investigating the behavior of clayey geomaterials subjected to a tensile (negative) total stress. The method includes the use of high-capacity tensiometers to measure the pore–water pressure of the test specimen, an aspect which has not been demonstrated in any other direct tensile testing method. This addition allows interpretation of failure data in terms of effective stress rather than total stress, which is the approach that should be pursued in the saturated range. The test specimen shape and loading method have been modified from those commonly seen in existing literature to ensure that the direction of the major principal stress in the failure zone coincides with the direction of the externally applied tensile force, allowing for a more accurate analysis of tensile failure. Results are shown for saturated specimens and compared to results obtained for the same soil in uniaxial compression, using a modified version of the presented uniaxial tensile method, and a triaxial compression test. It is demonstrated that crack initiation occurs by shear failure if the data are interpreted in terms of effective stress rather than total stress and that the failure mechanisms under tension do not differ from compression.

Effects of temperature on the shear strength of saturated sand

The effect of temperature on the shearing response of saturated, dense sand was investigated using a series of temperature-controlled, isotropically-consolidated, hollow cylinder triaxial compression tests, where specimens were heated in drained conditions followed by shearing in undrained conditions. As expected, the deviatoric stress at the peak state (i.e., the undrained shear strength) was observed to increase with increasing initial mean effective stress. However, it was observed to decrease linearly with increasing temperature. The effects of temperature on the deviatoric stress at the peak state were attributed to a linear increase in the magnitude of negative shear-induced pore water pressure at the peak state with temperature. The relationship between the undrained shear strength and the pore water pressure with changes in temperature was represented well by linear equations. When the shear strength was interpreted in terms of the critical state, no obvious changes in the critical state line in the p′-q plane were observed, and the critical state friction angle was unaffected by temperature. During drained heating, the dense sand specimens were observed to expand volumetrically, causing the normal consolidation line in the e-p′/pa0.5 plane to shift upward with increasing temperature without a change in the slope. The negative pore water pressure during undrained shearing caused the state paths of the dense sand specimens to move to the right. As the magnitude of negative pore water pressure increased with increasing temperature, no obvious effects on the critical state line in the e-p′/pa0.5 plane were observed.

Mechanism of electro-osmotic chemical for clay improvement: Process analysis and clay property evolution

Electro-osmotic chemical methods were applied in a series of geotechnical studies in which various solutions were injected into clay samples, increasing the undrained shear strength of the clay. Such methods resulted in a series of phenomena and in clay property evolution. In this study the current, temperature, settlement, drainage volume, and electrical potential distribution during the electro-osmotic experiments were monitored as process data, and the electrical conductivity in clay, calcium content, pH, water content, and undrained shear strength after the electro-osmotic chemical experiments were measured as clay property evolution data. Based on the process data, clay property data, and relevant theories, the mechanism of clay reinforcement with electro-osmotic chemicals was comprehensively explained. The development of an electro-osmotic permeability coefficient difference and the permeable drainage boundary condition resulted in a negative pore water pressure distribution shaped as an arch, which resulted in the water content having the same distribution and induced settlement. Because of the gathering of Ca2+ and H+ near the anode, a chemical cement reaction occurred near the cathode. The electrical potential gradient near the anode was lower than that near the cathode. Because of the chemical cement reaction between Ca2+, OH−, and SiO32−, the undrained shear strength near the cathode was much higher than that near the anode. In addition, a coupling analysis of the electro-osmotic chemical process and clay property evolution is presented at the end of the discussion.

Frost heave and frost heaving-induced pressure under various restraints and thermal gradients during the coupled thermal–hydro processes in freezing soil

Studies of frost heaving-induced pressure (FHIP) have been gaining increasing attention for applications using the freezing method to strengthen soils. This paper demonstrates a technique for measuring the FHIP when heaving is constrained. A series of freezing tests were conducted under various restrained stiffnesses and associated with a thermal gradient. The evolution of frost heave and the FHIP during coupled hydro–thermal interaction were examined. From this study, it was found that restraint prevents frost heave by impeding formation of the ice lens. A thermal gradient is a necessary condition for both water flow and frost heave, since pore water solidifies into ice and thus causes suction (negative pore water pressure) at the base of the ice lens. The pore structure and flow properties of freezing soil vary, since ice crystals progressively block the flow of water, whilst discontinuous ice lenses result in variation of water distributions. The increase of the FHIP appeared to cease when the ice pressure reached a maximum value, based on the microscopic analysis of equivalent water pressure. Moreover, the stable stage for the FHIP lagged behind the stabilization temperature. A macroscopic analysis of the different FHIPs under various different restraints was also carried out. It was found that increased restrained stiffness caused increased deformation and resulted in an increase of the observed FHIP. The coupled hydro–thermal behaviors analyzed in this study enable a better understanding of heat transfer and fluid flow in freezing granular media (soils).

Analytical solutions of pore-water pressure distributions in a vegetated multi-layered slope considering the effects of roots on water permeability

New analytical solutions of pore-water pressure (PWP) distributions in a vegetated multi-layered slope are derived considering the effects of roots on water permeability. PWP distributions are significantly affected by root-induced reductions in water permeability due to increases in the root volume ratio (Rv). Under drying conditions, negative PWP increases with Rv. The negative PWP induced by root water uptake increases with the desaturation coefficient (αi) or the ratio of the transpiration rate to saturated water permeability (Tp/ksi). As the overall water permeability underneath the vegetation layer decreases, the negative PWP induced by root water uptake increases.

Field instrumentations and monitoring of GeoBarrier System for steep slope protection

Abstract This paper illustrates the application of capillary barrier system as protection of steep slope from rainfall-induced slope instability. In this regard, the mechanical instability of the slope is protected by a reinforced soil system with geobag as facing and geogrids as slope reinforcement. Combination of slope reinforcement and capillary barrier system to protect steep slope forms a GeoBarrier System (GBS). The capillary barrier was constructed using recycled concrete aggregate (RCA) of different particle sizes as fine- and coarse-grained layers. The fine-grained material was encapsulated in a geobag while the coarse-grained material is laid between the reinforced soil and the geobag. A pilot study was carried out in Singapore to evaluate the performance of the GBS when subjected to rainfall infiltration. The site was instrumented with a rain-gauge and two piezometers to monitor rainfall and groundwater response to rainfall. The slope was instrumented with tensiometers and soil moisture sensors as well as earth pressure cells to monitor the response to rainfall infiltration in terms of matric suction and deformation. An adjacent original slope was also instrumented to investigate the effectiveness of the GBS in reducing rainfall infiltration and maintaining the negative pore-water pressure. The detailed installation and instrumentation of the GBS and original slopes are discussed in this paper. Results show that the GBS was effective in maintaining the negative pore-water pressure; therefore, in maintaining the stability of the slope. On the other hand, the negative pore-water pressure and lateral earth pressure in the original slope were more affected by rainfall infiltration.

The Behavior of Bushehr Carbonates Sand in the Persian Gulf Under Different Intermediate Principal Stresses

The design of geotechnical structure acting under multi-axial loading requires knowledge about the soil behavior under intermediate principal stress. Even though conducting the numerous experiments and numerical researches about the behavior of common standard silica soils under 3-D loading, few investigations are made for soils with the dilative behavior. This paper aims to study the behavior of Bushehr carbonate sand from a north shelf of Persian Gulf under various intermediate principal stresses by conducting the hollow torsional shear tests. The test result implies that the plane strain condition occurs when an intermediate principal parameter (b) is equal to 0.3. Under this condition, soil gets the maximum value in ultimate negative pore water pressure, ultimate deviatoric strain, effective frictional angle and the radial strain becomes close to zero. Maximum radial strain and minimum deviatoric strain occur under the triaxial condition where the b is 0. Besides, under b = 0 condition, soil has the lowest value of ultimate negative pore water pressure and effective friction angle. The considerable effect of intermediate principal stress on the soil behavior is found in samples packed with loose state.

Cavitation in high-capacity tensiometers: effect of water reservoir surface roughness

High-capacity tensiometers (HCTs) are sensors made to measure negative pore water pressure (suction) directly. In this paper, a new approach is proposed to expand the range and duration of suction measurements for a newly designed HCT. A new technique is employed to reduce significantly the roughness of the diaphragm’s surface on the water reservoir side in order to minimise the possibility of gas nuclei development and the subsequent early cavitation at the water–diaphragm interface. The procedures employed for the design, fabrication, saturation and calibration of the new tensiometers are explained in detail. Furthermore, the performance of the developed HCTs is examined based on a series of experiments carried out on a number of unsaturated clay specimens. An improvement in maximum sustainable suction in the range of 120–150% of their nominal capacity was obtained from different surface treatment methods. Moreover, the results show an improvement of up to 177% for the long-term stability of measurements, compared to the developed ordinary HCTs with untreated diaphragms.

Excess pore water pressure due to ground surface erosion

• The pore water pressure in an aquifer under an eroding surface with an initial unsaturated zone is found in closed form. • A method based on the Laplace transform is used to handle the moving upper boundary of the domain. • Over an impermeable layer the pressure decreases for low erosion rates. • For high erosion rates it, the pressure can increase temporarily before decaying. The Laplace transform is applied to solve the groundwater flow equation with a boundary that is initially fixed but that starts to move at a constant rate after some fixed time. This problem arises in the study of pore water pressures due to erosional unloading where the aquifer lies underneath an unsaturated zone. We derive an analytic solution and examine the predicted pressure profiles and boundary fluxes. We calculate the negative pore water pressure in the aquifer induced by the initial erosion of the unsaturated zone and subsequent erosion of the aquifer.

THE EFFECT OF MATRIC SUCTION ON THE SHEAR STRENGTH OF UNSATURATED SANDY CLAY

The shear strength parameters of the soil is the key engineering property required for the designof geotechnical structures and stability analysis. The shear strength of an unsaturated soil is controlled by matricsuction. This research examined the effect of matric suction on the shear strength behavior of sandy clay usinga series of experimental tests in the laboratory. The contact filter paper method was used to measure matricsuction and the SoilVision knowledge-based system was used to give an approximation of the entire soil watercharacteristic curve (SWCC). The shear strength of the unsaturated sandy clay was examined using a series ofunconsolidated undrained (UU) triaxial tests to identify the relationship between shear strength, cohesion,internal friction angle, and matric suction. The result shows that the shear strength of the soil increases asmatric suction increases. It was found that when the matric suction is smaller than air entry value (AEV), theshear strength of the soil increases as matric suction increases and the increment is linear because the soil is insaturated conditions. At a certain matric suction value (on soil samples occurred at 40-600 kPa matric suctionvalues), there is a significant increase in the shear strength of soil associated with the inter-particle forceproduced due to negative pore water pressure. Furthermore, at a high matric suction, the relationship betweenshear strength and matric suction tend to be stable as in a low water content, the matric suction is not transmittedeffectively to the contact point of soil particles.

Hydraulic and mechanical characteristics of compacted sand–bentonite: tyre chips mix for its landfill application

Compacted sand–bentonite mixtures are generally placed at waste dumping places as a means of avoiding contaminant movement to reduce or remove the potential for groundwater pollution. Owing to their almost impermeable nature and high pollutant adsorption tendency, bentonite generally treated as a liner material to avoid subsurface contamination. Cracks are formed in the landfill liner due to the desiccation, large interlayer shrinkage and differential settlement of bentonite. Fibres are commonly added to sand–bentonite mixture to act as a reinforcing material to eliminate the development of tensile cracks. Waste tyre chips are derived from the tyre retreated technique, and it can be included as short fibre in sand–bentonite mixture for the use as landfill barrier material. Hydraulic conductivity and shear strength are the two important parameters of the liner material, which are considered for hydraulic, and stability analysis in landfill application. In this context, consolidation and consolidated undrained tests were performed to study the hydraulic and mechanical behaviour of sand–bentonite mixture mixed in a proportion of 80:20 and mixed with 0, 5, 10 and 15% of waste tyre chips. From the consolidation test, swelling pressure and swelling potential reduced maximal at 15% of tyre chip content. Hydraulic conductivity and negative pore water pressure altered significantly with the application of tyre chips. Effective internal angle of friction enhanced from 24.9° to 30.5° and critical state parameter improved from 0.738 to 1.429 with the presence of 15% of tyre chips. Test result suggested that maximum of 10% of tyre chips can be used as a landfill liner and 15% tyre chips would be benefited for landfill cover.

Ultimate bearing capacity of reinforced saturated ground

Results of model tests on saturated horizontal ground reinforced with geosynthetic reinforcement are analysed using an extended formula for the ultimate bearing capacity ratio (BCRr). Three components are considered in this formula, namely the bearing capacity ratio induced by the deep-footing mechanism (BCRd), that induced by the wide-slab effect (BCRs) and that induced by apparent cohesion (or negative excess pore water pressure) (BCRc). The calculated BCRr values for saturated reinforced ground agree well with measured values (BCRm) within a maximum error of 12·5%. For a low reinforcement placement density (Rd = 2), values of BCRd can be as high as 70% of the BCRr, suggesting the importance of the deep-footing effect in a lightly reinforced ground. In the case of high Rd (= 12), the contributions of BCRs and BCRc to BCRr become significant for saturated reinforced grounds.

Response of a loess landslide to rainfall: observations from a field artificial rainfall experiment in Bailong River Basin, China

Rainfall-induced landslides are a significant hazard in many areas of loess-covered terrain in Northwest China. To investigate the response of a loess landslide to rainfall, a series of artificial rainfall experiments were conducted on a natural loess slope, located in the Bailong River Basin, in southern Gansu Province. The slope was instrumented to measure surface runoff, pore water pressure, soil water content, earth pressure, displacement, and rainfall. The hydrological response was also characterized by time-lapse electrical resistivity tomography. The results show that most of the rainfall infiltrated into the loess landslide, and that the pore water pressure and water content responded rapidly to simulated rainfall events. This indicates that rainfall infiltration on the loess landslide was significantly affected by preferential flow through fissures and macropores. Different patterns of pore water pressure and water content variations were determined by the antecedent soil moisture conditions, and by the balance between water recharge and drainage in the corresponding sections. We observed three stages of changing pore water pressure and displacement within the loess landslide during the artificial rainfall events: Increases in pore water pressure initiated movement on the slope, acceleration in movement resulting in a rapid decrease in pore water pressure, and attainment of a steady state. We infer that a negative pore water pressure feedback process may have occurred in response to shear-induced dilation of material as the slope movement accelerated. The process of shear dilatant strengthening may explain the phenomenon of semi-continuous movement of the loess landslide. Shear dilatant strengthening, caused by intermittent or continuous rainfall over long periods, can occur without triggering rapid slope failure.

Settlement and load transfer mechanism of a pile group adjacent to a deep excavation in soft clay

Three-dimensional coupled consolidation analysis is conducted to gain insight into the response of a 2×2 floating pile group adjacent to deep excavation in soft clay. By using a validated finite element model, the influence of the excavation depth, pile length, pile group location from excavation, the supporting system stiffness, soil state and permeability, and working load are systematically studied. The analysis revealed that the maximum settlement occurs when the pile group is founded at the excavation level and at a distance of 0.75-times the excavation depth, although the induced bending moment is minimum. In contrast to pile group settlement, tilting is maximum when it gets closer to the wall and minimum at a distance of 0.75-times the excavation depth. It is also observed that the influence of system stiffness is more pronounced for the flexible wall and when the pile group is located at the excavation level. Applied working load influences pile group settlement but has relatively minor effects on pile group tilting. Excess negative pore water pressures are generated in the soil elements due to excavation. Pile group experiences progressive long term settlement with the dissipation of excess negative pore water pressure.

The Effect of Drying-Wetting Cycle’s Repetition to the Characteristic of Natural and Stabilization Residual Soils Jawa Timur - Indonesia

Indonesia, which located in tropical region, continuously undergoes wetting and drying cycles due to the changeable seasons. An important role in activating the clay minerals on tropical residual soils is the main factor that affects the static and dynamic properties, such as: volume change, soil suction and dynamic modulus. The purpose of this paper is to evaluate the effect of drying-wetting cycles repetition on volume change, soil suction and mechanical characteristics of natural and stabilization of residual soils from Jawa Timur - Indonesia. The natural undisturbed and stabilized residual soil sample was naturally and gradually dried up with air to 25%, 50%, 75%, and 100 % of the initial water content. The wetting processes were carried out with the gradual increment water content of 25 %(wsat − wi), 50 %(wsat − wi), 75 %(wsat − wi), up to 100 %(wsat − wi). The Direct Shear test is used to measure the mechanic properties, and Whatman filter paper No. 42 is used to measure the soil suction. The drying-wetting processes were carried out for 1, 2, 4, and 6 cycles. The laboratory test results showed that, the void ratio decreased, the unit weight, cohesion and the internal friction angle were increasing due to stabilization. Drying-wetting cycle repetition reduces void ratio, negative pore-water pressure, cohesion and internal friction angle of natural and stabilized soils. Briefly, the decreased of mechanical soil properties was proven from the physical properties change observation.

Uncertainty of the Soil–Water Characteristic Curve and Its Effects on Slope Seepage and Stability Analysis under Conditions of Rainfall Using the Markov Chain Monte Carlo Method

It is important to determine the soil–water characteristic curve (SWCC) for analyzing slope seepage and stability under the conditions of rainfall. However, SWCCs exhibit high uncertainty because of complex influencing factors, which has not been previously considered in slope seepage and stability analysis under conditions of rainfall. This study aimed to evaluate the uncertainty of the SWCC and its effects on the seepage and stability analysis of an unsaturated soil slope under conditions of rainfall. The SWCC model parameters were treated as random variables. An uncertainty evaluation of the parameters was conducted based on the Bayesian approach and the Markov chain Monte Carlo (MCMC) method. Observed data from granite residual soil were used to test the uncertainty of the SWCC. Then, different confidence intervals for the model parameters of the SWCC were constructed. The slope seepage and stability analysis under conditions of rainfall with the SWCC of different confidence intervals was investigated using finite element software (SEEP/W and SLOPE/W). The results demonstrated that SWCC uncertainty had significant effects on slope seepage and stability. In general, the larger the percentile value, the greater the reduction of negative pore-water pressure in the soil layer and the lower the safety factor of the slope. Uncertainties in the model parameters of the SWCC can lead to obvious errors in predicted pore-water pressure profiles and the estimated safety factor of the slope under conditions of rainfall.