Effect Of Overburden Pressure Research Articles

Effect of overburden stress and surcharge pressure on the liquefaction response of sands

The physics and mechanism of soil liquefaction is understood sufficiently well by researchers now. However, the effect of various external factors such as the weight of the structure and overburden pressure on the liquefaction behavior of cohesionless soils is still under investigation. This paper presents a comprehensive study consisting of shaking table experiments on saturated sand beds to understand the effect of overburden pressure for free field conditions and the effect of surcharge weight for under the structure conditions on the liquefaction response of the sand beds. Pore water pressure was monitored continuously and the loss of effective stress with dynamic time was calculated for all the tests. Liquefaction occurred for shaking under specific ground motion conditions, which was visibly seen in the shaking table tests when the sand bed started to shake like a liquid and sand boils were formed on the bed. The number of cycles of dynamic shaking required for liquefaction and the pore water pressure were compared for different models. It was observed that increase in overburden pressure increases the liquefaction resistance of the soil. For sand beds under building pressure, the liquefaction response was observed to depend a great deal on the dynamic loading parameters. In the case of sand beds under surcharge loads, the response was complicated because of the complex soil–structure interaction under dynamic loading conditions.

Effect of overburden pressure and degree of saturation on compressibility characteristics

The settlement of a structure founded on clay comprises of immediate and consolidation settlements. In the case of clays, consolidation settlement is more than immediate settlement. The parameters influencing consolidation settlement of a normally loaded clay layer are degree of saturation (S%), void ratio (e) of soil prior to excavation, amount of overburden (σo) removed, amount of rebound and intensity of loading (σ) upon building superstructure. This paper presents the effect of the above parameters on compressibility characteristics of a clay. The different steps undertaken for footing construction were simulated in the laboratory. Remoulded clay samples were prepared from oven-dry fraction passing 425 μm sieve. The density of the samples was kept constant at 13 kN/m3. Degree of saturation was varied at 25%, 50%, 75% and 100%. The surcharge on the samples (overburden in the case of field clay layer) was varied as 25 kPa, 50 kPa and 100 kPa. Compressibility characteristics such as initial compression under the applied surcharge (overburden), rebound upon removal of surcharge and recompression were studied through one-dimensional consolidation tests. The rate and amount of initial compression, rebound, e-σ and e-log σ curves, compression index (Cc), coefficient of compressibility (av) and coefficient of volume compressibility (mv) were studied.

Clogging of finger drain systems in MSW landfills

A numerical model ‘BioClog’ is used to examine clogging of granular drainage material within finger drains at the base of landfills. The reduction in porosity of drainage material is evaluated as the accumulation of clog mass reduces the pore spaces of granular media. The leachate mounding within the landfills caused by the reduction in hydraulic conductivity of finger drain is modeled. The effect of overburden pressure on the engineering properties of waste material and consequent leachate mounding is considered. The calculated rates of leachate mound development for two landfills with finger drain leachate collection are shown to be in encouraging agreement with the observed data at the times when data were available. It is demonstrated that a significant leachate mound can be expected to develop over time when the leachate collection system is comprised of finger drains; however the rate of increase and the magnitude at a given time will depend on a number of factors including the grainsize, thickness and spacing of the finger drains, and the thickness of waste as well as the leachate generation rate.

Evaluation of deformation parameter for deep excavation in sand through case histories

This study back analyzed deformation parameters of in situ sand through two excavation case histories in Kaohsiung, Taiwan. Two main features are highlighted; deformation prediction based on monitoring data at the first excavation stage and in situ Young’s modulus evaluation for sand considering monitoring data at the overall excavation stages. The former tends to establish a reliable method to predict the wall deflection at the critical stage based on the data at the first stage and the latter to enrich the limited database of Young’s modulus correlation for sand, specifically applicable for deep excavations analysis. The two constitutive models, linear elastic perfectly plastic and non-linear stress–strain constitutive models, were selected. The stiffness parameters of the models were discretely distributed along the subdivided soil body mesh to reflect the effect of overburden pressure on the in situ soil. In addition, relationship between Standard Penetration Test value (SPT-N value) and Young’s modulus and relationships for estimating the in situ Young’s modulus of the Kaohsiung sand as a function of depth were evaluated. The results greatly enhanced a framework for estimating the in situ Young’s modulus of sand.

Applicability of T-Bar and Ball Penetration Tests to Soft Clayey Grounds

ABSTRACT T-bar and Ball penetration tests (TPT and BPT, respectively) were carried out at three sites consisting of soft clays. These penetration tests have several advantages over the conventional cone penetration test (CPT): much larger project area and no effects of overburden pressure for obtaining soil parameters such as undrained shear strength. Therefore, TPT and BPT are considered to be suitable in-situ tests for very soft ground. In this paper validity of the assumed mechanism of these cones are examined, using ball cones with different diameters of the ball. In addition, the cone factors for CPT, TPT and BPT are compared based on undrained shear strengths (su) measured from field vane and direct shear tests. Cyclic BPT was carried out to measure the sensitivity of soil layers. It is revealed that reduction in the tip resistance with the increase in the number of cyclic penetration can be correlated with the sensitivity measured by the field vane tests. Finally, the possibility of estimating the in-situ effective overburden pressure (σ′vo) is discussed for an artificial island filled by dredged clayey soil, where consolidation due to self weight of the filling is still continuing.

Shear strength characteristics of sand-gravel mixtures

Most of the previous constitutive models used for investigation of the shear behavior of sandy soils have been developed without consideration of the oversize particles. The size of experimental samples is an important limitation in this case. In this research a laboratory study has been planned to investigate the shear strength of sandy soils containing different gravel contents. The tests conducted using a large scale direct shear apparatus with a 300 mm x 300 mm x 170 mm shear box. All the tests have been performed on dry samples in drained condition. The gravel contents of 20, 40 and 60 percent were used in sample preparation. The tests conducted in three different densities of 35, 60 and 85 percent and three different surcharges of 150, 300 and 450 kN/m². It was observed that the increase in gravel content and relative density results to an increase in the shear strength and dilation of the mixture. However, the gravel content has a more visible effect on the shear strength rather than the relative density. The brittleness and softening of the shear behavior also increases as the gravel content increases in the mixture. It was determined that the minimum void ratio of the mixture is a useful parameter to investigate the effects of gravel grains on the shear strength of sandy soil. The effect of overburden pressure on this relationship was also considered in this study.

The effect of fine soil content on the filtration characteristics of geotextile under cyclic flows

The use of geotextiles as filters for revetments or embankments has become popular. The geotextile filter must be permeable enough to drain water effectively, but it also has to retain the soil particles to prevent them from being washed out. In the research on the filtration function of geotextiles, retention and permeability criteria have been proposed. Most of the research has considered conditions that are simpler than real in situ conditions, such as cyclic flows due to the fluctuation of water table caused by sea waves, boats, or periodic drawdown of water for irrigation purposes. Moreover, soils with different fines contents will cause dissimilar soil erosion behaviour in a revetment that uses geotextiles. For this reason, a cyclic flow apparatus is adopted in this research to take account of the effect of overburden pressure and cyclic flows on soils. The soil samples are composed mainly of sand, with 0–10% fines content of silts and clays. The applied cyclic flow periods range from long to short, in order to simulate a range of waves. The results show that the fines content has a significant effect on the filtration behaviour of a soil–geotextile system. In addition, the normal pressure and cyclic flow period have an influence on the performance of the geotextile filter.

Effect of effective overburden pressure on geomembrane/soil interface transmissivity

Fluid flow through a defect in the geomembrane of a composite geomembrane/soil liner (flow rate tests) and interface transmissivity tests between a geomembrane and a soil layer were conducted. The results are compared, and the effect of overburden pressure p′ on the geomembrane/soil interface transmissivity θ and interface thickness tin is investigated. For flow rate tests, the values of θ are back-calculated using the Rowe and Touze-Foltz et al. solutions. With known values of θ, tin is calculated using Newton's viscosity law for fluid flow between two parallel plates. For the conditions considered, it is shown that θ and tin reduce significantly with increasing p′. Comparisons are made between the calculated values of θ and tin and those reported in the literature. It is shown that ignoring the effect of p′ on θ and tin can lead to gross overestimation of these parameters.

Evaluation of the water retention curve for geosynthetic clay liners

Geosynthetic clay liners (GCLs) are frequently used as part of composite base liners for landfills. Heat generated by the waste can potentially cause drying of the hydrated GCL. In order to better understand the performance of GCLs under such conditions, an assessment of the water retention characteristics under drying conditions is required. To date, such an investigation has not been undertaken for GCLs, although some data exists for compacted bentonite. This paper presents the results of laboratory testing aimed at obtaining water retention curves (WRCs) for two GCL materials representative of those commonly used in practise. Pressure plate and pressure membrane extractors were used to assess the relationship between the degree of saturation and the applied capillary pressure (suction) for a range of capillary pressures between 10 and 10 000 kPa. The effect of overburden pressure was investigated, as was the relationship between capillary pressure and bulk GCL void ratio. Volume change behaviour was found to have a significant effect on the WRCs. Representative parameters were obtained by fitting the predictions of three WRC models to the experimental data. Reasonable fits were achieved and the parameters obtained may be used for numerical modelling.

Shear waves in surficial sediments: Revisited

Numerous measurements of compressional and shear wave speed and attenuation have been made since Ed Hamilton’s classic works on sediment geoacoustic properties (1955–1987). Recent measurements of shear wave speed and attenuation are summarized; regressions between shear wave speed and attenuation and easily measured sediment physical properties are used to demonstrate the predictability of these geoacoustic properties. The effect of overburden pressure (effective stress) on shear wave speed is also discussed. [Work supported by ONR.]

Influence of Stress on the Characteristics of Flow Units in Shaly Formations

Abstract Reservoir evaluation of shaly formations and enhancement of reservoir characterization has long been a difficult task. This study is devoted to developing relationships for interpreting the combined effects of subsurface stress and shale on petrophysical properties of reservoir rocks. These new relationships are used to: (1) identify shale types, (2) characterize flow units in shaly formations using in situ measurements, and (3) study the effect of stress on reservoir quality index and fluid flow paths of shaly formations. Several flow unit models have been developed to achieve these goals. The proposed flow unit models are based on current shale models appearing in the literature. These flow unit models introduce unique parameters for reservoir characterization of shaly formations. These parameters include the slope of a traight line (that defines the flow unit) and the stress factor on a log-log plot of in situ reservoir quality index versus in situ porosity of the shaly formations. Finally, these models are used to identify shale types and select a suitable water saturation model for shaly formations. The models are validated using simulated data of porosity and pressure. The new models, in combination with the methodologies developed in this study, represent an effective tool for an enhanced reservoir description of shaly reservoirs. Introduction Characterization of shaly formations has been a difficult task for several reasons. One key factor of this difficulty is that the use of conventional core test data simply does not work. Special equipment or procedures are required to determine the low permeabilities and porosities. A serious, and more fundamental, problem related to studying shaly formations is that of the very heterogeneous nature of shales. In addition, the consideration of the subsurface stress effect will make investigation and characterization of shaly formations much more difficult. With respect to the stress effect, Hilchie(1) used six brine saturated porous samples under simulated conditions of overburden and temperature. The simulated overburden pressure used was up to 68.45 mPa (10,000 psi) and the simulated temperature conditions were up to 232 °CDATA[C (450 °CDATA[F). The results showed that the increase of stress caused an increase of the formation resistivity factor. Longeron(2) investigated the effect of overburden pressure on the electrical properties of sandstone. The results showed that the formation resistivity factor increased by about 15﹪ for sandstone when stress ranging from 276 mPa to 20 mPa (400 psi to 2,900 psi) was applied. Lewis et al.(3) investigated the effects of stress and wettability on the water saturation exponent "n" and the cementation exponent "m." The results showed that changes in stress have a relatively minor effect upon the water saturation exponent "n" and the cementation exponent "m," but trends having an increase in these exponents with increasing stress levels have been observed. Lewis et al. data also showed a slight decrease in these exponents with decreasing stress, but the effectf stress on the water saturation exponent "n"was less clear than that of the cementation exponent "m."Therefore, changes in the cementation and water saturation exponents are probably small enough that they can be neglected.

Distribution of ground rigidity and ground model for seismic response analysis in Hualian project of large scale seismic test

An international joint research program called HLSST is proceeding. HLSST is large-scale seismic test (LSST) to investigate soil-structure interaction (SSI) during large earthquake in the field in Hualien, a high seismic region in Taiwan. A 1 4 - scale model building was constructed on the gravelly soil in this site, and the backfill material of crushed stone was placed around the model plant after excavation for the construction. Also the model building and the foundation ground were extensively instrumental to monitor structure and ground response. To accurately evaluate SSI during earthquakes, geotechnical investigation and forced vibration test were performed during construction process namely before/after base excavation, after structure construction and after backfilling. And the distribution of the mechanical properties of the gravelly soil and the backfill are measured after the completion of the construction by penetration test and PS-logging etc. This paper describes the distribution and the change of the shear wave velocity V s measured by the field test. Discussion is made on the effect of overburden pressure during the construction process on V s in the neighbouring soil and, further on the numerical soil model for SSI analysis.

Stress Strain Characterization of Sand Packs Under ?Uniform Loads As Determined From , . Computer Assisted Tomography

Abstract The local changes in the density and. porosity of dry sand packs as a function of the imposed overburden were determined using Computer Assisted Tomography. Pressure cycles were imposed on sands with different characteristics. The density and porosity was measured at a resolution of 0.075 cm × 0.075 cm × 0.3 cm. Hysteresis in the densityand porosity was observed. The imposed overburden and pore volume changes were translated to stress and strain, respectively. The data was used to calculate material properties such as the bulk compressibility and the formation compaction. The results were compared with information available in the literature. Good agreement was obtained. Introduction and Theory Computer Assisted Tomography (CAT) scanning has been use in the characterization of unconsolidated core material(1, 2) and has been found very useful in identifying core heterogeneities and areas of formation damage. Physical properties such as bulk density and porosity can be calculated using simple models(3, 4). However, the porosity measurements will be in error if the core is not compressed back to reservoir pressure. With this in mind, it was decided to perform a series of tests that would demonstrate the effect of overburden pressure on porosity. At the same time, methods using CAT scanners in the study of fracturing and the measurement of material properties appeared in the literature(5). The data collected from the porosity under various overburden pressures could then be translated into stress strain data and physical properties such as the bulk modulus could actually be measured(5, 6). The CAT scanner provides normalized values of the linear attenuation coefficient of each volume element (voxel) which are usually called CT numbers (CTn) at a resolution that varies depending on the scanner used. The resolution of the measurements in this work is 0.075 cm × 0.075 cm × 0.3 cm. The bulk density is calculated from the CT number from the following equation: (1) ρ b = 1.119 + 0.00163 * CT n The porosity of a dry sand pack is calculated from the bulk density through: Equation (2) Available In Full Paper. where 2.649 is the grain density of quartz. Therefore, from a single scan of a given cross section, the bulk density and porosity maps of the cross section can be derived at the given resolution. If the sand pack is saturated with a single liquid (e.g., water) the bulk density can still be calculated using Equation (1). The porosity will be given through: Equation (2a) Available In Full Paper. where the subscript L denotes the liquid phase. When doing a compaction test, uniaxial stress or hydrostatic pressure is applied to the sample while images of the sample are taken before and after each change of pressure. Since density and porosity are mapped in each pressure step, density and porosity change (compression) curves can be generated for a given sand as a function of the imposed pressure. Density changes can be related to volume changes through the following equation: Equation (3) Available In Full Paper.

The fabric of a clay soil under controlled mechanical and hydraulic stress states

Fabric determination is fundamental to the understanding of several mechanims controlling a clayey soil behavior. Two techniques; namely, mercury intrusion tests and transmission electron microscopy (TEM) were used to study the soil fabric of a remoulded Boom clay. The effect of overburden pressure, excavation and desaturation on the soil fabric are studied. Results demonstrate that the nature of soil fabric is dependent on both the applied mechanical and hydraulic stress state conditions.

An empirical method to estimate shear wave velocity of soils in the New Madrid seismic zone

In this study, a set of charts are developed to estimate shear wave velocity of soils in the New Madrid seismic zone (NMSZ), using the standard penetration test (SPT) N values and soil depths. Laboratory dynamic test results of soil samples collected from the NMSZ showed that the shear wave velocity of soils is related to the void ratio and the effective confining pressure applied to the soils. The void ratio of soils can be estimated from the SPT N values and the effective confining pressure depends on the depth of soils. Therefore, the shear wave velocity of soils can be estimated from the SPT N value and the soil depth. To make the methodology practical, two corrections should be made. One is that field SPT N values of soils must be adjusted to an unified SPT N′ value to account the effects of overburden pressure and equipment. The second is that the effect of water table to effective overburden pressure of soils must be considered. To verify the methodology, shear wave velocities of five sites in the NMSZ are estimated and compared with those obtained from field measurements. The comparison shows that our approach and the field tests are consistent with an error of less than of 15%. Thus, the method developed in this study is useful for dynamic study and practical designs in the NMSZ region.

Gravitational Equilibrium Moisture Profiles in Swelling Soils

Extension of the work of Philip (1969a, 1972) has shown that the effect of overburden pressure on volume change of swelling soils produces equilibrium moisture profiles entirely different from those predicted without considering the overburden pressure effects. Volume change of swelling soils accompanied by moisture changes also affects the overburden pressure. The total volume change under an overburden pressure occurs in three different shrinkage phases from saturation to oven dry. These volume change characteristics are usually described on the shrinkage surface, a surface drawn as void ratio e (volume of voids/volume of solids) versus moisture ratio Θ (volume of water/volume of solids) for different overburden pressures. A relationship for the moisture gradient is developed by assuming that the overburden potential, a component of the total potential of soil water, is a function of moisture ratio only across a small soil element of a long swelling soil column which consists of large numbers of finite soil elements. The moisture gradient has complex behavior based on the properties of the three shrinkage phases on the shrinkage surface. Distribution of equilibrium moisture paths is then explained by evaluating the moisture gradients at points on an idealized shrinkage surface. It is shown that the soil at great depths could either be saturated or unsaturated at equilibrium. (These depths could be over 250 m for some sodium montmorillonite clays).

Effect of overburden pressure on liquefaction resistance of sand : V.S. Pillai & P.M. Byrne, Canadian Geotechnical Journal, 31(1), 1994, pp 53–60

Effect of overburden pressure on liquefaction resistance of sand : V.S. Pillai & P.M. Byrne, Canadian Geotechnical Journal, 31(1), 1994, pp 53–60

Effect of overburden pressure on liquefaction resistance of sand

The effect of overburden pressure on liquefaction resistance of sand is studied and results of a site-specific investigation are presented. When estimating liquefaction resistance of sand from the indirect approach using the chart suggested by Seed et al. (1984) a correction factor Kσ is applied to account for vertical effective overburden stresses larger than 1 tsf. Published data indicate a decrease in Kσ with increased confining stress but with a wide range of Kσ values for the same confining stresses, predicting significantly differing liquefaction resistance. The effect of confining stresses on liquefaction resistance was investigated as part of a comprehensive seismic assessment of Duncan Dam in British Columbia. The results indicate that Kσ is dependent on confining stresses and the relative density of the soil, and values are generally significantly higher than much of the previously published data. Key words : liquefaction, sand, confining stress, density, cyclic resistance ratio.

Swell Potential Versus Overburden Pressure

Abstract Data were collected and reviewed pertaining to one-dimensional vertical free-swell tests performed on existing fill soils from various projects located in Southern California. The purpose of reviewing the data was to evaluate the effect of overburden pressure on the swelling potential of expansive soils. The swell tests were performed on relatively undisturbed specimens at the in-place moisture content and density under the actual overburden pressure. The data indicate that at an overburden pressure of approximately 1400 lb/ft2 (67 kPa) or greater, the swell potential generally decreases to less than 1%.

A model for water transport and ice lensing in freezing soils

A one‐dimensional model for water transport and ice lensing in incompressible saturated and solute‐free soil specimens is proposed for the simulation of small‐scale frost heave tests in the laboratory. The model considers (1) open‐system freezing in which the variables T and Pw are independent, (2) an ice lens that continues to grow as long as enough energy is available in the frozen fringe to produce the work required for mass transfer to the ice lens, and (3) a new ice lens that forms when the vertical strain in the frozen soil reaches the instantaneous tensile failure strain. The proposed frost heave model is amenable to computer simulation procedures which provide predictions of rate of frost heave and rate of pore freezing front penetration as functions of applied load, thermal and water flow regimes, and soil properties. Position, time of initiation, and ultimate thickness of individual ice lenses emerge also as a part of the solution. A quantitative comparison of observed and simulated responses for various boundary conditions is shown for a reference soil known as Devon silt. The model was found to exhibit many of the characteristics observed in frost heave tests on Devon silt and predicted the effects of overburden pressure on frost heave rate.