Modulus-Suction-Moisture Relationship for Compacted Soils in Postcompaction State

Despite clear evidence, changes in mechanical properties (i.e., stiffness or modulus) of compacted subgrades in response to subgrade moisture regime changes after construction have rarely been investigated in the geotechnical profession. In particular, when in-service assessment of pavement subgrade is made, the modulus-moisture variation should be addressed on the basis of unsaturated soil mechanics. This study presents the unsaturated small-strain modulus behavior of five predominately fine-grained compacted subgrade soils. The small-strain shear modulus (Go) of saturated compacted specimens subjected to a desorption soil-water characteristic curve (SWCC) was evaluated using bender elements. A test apparatus was designed to apply two stress state variables, the net confining pressure and matric suction, during the Go measurements. The relationship between Go and the SWCC under a constant mean net stress was developed. Additionally, the effect of compaction moisture content, compaction energy, and soil type on the Go-SWCC relationship was investigated. Finally, a relationship describing the small-strain modulus behavior of unsaturated compacted soils is proposed.     

Assessing the Potential of Internal Erosion and Suffusion of Granular Soils

This study presents a new empirical criterion for assessing the potential of internal erosion and suffusion of granular soils. This method considers the bimodal structure of a soil having a primary coarse fabric and loose finer particles based on the porosities influenced by the particle size distribution and the degree of compaction. By comparing the representative particle size of a loose finer fraction with the controlling constriction size of a primary coarse fabric, a distinct boundary between internally stable and unstable soils with respect to internal erosion may be found.     

Design of Compacted Lateritic Soil Liners and Covers

Laboratory tests were conducted on three lateritic soil samples to illustrate some pertinent considerations in the design of compacted lateritic soil liners and covers. The three design parameters investigated are hydraulic conductivity, desiccation-induced volumetric shrinkage, and unconfined compressive strength. Test specimens were compacted at various molding water contents using four compactive efforts. The compaction conditions were shown to have some relationship with soil compaction using either the plasticity modulus or the plasticity product (i.e., clay index). For construction quality assurance purposes, the traditional approach was compared with the modern criterion. Deficiencies associated with the traditional approach for soil liners found in literature also apply to lateritic soils. Overall acceptable zones were constructed on the compaction plane to meet design objectives for hydraulic conductivity, volumetric shrinkage strains, and unconfined compressive strength. The line of optimums was identified as a suitable lower bound for overall acceptable zones of lateritic soils. The volumetric shrinkage strain was also identified as the second most important design parameter for lateritic soils. The shapes of the acceptable zones were affected by the fines contents of the soils.     

Suction-Controlled Laboratory Test on Resilient Modulus of Unsaturated Compacted Subgrade Soils

Conventionally, the resilient modulus test is conducted in the laboratory under different moisture content in which matric suction is unknown during the test. To investigate the influence of the matric suction on the resilient modulus, this study integrated the suction-controlled testing system and developed a modified testing procedure for the resilient modulus test of unsaturated subgrade soils. Based on the axis-translation technique, two cohesive soils were tested to investigate the effect of matric suction on resilient modulus. In the modified testing procedure, in order to fulfill the equilibrium in matric suction, the number of load cycles at each loading sequence of the resilient modulus test (AASHTO T 292-91) needs to be increased significantly. Experimental data indicate that matric suctions measured in the specimen after consolidation and resilient modulus tests are consistent with the matric suctions deduced from the soil-water characteristic curve corresponding to the same moisture content. In general, the resilient modulus obtained by the suction-controlled resilient modulus test appears to be reasonable. The trends of resilient modulus obtained by the suction-controlled resilient modulus test are consistent with those obtained by the conventional resilient modulus test. However, the suction-controlled resilient modulus test provides better insights that can help in interpreting the test results.     

Shear Strength of Compacted Soil under Infiltration Condition

Landslides in residual soil slopes are commonly induced by rainfall infiltration. These residual soils are typically in an unsaturated state with negative pore-water pressures or matric suctions since the groundwater tables in steep slopes are often deep. The net normal and shear stresses of the soil remain essentially constant during rainwater infiltration into the slope. Failure of the slope during rainfall can be primarily associated with the decrease in the matric suction of the soil. The objective of the study was to investigate the strength and deformation characteristics of a residual soil of the Bukit Timah Granitic Formation during infiltration that leads to slope failure. There were two modified direct shear apparatuses used. One apparatus was used for the determination of shear strength under controlled suction conditions while the other apparatus was used for shearing-infiltration tests. The shearing-infiltration test results were compared with the shear strength values obtained from the shearing tests under constant suction. The shearing-infiltration test results indicate a close relationship between the decreasing matric suction and the increasing displacement rate of the soil specimen. At the initial part of the infiltration process, there is a rapid reduction in matric suction that is accompanied by little movement in the soil. When failure of the soil is imminent, the soil movement will accelerate.     

Threshold Shear Strain for Cyclic Pore-Water Pressure in Cohesive Soils

Threshold shear strain for cyclic pore-water pressure, γt, is a fundamental property of fully saturated soils subjected to undrained cyclic loading. At cyclic shear strain amplitude, γc, larger than γt residual cyclic pore-water pressure changes rapidly with the number of cycles, N, while at γc<γt such changes are negligible even at large N. To augment limited experimental data base of γt in cohesive soils, five values of γt for two elastic silts and a clay were determined in five special cyclic Norwegian Geotechnical Institute (NGI)-type direct simple shear (NGI-DSS), constant volume equivalent undrained tests. Threshold γt was also tested on one sand, with the results comparing favorably to published data. The test results confirm that γt in cohesive soils is larger than in cohesionless soils and that it generally increases with the soil’s plasticity index (PI). For the silts and clay having PI=14–30, γt = 0.024–0.06% was obtained. Limited data suggest that γt in plastic silts and clays practically does not depend on the confining stress. The concept of evaluating pore water pressures from the NGI-DSS constant volume test and related state of stresses are discussed.     

Critical Shear Stress of Bimodal Sediment in Sand-Gravel Rivers

A new model for the critical shear stress and the transport of graded sediment is presented. The model is based on the size distribution of the bed surface and can be used to compute sediment transport rates in numerical simulations with an active layer model. This model makes a distinction between unimodal and bimodal sediments. It is assumed that all size fractions of unimodal sediments have the same critical shear stress while there is selective transport for the gravel fractions of bimodal sediments. A recently published laboratory transport data set is used to calibrate our model.     

Axial Compression of Footings in Cohesionless Soils. I: Load-Settlement Behavior

The results of 167 full-scale field load tests were used to examine several issues related to the load-displacement behavior of footings in cohesionless soils under axial compression loading, including (1) method to interpret the “failure load” from the load-settlement curves; (2) correlations among interpreted loads and settlements; and (3) generalized load-settlement behavior. The L1-L2 method was found to be more appropriate than the “tangent intersection” and “10% of the footing width” methods for interpreting the failure load. The interpreted loads and displacements indicate that footing load-settlement behavior is less elastic and more nonlinear than that of drilled foundations. The results show that the footing behavior will be beyond the elastic limit for designs where a traditional factor of safety between 2 and 3 is used. A normalized curve was developed by approximating the load-settlement curve for each load test in the database by hyperbolic fitting, and the uncertainty in this curve was quantified. This normalized curve can be used in footing design that considers capacity and settlement together. Where possible or warranted, the normalized curve can be subdivided as a function of initial soil modulus.     

Experimental Investigation on Effectiveness of a Vertical Drain under Cyclic Loads

This study examines the effectiveness of a prefabricated vertical drain (PVD) installed in reconstituted soft clay, where a large cyclic triaxial device was used to simulate repeated loading representing a rail track environment. The experimental data demonstrate that PVD successfully prevents excessive buildup of pore water pressure during cyclic loading, and that dissipation continues to occur after the load is removed. The findings of this study have direct relevance to rail track environments, e.g., in coastal Australia, where PVD installation beneath rail tracks constructed on low-lying estuarine soils has been recently introduced.     

Numerical Investigation of the Role of Turbulent Bursting in Sediment Entrainment

In this work we incorporate a Gram–Charlier-type joint probability distribution of near-bed two-dimensional instantaneous velocities into a simple mechanistic model to investigate the role of turbulent bursting in sediment entrainment. The results reveal that under typical values of bed-shear stress (>3?Pa), the time fractions of Quadrants 1–4 (Q1–Q4) remain constantly as 16, 34, 19, and 31%, respectively. Entrainment of the fine sediment mixtures is dominated by the lifting mode, whereas entrainment of the coarse ones is dominated by rolling. Sweeps (Q4) are consistently the most significant contributor to entrainment under various types of sediment mixtures. As the standard deviation of grain-size distribution increases, the hiding effect exerted on the finer grains of the mixture is reduced, leading to the elevated correction factors for effective hydrodynamic forces, and thus the reduced threshold velocities for entrainment. The reduced thresholds would, in turn, enhance the fractional contributions of ejections and inward interactions (Q2 and Q3), which are associated with negative longitudinal velocity fluctuations, such that the fractional contribution of outward interactions (Q1) would become less significant.     

Laboratory Investigation on Assessing Liquefaction Resistance of Sandy Soils by Shear Wave Velocity

Shear wave velocity (Vs) offers engineers a promising alternative tool to evaluate liquefaction resistance of sandy soils, and the lack of sufficient in-situ databases makes controlled laboratory study very important. In this study, semitheoretical considerations were first given based on review of previous liquefaction studies, which predicted a possible relationship between laboratory cyclic resistance ratio (CRRtx) and Vs normalized with respect to the minimum void ratio, confining stress and exponent n of Hardin equation. Undrained cyclic triaxial tests were then performed on three reconstituted sands with Vs measured by bender elements, which verified this soil-type-dependent relationship. Further investigation on similar laboratory studies resulted in a database of 291 sets of data from 34 types of sandy soils, based on which the correlation between liquefaction resistance and Vs was established statistically and further converted to equivalent field conditions with well-defined parameters, revealing that CRR will vary proportionally with (Vs1)4. Detailed comparisons with Vs-based site-specific investigations show that the present lower-bound CRR–Vs1 curve is a reliable prediction especially for sites with higher CSR or Vs1. The framework of liquefaction assessment based on the present laboratory study is proposed for engineering practice.     

Erosion of Cohesive Sediments: Resuspension, Bed Load, and Erosion Patterns from Field Experiments

New field data on cohesive sediment erosion is presented and discussed, with particular focus on partitioning the total erosion into resuspension and bed load. The data were obtained using a recently developed in situ flume of the National Institute of Water and Atmospheric Research, New Zealand. The erosion rate is estimated from direct measurements of bed surface elevations by acoustic sensors, whereas resuspension rate is obtained using data on sediment concentrations measured by optical backscatter sensors. The bed- load contribution to the total erosion rate is evaluated from the conservation equation for sediments. To test repeatability, the data from the in situ flume are compared with those from a previous version of the flume. The results show that comparative studies of in situ flumes and standardized deployment procedures enable direct comparison of experimental data on cohesive sediment erosion. Overall, the data show that a commonly used assumption that the erosion rate is equal to the resuspension rate is not always valid as bed load plays a significant role in cohesive sediment erosion. The data also highlight the importance of clay content and other sediment physical characteristics in the sediment mixture.     

Evaluation of Cyclic Shear Strength of Two Cemented Calcareous Soils

The mechanism controlling the cyclic shear strength of cemented calcareous soils was investigated based on the results obtained from monotonic and cyclic triaxial tests on two different types of calcareous soil. Undrained cyclic triaxial tests performed on artificially cemented calcareous soils with different loading combinations showed that the effective stress path moved towards or away from the origin, due to the generation or dissipation of pore pressure with progressive cycles. Previous investigations have shown that the Peak Strength Envelope or the State Boundary Surface or the Critical State Line forms a boundary beyond which effective stress paths during cyclic loading cannot exist. However, in this study it was observed that the maximum stress ratio (ηmax) obtained from monotonic tests defined the boundary for the cyclic tests. Based on the information obtained from this study, an approach for evaluating the cyclic shear strength is proposed. It was observed that the modified normalized cyclic shear strength had a strong linear relationship with the logarithm of the number of cyclic to failure irrespective of confining pressure, type of consolidation and stress reversal.     

Penetration Type Field Velocity Probe for Soft Soils

The assessment of the shear stiffness of dredged soft ground and soft clay is extremely difficult due to soil disturbances caused during sampling and field access. Several in situ methods such as spectral analysis of surface waves, multichannel analysis of surface wave, cross hole, and downhole methods have been developed to measure the shear-wave velocity, but a few disadvantages hinder the adoption of existing methods to soft ground. This study presents a new apparatus, the penetration type field velocity probe (FVP), which overcomes several of the limitations of commonly used shear-wave measurement methods in the field. Design concerns of the FVP include the tip shape, soil disturbance, transducers, self acoustic insulation, protectors, and the electromagnetic coupling from transducer-to-transducer and cable-to-cable. The crosstalk between cables is eliminated by grouping and extra grounding of the cables. The shear-wave velocity of the FVP is directly calculated, without any inversion process, by using the travel distance and the travel time. After calibration tests are carried out in the laboratory, application tests in the field are conducted up to 29 m in depth. Calibration results show the velocity profiles obtained by the FVP and by the rods fitted with transducers are similar to each other. The experimental results obtained in the field show that the FVP can produce reasonable and detailed shear-wave velocity profiles in soft clay. This study suggests that the FVP may be an effective technique for measuring the shear-wave velocity in soft ground.     

Effects of Lifting Force on Bed Topography and Bed-Surface Sediment Size in Channel Bend

In bed-load sediment transport, the lifting force plays an important role in reducing the friction between sediment particles and the bed surface, and it makes particle transportation by the shear force easier. Because the lifting force is related to vorticity, a three-dimensional (3D) numerical model incorporating large eddy simulations was applied to simulate the vorticity field in a channel bend. The results show that the distribution of vorticity is highly nonuniform, and it can lead to significant variations in lifting force and bed-load sediment transport per unit width in a channel bend. Relevant theories are modified on the basis of physical reasoning and then incorporated into numerical models to investigate the lifting-force effects on the bed topography and bed-surface sediment size gradation in a channel bend. With the lifting-force effects considered, it is shown that the errors in simulated bed topography can be reduced by approximately 40% and in bed-surface sediment size by 50%.     

Surface-Based Fractional Transport Predictor: Deterministic or Stochastic

Stochastic bed load transport formulas for nonuniform sediment exist, but most of them do not account for the composition of surface material to predict fractional transport rate. This study transformed a surface-based bed-load transport predictor to a stochastic one by approximating the fluctuation of bed-shear stress with a standard log-normal distribution. The deterministic predictor underpredicts fractional transport rate at low values of bed-shear stress and Reynolds number. The modified stochastic predictor predicts fractional transport rate more accurately and converges to the deterministic one at high shear stresses.     

Experimental and Numerical Investigation of Bed-Load Transport under Unsteady Flows

The dynamic behavior of bed-load sediment transport under unsteady flow conditions is experimentally and numerically investigated. A series of experiments are conducted in a rectangular flume (18?m in length, 0.80?m in width) with various triangular and trapezoidal shaped hydrographs. The flume bed of 8?cm in height consists of scraped uniform small gravel of D50 = 4.8??mm. Analysis of the experimental results showed that bed-load transport rates followed the temporal variation of the triangular and trapezoidal hydrographs with a time lag on the average of 11 and 30?s, respectively. The experimental data were also qualitatively investigated employing the unsteady-flow parameter and total flow work index. The analysis results revealed that total yield increased exponentially with the total flow work. An original expression which is based on the net acceleration concept was proposed for the unsteadiness parameter. Analysis of the results then revealed that the total yield increased exponentially with the increase in the value of the proposed unsteadiness parameter. Further analysis of the experimental results revealed that total flow work has an inverse exponential variation relation with the lag time. A one-dimensional numerical model that employs the governing equations for the conservation of mass for water and sediment and the momentum was also developed to simulate the experimental results. The momentum equation was approximated by the diffusion wave approach, and the kinematic wave theory approach was employed to relate the bed sediment flux to the sediment concentration. The model successfully simulated measured sedimentographs. It predicted sediment yield, on the average, with errors of 7% and 15% of peak loads for the triangular and trapezoidal hydrograph experiments, respectively.     

Axial Compression of Footings in Cohesionless Soils. II: Bearing Capacity

An extensive database of full-scale field load tests was used to examine the bearing capacity for footings in cohesionless soils. Each load test curve was evaluated consistently to determine the interpreted failure load (i.e., bearing capacity) using the L1-L2 method. This test value then was compared with the theoretical bearing capacity, computed primarily using the basic Vesi? model. The comparisons show that, for footing widths B>1?m, the field results agree very well with the Vesi? predictions. However, for B<1?m, the results indicated a relationship between B and the predicted-to-measured bearing capacity ratio. Accordingly, a simple modification was made to the bearing capacity equation, and the resulting predictions are very good.     

Near-Bed Sediment Concentration Distribution and Basic Probability of Sediment Movement

Sediment concentration distribution and the basic probability of sediment movement near the channel bed are two of the most important and fundamental issues in the study of sediments. Based on statistical analysis and considering the transport mechanisms, the rules of sediment concentration distribution near a channel bed are studied. Analytical expressions for the near-bed sediment concentration distribution and mean sediment concentration are derived, and the expression for the mean sediment concentration near the bed is verified by measured data, which were obtained from previous experiments. With the help of statistical theory, the expressions of basic probabilities, i.e., rolling, saltating, and suspending probabilities, for sediment movement near the bed are also derived. The expression for starting probability is verified by the measured data. The verification shows that the results from the proposed expression agree well with the measured data. This research has both theoretical and practical significance for further investigation concerning rules of bed load and suspended sediment transport.     

Evaluation of Multidimensional Transport through a Field-Scale Compacted Soil Liner

A field-scale compacted soil liner was constructed at the University of Illinois at Urbana-Champaign by the U.S. Environmental Protection Agency (USEPA) and Illinois State Geological Survey in 1988 to investigate chemical transport rates through low permeability compacted clay liners (CCLs). Four tracers (bromide and three benzoic acid tracers) were each added to one of four large ring infiltrometers (LRIs) while tritium was added to the pond water (excluding the infiltrometers). Results from the long-term transport of Br? from the localized source zone of LRI are presented in this paper. Core samples were taken radially outward from the center of the Br? LRI and concentration depth profiles were obtained. Transport properties were evaluated using an axially symmetric transport model. Results indicate that (1) transport was diffusion controlled; (2) transport due to advection was negligible and well within the regulatory limits of ksat ? 1×10?7?cm/s; (3) diffusion rates in the horizontal and vertical directions were the same; and (4) small positioning errors due to compression during soil sampling did not affect the best fit advection and diffusion values. The best-fit diffusion coefficient for bromide was equal to the molecular diffusion coefficient multiplied by a tortuosity factor of 0.27, which is within 8% of the tortuosity factor (0.25) found in a related study where tritium transport through the same liner was evaluated. This suggests that the governing mechanisms for the transport of tritium and bromide through the CCL were similar. These results are significant because they address transport through a composite liner from a localized source zone which occurs when defects or punctures in the geomembrane of a composite system are present.