Journal of Geotechnical and Geoenvironmental Engineering

When designing embankments over soft soils, geotechnical engineers face many challenges. These include potential bearing failure, intolerable settlement, and global or local instability. Pile-supported embankments have been emerged as an effective alternative successfully adopted worldwide to solve these problems. This paper focuses on three cases in which pile-supported embankments were used for constructing highways in the eastern coastal region of China. Each case provides a description of the soil profile, construction procedure, and field monitoring of the settlements, earth pressures, and pore-water pressures. Field monitored data from contact pressures acting on the piles and the soils, to the settlements of the piles and the soils are reported and discussed. The development of the earth pressures both on the piles and the soils shows that there was a significant soil arching in the embankment, and the measured earth pressures acting on the piles are much higher than that acting on the soils between the piles. The measured differential settlements between the piles and the soils are small. The pore-water pressures are not significant compared to the embankment loads. Most pore-water pressure dissipated after the complete of filling. The measurements have proven the effectiveness of this technique in reducing total and differential settlements. Finally, the available design standards and approaches are verified by three cases.

This paper focuses on the design and performance of a very tall mechanically stabilized earth (MSE) wall. Expansion of Seattle-Tacoma International Airport called for the construction of a third runway west of the two existing runways. A significant volume of compacted earth fill was required to raise the grade as much as 50 m to meet the level of the existing airfield. Nominal 2H:1V fill slopes were used where possible, but MSE retaining walls were used where fill slopes would have encroached into existing wetlands. Consequently a four-tier 46-m-tall MSE wall was constructed along a portion of the western edge of the embankment. Performance monitoring included strain gauge-instrumented reinforcing strips, inclinometer installations with sondex settlement rings, optical survey of the wall facing for vertical and lateral movements, and piezometers. This paper describes wall design issues, aspects associated with the instrumentation of the wall, and the observed performance. Monitoring indicates satisfactory performance of the MSE wall and compares reasonably well with predicted performance.

In this paper, an analytical study considering the effect of uncertainties in the seismic analysis of geosynthetic-reinforced soil (GRS) walls is presented. Using limit equilibrium method and assuming sliding wedge failure mechanism, analysis is conducted to evaluate the external stability of GRS walls when subjected to earthquake loads. Target reliability based approach is used to estimate the probability of failure in three modes of failure, viz., sliding, bearing, and eccentricity failure. The properties of reinforced backfill, retained backfill, foundation soil, and geosynthetic reinforcement are treated as random variables. In addition, the uncertainties associated with horizontal seismic acceleration and surcharge load acting on the wall are considered. The optimum length of reinforcement needed to maintain the stability against three modes of failure by targeting various component and system reliability indices is obtained. Studies have also been made to study the influence of various parameters on the seismic stability in three failure modes. The results are compared with those given by first-order second moment method and Monte Carlo simulation methods. In the illustrative example, external stability of the two walls, Gould and Valencia walls, subjected to Northridge earthquake is reexamined.

An approach to quantify nonuniformity of compacted earth materials using spatially referenced roller-integrated compaction measurements and geostatistical analysis is discussed. Measurements from two detailed case studies are presented in which univariate statistical parameters are discussed and compared to geostatistical semivariogram modeling parameters and analysis. The univariate and geostatistical parameter values calculated from the roller-integrated measurements are also compared to traditional spot test acceptance criteria. Univariate statistical parameter values based on roller-integrated measurement values provide significantly more information than traditional point measurements, while geostatistics can be used to identify regions of noncompliance and prioritize areas for rework.

The paper presents the performance of four long-span deep-corrugated steel box culverts with spans of 8- and 14-m during backfilling, as well as comparisons with finite-element modeling and design codes. Two of the culverts were stiffened at the crown arch. The test results show that the stiffening applied on the culverts is quite effective. The crown rises of the respective stiffened culverts were found to be half those of the not-stiffened culverts. The influence of the structure geometry on the soil-passive earth pressure was confirmed, as well as the sensitivity of box culverts to soil loads with increasing spans. The results showed that the influence of the size and shape of the box culverts on the amount of thrusts must be better implemented in the design method. The finite-element analysis results were conservative when live loading was concerned but the crown displacements and thrust during backfilling were underestimated.

The comfort and safety of a moving train is largely determined by the dynamic response of the railway track and its foundation (i.e., subgrade). To study the dynamic stability of a silt subgrade subjected to train traffic loading with increasing speed, cyclic triaxial tests were conducted for compacted silt specimens with varying dry density, water content, dynamic stress, and load frequency. The laboratory test results and field measurements of the subgrade dynamic stress under train loading indicate that with increasing train speed, an increase in dynamic stress and load frequency does not impair the stability of the silt subgrade, provided the subgrade is in sound physical condition (i.e., its natural water content approximates the optimal water content) and the relative compaction is at least 90%. However, if the relative compaction is 85%, the subgrade is stable only at a dynamic stress level that is below 70 kPa, and the subgrade may suffer shear failure at a higher dynamic stress level. The elastic deformation of the subgrade linearly increases with an increase in train speed. However, if the degree of saturation of the silt subgrade increases, the thresholds of both the dynamic stress and resilient modulus decrease markedly, accompanied by sharp increases in elastic deformation and cumulative deformation and can even result in the shear failure of the subgrade. These conditions are unfavorable for the high speeds and stability needed for trains; therefore, train speeds should be limited in wet conditions to reduce subgrade dynamic stress and load frequency.

Flow liquefaction is a major design issue for large soil structures such as mine tailings impoundments and earth dams. If a soil is strain softening in undrained shear and, hence, susceptible to flow liquefaction, an estimate of the resulting liquefied shear strength is required for stability analyses. Many procedures have been published for estimating the residual or liquefied shear strength of cohesionless soils. This paper presents cone penetration test-based relationships to evaluate the susceptibility to strength loss and liquefied shear strength for a wide range of soils. Case-history analyses by a number of investigators are reviewed and used with some additional case histories. Extrapolations beyond the case-history data are guided by laboratory studies and theory.

A weighted-residual approach for differentiating one-dimensional discrete data is presented and applied to an experimental program in which distributions of bending moment were measured along a model pile foundation in a centrifuge test. The weighted-residual approach is validated by first differentiating a sinusoidal bending moment distribution, and errors in first and second derivatives associated with various ratios of wavelength to sampling interval are computed. A bending moment distribution from a finite-element simulation of a pile foundation is differentiated using the weighted-residual technique, by fitting cubic splines, and by polynomial regression, and second derivatives are compared with the recorded subgrade reaction distributions. The influence of adding noise to the sampled bending moment distribution prior to differentiation is explored and is found to be most influential when sampling intervals are small. Bending moment data recorded during the centrifuge experiment are double differentiated and uncertainty in strain gauge calibration factors and position are incorporated using a Monte Carlo simulation to assess potential errors in the computed second derivatives.

One important component in the design of tunnels in urban areas is a correct assessment of the interaction between the underground excavation with other structures in the vicinity. In this sense a correct stress-strain response by the model representing the rock mass behavior is essential. The shear and normal displacement of rock discontinuities and their shear and normal stiffness control the distribution of stress and displacement within a discontinuous rock mass. In conditions where an equivalent continuum based approach is not applicable, the joint material model should be able to describe important mechanisms such as asperity sliding and shearing, post-peak behavior, asperity deformation, and the effect of soft infilling. The distinct element code UDEC was used to simulate the direct shear tests on a natural joint profile, and the prediction of two existing models of discontinuity strength and deformation were then compared with a new soil-infilled joint model and with experimental data for clean and soil-infilled rock joints. A numerical modeling of a cavern excavated in a jointed medium is also presented to illustrate the response of different models. The proposed soil-infilled joint model described more comprehensively the occurrence of dilation and compression with lateral displacements and also better represented the double peak shearing in relation to the adopted squeezing mechanism that could not be captured by the two existing models.

Piles composed of more than one material in their cross section have been used for more than 100 years. Originally this was limited to driven steel shell or pipe piles filled with portland-cement concrete. More recent developments include various types of drilled elements such as micropiles that consist of various combinations of steel shells, portland-cement grout, and steel reinforcing bars. The structural analysis or design of piles with multicomponent cross sections under axial load requires that the axial stress be apportioned to the various components. Traditionally this has been done using an approximate one-dimensional model that implies the components interact with each other only axially, not radially, and that there is no radial interaction with the ground around the pile. This note presents a new three-dimensional model that explicitly and rigorously considers not only the Poisson effects caused by axial load and the triaxial stress field that develops within and between components of a pile as a result but also how this stress field is affected by radial stresses in the adjacent ground. This new model is based on the theory of linear elasticity and yields a closed-form solution that can be either evaluated independently or incorporated within a more general analytical model for axial pile capacity. Examples of calculated results obtained using this new model are presented and suggest that Poisson effects are relatively small in magnitude so that the traditional one-dimensional model is adequate for routine use in most cases.

The seismic stability of slopes reinforced with a row of piles is analyzed using the kinematic theorem of limit analysis within the framework of the pseudostatic approach. An existing method which is based on the theory of plasticity is used to determine the lateral forces provided by the piles. Expressions for calculating the yield acceleration coefficient are derived. Then, based on Newmark’s sliding block concept, the permanent displacement induced by an earthquake shocking can be calculated by the integrals of seismic records. An example is investigated to illustrate the validity of this method and the effects of piles on a restraining slope’s dynamic deformation.

High-pressure isotropic compression tests were carried out on reconstituted sand samples that were reinforced with cement, randomly distributed fibers, or both, making comparisons with the unreinforced sand and conducting tests from a variety of initial specific volumes. The results indicated changes in the isotropic compression behavior of the sand due to the inclusion of fibers and/or cement. Cementitious bonds are sufficiently strong relative to the particles to allow the cemented samples to reach states outside the normal compression line (NCL) of the uncemented soil, but the effectiveness of cemented fiber-reinforced specimens is even larger due to the control of crack propagation in the cemented sand after the inclusion of fibers. Distinct NCLs were observed for the sand, fiber-reinforced sand, cemented sand, and fiber-reinforced cemented sand. Both fiber breakage and fiber extension were observed in fibers measured after testing indicating that fibers individually have worked under tension, even though in the macroscopic scale, isotropic compressive stresses were applied. Fiber reinforcement was found to reduce the particle breakage of both the uncemented and cemented sands.