Journal of Geotechnical and Geoenvironmental Engineering

Estimation of soil behavior type from cone penetration testing, and the interpretation of dissipation tests, is complicated in intermediate soil types, such as silty sands, sandy silts, etc., where partial consolidation occurs during penetration. This issue is investigated in this paper using results from cavity expansion and finite element analyses as well as field and centrifuge piezocone data. The implications for soil classification are examined using analytical expressions to explore the effect of normalized shear strength, rigidity index, and overconsolidation ratio relative to the influence of partial consolidation and viscous effects under fully undrained conditions. It is shown that partial drainage conditions can affect where data plots on soil behavior charts, thus complicating soil classification. The effect on dissipation tests following partial consolidation during cone penetration is shown to create errors in interpretation using experimental and numerical data. A new approach is developed based in part on manipulation of solutions for pore pressure dissipation (following undrained penetration) to account for these errors when interpreting dissipation tests. Errors can become significant during standard cone penetration testing when the $t_{50}$ dissipation time is less than about 50 s. Guidelines, including equations and a chart, are presented for practical use. Finally, implementation of this approach is demonstrated in a brief case study.

When a pile is embedded in a consolidating ground (e.g., newly reclaimed land), soil may settle more than the pile, thus generating negative skin friction along the pile shaft. This negative friction induces additional axial load to the pile (dragload) and pulls the pile further downward (downdrag). In this paper, the problem is investigated numerically with the finite-element package ABAQUS. It was found that the package defaults an interface model that models the mobilized interface strength in a way that the effect of water pressure was overlooked. Therefore, a modified numerical algorithm is proposed in this study. It amends the model by correctly bringing pore water pressure into the calculation steps. which properly simulates the effective stress-dependent nature of the shear strength at the soil-pile interface. The algorithm is then verified by a self-contained simple to understand simulation. A case history of two piles (one of them coated with bitumen) embedded in a consolidating soft ground is then back analyzed with the proposed algorithm. Fully coupled consolidation and geometric nonlinearity are also considered in the analyses. The transient response of the problem is investigated, including the development of dragload, downdrag, and neutral plane with time. The simulation generally fits well with the field measurements. Parametric studies of the effects of pile head loading reveal that the position of the neutral plane depends not only on the magnitude of the applied pile head loading, but is also affected when the load is applied with respect to the consolidation process.

This paper presents the results of a series of dynamic centrifuge tests that were conducted for $2\times 2$ pile groups in a three-layer laterally spreading soil profile consisting of a nonliquefiable cohesive crust overlying loose, liquefiable sand, overlying dense sand. Two main variables are considered, both of which received little attention in previous work on piles in laterally spreading soils, namely (1) the axial load carried by the foundation, and (2) whether the slope boundary conditions are infinite or finite. The data show that the presence of axial load reduces the lateral stiffness of the foundation resulting from $P\mathrm{\text{-}}\Delta$ effects and reduces their capacity to resist lateral kinematic loads from spreading soil. This degradation in lateral response (bending) may be accompanied by substantial settlement of the foundation as a competing failure mode that must also be considered in design. Furthermore, the mechanical response of the liquefied soil appears to vary greatly with the slope boundary condition. This is particularly true at the interface between the liquefied sand and the cohesive crust, where the downslope displacement of the crust for infinite slopes is much greater than the underlying sand, with the reverse being true for finite slopes. The data also suggest an alternative mechanism to the water film concept that has been used previously to account for the large downslope movements of low permeability crustal layers. This fundamental difference in mechanical response provides insight that may lead to the improvement of simplified empirical methods for estimating surficial displacements caused by lateral spreading.

Construction effects on pile-soil systems arise from the process of installing displacement piles. This study conducted a comprehensive field test program complemented with laboratory tests to observe the performance of jacking open-ended concrete pipe piles into silt deposits. The jacked piles were examined in a plugged mode during installation. Direct observation on the soil plugs reveal that their formation generally accords with the stratigraphic nature of the layered soils. Soil-arching behavior during pile penetration causes the soil in the shear zone along the inner pipe wall to mainly come from the uppermost layer of the deposit. Laboratory tests on the soil plug shows evident compaction and the tendency increased strength over time. The buildup of the excess pore pressure and radial total stress in the soil is sensitive to the jacking installation procedure. By taking into account the soil displacement related to the plugging degree, the captured peak excess pore pressure at a given horizon can be modeled by the cavity expansion theory that normally adapts to closed-ended pile. The jacking annulus resistance normalized by the cone tip resistance is independent of the penetration depth and the degree of plugging. A considerable portion of the annulus resistance is locked in the pile after installation, decreases a little during adjacent pile installation, and remains stable over a long period.

Present methods for assessing the potential for unsatisfactory levee performance because of underseepage consist of deterministic seepage analyses and simplified reliability methods. Deterministic methods consist of calculating factors of safety based on the ratio of the critical gradients of the soil and hydraulic exit gradients without taking into account high levels of uncertainty in soil properties and subsurface geometry that are inherent to many levee analyses. The most common simplified reliability approaches currently being used to analyze levees against underseepage apply the first-order second-moment Taylor series method, using the U.S. Army Corps of Engineers blanket theory equations as the performance functions. In many cases, these methods do not realistically reflect the geometry of the levee’s foundation soils and the uncertainty associated with their performance. This study proposes a new application for the response surface method that allows modeling the initiation of erosion process with more accurate failure mechanisms and more complex subsurface geometry. The response surface–Monte Carlo (RSMC) simulation method uses finite-element analyses to develop a series of equations that define the relationship between the variables and the factor of safety ($F$). Using these equations, probability density functions (PDF) for variables, and the computer program @Risk, a Monte Carlo simulation is performed to calculate the conditional probability of unsatisfactory performance because of underseepage for a given river flood level. Two examples are presented to illustrate the proposed procedure. Multiple regression analyses are performed to assess the relative effect that changes in the input variables have on the $F$ for the various analyses. The results suggest that uncertainty in the levee geometry has the greatest effect on the variation of the $F$ for the cases studied.

A study on liquefaction resistance of Solani sand reinforced with geogrid sheet, geosynthetic fiber, and natural coir fiber is reported. Tests were carried out on shake table (vibration table) with sand samples prepared at a relative density of 25%, with and without reinforcements. Synthetic geogrid sheets were used in three different combinations of three, four, and five layers. In case of fibers, the percentage of fibers by weight of dry sand were taken as 0.25, 0.50, and 0.75% and mixed randomly with the sand sample. The liquefaction parameters, such as the maximum pore water pressure ($U_{\max }$), maximum pore water pressure built-up time, stay time for $U_{\max }$, and pore water pressure dissipation time were measured corresponding to various levels of accelerations varying from 0.1–0.4 g. The frequency of the dynamic load was kept constant at 5 Hz. The liquefaction resistance of sand was evaluated in terms of maximum pore water pressure ratio ($r_{u\max }$). Test results indicated that on inclusion of fibers and geogrid sheets into the sand samples, the $r_{u\max }$ decreased. On increasing the fiber content and number of geogrid sheets, $r_{u\max }$ decreased further, and this decrease is significant at a small amplitude of excitation. The average increase in liquefaction resistance of sand reinforced with synthetic and coir fibers was found to be 88 and 91%, respectively, for 0.75% fiber content, whereas for five layers of geogrid sheets, this increase was about 31% at 0.1 g acceleration.

The use of new materials for soil strengthening is crucial for geotechnical engineering, especially in foundation construction. Our main objective was to investigate the potential use of two-component water soluble epoxy resins to improve the physical and mechanical properties of medium sand, because the efficacy of these resins on soil strengthening has not yet been properly investigated. Experiments were conducted using resins with different epoxy resin-to-water (ER/W) ratios. The results indicate that the epoxy resins improve the physical and mechanical properties of sand significantly, and if successfully grouted into a formation, the resins could provide a suitable solution for the stabilization of foundation material. Based on the experimental results, a nonlinear regression analysis was performed to correlate the mechanical properties and permeability with curing time and ER/W ratio.

This paper describes a method to compute the mobilized shear modulus, $G$, and damping ratio, $D$, using the nonlinear fit of experimental transfer functions obtained at different depths in centrifuge models, with the analytical expression of the amplification function for a viscoelastic soil layer on a rigid base. The corresponding shear strain, $\gamma$, is computed as a function of the particle velocity and shear wave velocity. The sources of potential error in the determination of $G$, $D$, and $\gamma$ embedded in the proposed method are identified and discussed in comparison with two other methods that have been proposed in the literature, based either on the determination of the time lag of accelerations between two accelerometers or on the evaluation of the shear stress-strain cycles from acceleration time histories recorded at different depths in the model. The performance of the three methods is evaluated using the experimental data obtained from nine centrifuge tests on dry sand. The values of $G$ obtained by the proposed method compare well with the results of laboratory and literature data; $D$ values are more dispersed and slightly higher than the literature data.

Most clays, either naturally deposited or man-made, possess a certain degree of overconsolidation owing to tamping, cyclic loading, erosion, excavation, and/or changes in groundwater tables. An easy-to-use constitutive model for overconsolidated clays is useful for relevant engineering applications. In this paper, a simple model is proposed for overconsolidated clays based on the unified-hardening (UH) model. To evaluate the potential peak stress ratio of overconsolidated clays, a parabolic Hvorslev envelope rather than a straight envelope (used in the original UH model) is adopted. The proposed parabolic Hvorslev envelope passes through the origin of the mean stress-deviatoric stress plane. It has a slope of 3 as the overconsolidation ratio (OCR) approaches infinity and intersects with the critical state line as the OCR reaches unity. This modification leads to more realistic predictions for highly overconsolidated clays than does the original UH model with a straight Hvorslev envelope and is consistent with the critical state soil mechanics in which the higher peak stress ratio in overconsolidated clays is a result of interlocking (or dilatancy) rather than cohesion. The modified UH model retains the same parameters as those in the modified Cam-clay model. Reasonable agreement between the model predictions and experimental data demonstrates that the modified model is capable of addressing the fundamental behavior of overconsolidated clays. The present model is developed for reconstituted clays with an isotropic fabric. The potential improvement of the model, taking into account anisotropy and structural effects, is discussed.

Numerical analyses for soil structures under monotonic and cyclic loading using nonlinear models have developed rapidly in recent years. Effective stress-based soil models are incorporated in the analyses to predict soil deformation and/or liquefaction. However, in applications to engineering projects, model parameter calibration often becomes an obstacle to the practical use of the model because of the uncertainties and lack of appropriate laboratory test results. This article proposes a procedure to calibrate a soil model for simulating liquefaction and ultimate failure under monotonic and/or cyclic loading, using data based on in situ measurements in the standard penetration test (SPT). The ultimate failure state that limits soil dilation is based on the correlation of residual strength to void ratio, and the state-dependent dilatancy concept. A relationship between soil liquefaction resistance and equivalent number of cycles is developed based on published SPT-based liquefaction triggering charts, together with correlations between a magnitude scaling factor and number of equivalent cycles to liquefaction. Examples to illustrate the application of these concepts are also presented.