Journal of Geotechnical and Geoenvironmental Engineering

The prediction of liquefaction-induced damage to structures can be aided in part by an understanding of the magnitude of inherent spatial variability of the underlying liquefaction-susceptible soils. However, little is known about the role of clean sand and overburden stress correction of the cone penetration resistance on the magnitudes of the random field model (RFM) parameters commonly used to describe spatial variability. Using the results of an extensive investigation of a level ground-level stratum test site with liquefaction-susceptible sand and silty sand, clean sand and overburden stress correction was shown to produce little impact to the autocorrelation length of the sands and silty sands investigated in this study. However, the inherent variability and depth-dependent trends in the corrected cone tip resistance are significantly impacted by conditioning commonly required for liquefaction analyses. The RFM parameters developed in this study were used to generate a calibrated, kriged volume of the test site and used to illustrate the roles of the intensity of shaking, the governing scale of a structure, and the spatial variability in the magnitude of liquefaction-induced differential settlement and angular distortion. Owing to the preferential liquefaction of looser materials before denser materials, it is shown that the lower intensities of shaking can produce larger differential settlements and poorer structural performance than higher levels of shaking in level-ground level-stratum sites. This work points to the continued need to study the role of spatial variability on the performance of structures in liquefaction-susceptible soils.

In estimating the vertical bearing capacity of spudcan foundation, the effect of the realistic lattice leg extending from the top of the spudcan is often ignored. This paper examines both the lattice leg and sleeve effect on the transient vertical bearing capacity of a spudcan foundation using centrifuge model tests and the numerical method. The lattice legs and sleeves are characterized by two parameters, namely, the opening ratio and area ratio. The experimental results indicate that the use of lattice legs and sleeves tends to increase the spudcan bearing capacity, compared with a spudcan without legs, in both normally consolidated and overconsolidated clay deposits. This effect appears to vary with the opening ratios and area ratios of the leg. The finding shows that the lattice legs and sleeves affect the transient vertical bearing capacity in three ways. First, it restricts soil backflow to the region of the spudcan footprint. Second, a small amount of the sleeve friction resistance is induced during spudcan penetration. Finally, a larger vertical influence zone of the soil beneath the spudcan footing is mobilized. All these effects contribute to an increase in the transient spudcan bearing capacity. It may provide a way to enhance the bearing capacity and uplift resistance of spudcan foundation by changing the configuration of the existing lattice leg.

Four medium-scale experiments were conducted in a laboratory setting to better understand the effect of soil layering on the increase in pile resistance as a function of time known as pile setup. Each experiment entailed compacting a soil profile consisting of cohesionless and/or cohesive soil in a large manhole and driving a steel pile model instrumented with strain gauges and pile driving monitoring system sensors. Piezometers were placed in selected soil layers to monitor pore-water pressure. Two experiments performed were entirely cohesive and cohesionless soil, and the other two experiments were conducted with mixed soil profiles simulating single- and double-drainage conditions. Time-dependent pile resistances were estimated by pile restrikes using the monitoring system with subsequent signal matching analyses. Static load tests (SLT) were also performed at the end of the last restrike. Cone penetration tests (CPT) were performed before pile installation and after the static load test. This study showed that pile setup occurred in cohesive soil layers, followed the logarithmic time scale, and is contributed mainly from the gain in shaft resistance. No pile setup was experienced in cohesionless layers, regardless of their locations along a test pile and the soil stratigraphy. The presence of a double-drained condition in an intermediate cohesive soil facilitates the dissipation of excess pore-water pressure and pile setup. The pile setup trend follows the pattern of pore-water pressure dissipation in a cohesive soil.

In certain field conditions, the soil element is subjected to cyclic stress paths with both principal stress rotation and deviatoric stress variation. To better understand the response of soft clay under this type of stress path, 19 cyclic tests were conducted on undisturbed Wenzhou soft clay through hollow cylinder apparatus. By using defined waveforms, a series of cyclic hollow cylinder experiments that mimic cardioid-shaped stress paths were carried out under different cyclic axial stress and shear stress levels. The development of axial strain, axial stress-strain relationship, and dynamic modulus are discussed. Results show that when the cyclic stress ratio (CSR) is below a threshold value of cyclic stress ratio (TCSR), the shear stress level has little effect on the dynamic response of soil. With increasing CSR, the shear stress level has a more and more significant effect on the cyclic behavior of soft clay. Based on the test results, a new model for degradation index considering the principal stress rotation and deviatoric stress variation is established. Under the shakedown state, a linear relationship between them in semilog plot is presented and verified by previous existing experimental data.

A general approach for reliability-based analysis and design for pullout and tensile failure internal limit states of soil nail walls is presented. Reliability index values are computed using a closed-form solution that captures the influence of nominal load and nominal resistance model type and accuracy (method bias), bias dependencies, uncertainty in nominal load and resistance values, and possible cross-correlation (dependency) between nominal nail load and resistance terms. Maximum nail loads under operational conditions for the two limit states are calculated using the current Federal Highway Administration (FHWA) simplified method and an improved version recently published by the authors. Nail pullout capacity is calculated using the effective stress method used in Hong Kong and a modified version that has been empirically adjusted to improve model accuracy for soil nails installed in two different Hong Kong soils. Example designs with three different nail length patterns are used to illustrate the design approach and the assessment of margins of safety using factor of safety and reliability index for the pullout limit state. The results of parametric analyses and design examples demonstrate that for the same target reliability index, the combination of improved load and resistance models gave better solutions based on total length of soil nails.

Engineering geological characterization, subject to spatial variability of soil properties, is a three-dimensional (3D) problem in reality, although it is often simplified as one- or two-dimensional. Direct characterization of 3D spatial variability is a challenging task due to the scarcity of geotechnical data and a satisfactory characterization method. To address such a problem, this paper develops a cone penetration test (CPT)–based probabilistic approach for characterizing 3D spatial variability underlying the framework of maximum likelihood estimation (MLE). A matrix decomposition technique is applied to enhance the practical application of MLE for high-dimensional and spatially correlated data. Results of a case study and three virtual site analyses indicate that MLE provides more accurate estimates of random field parameters with smaller statistical uncertainty than the commonly used method of moments with best fitting, particularly for the estimation of scale of fluctuation. In addition, simultaneous vertical and horizontal characterization based on multiple CPTs is a feasible way for 3D spatial variability characterization in the presence of limited data, such as the limited sounding issue and the thin layer issue. The sampling strategy having some closely located CPTs is preferable for 3D spatial variability characterization.

In an earlier paper, the writers proposed modifications to the AASHTO simplified method to address shortcomings in the method and to improve reinforcement load predictions for walls under operational conditions. The new method is called the simplified stiffness method to emphasize the importance of reinforcement stiffness on reinforcement loads rather than strength as in the current AASHTO approach. The method provides a seamless transition for walls built with soil reinforcing elements ranging from inextensible steel to relatively extensible geosynthetics. The accuracy of the new approach is examined statistically and predicted loads are demonstrated to be more accurate than loads computed using the AASHTO simplified method and a log spiral limit equilibrium–based approach. The paper shows how the simplified stiffness method can be applied within a load and resistance factor design (LRFD) framework. To ensure that the reinforced soil zone remains within working stress conditions, a new limit state to address failure of the soil within the reinforced soil zone is introduced and applied within the LRFD framework. Step-by-step design examples for three different wall cases constructed with cohesionless backfill soils are provided in the Supplemental Data for the paper.

One-dimensional self-weight consolidation of soil and other compressible porous media is important for a variety of engineering applications. This note presents a benchmark problem and numerical solutions for large strain self-weight consolidation of a layer of saturated soil slurry. The solutions provide the values of settlement, excess pore pressure, and void ratio, which can be used to validate other analytical and numerical models. The solutions include effects of vertical strain, soil self-weight, nonlinear constitutive relationships, and changing material properties during the consolidation process.