Journal of Geotechnical and Geoenvironmental Engineering

Laboratory testing shows that microbially induced carbonate precipitation (MICP) through microbial denitrification can improve the mechanical properties of a sand without inducing significant interparticle cementation. Consolidated isotropically undrained triaxial compression testing of Ottawa 20–30 sand treated with denitrifying microorganisms shows that, even at low carbonate contents and with no observed cementation, soil treated through MICP exhibits significantly improved stiffness and dilatant behavior. These improvements are also evident when the treated soil is dried, reconstituted, and retested, indicating that the stiffness and dilatant properties of the soil can be improved by MICP in the absence of interparticle cementation, particularly at low strains. However, these improvements may be reduced or eliminated when the soil is reconstituted and tested multiple times. These results indicate that small amounts of MICP can induce significant improvement in treated soils, potentially leading to savings in time and money if this technology is applied in the field.

The interpretation of pressuremeter test results in cohesive soils assumes that the soil is saturated and the rate of loading is such that undrained conditions apply. Pressuremeter tests in compacted soils or heavily overconsolidated clays may have a degree of saturation that is less than 100%. Consequently, the soil may undergo volume changes during the initial stages of loading. Loading of the soil will result in the compression of occluded air and consequently the initial circumferential strains may be a function of volumetric changes and not a result of borehole expansion alone. A method to determine the applied radial total stress that will result in saturation for the pressuremeter test is developed based on Hilf’s method of pore water pressure prediction during compressive loading of an unsaturated soil. A method to quantify the volume change that will occur within the plastic region around the pressuremeter probe prior to saturation is also developed.

This paper has two objectives. The first is to evaluate the cyclic shear strain needed to trigger liquefaction, ${\gamma }_{cl}$, in clean and silty sands in the field during earthquakes as an alternative to the currently used cyclic resistance ratio (CRR). The second objective is to explore the effect of a high effective overburden pressure, ${\sigma }_{v0}^{\prime }$, on the value of CRR. The first objective is accomplished mainly by using an equation relating ${\gamma }_{cl}$ and CRR valid for shear wave velocity–based liquefaction charts. This equation is supplemented with laboratory results from undrained cyclic strain-controlled tests as well as large-scale and centrifuge model shaking experiments. It is shown that for recent uncompacted clean and silty sand deposits and earthquake magnitude $M_w=7.5$, ${\gamma }_{cl}\approx 0.03\%$, with this value increasing to ${\gamma }_{cl}\approx 0.3\%$ (in some cases to 0.6%) in denser and overconsolidated, preshaken, and compacted sands. These small values of ${\gamma }_{cl}$ in the field are controlled by two factors: excess pore pressure buildup in the soil due to the cyclic straining and the redistribution of excess pore pressures and upward water flow that occurs during shaking. The available evidence suggests that ${\gamma }_{cl}$ is either constant with ${\sigma }_{v0}^{\prime }$ or increases slowly with it. Therefore, in the second objective the simplest assumption is made in the paper that the small ${\gamma }_{cl}$, valid for the low confining pressures covered by the liquefaction charts [${\sigma }_{v0}^{\prime }<200\mathrm{kPa}$ ($\approx 2\mathrm{atm}$)] can be extrapolated without change to confining pressures as high as ${\sigma }_{v0}^{\prime }=800$ or 1,000 kPa ($\approx 8\mathrm{or}10\mathrm{atm}$). This assumption allows derivation of a simple expression for the overburden pressure factor, $K_{\sigma }$. The expression predicts a decrease in $K_{\sigma }$ with confining pressure that is very similar to some curves of $K_{\sigma }$ versus ${\sigma }_{v0}^{\prime }$ proposed in the literature. $K_{\sigma }$ curves are also calculated assuming that ${\gamma }_{cl}$ is proportional to ${({\sigma }_{v0}^{\prime })}^{\beta }$, where $0\le \beta \le 0.5$. Additional experimental research is needed to establish the exact variation of ${\gamma }_{cl}$ with ${\sigma }_{v0}^{\prime }$ for different sands and different parts of the liquefaction charts and to clarify some of the remaining discrepancies between the different approaches.

Although commonly ignored in classical earth pressure methods, precipitation-induced failures in retaining structures highlight a need for explicit consideration of negative pore pressures or matric suction. Changes in the degree of saturation in the backfill can significantly affect active earth pressures. This paper presents an analytical framework for calculating the thrust of active earth pressures under unsaturated steady flow conditions. The proposed method addresses limitations associated with the collapse mechanism and effective stress representation in alternative analytical methods. The formulation was derived by implementing an analytical solution for one-dimensional (1D) steady flow into a limit equilibrium–based effective stress analysis. A closed-form solution for the thrust was obtained by employing a log spiral surface, considering the effect of tension cracks, backslope inclination, and batter for $c^{\prime }\text{-}{\varphi }^{\prime }$ soils with variable backfill saturation. A unified effective stress representation was used to describe the state of stress in partially saturated soils. The method was evaluated for different hypothetical soil types, and the effect of infiltration on lateral earth pressures was demonstrated. This approach presents a useful tool for explaining the true service state behavior of walls and evaluating the forensics of failed structures.

The Simplified Method as reported in AASHTO and Federal Highway Administration (FHWA) manuals has been demonstrated to give poor predictions of unfactored reinforcement loads and strains, especially for geosynthetic reinforced soil walls. The writers have proposed the K-stiffness Method to improve the load prediction accuracy for walls under working stress (operational) conditions. However, it has also been recognized in recent publications by the writers and others that further improvements to the K-stiffness Method are needed. Furthermore, acceptance of the K-stiffness Method has been hindered due to its perceived complexity and the use of the plane strain friction angle to quantify the strength of the reinforced soil. This paper takes a fresh look at both methods and uses lessons learned from the K-stiffness Method development to improve the accuracy of the AASHTO/FHWA Simplified Method. Key parameters introduced during the development of the K-stiffness Method are applied to the Simplified Method and updated to further improve load prediction accuracy. Additional wall case histories have been added to the database used for the original K-stiffness Method to calibrate the new model and to broaden its utility. An important improvement is a single model that allows for seamless load prediction across a range of walls constructed with relatively extensible geosynthetic reinforcement and inextensible steel reinforcement materials. The quantitative improvement of the new model (Simplified Stiffness Method) compared to the current AASHTO/FHWA Simplified Method is demonstrated through statistical analysis of load bias values (i.e., the ratio of measured to predicted reinforcement load).

A constitutive model that represents soil behavior under a wide range of loading conditions is necessary for the simulation of complex boundary value problems. However, most laboratory tests are interpreted with an assumption of uniform stresses and strains within the tested soil specimen even when the specimen is known to experience nonuniform stress-strain distribution as in the direct simple shear (DSS) test. Numerous tests are often needed to fully characterize a soil’s nonlinear and anisotropic behavior and to develop an appropriate soil model. This paper utilizes an evolutionary inverse analysis approach to extract nonuniform stresses and strains within ${\mathrm{K}}_0$ consolidated-undrained direct simple shear (${\mathrm{CK}}_0\mathrm{UDSS}$) test specimens on Boston blue clay (BBC) and directly develop soil constitutive models. The extracted soil behavior is consistent with known behavior of BBC including anisotropic stress-strain response and small strain nonlinearity obtained under complex laboratory loading conditions. The developed soil models from DSS tests are applied to a deep excavation case history. The analysis results show that global responses, such as lateral wall deflections and vertical ground surface settlements, approximately match measured response. The proposed approach represents a major shift in our ability to efficiently bridge numerical modeling and laboratory testing and changes the way soil characterization and constitutive model development is approached. It is possible to use very few laboratory tests to directly develop versatile material models that can be used in the solution of geotechnical field problems without the need for complex formulations or length development processes.

Modeling of the one-dimensional consolidation process for soil and other compressible porous media is important for a variety of engineering applications. This paper presents solutions for two benchmark problems for large strain consolidation of a saturated soil layer that can be used to check other analytical and numerical analysis methods. The solutions include effects of vertical strain, soil self-weight, nonlinear constitutive relationships, and changing material properties during the consolidation process. High-accuracy data are presented for each problem and include changes of settlement, void ratio, and excess pore pressure with time.