Journal of Geotechnical and Geoenvironmental Engineering

The tunnel seismic ahead-prospecting method is used to estimate adverse geology ahead of the tunnel face. Accurately realizing this geology helps to guarantee safety during construction using a tunnel boring machine (TBM). The paper proposes a three-dimensional (3D) seismic ahead-prospecting method by optimizing a filtering method and an imaging algorithm. In this paper, a united filtering method based on $f–k$ and $\tau –p$ filtering is introduced. First, the waves from the interfaces behind the tunnel face are removed and P- and S-waves are separated. Then the interference waves are further eliminated by $\tau –p$ filtering. Mirrored artifacts resulting from the equitravel time plane algorithm are suppressed by the directional weighting integrated into the algorithm. Imaging results indicate that the new method suppresses mirrored artifacts better and produces accurate results. Moreover, the improved algorithm produces reflections that are more consistent with the interfaces than those produced by the original algorithm without directional weighting. Field studies using the 3D seismic ahead-prospecting method have accurately identified and located the main fractured zones, proving the reliability and superiority of the method.

A new service limit state (SLS) design procedure for drilled shafts is described, which utilizes strength resistance factors calibrated to achieve a target probability of failure. The new procedure can be used for varying values of the target probability of failure, and allows the shaft dimensions, soil strength, variability of soil strength, loads, and allowable settlement to be adjusted freely in the design. The procedure eliminates the need to establish resistance factors for the SLS on a case-by-case basis, which is a current practical issue for load and resistance factor design (LRFD). Sensitivity analyses presented indicate that resistance factors for a broad set of SLS design conditions can be reasonably established considering four parameters: the target probability of failure, normalized load, normalized shaft dimension, and the variability of soil/rock strength. Calibrated resistance factors are presented as a function of these four parameters in the form of resistance factor charts, as well as in the form of analytical equations developed through regression of the rigorously calibrated resistance factors. The proposed resistance factors range from 0.1 to 0.4, primarily because of the significant variability and uncertainty associated with side resistance for drilled shafts in shale.

Gassy soils are a special case of unsaturated soils, for which the gas phase exists in the form of occluded bubbles of a size that allows the bubbles to fit within the void spaces without distorting the soil structure. The applicability of the original form of Terzaghi’s effective stress principle was tested for this class of soils. A series of globally undrained tests on gassy specimens, as well as drained effective stress path (ESP) tests on fully saturated specimens, was presented to validate this hypothesis. In a gassy test, the effective stresses were computed with the assumption that the Terzaghi’s effective stress principle was valid. In an ESP test, the effective stress path obtained from the experiment on an identical gassy specimen was applied to a saturated specimen with the drainage lines opened. Two groups of tests covering a degree of saturation ($S_r$) from 92 to 100% were performed. The application of the same effective stress path in the gassy and the ESP tests resulted in the same axial versus volumetric strain response, and therefore it was in accordance with the postulated validity of the Terzaghi’s effective stress principle under gassy states. Such results were found to be valid within the entire range of degrees of saturation explored in this study, and they provide new support for the use of the original definition of Terzaghi’s effective stress in geotechnical analyses for gassy deposits.

Increasing awareness of the role of spatial variability in the assessment of liquefaction susceptibility has driven the need to characterize the vertical and horizontal spatial variability of liquefiable soils. Specifically, the variability of the standard penetration tests and cone penetration tests (CPTs) and the fines content are needed to evaluate liquefaction triggering. Although the analysis and determination of the vertical inherent variability of sandy soils has been conducted in several prior studies, very little prior work has been conducted to characterize the typical horizontal component of spatial variability. This study employs a comprehensive data set developed from a test site established to evaluate liquefaction susceptibility and mitigation to characterize the vertical and horizontal spatial variability in CPT cone tip and sleeve resistance and the fines content. Random field model (RFM) parameters, such as the vertical and horizontal scale of fluctuation, coefficient of inherent variability, and trend functions for each measurement are established as well as the variability in these parameters. A site-specific CPT-based fines content correlation is employed within kriging simulations performed using calibrated geostatistical models and demonstrated the effect of the assumed variogram model on the accuracy of fines content measured in separate independent boreholes. This paper shows that calibrated kriging simulations can sufficiently predict the fines content in silty beach sands and that the spatial variability of fines content can be estimated using RFM parameters developed from CPTs.

This paper focuses on desaturation due to the microbially mediated dissimilatory reduction of nitrogen, or denitrification, for mitigating the potential for earthquake-induced soil liquefaction. Denitrification has the potential to mitigate earthquake-induced liquefaction as a two-stage process referred to as microbially induced desaturation and precipitation (MIDP). In MIDP, desaturation provides mitigation in Stage 1, and microbially induced carbonate precipitation (MICP) provides mitigation in Stage 2. Denitrifying bacteria, when stimulated, have the ability to rapidly desaturate a saturated soil by producing nitrogen and carbon dioxide gases. Small amounts of desaturation can significantly increase the cyclic resistance of saturated soil. As this desaturated condition may not last indefinitely, desaturation is relied upon only to provide temporary mitigation. However, in addition to desaturating the soil, denitrifying bacteria can induce MICP by increasing the pH and carbonate alkalinity of the pore fluid. Interparticle cementation, void filling, and particle roughening induced by MICP improve the cyclic resistance and dilatant behavior of granular soil, thereby providing long-term mitigation of liquefaction. This paper describes the use of desaturation via denitrification for short-term mitigation of earthquake-induced liquefaction. The use of MICP via denitrification as a long-term solution is presented in a companion paper.

This paper focuses on the potential for microbially induced carbonate precipitation (MICP) via dissimilatory reduction of nitrogen, or denitrification, to mitigate the potential for earthquake-induced soil liquefaction. Denitrification has the potential to provide mitigation of earthquake-induced soil liquefaction as a two-stage process referred to by the authors as microbially induced desaturation and precipitation (MIDP). In MIDP, desaturation provides mitigation in Stage 1 and MICP provides mitigation in Stage 2. Denitrifying bacteria, when stimulated, have the ability to rapidly desaturate a saturated soil via gas production, thereby significantly increasing the cyclic resistance of the soil. However, because this desaturated condition may not last indefinitely, desaturation is only relied upon to provide temporary mitigation in MIDP. In Stage 2 of MIDP, interparticle cementation, void filling, and particle roughening as a result of MICP significantly improve the strength, stiffness, dilatant behavior, and cyclic strength of the soil, thereby providing long-term liquefaction mitigation (i.e., after denitrification has ceased). This paper describes the use of MICP via denitrification for the long-term mitigation of earthquake-induced liquefaction. Desaturation via denitrification for the short-term mitigation of liquefaction potential is presented in a companion paper.