Journal of Geotechnical and Geoenvironmental Engineering

A new strain-based model is presented in this paper for assessing the residual excess pore-water pressure ($u_{\mathrm{e}}$) buildup in fully saturated sands. In this model, the generation of residual $u_{\mathrm{e}}$ is quantified by the volumetric strain changes owing to cyclic shearing. The concept of threshold shear strain is introduced into the model. To characterize the generation of residual $u_{\mathrm{e}}$ in this model, a number of tests, including one resonant column test, a series of undrained and drained multistage, and single-stage strain-controlled cyclic triaxial (CTX) tests, are performed with fully saturated fine sand samples. The cyclic shear-volume coupling equation and the correlation between the progressive increase of residual $u_{\mathrm{e}}$ during undrained CTX test and the accumulated volumetric strain during drained CTX test are established. Furthermore, a simple bulk modulus equation is derived by differential method. Finally, the general applicability of the proposed model is validated by the experimental data for the same sand, and independent confirmations are also demonstrated with the original experimental data for four sands, obtained from the literature. The proposed pore-water pressure model provides new insights into the mechanics of residual excess pore-water pressure buildup under undrained cyclic loading conditions.

The small-strain damping ratio (${\mathrm{D}}_{\min }$) is a key parameter in site response models and using values from laboratory tests tends to overpredict the site response because laboratory tests cannot capture the wave scattering effects that are present in the field. In this study, earthquake motions from four downhole array sites are used to investigate the increase in ${\mathrm{D}}_{\min }$, as quantified by the ${\mathrm{D}}_{\min }$ multiplier applied to the laboratory based ${\mathrm{D}}_{\min }$, required to match the site response. Empirical observations from the downhole array data are compared with theoretical results from linear-viscoelastic, one-dimensional site response analysis. Different measures of ground response are considered when evaluating the site response: the acceleration transfer function (TF), the spectral acceleration amplification factor (AF), the surface motion peak ground acceleration (PGA), peak ground velocity (PGV), and Arias Intensity (Ia), and the change in the high-frequency spectral decay parameter ($\mathrm{\Delta }\kappa$) between the downhole and surface sensors. It is recommended that the ${\mathrm{D}}_{\min }$ multiplier for a site be selected to best match the Ia rather than the TF. Across the four sites, the required ${\mathrm{D}}_{\min }$ multiplier varies from 1.5 to 5.5. It is hypothesized that the magnitude of the ${\mathrm{D}}_{\min }$ multiplier may be related to the geologic depositional environment of the site, with larger ${\mathrm{D}}_{\min }$ multipliers associated with more spatially variable geologic conditions. These conditions have more variation in shear wave velocity across a site, which leads to more wave scattering and larger ${\mathrm{D}}_{\min }$ multipliers.

Fine refuse from coal processing is frequently disposed of as slurry in an impoundment supported by an embankment consisting of coarse refuse. Flow failure of coal-waste impoundments and metal-tailings dams under static and dynamic impact loading has occurred many times in the last few decades in the form embankment or dam instability or breakthrough of impounded coal-waste slurry into underground mines. Such failures can release huge amounts of flowable waste and cause severe property damage, loss of life, and significant environmental degradation. Until recently, there has been no comprehensive study reported on the properties and flow behavior of impounded coal refuse. In this study, laboratory experiments were performed on samples collected from the Appalachian coal field to measure the geotechnical properties of fine coal refuse. A comprehensive investigation was carried out on the flow behavior of impounded fine-coal-refuse slurry under static and impact loading by using a group of small-scale and centrifuge model tests. The relationships between water content and important parameters in flow behavior were established, and the flow behavior of slurry with different water contents was analyzed under static and impact loading. In addition, the employment of slurry cells as a potential countermeasure to flow failure was evaluated in centrifuge model tests.

This paper reports on a series of laboratory hydraulic tests on a select range of granular soils compacted at relative densities between 0% and 100%. The critical hydraulic gradient at the onset of seepage failure (i.e., heave and suffusion) is considerably smaller than unity for internally unstable (i.e., nonuniform) sand–gravel mixtures due to stress reduction in their finer fraction. For example, stable uniform fine sands have been shown to exhibit heave at hydraulic gradients $\ge 1.0$, whereas sand–gravel mixtures suffer from suffusion at hydraulic gradients $\ll 1.0$. The boundary friction from the cell walls of test equipment would influence the development of heave, while suffusion is controlled by interparticle friction. In this study, the critical hydraulic gradient is modeled theoretically by considering the effects of interparticle and boundary frictions, and stress reduction in the soil. The experimental results from both this and past studies are used to verify the proposed model, which showed good agreement with experimental observations with less than 5% standard error.

The erroneous load transfer curve calculated using the nominal cross-sectional area ($A_C$) and elastic modulus of a concrete specimen ($E_{\mathrm{CU}}$) was modified by considering the nonuniform cross-sectional area ($A_{\mathrm{CR}}$) and nonhomogeneous elastic modulus ($E_{\mathrm{CV}}$) of concrete. A drilled shaft instrumented with strain gauges and pipes for a crosshole sonic logging (CSL) test was constructed as the test pile. To estimate $A_{\mathrm{CR}}$, the incremental volume of the placed concrete was recorded. After curing the concrete, the CSL test was performed to estimate $E_{\mathrm{CV}}$ based on the compressional wave (P-wave) velocity. In addition, a bidirectional load test was conducted, and the load transfer curve was calculated. The results of the analyses demonstrated that although the erroneous load transfer curve was partially modified by considering either $A_{\mathrm{CR}}$ or $E_{\mathrm{CV}}$ individually, the modification was enhanced further by adopting both $A_{\mathrm{CR}}$ and $E_{\mathrm{CV}}$. This study demonstrates that the estimation of $A_{\mathrm{CR}}$ and $E_{\mathrm{CV}}$ may be used to calculate reasonable load transfer curves for drilled shafts.

When subjected to axial loading, the response of a pile in a pile group is different from that of an isolated single pile, all other factors being the same. Three-dimensional finite-element analyses were performed to study the response to vertical loads of nondisplacement pile groups in sand. The analyses were done with an advanced bounding-surface-plasticity constitutive model. The mechanical response of soil elements at various locations between the group piles was thoroughly investigated to reveal the underlying pile–soil–pile interaction mechanisms. Pile group efficiency was found to be a function of the pile head settlement, number of piles in the group, pile-to-pile spacing, and relative density of the sand. The effect of cap–soil contact on the pile group resistance development was also studied. For small pile groups, the group efficiency is close to or greater than 1 at settlement levels likely to be of interest in design.

The soil water retention curve (SWRC) is one of the most fundamental characteristics of unsaturated soils. Because unsaturated soils are subjected to a range of natural processes and engineered treatments, a thorough understanding is needed of how their retention properties change when exposed to each separate treatment. In recent years, several biological treatment methods, such as microbial-induced calcite precipitation (MICP), have been introduced as environmentally friendly techniques. This study investigates the effect of biological treatment by means of Bacillus sphaericus on the SWRC of both coarse-grained and fine-grained soil samples. Pore fluid pH, X-ray diffraction, and scanning electron microscopy tests are used to better delineate the mechanisms affecting biologically induced alterations of SWRCs. Results showed that higher bacterial concentrations produced higher air-entry values in coarse-grained soils. However, the fine-grained samples showed an initial increase followed by a decrease in the air-entry value when the bacterial concentration increased. Changes in the soil porous structure and double-layer thickness are thought to be main reasons causing the different behavior of fine-grained soils.

This study presents an estimating method to calculate the static and the dynamic stresses in the geosynthetic-reinforced pile-supported track-bed (embankment), based on a series of full-scale model tests under train moving loads. The testing results indicate the distinct soil arching effect for both the static and the dynamic stresses, with the same height of soil arching and similar values of pile efficacy. The pile efficacy for the dynamic stress in the proposed method is thus estimated by the existing analytical model for the static stress. The distributions of the static and the dynamic stresses estimated by this method are evaluated by comparisons with the testing data in the present model test. The results show that above the height of soil arching, the static and the dynamic stresses follow the variations of the self-weight of the track-bed (including static surcharge) and the Boussinesq solution (strip load; 3 m width as that of the concrete base; applied dynamic stress at the specific train speed), respectively. In the soil arching area, the predictions by this method are consistent with the testing data above the pile cap, whereas this method provides slightly higher values above the surrounding subsoil than the experimental results. Generally, this method appears reasonable, showing the difference of the dynamic stress no more than 5 kPa between the tests and the estimations.