Journal of Geotechnical and Geoenvironmental Engineering

A comprehensive experimental investigation was conducted to investigate the effects of loading condition and confining pressure on strength properties and localization phenomena in sands. A uniform subrounded to rounded natural silica sand known as F-75 Ottawa sand was used in the investigation. The results of a series on conventional triaxial compression (CTC) experiments tested under very low-confining pressures (0.05–1.30) kPa tested in a microgravity environment abroad the NASA Space Shuttle are presented in addition to the results of similar specimens tested in terrestrial laboratory to investigate the effect of confining pressure on the constitutive behavior of sands. The behavior of the CTC experiments is compared with the results of plane strain experiments. Computed tomography and other digital imaging techniques were used to study the development and evolution of shear bands.

Braced excavations for the Central Artery/Tunnel project in Boston, Mass. include several instrumented sections for monitoring excavation and ground response to soil removal and strut installation. Strain in the struts is measured using thermally matched vibrating wire strain gauges (VWSGs). This paper presents issues related to strut load interpretation using VWSG data and the difficulties and errors associated with the use of field data. The paper presents a procedure to separate earth load from thermal load induced in the struts due to temperature variation throughout construction. The procedure is an extension of a method that uses measured temperature and corresponding load change to compute a thermal (temperature change induced) load coefficient. The procedure emphasizes the dependency of the thermal load coefficient on construction stage and support configuration. A numerical analysis is presented to support the proposed procedure. Strut loads and earth pressure diagrams are presented using the proposed procedure. Comparisons are made between measured loads and the design load envelope developed prior to construction.

The scope of this paper is to develop a new approach to the two-part wedge method for translational failure analysis of landfills. The upper and lower bound solutions for the landfill stability, i.e., ${\mathrm{FS}}_{\max }$ and ${\mathrm{FS}}_{\min }\text{,}$ can be readily determined using this method. Using an average factor of safety, ${\mathrm{FS}}_{\mathrm{ave}}\text{,}$ to replace the true factor of safety, ${\mathrm{FS}}_{\mathrm{true}}\text{,}$ the differences between ${\mathrm{FS}}_{\mathrm{ave}}$ and ${\mathrm{FS}}_{\mathrm{true}}$ are within 5% for most considered cases. The solutions from the new method ensure that the waste strength is not exceeded anywhere within the waste mass. The calculated results agreed well with the results computed from the computer code PCSTABL6. The ability of the new method to calculate the translational failure of waste mass with a predetermined sliding failure face is demonstrated. Also, a waste filling sequence to keep the factor of safety above a stipulated value during the operation phase of the landfilling process can be generated.

This paper presents an innovative, practical methodology to establish priorities for slope monitoring and the planning of stabilization works in large hilly areas. The study is based on the analysis of geological and geomorphological maps, identification of catchment areas, and the division of catchment areas into primary (with main stream) and secondary (without main stream) sub-basins. Attention was focused on secondary sub-basins and their characteristics: lithology; an ad hoc parameter q (related to the average rainfall, the size of the basin, and the width of the toe section), and the average slope inclination i. The technique presented enables the “landslide hazard curve,” unique for a given formation in homogenous areas with similar rainfall conditions and seismicity, to be drawn in a chart which features q and i. All the ascertained and assumed unstable areas represented in the chart are located above the landslide hazard curve. The study was carried out scientifically by taking into account engineering aspects (geological, geomorphological, hydrological, and geotechnical), which influence slope stability. Groundwater levels, important in mapping out landslide prone areas, were considered in the study by means of the parameter q.

In situ field testing has been performed over the past several years at a silty sand site in Austin, Tex. using the borehole spectral analysis of surface waves (SASW) tool to develop the technique and assess the validity of the method. The borehole SASW tool is an inflatable pressuremeterlike device that allows surface wave measurements to be performed along the wall of an uncased borehole while varying the in situ states of stress. Field results demonstrate the applicability of borehole SASW testing as a method to characterize soil sites and provide information about in situ shear wave velocity and the relationship between shear wave velocity and state of stress. Results from a borehole SASW test conducted at the Austin site are presented herein to demonstrate the applicability and validity of the method.

The effects of the shear strain rate $\dot{\gamma }=d\gamma /\mathrm{dt}$ on the secant shear modulus $G_s$ of three clays and three sands at small cyclic shear strain amplitudes ${\gamma }_c$ under simple shear loading conditions are described. The amplitude ${\gamma }_c$ varied between 0.0003 and 0.02% and $\dot{\gamma }$ between 0.0002 and 0.04 %/s. For all six soils $G_s$ increases with $\dot{\gamma }\text{,}$ such that the $G_s$ versus $\log \dot{\gamma }$ data plot approximately along a straight line. The slope of this line is ${\alpha }_G$=strain-rate shear modulus parameter, while its normalized value ${\alpha }_G/G_s=N_{\dot{\gamma }-G}$=shear strain-rate modulus factor. For the clays tested ${\alpha }_G$ ranges between 2.5 and 7.5 MPa and $N_{\dot{\gamma }-G}$ between 2.0 and 11.5%. For the sandy soils ${\alpha }_G$ ranges between 0.2 and 3.0 MPa and $N_{\dot{\gamma }-G}$ between 0.2 and 6.0%. Both ${\alpha }_G$ and $N_{\dot{\gamma }-G}$ decrease moderately with increasing ${\gamma }_c\text{,}$ i.e., the largest ${\alpha }_G$ and $N_{\dot{\gamma }-G}$ were obtained at the smallest ${\gamma }_c\text{.}$ For five soils the families of the normalized modulus reduction curves $(G_s/G_{\max })-\log {\gamma }_c$ were constructed, such that each curve pertains to a constant $\dot{\gamma }\text{.}$ It was found that $\dot{\gamma }$ has essenlially no effect on the shape of the $\mathrm{constant}-\dot{\gamma } (G_s/G_{\max })-\log {\gamma }_c$ curve. A brief review of previous experimental studies is included.

Weathered soils are used extensively as fill materials in slope construction in tropical and subtropical cities such as Hong Kong. The mechanical behavior of loose decomposed fill materials, particularly in the unsaturated state, has not often been investigated and is not yet fully understood. The objective of this study was to understand the mechanical behavior of loose unsaturated decomposed granitic soil and to study the effects of the stress state, the stress path and the soil suction on the stress–strain relationship, shear strength, volume change, and dilatancy via three series of stress path triaxial tests on both saturated and unsaturated specimens. It was found that loose and saturated decomposed granitic soil behaves like clean sands during undrained shearing. Strain-softening behavior is observed in loose saturated specimens. In unsaturated specimens sheared at a constant water content, a hardening stress–strain relationship and volumetric contractions are observed in the considered range of net mean stresses. The suction of the soil contributed little to the apparent cohesion. The angle of friction appeared to be independent of the suction. In unsaturated specimens subjected to continuous wetting (suction reduction) at a constant deviator stress, the volumetric behavior changed from dilative to contractive with increasing net mean stress and the specimen failed at a degree of saturation far below full saturation. It was revealed that the dilatancy of the unsaturated soil depends on the suction, the state, and the stress path.

To evaluate the uplift behavior of anchors installed by the blade underreaming system, a numerical model for anchors in silty sand has been developed in this study and the calculated results are compared to the results of full scale anchor pullout tests. Although the blade-underreamed anchor tends to be irregular in shape due to possible collapse of the borehole, the excavated anchor showed an underreamed body of approximately multiple-stepped shape. Despite the difference in shape, the numerical results indicate that the difference between the load–displacement curve of the multiple-stepped anchor and that of the conical shaped anchor is small. In addition, the anchorage behavior of conical shaped anchors calculated from this numerical model was in good agreement with those of full scale anchor tests. No sign of progressive soil yielding along the underreamed body was found from the numerical analysis. So, the pull-out capacity of this underreamed anchor increases more than linearly with the length of the underream. Since only a small underream angle is needed to generate a substantial increase in anchor pull-out resistance, the ultimate pull-out capacity of the blade-underreamed anchor is found to be higher than that of straight shaft anchor in silty sand.