Journal of Geotechnical and Geoenvironmental Engineering

The time to rupture of a 1.5-mm-thick high-density polyethylene geomembrane aged in a typical landfill composite liner configuration is investigated under a pressure of 250 kPa at 85°C. The geomembrane was underlain with a geosynthetic clay liner and had a $560\hspace{0.17em}\text{g}/{\text{m}}^2$ needle-pinched nonwoven geotextile protection layer separating it from 50 mm of drainage gravel containing leachate. Seventeen (0.6-m-diameter) tests were conducted. In addition to 9 months required to deplete antioxidants (Stage I), the tests indicated a lag period (Stage II) of 5.5 months and a time from the start of degradation to rupture (Stage III) of 20 months, giving a total inferred time to rupture of 34.5 months (2.9 years). There were up to 61 brittle ruptures per sample (i.e., $>2\hspace{0.17em}\text{million cracks per hectare}$). The ruptures were predominately oriented in the machine direction and located (1) directly beneath a gravel contact, (2) at the side of a gravel indentation, or (3) between gravel indentations. The ruptures between gravel indentations were the least frequent but largest. The calculated strains perpendicular to the rupture direction were $24\pm 6\%$. Rupture occurred, although the average stress-crack resistance for all ruptured geomembrane samples still was approximately $760\pm 200\hspace{0.17em}\text{h}$, with a minimum of 360 h. These results indicate the importance of minimizing tensile strains in the geomembrane in the design of a liner system.

A three-dimensional (3D) numerical model simulating a twin tunnel excavated by a tunnel-boring machine (TBM)–earth-pressure balance (EPB) is presented. All relevant components are taken into account, and the influence of the parameters adopted in the TBM-EPB method on the induced ground settlements is investigated. The sensitivity to variations of the face support, the pressure applied to the steering gap slurry, and the tail gap grouting were also evaluated within the framework of parametric analyses. The effects from and on a building located at or beyond the greenfield trough width were also investigated. The greenfield volume loss at the ground surface was found to be 0.85%, increasing to 0.97% at the building foundation level. These values were further increased by 6% at the cross section where the building is located. Furthermore, a full soil-structure interaction using 3D numerical analysis was carried out to evaluate the ability of numerical methods to provide accurate displacement prognosis for buildings adjacent to tunnels excavated by the TBM-EPB method. Conclusions regarding the effects of relevant parameters, which have been drawn based on the application of the method to the ongoing Thessaloniki subway, could be useful for the future design and construction of shallow tunnels by the TBM-EPB method.

This paper describes a case history of a deep cement mixed (DCM) column–supported embankment that is part of the Ballina Bypass section of the Pacific Highway Upgrade project in Australia. Measured settlements during and after construction of the embankment were significantly greater than the predicted settlements and suggested that the DCM columns were yielded. The case history was analyzed using a finite-element model based on the coupled theory of nonlinear porous media. Two cases were analyzed: with and without the strain-softening behavior of DCM columns caused by breakage of the soil-cement structure. The computed settlements, excess pore-water pressures, and lateral deformations were compared with field measurements. Results show that there was good agreement between the measured and the computed parameters when the strain-softening behavior of the columns was included. These results clearly show that consideration of the strain softening of DCM columns in the analysis is important if yielding occurs during or after embankment construction.

This paper presents the results of monotonic and cyclic triaxial tests performed on gassy, medium-dense sand specimens. These sands are representative of conditions in a loose-sand deposit that was densified using multiple blasting passes at a site in South Carolina. The equipment and laboratory testing procedures used to reproduce the postblast densification conditions observed at a field test are described in detail. Results of undrained and drained compression tests showed that the gassy-specimen responses were bounded by the fully saturated drained and undrained responses. The undrained test results showed that the presence of gas, even in small amounts, made the soil more compressible and restricted the buildup of excess pore-water pressures as compared with those pressures observed in fully saturated specimens. Additionally, the shear strengths of gassy, medium-dense sand specimens were similar to those observed in saturated, loose sands sheared under drained conditions. For a given cyclic stress ratio and number of cycles, the axial strains and excess pore-water pressures were significantly smaller than those for a saturated specimen with the same void ratio. Implications of the behavior of such blast-densified sands are discussed.

The mechanical behaviors of rockfill materials at a comparatively low pressure are very important (e.g., for the stability of the top part and the slope of rockfill dams). In addition, the stress conditions of rockfill dams are unlikely to be axisymmetric but are three-dimensional. Because of this, the influence of the intermediate principal stress has to be taken into consideration in design. This paper presents a study on the effect of intermediate principal stress on the strength and dilatancy behavior of rockfill material at a comparatively low pressure. A series of constant $b\text{-value}$ tests were carried out on rockfill materials commonly used for earth dam construction. Using the test data, the influences of the $b\text{-value}$ on the peak friction angle, critical-state friction angle, and maximum dilatancy angle of rockfill materials were investigated. A revised relative dilatancy index was proposed for rockfill material to describe the change in the shear strength with the intermediate principal stress. The relationship among the incremental friction angle, the maximum dilatancy angle, and the $b\text{-value}$ was examined. A linear relationship between the incremental friction angle and the revised relative dilatancy index was established by considering the effect of the $b\text{-value}$. The predictions given by this relationship agree well with experimental data. Furthermore, the $b\text{-value}$ can influence the particle breakage of rockfill materials, indicating that such breakage is dependent on the stress path.

Sensitive clay subjected to cyclic loading may experience gradual loss of its shear strength, which may lead to a significant reduction in its bearing capacity, excessive settlement, and perhaps catastrophic failure. This may be caused by the cyclic loading, which acts as a disturbing agent and a major cause for increasing pore-water pressure, leading to leaching of salt content and the failure of the card house structure of the sensitive clay. This paper presents the results of an experimental investigation on sensitive clays subjected to cyclic loadings under undrained and drained conditions. Tests were conducted to examine the effects of cyclic loading, deviator stress, load frequency, preconsolidation pressure, overconsolidation ratio, confining pressure, sensitivity, liquidity index, and degree of saturation on the shear strength of sensitive clay. Depending on the combined effect of governing parameters, the shear strength of the clay may significantly be reduced, and the clay may reach a liquefiable state. This study introduces the concept of a safe zone to examine existing foundation conditions or provide guidelines for the design of new foundations on sensitive clay.

The electrical characteristics of soil-water mixtures reflect the soil type, ionic concentration, surface conduction, fluid saturation, porosity, and pore connectivity of the mixtures. Archie’s law commonly is used to analyze the electrical resistivity measurement results of soil-water mixtures. This paper explores the pore-fluid effect on Archie’s law. Experimental tests were performed on sand and clay specimens to measure the variation in their electrical resistivity at different porosities and for different electrical resistivities of the pore fluid. The results demonstrate that the sand specimens (i.e., low specific surface soil) have a unique Archie’s law curve; however, the clay specimens (i.e., high specific surface soil) have an inconsistent Archie’s law curve owing to the surface conduction induced by the double layer of clay particles. In particular, Archie’s law should be applied and analyzed cautiously when high specific surface soils are subjected to pore fluids with high electrical resistivities (i.e., low electrolyte conductivity).