Journal of Geotechnical and Geoenvironmental Engineering

Geosynthetic clay liners (GCLs) were exhumed from composite barriers, (i.e., geomembrane over GCL) in final covers at four sites after 4.7 to 6.7 years to evaluate the in-service condition. Monovalent bound cations were replaced by divalent cations in all GCLs, with near complete exchange at two-thirds of the sampling locations. Hydraulic conductivity was measured using two dilute solutions commonly used as permeant water: standard water (SW, $0.01M$ ${\text{CaCl}}_2$ solution) and type II deionized water (DW). Hydraulic conductivities to SW varied over four orders of magnitude, whereas identical specimens (i.e., from same sample) had hydraulic conductivities to DW consistently $\le 3\times {10}^{-10}\text{m}/\text{s}$ . Higher hydraulic conductivities and sensitivity to permeant water did not correspond directly to the amount of cation exchange. Exhumed GCLs with higher gravimetric higher water contents $\left(>50\mathrm{\%}\right)$ exhibited a gel structure indicative of osmotic hydration and had lower hydraulic conductivities to both SW and DW, regardless of the amount of sodium (Na) replaced by divalent cations. These GCLs with higher water contents were placed on subgrade having water content in excess of optimum water content (standard Proctor). Conditions that promote rapid hydration and osmotic swell in a GCL are recommended to ensure that a GCL in a composite barrier maintains low hydraulic conductivity $\left(\le 5\times {10}^{-11}\text{m}/\text{s}\right)$ , even if the native Na is ultimately replaced by divalent cations. Subgrade with water $\text{content}\ge \text{optimum}$ water content is recommended.

Full-flow penetrometers (the T-bar and ball) are increasingly used on sites with thick deposits of soft clays, particularly prevalent offshore. Full-flow penetration tests were performed at five international well-characterized soft clay test sites to assess the use of full-flow penetrometers to estimate undrained shear strength. Field vane shear data were used as the reference undrained strength. Statistical analyses of strength factors indicates that full-flow penetrometers provide an estimate of undrained shear strength at a similar level of reliability compared to the piezocone. Relationships for estimating the strength factor and soil sensitivity using only full-flow penetrometer data obtained during initial penetration and extraction are developed. A strong dependence of the strength factor on sensitivity was identified and can be used for the estimation of undrained strength. The effectiveness and use of the developed correlations are demonstrated through their application at an additional test site.

Very large quantities of mined ore materials are processed annually to obtain the various types of minerals that are essential to industrial processes. Due to its execution simplicity and low cost, upstream tailings disposal has been the most common method used for the surface disposal of tailings produced from processing the mined ore materials in spite of it being the most vulnerable to failure. The complex hydromechanical behavior of the upstream tailings disposal facilities (UTDFs) during their staged construction makes the traditional approaches to consolidation, stability, and seepage analyses inadequate for producing accurate and, in many situations, correct design and evaluation of these facilities. Transient coupled nonlinear finite-element analyses are carried out in this paper using the general purpose code ABAQUS to investigate the hydromechanical behavior of the UTDF during its staged construction. The analyses simulate, in combination, a number of realistic features of the UTDFs during construction, including the transient partially saturated flow within the impoundment, the two-dimensional large consolidation and nonlinear material response of the tailings, and the staged construction loading with time. The UTDF response as predicted by the numerical analyses, which are carried out in the form of comparative study on three different impoundment tailings, is qualitatively comparable with the UTDF behavior trends observed and reported in the literature.

The analysis of steep slope and cliff stability in variably cemented sands poses a significant practical challenge as routine analyses tend to underestimate the actually observed stability of existing slopes. The presented research evaluates how the degree of cementation controls the evolution of steep sand slopes and shows that the detailed slope geometry is important in determining the characteristics of the failure mode, which in turn, guide the selection of an appropriate stability analysis method. Detailed slope-profile cross sections derived from terrestrial lidar surveying of otherwise inaccessible cemented sand cliffs are used to investigate failure modes in weakly cemented [unconfined compressive strength $\left(\text{UCS}\right)<30\text{kPa}$ ] and moderately cemented $\left(30<\text{UCS}<400\text{kPa}\right)$ sands and their role in the evolution of the geometry of the slopes. The results show that high-resolution slope topography, such as can be obtained with terrestrial lidar, is essential for identifying altogether different failure modes in weakly cemented (shear-mode) and moderately cemented (tensile-mode) sand slopes. Analyses show that the standard Culmann method for steep slopes is inappropriate for modeling the stability of cemented sand slopes since it tends to overpredict expected crest retreat and underestimate failure plane angle. Instead, a simplified analysis using infinite slope assumptions, but applied to a slope with finite dimensions subject to changing geometric conditions, such as toe erosion and slope steepening, is suggested for analysis of weakly cemented sand slopes. For moderately cemented sand slopes, a limit equilibrium analysis directly comparing the cliff tensile stress and cemented sand tensile strength is shown to reasonably predict failure conditions and timing as a result of either slope steepening or tensile strength loss, presumably from wetting in most cases.

In strong over soft two-layered clays, there is a potential for the footing to experience a punch-through failure, where the footing penetrates a large distance at a short time after the initial peak resistance is reached. Three-dimensional (3D) large deformation finite-element analyses using 3D RITSS method were conducted to simulate the penetration responses of square footings in strong over soft clays. The effects of surface soil heave and soil layer interface deformation during footing penetration were studied in weightless soils. Fitted equations were proposed to express the footing capacity response against the penetration depth. Based on the fitted equations, formulas to calculate footing peak bearing factor and the corresponding penetration depth were developed. The peak footing capacity factor and the corresponding penetration depth increases with the increasing of soil layer strength ratio, relative top soil layer thickness and soil weight factor, thus the potential of punch-through failure was reduced accordingly. It was also found that the soil weight effect can be a simple surcharge based on the formula developed in the weightless soil. Design charts for the peak footing capacity factor and the corresponding penetration depth were developed.

The behavior of pile foundations subjected to horizontal loading is typically evaluated using horizontal load tests. Although load tests are valuable to understand site-specific soil-structure interaction phenomena, validated predictive methods are also useful during the design phase. In this study, the results from horizontal load tests are compared with methods which predict the horizontal bearing capacity of piles using in situ measurements of soil behavior. Specifically, several horizontal load tests were performed in order to evaluate the behavior of two 12-m long Strauss piles and four bored piles with similar length, all installed in a lateritic soil profile. Two prediction methods were evaluated using $p\text{-}y$ curves computed from the results of Marchetti’s dilatometer test (DMT) results. The predictive methods using the $p\text{-}y$ curves from the DMT showed good agreement with the behavior observed in the pile loading test.

Level 1 reliability methods have been internationally accepted as the basis for development of the new generation of geotechnical design codes. A key requirement of this design approach is the identification and quantification of uncertainties associated with the geotechnical design under consideration. This paper presents four load test databases from South Africa for driven piles in noncohesive soils (29 tests), bored piles in noncohesive soils (33 tests), driven piles in cohesive soils (59 tests), and bored piles in cohesive soils (53 tests). The capacity model factor is defined as the ratio of the interpreted capacity (Chin-Davisson approach) and the predicted capacity (static pile design formula). The uncertainty in the capacity model factor is modeled as a lognormal random variable. The model factor statistics reported in this study are required for reliability-based ultimate limit state design. The uncertainty in the load-settlement behavior is characterized by fitting measured load-settlement data to a hyperbolic equation and then normalizing the hyperbolic curve with the interpreted capacity. The resulting exercise reduces uncertainties in a set of nonlinear continuous curves to uncertainties in two hyperbolic curve-fitting parameters. This approach is practical and grounded realistically on the load test database with minimal assumptions. The hyperbolic parameter statistics reported in this study are required for reliability-based serviceability limit state design.

Electrical properties of soils have commonly been used to estimate geotechnical properties. This paper introduces a new device, an electrical resistivity cone probe (ERCP), to determine the electrical resistivity of seashore soft soils and estimate void ratio in the field. The probe consists of inner and outer electrodes with a coaxial structure. The probe tip is conical to minimize disturbance during penetration. A four-terminal pair configuration is used to prevent electrical interference. The electrical resistance is measured during a consolidation test, penetration tests in a large-scale calibration chamber, and at two field sites. With the resistivity of soils and electrical resistivity of pore water extracted from undisturbed soils, the void ratio is estimated using Archie’s law. The void ratio estimated by the ERCP in an oedometer cell is almost the same as the volumetric void ratio of sand determined from consolidation tests. In addition, the void ratio profile obtained by the ERCP agrees well with the volume-based void ratio in a sand-clay mixture prepared in the calibration chamber. The void ratio profile estimated in the field is inversely proportional to the standard penetration testing $N$ -value and the cone-tip resistance of the cone penetration test. This paper demonstrates that the ERCP may be an effective device for the estimation of the void ratio of seashore soft soils.

This paper presents the performance of an excavation with cross walls based on field observations and numerical analysis results. A small strain constitutive model was used in the analysis. By comparing the movements of the real case with the same case but without cross walls installed, the effectiveness of cross walls was thus evaluated. Results indicate that the movements in the case history were substantially reduced by the installation of cross walls. The maximum wall deflection and ground settlement at a place where the cross wall was installed was reduced by about 75 and 82%, respectively. The movements near the diaphragm wall or cross wall corner were relatively small, compared with those far away from the corner. The diaphragm wall still had some deflection at the sections where cross walls were installed and the amount of deflection was close to that from linear elastic analysis. It is justified that slime between cross walls and diaphragm walls during trench excavation was fully cleaned.

Several geotechnical engineering constructions subject the soil to a partial stress reduction. In addition, in practice, often both axisymmetric and nonaxisymmetric conditions can apply in different locations in the ground affected by the construction. In general, the corresponding stress states at failure will be, therefore, overconsolidated and cover a wide domain of the space of the principal stresses. However, the classic failure criteria typically fail to reproduce the observed failure envelope in wide range of stresses. In this note, the original and modified Lade’s criteria are validated and compared against recent data on Pisa clay at general overconsolidated stress states, in compression and extension. A generalization of the modified Lade’s criterion at overconsolidated clays is also proposed.