Journal of Geotechnical and Geoenvironmental Engineering

Glass fiber–reinforced polymer (GFRP) materials provide practical solutions to corrosion and site-maneuvering problems for civil infrastructures using conventional steel bars as reinforcements. In this study, the feasibility of using GFRP soil nails for slope stabilization is evaluated. The GFRP soil nail system consists of a GFRP pipe installed by the double-grouting technique. Two field-scale pullout tests were performed at a slope site. Fiber Bragg grating (FBG) sensors, strain gauges, linear variable displacement transformers (LVDTs), and a load cell were used to measure axial strain distributions and pullout force-displacement relationships during testing. The pullout test results of steel soil nails at another slope site are also presented for comparison. It is proven that the load transfer mechanisms of GFRP and steel soil nails have certain difference. Based on these test results, a simplified model using a hyperbolic shear stress-strain relationship was developed to describe the pullout performance of the GFRP soil nail. A parametric study was conducted using this model to study some factors affecting the pullout behavior of GFRP soil nails, including nail diameter, shear resistance of soil-grout interface, and ratio of interface shear coefficient to the Young’s modulus of the nail. The results indicate that the GFRP soil nail may exhibit excessive pullout displacement and thus a lower allowable pullout resistance than with the steel soil nail.

Desaturation of ground by air injection attracts considerable attention in recent years as an innovative technique for a liquefaction countermeasure. Several research programs were conducted in laboratories regarding the related topics. This paper describes an in situ air-injection test that aims to examine the effectiveness of the air injection to desaturate ground and the validity of observation techniques to monitor the evolution of the unsaturated zone. In the test, air was injected from an air injector deployed in a targeted saturated-sand layer at a depth of 6 m. Observations revealed that the air-flow rate increased linearly with increasing air-injection pressure and the desaturated zone was generated within 4 m from the injection point. A 3-dimensional electric resistivity tomography technique was effective for evaluation of the desaturated zone. The degree of saturation of the in situ soil was observed by using high quality undisturbed samples obtained by the ground freezing method. The degree of saturation ranged from 68–98%, which was low enough to almost double the liquefaction resistance of the soil at the site. Numerical analyses were also conducted with a gas-liquid two-phase flow simulator to describe the evolution of the soil desaturation. Qualitatively, predictions show a relatively good agreement with the in situ measurements of the 3D electric resistivity tomography and are quantitatively compatible with the in-field degree of saturation measured indirectly by using the frozen soil samples. Actual liquefaction resistance was evaluated utilizing the undisturbed samples by conducting a triaxial test under cyclic shear conditions, which revealed that desaturated samples were indeed less susceptible to liquefaction compared with the fully saturated samples.

Landfill sites have been implicated in greenhouse warming scenarios as a significant source of atmospheric methane. In this study, the effects of extreme compaction on the two main soil-gas transport parameters, the gas diffusion coefficient ($D_p$) and the intrinsic air permeability ($k_a$), and the cumulative methane oxidation rate in a landfill cover soil were investigated. Extremely compacted landfill cover soil exhibited negligible inactive soil-air contents for both $D_p$ and $k_a$. In addition, greater $D_p$ and $k_a$ were observed as compared with normal compacted soils at the same soil-air content ($\epsilon$), likely because of reduced water-blockage effects under extreme compaction. These phenomena are not included in existing predictive models for $D_p(\epsilon )$ and $k_a(\epsilon )$. On the basis of the measured data, new predictive models for $D_p(\epsilon )$ and $k_a(\epsilon )$ were developed with model parameters (representing air-filled pore connectivity and water-blockage effects) expressed as functions of dry density (${\rho }_b$). The developed $D_p(\epsilon )$ and $k_a(\epsilon )$ models together with soil-water retention data for soils at normal and extreme compaction (${\rho }_b=1.44$ and $1.85\mathrm{g}{\mathrm{cm}}^{-3}$) implied that extremely compacted soils will exhibit lower $D_p$ and $k_a$ at natural field-water content ($-100\mathrm{cm}$ ${\mathrm{H}}_2\mathrm{O}$ of soil-water matric potential) because of much lower soil-air content. Numerical simulations of methane gas transport, including a first-order methane oxidation rate, were performed for differently compacted soils by using the new predictive $D_p(\epsilon )$ model. Model results showed that compaction-induced difference in soil-air content at a given soil-water matric potential condition is likely the most important parameter governing methane oxidation rates in extremely compacted landfill cover soil.

This paper uses a hybrid method for analysis and design of slope stabilizing piles that was developed in a preceding paper by the writers. The aim of this paper is to derive insights about the factors influencing the response of piles and pile-groups. Axis-to-axis pile spacing ($S$), thickness of stable soil mass ($H_u$), depth ($L_e$) of pile embedment, pile diameter ($D$), and pile group configuration are the parameters addressed in the study. It is shown that $S=4D$ is the most cost-effective pile spacing, because it is the largest spacing that can still generate soil arching between the piles. Soil inhomogeneity (in terms of shear stiffness) was found to be unimportant, because the response is primarily affected by the strength of the unstable soil layer. For relatively small pile embedments, pile response is dominated by rigid-body rotation without substantial flexural distortion: the short pile mode of failure. In these cases, the structural capacity of the pile cannot be exploited, and the design will not be economical. The critical embedment depth to achieve fixity conditions at the base of the pile is found to range from $0.7H_u$ to $1.5H_u$, depending on the relative strength of the unstable ground compared to that of the stable ground (i.e., the soil below the sliding plane). An example of dimensionless design charts is presented for piles embedded in rock. Results are presented for two characteristic slenderness ratios and several pile spacings. Single piles are concluded to be generally inadequate for stabilizing deep landslides, although capped pile-groups invoking framing action may offer an efficient solution.

A fundamental study of pile-soil systems subjected to lateral loads in clay soil was conducted by using experimental tests and a lateral load-transfer approach. The emphasis was on an improved wedge failure model developed by considering three-dimensional combination forces and a new hyperbolic $p\mathrm{\text{-}}y$ criterion. A framework for determining the $p\mathrm{\text{-}}y$ curve on the basis of both theoretical analysis and experimental load test results is proposed. The proposed $p\mathrm{\text{-}}y$ method is shown to be capable of predicting the behavior of a large-diameter pile under lateral loading. The proposed $p\mathrm{\text{-}}y$ curves with an improved wedge model are more appropriate and realistic for representing a pile-soil interaction for laterally loaded piles in clay than the existing $p\mathrm{\text{-}}y$ method.

Penetration resistance in fine-grained soils varies with the rate of penetration. Considering undrained behavior as a reference, as the rate of penetration is reduced, soil resistance increases because of the effects of partial consolidation and soil strengthening immediately ahead of the probe. Many penetration tests have been performed under different rates of penetration to identify the range of drainage characteristics of the soils used, correlating these conditions with laboratory interpretations and in situ tests. A backbone curve relates the variation of the normalized point resistance with the normalized rate of penetration. This work presents an analytical approach to the backbone curve equation used to fit test data. In addition, this paper presents a set of centrifuge tests with variable penetration rates performed with a soil classified as silty tailings, which has different geotechnical behavior from most of the soils used by previous researchers.

In recent years, the use of helical anchors has expanded beyond their traditional use in the electrical power industry. The advantages of rapid installation and immediate loading capability have resulted in their being used in more traditional civil engineering infrastructure applications. Unfortunately, our current understanding of these anchors is unsatisfactory, and the underlying theoretical framework adopted by engineers has proven to be largely inappropriate and inadequate. A better understanding of helical anchor behavior will lead to increased confidence in design, a wider acceptance as a foundation alternative, and more economic and safer designs. The primary aim of this research is to use numerical modeling techniques to better understand multiplate circular anchor foundation behavior in clay soils. A practical design framework for multiplate anchor foundations will be established to replace existing semiempirical design methods that are inadequate and have been found to be excessively under- or overconservative. This framework can then be used by design engineers to confidently estimate the pullout capacity of multiplate anchors under tension loading.

A series of centrifuge model tests of the lateral response of a fixed-head single pile in soft clay is reported. Both monotonic and cyclic episodes of loading are described, with varying amplitude and with intervening periods of reconsolidation. The soil conditions are characterized by cyclic T-bar penetrometer tests. The ultimate capacity under monotonic load for virgin and for postcyclic conditions was found to be comparable with calculations based on existing design methods, including theoretical plasticity solutions and empirical methods. The lateral stiffness was observed to degrade with cycles, with the rate of degradation being greater for larger cycles. The degradation pattern has been tentatively linked to the cyclic T-bar response, by considering the ‘damage’ associated with the cumulative displacement and remolding, in each case. This approach provides a consistent interpretation of the tests. Although episodes of pile movement and soil remolding led to a reduction in lateral resistance, intervening periods of reconsolidation led to a similar magnitude of recovery and a reduction in the level of softening in subsequent cyclic episodes. During an initial episode of cyclic lateral movement, the stiffness degraded by a factor of 2.3, which is comparable with the strength sensitivity derived from a cyclic T-bar test. In contrast, after five episodes of reconsolidation, the stiffness had recovered back to within 25% of the stiffness observed in the first cycle of the first episode, and it showed negligible degradation during subsequent cycling. This observation implies that, over a long period of cyclic loading, the lateral stiffness of a pile may tend towards a value that is independent of cycle number, and that represents a balance between the damaging effects of remolding and pore pressure generation and the healing effects of time and reconsolidation.