ICE - Institution of Civil Engineers - Virtual Library

A design chart is a graphical tool that provides solutions to different scenarios of a system. In this paper, two types of design chart are developed based on deterministic and reliability-based analyses for determining the interface shear strength required for stability of a cover system to achieve a target safety factor of 1.5 and failure probability of 1 × 10⁻². The deterministic design chart assists in the selection of different types of geosynthetic for lining materials based on the required interface shear strength for stability, and the reliability-based design chart enhances decision-making by taking into account the uncertainties in the design parameters, such as the variability of interface shear strength parameters. Additionally, the latter chart can also be used to determine the optimum slope angle for a containment facility that will satisfy both the target factor of safety and acceptable failure probability. Examples are provided to illustrate the use of the design charts in estimating the minimum required interface shear strength and their allowable variability for a given veneer cover, and the optimum slope inclination corresponding to different interface shear strengths and their associated variability.

This paper describes the interaction behaviour of soil/geogrid and soil/concrete interfaces of a reinforced soil-retaining wall system under monotonic and cyclic loading conditions. A sand-clay mixture was tested with geogrids, which were manufactured from polyester and polyvinyl alcohol, and with concrete mortar in a modified direct shear device. In the cyclic tests, a total of 15 loading cycles or more were applied under constant displacement and by controlling the shear stress at a level of 56–74% of the monotonic shear strength. The test results showed that the strength of soil/geogrid interfaces was not affected significantly by cyclic loadings. At the end of cyclic loading, a larger stiffness and stress was mobilised in the reloading curve owing to the quasi-preconsolidated behaviour of the sand–clay mixture. For the soil/concrete interfaces, the interaction behaviour was different between the fresh and used (polished) specimens: significant strength reduction of up to 30% occurred in the used concrete specimens compared with the corresponding fresh specimens. The limited test results suggest that interface shear strengths obtained from monotonic loading may be relevant for designing reinforced soil-retaining walls subjected to earthquake loading.

The current test method for quantifying geotextile pore structure for filtration design is the AOS test (ASTM D 4751). The AOS test quantifies structure using a single value, the O₉₅ or 95% opening size. The bubblepoint test (ASTM D 6767) is a method for calculating the pore size distribution (PSD) of a geotextile. Information on a number of pore sizes more completely describes a geotextile, and may allow for better filtration design criteria. An important step of the bubblepoint test involves saturating a geotextile sample with a wetting fluid and then expelling the fluid using controlled airflow. The data reduction of ASTM D 6767 supposedly accounts for the effect of the wetting fluid, and several different wetting fluids are allowed. Accurate knowledge of the contact angle between the wetting fluid and the geotextile is essential to computing accurate pore size distributions. Even with accurate contact angle measurement, the use of different wetting fluids yields different pore size distributions. This suggests a need to re-examine the data reduction of ASTM D 6767.

Fluid flow through a defect in the geomembrane of a composite geomembrane/soil liner (flow rate tests) and interface transmissivity tests between a geomembrane and a soil layer were conducted. The results are compared, and the effect of overburden pressure p′ on the geomembrane/soil interface transmissivity θ and interface thickness t _in is investigated. For flow rate tests, the values of θ are back-calculated using the Rowe and Touze-Foltz et al. solutions. With known values of θ, t _in is calculated using Newton's viscosity law for fluid flow between two parallel plates. For the conditions considered, it is shown that θ and t _in reduce significantly with increasing p′. Comparisons are made between the calculated values of θ and t _in and those reported in the literature. It is shown that ignoring the effect of p′ on θ and t _in can lead to gross overestimation of these parameters.

Dry jet mixing (DJM) and prefabricated vertical drains (PVDs) have been successfully used for soft ground improvement worldwide. They both have advantages and limitations, and have been used independently so far. In this study, DJM and PVDs were used in combination to enhance the performance of soft ground improvement. In the combined method, PVDs are first installed and then DJM columns are installed between the PVDs at larger spacing. This combined method improves the effectiveness of ground improvement and creates a more economical solution. This paper presents the concept, mechanisms, construction procedures, and field validation of this new method. In situ standard penetration tests were performed in dry mixed columns and cone penetration tests and field vane shear tests were performed in soils between the columns. Plate load tests were subsequently conducted on the treated ground. Excess porewater pressures were monitored during and after the installation of DJM columns with and without PVDs. Field validation was conducted through a pilot highway embankment on very soft clay in Jiangsu, China. Long-term settlement monitoring was carried out for the construction of this highway embankment. Field tests and monitoring clearly showed that the DJM-PVD method performed well in enlarged spacing as compared with the conventional DJM method with close spacing.

Although intact geomembranes are excellent barriers to fluid flow, there are still two possible pathways for the transport of organic contaminants into the environment: leakage through defects in the geomembrane, and diffusion through the intact geomembrane. Analytical, semi-analytical and numerical techniques for calculating leakage rates have been developed. However, no previous attempt has been made to rigorously model the interaction between leakage through a hole in a wrinkle and diffusion. To address this shortcoming, finite-element analyses in two-dimensional space are performed to simulate the transport of dichloromethane (DCM) through a leaking HPDE geomembrane (GM) for eight cases involving composite liners, including both compacted clay liners and geosynthetic clay liners. Equations of steady-state seepage and time-dependent diffusion-advection in saturated soils are solved. Transport through the GMs is simulated by a novel, mass-conserving equivalent boundary condition that renders the problem computationally more tractable. Results are compared with those obtained from widely used one-dimensional methods, namely the Rowe equation for leakage through a wrinkle and the POLLUTEv7 program for mass transport. It is shown that predictions of leakage and transport by the one-dimensional method (POLLUTEv7) are within a maximum 30% of two-dimensional values under conditions of perfect mixing in the aquifer. When no horizontal mixing is present, concentrations of DCM are found to increase by up to 43% in two-dimensional analyses. It is concluded that one-dimensional analyses are adequate for most practical purposes.