Optimising the Performance of Bentonite in Geosynthetic Clay Liners

Geosynthetic clay liners (GCLs), developed over the last quarter century, are innovative composite matting comprising of bentonite with two covering geosynthetics. The three-layered (2:1) clay mineral montmorillonite forms the dominant (approximately 75-90% by weight) clay mineral in the bentonite that is used in its manufacture. Quantitative mineralogical analyses with an assessment of the adsorbed cation regime need to be normally carried out as part of performance appraisal. The mineralogical and geotechnical differences between the preferred sodium and the less effective calcium montmorillonite are presented in the paper. The liners with its encapsulated high active clay minerals depend on the water balance between the sealing element and the surrounding soil layers. Assessment of long term hydraulic conductivities and clay-leachate compatibility assessment is also deemed necessary and the derogatory factors affecting the performance of the bentonite in GCLs placed in difficult construction and hostile chemical environments are discussed. The performance specifications for GCLs are identified in this paper and the need to enhance its cation exchange capacity with polymer treatment coupled with the need for factory prehydration of the untreated sodium bentonite is emphasised

Geotechnical Characteristics of Peat

Peat soil is encountered in many areas and generally originates from plant/animal remains and is considered partly as decomposed biomass (Adnan and Wijeyesekera, 2007). Due to this composition, the structure of this soil is very different when compared with inorganic soils like clay, sand and gravel. Peat has a high compressibility, low shear strength, high moisture content and low bearing capacity (Bujang, 2004, Adnan et al., 2007). The behaviour and composition of peats in different geographical areas are different from one another, accentuating the need in soil engineering for a useful geological classification of peat soils. This paper focuses on presenting a comparative overview of the characteristic geotechnical properties for these soils. It also examines and discusses the effects of composition on the basic properties and behaviour of each soil, supported by case studies from Malaysia

Rocker Pipe Solution to Alleviate Settlement Induced Distress in Flexible Pipes

Soil-pipe interaction studies generally recognise the significance of deformations in the pipe due to soil loading, but not differential ground and structure movements, which can induce excessive stress concentrations in the pipeline. Plastics pipes can suffer failure due to such movements, though their flexibility makes them less vulnerable than rigid pipes. This paper examines the settlements and how the redistribution of the soil sub grade reactions caused by the installation of a pipeline within a soil mass, can then be estimated by treating the pipeline as a beam on elastic foundation. Various case histories are summarised, demonstrating these effects, and pointing the way to possible solutions, which could be incorporated at the project design stage

Shear Modulus and Damping Properties of Peat Soils

Soils are subjected to dynamic loading of various forms. Some of these result from sources such as earthquakes, traffic loads and tidal action. To assess the level of the consequent ground motion of the soil, two parameters those are vital in dynamic analysis; shear modulus (G) and damping ratio (D) properties (Adnan et. al, 2007, Adnan and Wijeyesekera, 2008). Dynamic properties of soils such as sand, silt and clay have been studied for more time in the past (Chen et al., 2007, Hyde and Ward, 1985). However only a insignificant amount of work has been done on the dynamic properties of peat. This paper presents experimental results based on the undrained cyclic tests on different peats. Samples were collected from Holme Fen Post, Cambridgeshire and Solway Post, Carlisle. VJTech Cyclic Triaxial Testing Apparatus was used to measure these parameters. The significance of peat type, microstructure, loading frequency, confining pressure and index properties are also discussed

Geotechnical Challenges with Malaysian Peat

Malaysian peat is a tropical peat (Andriesse, 1988). This peat has unique characteristics, which makes it significantly different from other peat. In its natural state, this soil is normally dark reddish brown to black in colour and consists of partly decomposed leaves, branches, twigs and tree trunks with a low mineral content. These are formed through accumulation of disintegrated plant remains, which have been preserved under conditions of incomplete aeration and high water content. Hobbs (1986) stated that it was important to include and recognise the peat by not only its morphology but also by its basic engineering properties. The “special” characteristics for this soil are a high water content (>200%), high compressibility, high organic content (>75%), low shear strength (5-20kPa) and low bearing capacity (<8 kN/m2). These geotechnical characteristics make any form of construction on this soil very challenging in Malaysia (Zainorabidin and Bakar, 2003). This paper presents an overview of previous research carried out on peat soils (Mutalib et al, 1991; Jarrett, 1997; Bujang, 2004; Zainorabidin and Bakar, 2003). Edil (2003) demonstrated the variability of the peat properties even within a single sample, leading to it being far from homogeneous, which is a prime requirement in engineering soil mechanics. The compressibility of the individual peat particles invalidates one of the prime assumptions of particle incompressibility adopted in soil mechanics This paper also discusses the challenges that geotechnical engineers have faced in Malaysia when designing and managing construction on this soil

Mathematical and Physical Study of Pipe Lines Subjected to Differential Ground Movement

Soil-pipe interaction studies leading to the laboratory observations of the effects of differential ground movement between a heavy yielding structure and a pipeline firmly connected to it is presented in this paper. Such differential movements induce excessive stress concentrations on the pipeline. Plastics pipes fail as a consequence of such movements, though their flexibility should make them less vulnerable than rigid pipes. In order to evaluate the displacement, bending moment, shear force, vertical soil resistance at soil pipe interface under these conditions, innovative experimental techniques were developed and these are described in this paper. The soil resistance on a pipe section is characterised by the load-displacement behaviour of the embedded pipe section subjected to lateral displacement, vertical displacement, axial displacement along the axis of the pipe and rotation about the pipe axis. A mathematical analysis to complement the laboratory studies is developed and presented by treating the pipelines as a beam on elastic foundation. The magnitude and location of the maximum bending moments arising from yielding of the heavy structure is examined. The experimental observations of the behaviour of pipes subjected to such differential ground movement are compared with the results from the theoretical predictions. The provision of rocker pipe joints that entertain a permitted rotation helps to redistribute the adverse bending moments to acceptable levels and thereby alleviate distress in the pipeline. The paper gives results that demonstrate theoretically and experimentally the appropriateness of the use of flexibly jointed rocker pipes to prevent such failures. Field examples of the adoption of such joints is also presented and discussed culminating with the expression of the need for rational design procedures for pipeline foundations including rocker pipes to be incorporated into codes of practice such as EN 1295 is emphasised

Drying Kinetic of Prehydrated and Extruded Clay Mat

The isothermal drying of prehydrated and extruded clay mat was investigated on a laboratory scale over a temperature and relative humidity range from 20°C to 40°C and 15% to 70% respectively. Two sets of polymer enhanced clay mats were used in this study. By measuring the mass loss of the samples during the isothermal drying process basis dependencies necessary to describe the kinetic drying process was obtained. The most commonly used model, Page, Wang & Singh, Henderson & Pabis and Thin layer equation were used to fit experimental data using a nonlinear regression analysis. The fit quality of the proposed model was evaluated by using the standard error of estimate, relative percent error and coefficient of correlation. Results probed that the Page model was more appropriated to predict prehydrated and extruded clay mat for the range of temperatures and relative humidity studied. Applied model analysis enabled evaluation of the main transport properties: drying constant, drying rate, effective diffusion coefficient and exponential model parameter. It was found that the drying condition and type of polymer incorporated in the clay mat strongly influenced the drying kinetic and transport parameters

Moisture Desorption Isotherms and Thermodynamic Characteristic of Prehydrated and Extruded GCL

An experimental investigation into the drying of pre-hydrated and extruded GCL is presented in this paper. Moisture desorption isotherm at temperature from 20 to 40°C and water activity from 0.20 to 0.70 were determined using the static gravimetric method. The moisture isotherms were sigmoid shaped and were influenced by the thermal environment. The Guggenheim- Aderson-DeBoer (GAB) was used to fit experimental values using a non-linear regression analysis. GAB model was appropriate to predict the desorption equilibrium moisture content of pre-hydrated and extruded GCL for the range of temperatures and water activities studied. Finally, an expression for predicting the thermal property was developed

Flexibility of ‘Clay Mats’

This paper presents a scientific development to addressing the current absence of a convenient technique to identify the ductile to brittle transition of clay mats. The applicability of clay mats can be brought into question if they become brittle due to drying in hot weather / tropical climates. Bentonite clay mats produced with different liquid polymers and at different moisture content were used in the study. The dependence of flexural stiffness on moisture content is presented. All the specimens showed that the flexibility of the clay mat declined exponentially with decreasing moisture content. Often one would adopt the feeling from a finger pressure test to give a perception of the softness / stiffness of the material. The paper also presents an extension of this concept to adopt an appropriately modified Brinell hardness test for the clay mats. Concurrently, 3 point bending tests were carried out on samples of the clay mat to obtain a value for the elastic structural stiffness (EI). The paper further confirms a strong correlation between Brinell Hardness Test and the structural stiffness. This study helps to assess the performance of clay mats with different proportions of additives that have been introduced in the mat manufacture to delay the inevitable drying characteristics of the mat, when exposed to hostile thermal environments

Experimental investigation of hydrodynamic erosion of soils

Recent research studies revealed that most bridge piers and other hydraulic structures such as levees and embankments adopted in flood protection schemes in maritime environment have collapsed due to the failures attributed to scour associated with a soil–hydrodynamic interaction phenomenon (e.g. Shirole and Holt, 1991). Though erosion characteristics of coarse sands and gravels are relatively well known, soil-hydrodynamics interaction for finer sands such as silts and clays is not fully investigated. A series of small-scale laboratory experiments were conducted in a moveable sediment tank to investigate scour characteristics under various flow and soil conditions. The present paper discusses preliminary experimental observations and the analysis of hydrodynamic erosion of soils in line with the extensive research project „Mathematical and Physical Modelling of Hydrodynamic Erosion of Soils (HES)‟ carried out under the „UEL Promising Researcher Fellowship 2009-2010‟ funded by the University of East London (UEL)