The stress-strain behaviour of soil reinforced with a multiple cell geocell structure

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Laboratory studies on geocell mattresses

Several laboratory studies on the reinforcing effect of geocell mattresses have been performed over the last two to three decades. These studies were aimed at a wide variety of applications and the experimental procedures and setups differ considerably. Table 2.1 provides a summary of the relevant literature discussed in this section. Rea and Mitchell (1978) reported on laboratory tests to investigate the reinforcement of sand, using paper grid cells. Their study investigated the influence of the ratio of the diameter of the loading area to cell width, the ratio of cell width to cell height and the subgrade stiffness.
A mattress of square paper grid cells with a membrane thickness of 0.2 mm and a cell height of 51 mm was filled with a uniform fine quartz sand at its maximum density of 16.8 kN/m3 . The sand had a mean particle size of 0.36 mm and a coefficient of uniformity (Cu) of 1.45. Failure of the reinforced soil was sudden and well-defined and in some cases the cells burst open from the bottom along glued junctions. Figure 2.5 shows a sketch of the test setup. Tests were performed with the loading centred on the junction (x-test) and with the load centred on the cell (o-test) (Figure 2.6). Mhaiskar and Mandal (1992, 1996) investigated the efficiency of a geocell mattress over soft clay. The influence of the width and height of the geocells, the strength of the geocell membranes and the relative density of the fill material were investigated. Geocells of needle punched nonwoven and of woven slit film was used in the study. Mumbra sand with a minimum density of 16.05 kN/m3 , a maximum density of 18.1 kN/m3 and a Cu of 4.6 were used as a fill material. Tests were performed with the fill at a relative density of 15% and at 80%. Figure 2.7 shows a schematic sketch of the experimental setup used by Mhaiskar and Mandal (1992).

Published conclusions drawn from laboratory tests on geocell

reinforced mattresses Rea and Mitchell (1978) observed that the reinforcement resulted in a stiffening of the reinforced layer giving a raft like action to the layer. A raft like action of the geocell reinforced layer is also observed by Cowland and Wong (1993) for geocell reinforced layer under an embankment over soft clay. Other researchers mention the load spreading action of the reinforced layer and a subsequent reduction in the vertical stress in the layer underlying the geocell layer (Mhaiskar and Mandal, 1992; Bush et al., 1990). Dash et al. (2001) showed an increased performance on the footing over a buried geocell layer even with the geocell mattress width equal to the width of the footing.
The geocell mattress transfers the footing load to a deeper depth through the geocell layer. An increase in the bearing capacity of the geocell mattress with an increase in the ratio of cell height to cell width was observed by Rea and Mitchell (1978) and Mhaiskar and Mandal (1992). Dash et al. (2001) found that the load carrying capacity of the foundation bed increases with an increase in the cell height to diameter ratio, up to a ratio of 1.67, beyond which further improvements were marginal. The optimum ratio reported by Rea and Mitchell (1978) is around 2.25. Krishnaswamy et al. (2000) reported an optimum ratio of about 1 for geocell supported embankments constructed over soft clays. Dash et al. (2001) also noted that not only the aspect ratio of the cells but also the cell size (the cross sectional area of the cell compared to the loading area) had an influence on the performance of the geocell system. The increased load carrying capacity with decreasing pocket size is attributed to an overall increase in rigidity of the mattress and an increased confinement per unit volume of soil. A similar influence of the pocket size on the behaviour of the geocell reinforced soil was observed by Rajagopal et al. (1999) when performing triaxial tests on geocell reinforced soil samples. The research of Rajagopal et al. (1999) will be discussed in more detail in the next section.

is study aims to investigate the peak, as well as the pre-peak behaviour of

geocell-soil composite structures to further the understanding of the constitutive behaviour of geocell-soil composite structures. In order to achieve this goal, the constitutive behaviour of the fill and membrane material and the composite structures are investigated. Models are developed to describe the behaviour of the fill and membrane materials for the purpose of facilitating the understanding of the interaction of the components of the geocell-soil composite. In the investigation of the constitutive behaviour of the geocell-fill composite, consideration is first given to the behaviour of a single geocell composite structure after which the insights gained, are applied to multiple geocell structures. Due consideration is given to the volumetric behaviour of the fill and the non-uniform straining of the composite. This work advances the state of the art by addressing some of the shortcomings of the theories of Bathurst and Karpurapu (1993) and Rajagopal et al. (1999). A calculation procedure is developed to enable the calculation of the stress strain curve of a single cell geocell-soil structure, which facilitates the understanding of the interaction between the constituting components of the composite. This procedure incorporates the developed material models. This work for the first time presents a method for estimating the stress-strain behaviour of a granular soil reinforced by a single geocell.

Tests on the fill material

The fill material was obtained from Savuka Mine’s backfilling plant. Savuka mine is part of Anglo Gold’s operations near Carletonville. The mine operates mainly on the Ventersdorp Contact Reef and the Carbon Leader Reef of the Witwatersrand Complex. The tailings material is cycloned in the backfilling plant to reduce the < 40 µm fines contents and is normally referred to as classified tailings. Classified tailings are widely being used in mines as a backfill to provide regional support in mined stopes and is a logical choice for a fill material for support packs.

Microscopy on the material grains

Vermeulen (2001) pointed out that although it is convenient to simplify soils as continuum media for analytical purposes, it is the properties at particle level that ultimately control its engineering behaviour. Information on the particle shape and surface texture was gained by studying the material particles under optical and electron microscopes. A sample of the classified tailings material was separated into 10 size-ranges of which a specimen each was prepared for microscopic analyses (Table 3.1). The original soil sample was treated with a dispersant solution of Sodium hexametaphosphate and separated into a courser and finer section by washing it through the 63 µm sieve. The > 63 µm portion was wet sieved to separate it into the sizes shown in Table 3.1, while the < 63 µm portion was separated by settlement in water.

Table of contents :

• Summary
• Acknowledgements
• Table of contents
• List of tables
• List of figures
• 1 Introduction
  • 1.1 Background
  • 1.2 Objectives and scope of study
  • 1.3 Methodology
  • 1.4 Organisation of thesis
• 2 Literature review
  • 2.1 Introduction
  • 2.2 Geocell systems and applications
  • 2.3 Laboratory studies on geocell reinforcement
    • 2.3.1 Laboratory studies on geocell mattresses
    • 2.3.2 Published conclusions drawn from laboratory tests on geocell reinforced mattresses
  • 2.3.3 Studies aimed at the understanding of the membrane-fill interaction
  • 2.4 Conclusions drawn from the literature review
  • 2.5 Specific issues addressed in the thesis
• 3 Laboratory testing programme
  • 3.1 Introduction
  • 3.2 Tests on the fill material
    • 3.2.1 Basic indicator tests
    • 3.2.2 Material compaction
    • 3.2.3 Microscopy on the material grains
    • 3.2.4 Compression tests on soil
  • 3.3 Tests on membrane material
  • 3.4 Tests on geocell-soil composite – single geocell structure
  • 3.5 Tests on geocell-soil composite – multiple geocell structures
• 4 The strength and stiffness of geocell support packs
  • 4.1 Introduction
  • 4.2 Laboratory tests on fill material
    • 4.2.1 Basic indicator tests
    • 4.2.2 Microscopy on the material grains
    • 4.2.3 Compaction characteristics of the classified tailings
    • 4.2.4 Compression tests on soil
  • 4.3 The constitutive behaviour of the fill material
    • 4.3.1 Elastic range
    • 4.3.2 The strength and strain of the material at peak stress
    • 4.3.3 The material behaviour in terms of the stress-dilatancy theory
  • 4.4 Formulation of a constitutive model for the fill material
    • 4.4.1 The elastic range
    • 4.4.2 The yield surface
    • 4.4.3 The hardening behaviour and flow rule
    • 4.4.4 Obtaining parameters
    • 4.4.5 Comparison of model and data
  • 4.5 The behaviour of the HDPE membrane
    • 4.5.1 Interpretation of the test results
    • 4.5.2 Membrane behaviour
    • 4.5.3 Formulation of mathematical models for the membrane behaviour
    • 4.5.4 Model interpolation and extrapolation
  • 4.6 The constitutive behaviour of soil reinforced with a single geocell
  • 4.6.1 Implementation of the soil constitutive model into a calculation procedure
    • 4.6.2 Corrections for non-uniform strain
    • 4.6.3 Calculation of the stress state in the soil
    • 4.6.4 Calculation procedure
    • 4.6.5 Verification of the proposed calculational scheme
    • 4.6.6 Comparison with laboratory tests on soil reinforced with a single geocell
  • 4.7 The stress-strain behaviour of soil reinforced with a multiple cell geocell structure
• 5 Conclusions
  • 5.1 Introduction
  • 5.2 Geocell reinforcement of soil – general conclusions from literature
  • 5.3 Classified gold tailings
  • 5.4 HDPE membrane behaviour
  • 5.5 The behaviour of cycloned gold tailings reinforced with a single cell geocell structure
  • 5.6 The behaviour of cycloned gold tailings reinforced with a multiple cell geocell structure
• 6 References
  • Appendix A
  • Derivation of equations
  • A.1 Equation 3.2 – Correction factor for horizontal strain at the centre of a pack measurement with LVDT’s fixed at half of the original pack height
  • A.2 Equation 4.53 – The depth of the « dead zone »
  • A.3 Equation 4.55 – The relationship between the mean axial strain in and the overall strain of a cylinder of soil
  • A.4 Equation 4.56 – The relationship between the mean volumetric strain in and the overall volumetric strain of a cylinder of soil

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The Strength and Stiffness of Geocell Support Packs