INTRODUCTION 1.1 General
1.4 Actions and Design input parameters
2.2.4 Facing Units
2.2.6.2 Extensible Reinforcements
These reinforcements have two main inadequacies: they are susceptible to corrosion, King (1978) and hinder the development of active soil pressures due to their inextensibility, McGown et al (1968) and Yogarajah (1993). The lack of predictability of the extent of corrosion and the need to allow adequate soil movement for active pressure development has led these materials being in part replaced with materials made from synthetics polymers.
According to BS 8006 (1995) reinforcing elements, subject to design loads of a sustained nature, may be classified as
Extensible when the total axial tensile strains exceed I
%,
viz. Synthetic polymeric reinforcements, natural fibers. .
Generally, extensible reinforcements allow larger strains to occur without reinforcements rupturing. In these circumstances, the benefit ofthe mobilized tensile strength of the reinforcement is available even after the peak strength of the soil is reached, McGown et al (1978).
Relatively Extensible
reinforcements, that include almost all geosynthetic strips, sheets or grids, are thus defined as those which have rupture strains larger than the maximum tensile strain in the soil without reinforcement, under the same operational conditions. The properties of this type of reinforcement are time and temperature dependent. This is why, it has been suggested that long term sustained load [creep]
tests at the appropriate temperature are required to determine their load-strain-time- temperature behaviour, Kabir (1984), McGown et al (1984a), Murray and McGown (1987), Andrawes et al (1986) and many others.
The variety and availability of extensible materials have greatly increased in recent
years. Most are manufactured from polymeric synthetic materials. These
reinforcements have their own disadvantages such as being susceptible to long term
creep and stress relaxation. Further, they exhibit degradation due to ultraviolet light
when exposed to sunrays together with the decay caused by thermal cycling and
physical, chemical or biological attack. Despite these disadvantages, they are gaining
2.3 Geosynthetics
2. Thermosetting materials: Crosse linking of the long chain-like molecules
dominates their behaviour. This takes place during moulding under heat and
pressure. The resulting material is rigid when cooled and does not soften on
heating.
I. Thermoplastic materials: The features of these plastics are the relatively weak
Intermolecular Van-der -Waals forces. When these materials are heated, they
become soft and flexible and eventually, at high temperatures, exhibit a viscous
melt. When they are allowed to cool, they solidify again. This process of
softening by heating and solidifying on cooling may be repeated more or less
indefinitely.
Two most important plastics are:
wider acceptance due to economical benefits accrued from their use. The thesis
covers extensible synthetic polymeric (geosynthetics) reinforcements only. The
following section deals with the various materials under this category.
Geosynthetics are produced in various forms of plastics like sheets, strands, fibers,
etc., whiCh are linked and arranged in a variety of orientations. They are the produce
of polymeric materials whose long chain molecules are obtained by the process of
polymerization of monomers. In order to suit the purpose of use, their properties are
modified by mixing additives like plasticizers, reinforcements and stabilizers, etc.
2.3.1 Elasto- Visco-Plastic Nature of Geosynthetics
The majority of geosynthetics are manufactured using thermoplastic materials
(existing in the form of sheets, strands, fibers, etc.). These materials may exhibit a
wide range of mechanical behaviours; from brittle-elastic at low temperatures
through plastic, to visco-elastic or leathery, to rubbery and finally to viscous at high
temperature. Their behaviours are dictated by the nature of the constituent materials,
processing during manufacturing and their operative temperature.
If a constant load is applied, viz. sustained load (creep) test, the geosynthetics exhibit an instantaneous elastic strain followed by primary creep, secondary creep and tertiary creep, Figure 2.8. Primary creep is dominated by its quick rate and occurs in The more important is the fact that at normal operational temperatures, the load- strain-time-temperature behaviour of geosynthetics is dependent upon their micro and macro-structures. The principal micro-structural features that characterize all polymers are the molecular size, basic polymer network structure, chain stiffness and rigidity.
The behaviours of polymers are characterized by the Glass Transition Temperature [T8]' below which the material behaves like glass, i.e. it is hard and brittle. At a temperature greater than T
g• the Van der Waals bonds between the long polymer chains weaken due to the increase in the free volume in the polymers. This free volume makes it easier for the molecules to move past each other when a load is applied at a temperature higher than T
g. Consequently, the polymers strain more easily under a load at higher temperatures than T
g, compared to the temperatures lower than T
g. Alternatively, T
gmay be regarded as the threshold ten:tperatureabove which lesser energy is required to develop a particular strain over a certain period of time in the material, and below which more energy is required.
Mostly there are no network structures or cross-linking within engineering
thermoplastics; nevertheless, polymer-processing operations may result in small
amounts. The molecular state of the material is manipulated from the amorphous to
semi-crystalline and finally to crystalline state in this operation. Degree of
crystallinity within the thermoplastic components of geosynthetics is of great
significance. Most of the thermoplastic materials and processes, used to produce
various geosynthetics, lead to microstructures that are partly amorphous and partly
crystalline. Consequently, the mechanical behaviour of the components is often
elastic initially followed by visco-plastic which is the result of the long molecular
chains unfolding, (if chain-folded), or drawing out of the amorphous tangle, (if
glassy), together with straightening and aligning.
Thus, it may be appreciated that geosynthetics exhibit elasto-visco-plastic behaviour.
This means that their mechanical behaviour is in part similar to that of elastic solid, in part similar to that of a viscous liquid and in part similar to that of a plastic, with all these parts being temperature dependent. Therefore, when subjected to an
a very short time; whereas secondary creep occurs at a very slow rate and requires load to be applied over a longer time. Tertiary creep occurs at higher load levels where non-linear visco-elasticity becomes predominant. The material rupture under tertiary creep conditions is influenced by various factors and therefore it is difficult to predict.
Another important aspect of the effect of microstructure is the basic structure of the constituent polymer itself This structure defines the ability of molecular chains to slip past one another. In the case of polyesters, due to the relative inability of the molecular chains to slip past one another arising from the effective knots, they show little time dependent creep at low load levels. On the contrary, the smoother and straighter molecules in polypropylenes can slip past one another more easily, which is why they can exhibit a significant creep even at low load levels.
The degree of crystallinity is an important indicator in that it represents the thermoplastic component in the materia!. Under isothermal conditions, a 2% strained specimen is less crystalline than a 10% strained specimen, because the straightening and aligning of the long polymer molecules within it increase the degree of crystallinity.
In the final stage of manufacturing process of geosynthetics the various thermoplastic
components are linked and arranged geometrically to form the macro-structure,
which result in different generic types of geosynthetics including wovens, needle-
punched and melt bonded non-wovens, nets, grids and linear composites. These
various macro-structures further have a highly significant influence on the time
dependent load-strain behaviour of geosynthetics. Nevertheless, their temperature
dependent behaviour may not be highly influenced by the macro-structures, as this is
a microstructure issue.
2.3.2 Rheological and Mathematical Modelling of Geosynthetics Behaviour
