DECLARATION 1: PLAGIARISM
2. CHAPTER 2: LITERATURE REVIEW
2.6 Geosynthetics
The general definition of geosynthetic reinforcement is “A planar, polymetric (synthetic or natural) material used in contact with soil/rock and/or any geotechnical material in civil engineering applications” (Muller & Saathoff, 2015). Geosynthetics are basically sheets of woven materials used for reinforcement, separation, drainage etc. Geosynthetics also provide added stiffness if sufficient interlocking of soil particles takes place when support is lost.
Geosynthetics are manufactured from polymeric materials (the synthetic) used with soil, rock, or other geotechnical- related material (the geo) as part of a civil engineering project or system (Kae, 2003). Geosynthetics are made from synthetic polymers such as polypropylene, polyester, polyethylene, polyamide, PVC (Kae, 2003) . Geosynthetics are generally categorized into two main sections: Natural and synthetic. It can be permeable or impermeable, depending on the application (Rajagopal, n.d.).
Geosynthetic products can be geocells, geotextiles, geomembranes or geogrids (Kae, 2003).
• Geocells: Are 3D honeycomb structures generally used for support and erosion
control. Are also used to provide tensile reinforcement and shear resistance to increase the effective bearing capacity of the subgrade.
• Geotextiles: Used for reinforcement and are used for separation and filtration to
prevent contamination of the ballast and provide quick relief of pore water pressures in rail applications.
• Geogrids: are used to provide tensile reinforcement and shear resistance to increase the effective bearing capacity of the subgrade. They are also used to interlock with and confine the ballast, increasing its resistance to both vertical and lateral movement in rail applications.
• Geomembranes: Are used as fluid barriers due to their low permeability.
The investigation will consider the application of geosynthetics as soil reinforcement.
Geogrids and geotextiles are generally used for these applications and provide interlocking of soil fill.
The interactions between the soil and the geotextile interface results in the transfer of shear stress from the soil to the geotextile (Pinto, 2003). Using geosynthetic fabric as soil support over the cavity requires investigation into the load transfer ability of the material.
2.6.1 General principles:
The inclusion of geosynthetic reinforcement in the soil mass increases the bearing capacity of the fill layer. Geosynthetic reinforcement is based on the transfer of stresses between the soil and the reinforcement under tension. The stress is transferred to the geosynthetic by friction, the interlocking between the soil and the reinforcement causes this friction to occur.
Some of the key geosynthetic design factors are the required tensile strength, the depth at which the reinforcement must be installed and the number of layers that must be used. Geosynthetics are traditionally installed as basal reinforcement, where one layer of high tensile strength is placed at the required depth in the soil fill. A more comprehensive overview of the use of basal geosynthetic reinforcement is recorded in section 2.6.4.
2.6.2 Use of geosynthetics as reinforcement:
Geosynthetics are mainly used for the following purposes according to Han (n.d):
• Drainage
• Separation
• Reinforcement
• Erosion protection 2.6.2.1 Drainage
• Facilitate the flow of liquids or moisture
• Increased porosity to facilitate liquid movement 2.6.2.2 Separation
• Provide retention and prevent stones from punching through into soil
• Prevent soil fines from travelling into upper ballast or track (in rail applications)
2.6.2.3 Reinforcement
• Increase the interlocking between the soil and the reinforcement to increase the shearing resistance of the soil
• Increase the stiffness in the formation and minimise the formation settlements and deformations
2.6.2.4 Erosion protection
• Protect soil surfaces from erosion by rainfall/wind
• Retain the soil surface and minimise surface erosion
2.6.3 Geosynthetic characteristics:
The geosynthetic fabric characteristics vary in terms of the tensile strength, durability, and hydraulic properties (Han, n.d.).
2.6.3.1 Physical properties Mainly for quality control and assurance
• Type of material (Polymer, polyester etc)
• Roll length, width, diameter (mm)
• Thickness (mm)
• Mass per unit area (g/m2) 2.6.3.2 Hydraulic properties
The ability to provide drainage, containment, protection from fluid ingress, separation and containment.
• Opening characteristic, size of opening.
• Permeability
• Porosity
• In-plane flow capacity
2.6.3.3 Mechanical properties
This covers the strength requirements for geosynthetic reinforcement
• Tensile strength
• Creep resistance
• Penetration resistance
2.6.3.4 Durability properties Long term quality assurance
• UV resistance
• Chemical resistance
Geosynthetic properties were selected based on the function required of the geosynthetic. Since the application considers geosynthetic design to span subgrade voids, the mechanical properties of the geosynthetic formed the main design criteria. The basal reinforcement design properties were further considered in section 2.6.4 below.
2.6.4 Basal reinforcement
In the British Standard BS 8006:1995 the use of basal reinforcement is extensively covered.
The purpose of basal reinforcement is to prevent upward propagation of the sinkhole. Basal reinforcement was used in an investigation by Villard et al. (2009) as a safety measure to limit the deflections in the event of a sinkhole until the void material can be properly repaired.
Jaros et al. (2009) found that it is impractical to use basal reinforcement for applications where the diameter of the sinkhole is greater than 5m. This is because the strength and stiffness requirements for the geosynthetic in this application would be far too large hence a more economical option would be the use of multiple layers of geosynthetic reinforcement. In the case that basal reinforcement is used for this application, a supporting U-beam would still be required hence the less expensive multi-layered geosynthetic reinforcement must be considered.
2.6.5 Geosynthetic applications
In the case studies considered in chapter 2, geosynthetic reinforcement was used for various design situations, e.g. reinforcement for an embankment over soft clay (Bangkok), used in combination with cement stabilization (Germany, Kuwait, and U.S.A) to prevent collapse due
to void formation. The primary applications of geosynthetics in soil reinforcement considered in this investigation were for reinforcing embankments on soft soils and as a bridging system over sub grade voids.
2.6.5.1 Embankments on soft soil
Embankments constructed over soft soil face the threat of excessive settlement, due to the consolidation that the base layer undergoes. The basal reinforcement technique is generally used to prevent excessive settlements. Reinforcing it in this manner does not prevent settlement from taking place; instead it minimises the settlement and makes it more uniform.
2.6.5.2 Bridging of sub-grade voids
The German design guide EBGEO: 2011 regards two forms of geosynthetic stabilization:
a) Complete stabilization
The stability of the fill is guaranteed for the entire design working life, this is usually a geosynthetic used in combination with another type of reinforcement (e.g. cemented soils).
b) Partial stabilization
This is when local subsidence is allowed but it must not exceed the design limit. This is a temporary reinforcement technique used to prevent catastrophic collapse and maintain serviceability until a more permanent means of soil reinforcement can be installed.
2.6.6 Modelling of geosynthetic reinforcement
In order to model the geosynthetically reinforced fill an investigation into the current FEM geosynthetic design techniques was performed. In both Potts (2007) and Mifsud (2005) the geosynthetic layer is modelled as “special” membrane elements, specific to the ICFEP FEA program that was used. These were infinitesimally thin elements that allow stress to develop in their plane but not perpendicular to it (Mifsud, 2005). The reinforcement-soil interface was modelled as six-noded isoperimetric interface elements. The interface elements were put in place to prevent the reinforcement pulling-out from the soil fill.
In the laboratory model, the friction that occurs between the soil particles and the geotextile surface can result in variation of results and interlocking of soil grains. Messafer (1996) used a
combination of boundary element methods and discrete element analysis. The actual modelling of the geotextile was performed using a combination of a fabric surface and interface elements.
Importing the surface as a fabric represents the fabric strength of the geotextile. The interface elements account for the bonding and shear between the membrane and the soil.
In Ling & Lui (2003) a 2D model of a geosynthetically reinforced roadway was developed. The geosynthetic reinforcement was modelled as 3-node non-compression bar elements with linear elastic properties. In Bohagr (2013) the geosynthetic layer was modelled as an 8-noded axisymmetric element. In Perkins (2001) the interaction between the soil and the geosynthetic layer was modelled as nonlinear spring elements.
In this investigation the FEM modelling was performed using Strand7 FEM software, for the 2D analysis, the geosynthetic reinforcement is modelled as a 2-noded beam element and the 3D analysis, the geosynthetic is modelled as a 3D membrane element.