Geosynthetic Applications

Primary Functions Of Geosynthetics

Geosynthetics are generally designed for a particular application by considering the primary function that can be provided.  As seen in the accompanying table there are five primary functions given, but some groups suggest even more.

Separation is the placement of a flexible geosynthetic material, like a porous geotextile, between dissimilar materials so that the integrity and functioning of both materials can remain intact or even be improved.  Paved roads, unpaved roads, and railroad bases are common applications.  Also, the use of thick nonwoven geotextiles for cushioning and protection of geomembranes is in this category.  In addition, for most applications of geofoam, separation is the major function.

Reinforcement is the synergistic improvement of a total system’s strength created by the introduction of a geotextile, geogrid or geocell (all of which are good in tension) into a soil (that is good in compression, but poor in tension) or other disjointed and separated material.  Applications of this function are in mechanically stabilized and retained earth walls and steep soil slopes; they can be combined with masonry facings to create vertical retaining walls.  Also involved is the application of basal reinforcement over soft soils and over deep foundations for embankments and heavy surface loadings.  Stiff polymer geogrids and geocells do not have to be held in tension to provide soil reinforcement, unlike geotextiles.  Stiff 2D geogrid and 3D geocells interlock with the aggregate particles and the reinforcement mechanism is one of confinement of the aggregate.  The resulting mechanically stabilized aggregate layer exhibits improved load bearing performance.  Stiff polymer geogrids, with rectangular or triangular apertures, in addition to three-dimensional geocells made from new polymeric alloys are also increasingly specified in unpaved and paved roadways, load platforms and railway ballast, where the improved load bearing characteristics significantly reduce the requirements for high quality, imported aggregate fills, thus reducing the carbon footprint of the construction.

 

Identification of the Usual Primary Function for Each Type of Geosynthetic

 

Filtration is the equilibrium soil-to-geotextile interaction that allows for adequate liquid flow without soil loss, across the plane of the geotextile over a service lifetime compatible with the application under consideration.  Filtration applications are highway underdrain systems, retaining wall drainage, landfill leachate collection systems, as silt fences and curtains, and as flexible forms for bags, tubes and containers.

Drainage is the equilibrium soil-to-geosynthetic system that allows for adequate liquid flow without soil loss, within the plane of the geosynthetic over a service lifetime compatible with the application under consideration.  Geopipe highlights this function, and also geonets, geocomposites and (to a lesser extent) geotextiles.  Drainage applications for these different geosynthetics are retaining walls, sport fields, dams, canals, reservoirs, and capillary breaks. Also to be noted is that sheet, edge and wick drains are geocomposites used for various soil and rock drainage situations.

Containment involves geomembranes, geosynthetic clay liners, or some geocomposites which function as liquid or gas barriers.  Landfill liners and covers make critical use of these geosynthetics.  All hydraulic applications (tunnels, dams, canals, reservoir liners, and floating covers) use these geosynthetics as well.

Research On Geosynthetic Materials

Geosynthetic Applications

Geosynthetics are sheet polymeric materials used in civil engineering.  They have been used since the 1970s in geotechnical (soil) structures for functions such as separation, reinforcement, drainage, filtration, liquid containment and as gas barriers.  In practice this has included applications as diverse as reinforcement in the walls of the Pentagon, reservoir liners, canal liners, road reinforcement, retaining walls, sports fields, dams, landfill liners, embankment stabilization, tree containers, chemical tank liners, and as base and roofing membranes for new buildings.  There is an increasing trend to use recyclates in geosynthetics, particularly PET from bottle recovery.

Geosynthetics often play critical roles in civil engineering and it is important that the materials in use can withstand the physical and chemical pressures of the environment. These range from resistance to leachates from landfill to resistance to root damage in soil liners, as well as standard properties such as resistance to creep, oxidation and UV light, and tensile strength.  This has resulted in sets of test standards being developed by the EU, ISO, BSI and ASTM.

There are several main categories of geosynthetics: geotextiles, geomembranes, geosynthetic clay liners, geogrids and geonets.  This review discusses the polymers used in each type, production methods, test methods and applications.

Geotextiles are permeable fabrics comprising around 75% of all geosynthetics.  Globally, 1,400 million square metres are used each year and the trend in consumption is upwards.  Polypropylene comprises the bulk of this with polyester as the second most commonly used material, Polymer properties and economics decide on material choice. Natural fibers are being used where durability is less important.

Geomembranes are thin flexible sheets with very low permeability.  They are used as barriers to the passage of gases of liquids.  Butyl rubber was the first material used, but now PVC and polyethylene are the most common materials.  Uses include landfill odor control, facing of dams and reservoir liners.

Geosynthetic clay liners are structures containing a clay layer and used as water barriers.  Thus the main component is a clay mineral, bentonite.  They can be used instead of geomembranes or as a second line of defense to geomembranes.

Geogrids are sheets of tensile elements with a regular network of apertures, usually constructed of polyethylene, polypropylene or polyester.  The most common use is for reinforcement of unstable soil and waste masses.

Geonets are composite grid constructions used for drainage capabilities.  Usually a geotextile is used as the drainage core with an upper and lower section of geomembrane.

Advantages and Disadvantages Of Commonly Used Synthetic Geomembranes

Advantages & Disadvantages of Common Geomembrane Types

Geomembrane quality begins with base polymer resin selection.  It is important to select or specify high-grade polymer resins that have been manufactured to meet the specific, unique demands encountered by geomembranes.

Polymeric geomembrane properties are a function of the chemical structure of the base polymer resin, the molecular weight, the molecular weight distribution and the polymer morphology (e.g. the crystallinity).  Next, it is necessary to select the right combination of additives to protect the geomembrane, such as premium carbon black as well as antioxidant additives and stabilizer to ensure long life even in exposed conditions.  Finally, it is necessary to select the most appropriate geomembrane manufacturing & installation method.

Geomembrane	Advantages	Disadvantages
HDPE	Broad chemical resistance Good weld strength Good low temperature properties Relatively inexpensive	Potential for stress cracking High degree of thermal expansion Poor puncture resistance Poor multiaxial strain properties
LLDPE	Better flexibility than HDPE Better layflat than HDPE Good multiaxial strain properties	Inferior UV resistance to HDPE Inferior chemical resistance to HDPE
fPP	Can be factory fabricated and folded so fewer field fabricated seams Excellent multiaxial properties Good conformability Broad seaming temperature window	Limited resistance to hydrocarbons and chlorinated water
PVC	Good workability and layflat Behavior Easy to seam Can be folded so fewer field fabricated seams	Poor resistance to UV and ozone unless specially formulated Poor resistance to weathering Poor performance at high and low temperatures
CSPE	Outstanding resistance to UV and Ozone Good performance at low Temperatures Good resistance to chemicals, acids and oils	Cannot be thermally welded after ageing
EPDM	Good resistance to UV and ozone High strength characteristics Good low temperature performance Excellent layflat behaviour	Low resistance to oils, hydrocarbons and solvents Poor seam quality
Butyl rubber	Good resistance to UV and weathering Good resistance to ozone	Relatively low mechanical properties Low tear strength Low resistance to hydrocarbons Difficult to seam
Nitrile rubber	Good resistance to oils and fuels (but not biodiesel)	Poor ozone resistance unless properly formulated Poor tear strength

Landfill Liner Systems

Environmental Containment Systems

Liner systems are containment elements constructed under the waste to control infiltration of contaminated liquids into the subsoil or groundwater.  The contaminated liquid, or leachate, may be part of the waste itself or may originate from water that has infiltrated into the waste.

Liner systems consist of multiple layers which fulfill specific functions.  The description presented below refers specifically to landfill liner systems.  However, the main characteristics of liner systems are similar for other applications.  Landfill liner systems may consist, from top to bottom, of the following functional layers:

Protective layer
This is a layer of soil, or other appropriate material, that separates the refuse from the rest of the liner to prevent damage from large objects.

Leachate collection layer
This is a high-permeability layer, whose function is to collect leachate from the refuse and to convey it to sumps from where it is removed.  Frequently the functions of the protective layer and the leachate collection layer are integrated in one single layer of coarse granular soil.

Primary liner
This is a low-permeability layer (or layers of two different low-permeability materials in direct contact with each other).  Its function is to control the movement of leachate into the subsoil.

Secondary leachate collection layer or leakage detection layer
This is a high-permeability (or high transmissivity, if geosynthetic) layer designed to detect and collect any leachate seeping through the primary liner.  This layer is used only in conjunction with a secondary liner.

Secondary liner
This is a second (or backup) low-permeability layer (or layers of two different low-permeability materials in direct contact with each other).  Not all liner systems include a secondary liner.

Drainage layer
In cases where the liner system is close or below the water table, a high-permeability (or high transmissivity, if geosynthetic) blanket drainage layer is generally placed under the liner system to control migration of moisture from the foundation to the liner system.

Subbase
This layer is generally of intermediate permeability. Its function is to separate the liner system from the natural subgrade or structural fill.

These layers are normally separated by geotextiles to prevent migration of particles between layers, or to provide cushioning or protection of geomembranes.

As indicated above, liner systems may have a primary liner only or may include primary and secondary liners.  In the first case it is called a single-liner system, and if it has a primary and a secondary liner it is called a double-liner system.  Also, each of the liners (primary or secondary) may consist of one layer only (low-permeability soil, geomembrane, or GCL) or adjacent layers of two of these materials, in which case it is called a composite liner.  There are multiple combinations of these names, some of which are given below as examples (obviously there are many more combinations):

• Single synthetic liner: primary liner only, consisting of a geomembrane.

• Single soil liner: primary liner only, consisting of a low-permeability soil layer.

Leachate Collection Systems Part 1

Components Of Leachate Collection Systems

There are many components to a collection system including pumps, manholes, discharge lines and liquid level monitors.  However, there are four main components which govern the overall efficiency of the system.  These four elements are liners, filters, pumps and sumps

Liners

Natural and synthetic liners may be utilized as both a collection device, and as a means for isolating leachate within the fill to protect the soil and groundwater below.  The chief concern is a liners ability to maintain integrity and impermeability over the life of the landfill.  Subsurface water monitoring, leachate collection, and clay liners are commonly included in the design and construction of a waste landfill.  To effectively serve the purpose of containing leachate in a landfill, a liner system must possess a number of physical properties.  The liner must have high tensile strength, flexibility, and elongation without failure.  It is also important that the liner resists abrasion, puncture, and chemical degradation by leachate.  Lastly the liner must withstand temperature variation, be black (to resist UV light), easily installed, and economical.

There are several types of liners used in leachate control and collection.  These types include geomembranes, geosynthetic clay liners, geotextiles, geogrids, geonets, and geocomposites.  Each style of liner has specific uses and abilities.  Geomembranes, are used to provide a barrier between mobile polluting substances released from wastes, and the groundwater.  In the closing of landfills, geomembranes are used to provide a low-permeability cover barrier to prevent the intrusion of rain water.

Shown here is a leachate evaporation pond in a landfill site 

Leachate Collection Systems Part 2

Geosynthetic clay liners (GCLs) are fabricated by distributing sodium bentonite in a uniform thickness between woven and non-woven geotextiles.  Sodium bentonite has a low permeability which makes GCLs a suitable alternative to clay liners in a composite liner system.  Geotextiles are used as separation between two different types of soils to prevent contamination of the lower layer by the upper layer.  Geotextiles also act as a cushion to protect synthetic layers against puncture from underlying and overlaying rocks.  Geogrids are structural synthetic materials used in slope veneer stability to create stability for cover soils over synthetic liners or as soil reinforcement in steep slopes.  Geonets are synthetic drainage materials, which are often used in lieu of sand and gravel.  Geonets can replace 12 inches of drainage sand, thus increasing the landfill space for waste.  Geocomposites are a combination of synthetic materials ordinarily used singly.  A common type of geocomposite is a geonet heat bonded to two layers of geotextile, one on each side.  The geocomposite serves as a filter and drainage medium.

Geosynthetic clay liners are a type of combination liner.  One advantage to using a geosynthetic clay liner (GCL) is the ability to order exact amounts of the liner.  Ordering precise amounts from the manufacturer prevents surplus and over-spending.  Another advantage to GCL’s is the liner can serve appropriately in areas without an adequate clay source.  Conversely, GCL’s are heavy, cumbersome, and installation is very labor intensive.  In addition to be arduous and difficult under normal conditions, installation can be cancelled during damp conditions because the bentonite absorbs the water making it even more burdensome and tedious.

Geosynthetics – Geomembrane Liners

Geomembrane Liners

Geomembrane liners are impermeable sheets of polymer that are utilized in containment applications.  They are available in large custom panels or rolled goods that can be field seamed to form a barrier system.

Applications

• High Density Polyethylene (HDPE), PVC, EPDM, Reinforce
• polypropylene (RPP) & XR-5 liners
• secondary containment systems
• tank liners
• landfill liners
• pond liners
• stud liner systems
• golf course pond liners
• pipe liners
• leachate collection
• potable water
• lagoon liners

Long-term Performance Of HDPE Geomembranes As Landfill Liners

Geomembrane Liners are impermeable membranes used widely as cut-offs and liners.  Until recent years, liners were used mostly as pond liners; however, one of the largest current applications is to the containment of hazardous or municipal wastes and their leachates.

High density polyethylene (HDPE) geomembranes are normally used as part of a composite liner for waste containment facilities such as municipal solid waste (MSW) landfills and heap leach pads.  Field conditions, which include physical stresses on the geomembrane, elevated operating temperatures, and contact with leachate constituents, have the potential to affect the service life of the HDPE geomembranes.  This thesis examined the long-term performance of different HDPE geomembranes based on both conventional laboratory accelerated immersion tests and simulated landfill liner tests.  A 1.5mm HDPE geomembrane was immersed in different synthetic leachates at different temperatures in order to evaluate the effects of leachate chemical constituents on the depletion of antioxidants.  The results showed that a basic leachate with trace metals, surfactant, and a reducing agent was the most appropriate for evaluating the potential degradation of HDPE geomembranes.  A similar immersion test was performed to evaluate the effects of thickness on the aging of HDPE geomembranes.

Three commercially available HDPE geomembranes having nominal thicknesses of 1.5, 2.0, and 2.5mm were immersed in a synthetic leachate at four different temperatures in this experiment.  The results showed that a thicker geomembrane may have a longer service life if other things are similar.  The depletion of antioxidants from a 1.5mm thick HDPE geomembrane was examined by conducting accelerated aging tests at 55, 70, and 85oC under simulated landfill liner conditions.  The results showed that the antioxidant depletion rate was consistently lower for the simulated landfill liner tests compared to the leachate immersion tests.

The effectiveness of the aged HDPE geomembrane on the migration of volatile organic compounds (VOCs) was examined by conducting diffusion and partitioning tests using both unaged and aged HDPE geomembranes.  The results showed that the aging of HDPE geomembranes did not increase diffusive migration of organic contaminants, provided that the geomembrane remained intact.  A new method was developed to estimate the service life of the HDPE geomembrane based on the landfill liner temperature history.  The service lives of the HDPE geomembranes were calculated to be between 20 and 4700 years, depending on the geomembrane type, exposure conditions, and the time-temperature history examined.

Geomembrane Liners

Geomembrane Liners

Geomembrane Liners can be installed for the following applications:

• High Density Polyethylene (HDPE), PVC, EPDM, Reinforce
• polypropylene (RPP) & XR-5 liners
• secondary containment systems
• tank liners
• landfill liners
• pond/lagoon liners
• stud liner systems
• golf course pond liners
• pipe liners

Keep in mind, experience in fabrication, repair and project management, are all critical to success of any project.