Geosynthetics: How Do They Prevent Landslides
Landslides are the most widespread geological event; in fact, they are responsible for more than 18,000 deaths between 1999 and 2017. They also affected more than 4.8 million people worldwide during the same period of time. As technology advances, ways to prevent landslides are increasing in number and effectiveness. First, let’s take a look at what exactly a landslide is.
Landslides: What Are They and What Causes Them?
According to the United States Geological Survey (USGS), a landslide is defined as the movement of a mass of rocks or earth down a slope. There are five classes of slope movement:
Landslides occur when large amounts of geological materials, such as debris, earth, or bedrock, move down a slope. They can be initiated by rain, snow, human development, water level, earthquakes, volcanoes or changes in groundwater.
Preventing landslides is important because they are a very serious geologic hazard to humans and the environment. Counties, cities, and towns should properly plan for how natural disasters, new construction, and land-use will affect them, especially when it comes to sloped areas. In order to plan appropriately and mitigate landslides, geosynthetic materials will be needed.
Geosynthetics: What Are They?
In the late 1950s, the president of Carthage Mills experienced storm damage to his beach home in Florida. After the tropical storm passed through, he noticed severe beach erosion to the area around his home and surrounding areas. He decided he needed to protect his home and the environment from this happening again. In collaboration with the Coastal Engineering Lab at the University of Florida, he and the engineers developed a filtering fabric that was made from synthetic woven material. This fabric was noted as having filtering properties with the capability of holding back sand.
By 1958, interlocking concrete blocks along with the newly created woven material was used for the first time in preventing beach erosion for a waterfront structure. It wasn't until the late 70s that geotechnical fabrics, as they were called back then, were used in other applications. From 1978 to 1979, what we now know as geosynthetics were used in a test section of an embankment at Pinto Pass in Mobile, AL. From this application, tests were conducted that helped to develop design criteria and construction techniques for the first woven filtration fabrics. These durable, fibrous filter fabrics, also known as geotextiles or geosynthetics, are now used extensively in geotechnical engineering for soil reinforcement as well as the following:
- Separate, confine, distribute loads
- Piping - prevent soil movement while letting water move through the material
- Drainage - control water pressure allowing flow through the material
There are two types of geosynthetic materials: geofabrics and geoblocks. Geofabrics are essentially two dimensional sheets that separate, reinforce, filter, and drain. The second type, geoblocks or geogrids, are polyethylene interlocking paving units. Geoblocks are three dimensional and commonly used for soil strengthening under roads. There is a large, varying number of geosynthetics, but if they were to be classified they would be listed as:
- Woven and Non-woven
- 3D Mats
From grids to nets and membranes, each geosynthetic material is made differently and serves a variety of applications. Below each kind has been explained, noting their make up and capabilities.
Geotextiles are permeable fabrics used for reinforcement, protection, filtration, drainage and separation, and they are commonly used in conjunction with soil or rock. Geotextiles fall into two categories, woven and non-woven. Woven geotextiles use perpendicular fibers woven to make up a cloth like material. The fibers of non-woven geotextiles are more random and are often made of synthetic polymer.
Used for stabilization and reinforcement, geogrids are open, mesh type materials commonly used to reinforce retaining walls or subsoils below roads. Geogrids can be either uniaxial (strength in one direction) or biaxial (strength in both directions) and are used in place of traditional gravel for soil stabilization
Geonets are similar in structure to geogrids but come in biplanar (two sets of intersecting ribs) and triplanar (parallel central ribs with intersecting smaller sets). These polymeric nets are used for in plane drainage, erosion control, foundation wall drainage or leak detection for landfills or solid waste.
Geomembranes are liners with low permeability that control gas or liquid migration. This type of geosynthetic material is used for water containment, landfills, and mining. These liners are made out of thin polymeric sheets infused with polymer sprays, asphalt, or elastomer.
- Prefabricated Vertical Drains
Often called earthquake drains or wick drains, prefabricated vertical drains (PVDs) have a plastic core encapsulated by a geotextile material. The purpose of these drains is to pull water from soft, compressed soil so it consolidates faster.
- Geosynthetic Clay Liner
A geosynthetic clay liner (GCL) is a barrier made up of bentonite clay that is sandwiched between geotextiles. The function of a geosynthetic clay liner is to act as a hydraulic barrier to leachate, water, other liquids, and sometimes gases. This highly impermeable liner is cost effective and used in place of thick layers of clay, which is expensive.
Geocells, also called cellular confinement systems (CCS), is webbing with a cavity-like structure. This material is used for stabilization, flood defense, erosion control, retaining walls and tree root protection. Geocells are made of polyethylene, which is lightweight and easily handled.
This type of geosynthetic material is made up of a combination of a geotextile along with a geonet, geogrid, or geomembrane. By combining several materials, a geocomposite is capable of strengthening the soil while allowing for proper drainage. Geocomposites can help address several specific problems within an application.
The manufacturing of geosynthetic materials involves the use of thermoplastics. Although some specialized geosynthetics can have steel wire or biodegradable fibers incorporated into them, most are made with raw materials such as the following.
- Ethylenecopolymer Bitumen
Geosynthetics have been used successfully in stabilizing steep slopes of residual soil and weathered rock. Geotextiles placed horizontally in layers have helped to prevent landslides, which are generally induced by heavy rainfall. The tensile reinforcement and filtration help to stabilize the slopes or embankments. Through slope classification and geotechnical engineering reports, it can be determined what reinforcement type is needed to prevent landslides.
Slopes are classified into 3 classes: low risk, medium risk and high risk. The risk of the slope or embankment is based on the geometry of the given slope. Other factors included in the risk classification are geological features, ground water level and soil/rock make up. Geotechnical engineers will read soil mechanics, as well as conduct geotechnical investigations so as to understand the best geosynthetic to use and what its future maintenance and life cycle will be.
Geosynthetic Maintenance: How is It Done?
Geosynthetic material is either biodegradable or non-biodegradable. Biodegradable geotextiles are made from natural fibers such as raffia palm fronds, whereas non-biodegradable geotextiles are made from synthetic materials such as polyester, polypropylene or polyethylene. As technology has advanced, the concept and function of non-biodegradable geotextiles and other geosynthetics has become the preferred choice.
By repeated field studies, the life cycle of geosynthetics has been observed under UV, chemical and biological testing. Due to their make up of polymers (polyester, polypropylene or polyethylene), geotextiles have become a practical, easily maintained product used in geotechnical engineering applications. In a variety of industries, geosynthetic materials are maintained in various ways. From visual examinations and soil tests to road inspections, geosynthetic maintenance has proven that geosynthetic materials have a long lifecycle, especially those that are non-biodegradable.
Geosynthetics vs Traditional Material: How Are They Cost Effective?
Geosynthetics can have an overall lower product cost than that of traditional substances in most applications. Through life cycle cost analysis, geosynthetics have been a well accepted product used in natural disaster reconstruction, site development and the creation of roads for over 30 years. There are several advantages to using geosynthetics and they are:
- Space savings
- Avoid costly remove and replace methods
- Enables the use of local backfill material
- Can be used in conjunction with lower quality backfill materials
- Low maintenance due to long life cycle
Time and labor costs are lowered as well, because geotextiles do not generally require specialized equipment or methods. For example, geogrids can help a project's probability margins greatly due to cost and ease of use. Geogrids are easy to install, use less backfill material and require no special equipment. Understanding the available geosynthetic technology and what the application at hand needs, is critical to selecting the appropriate material.
A common mistake that is made in selecting a geosynthetic material, is looking at the unit cost for one component rather than the cost of all units involved. Let's say the installed cost of backfill material is $30 per ton, but you reduce the thickness by 1 inch. This provides a savings of a dollar per square yard. Now introduce a geosynthetic material that costs $4 per square yard, but it reduces the thickness of the gravel by 8 or 9 inches, your cost for the solution has now been reduced by $5 per square yard. To estimate costs when using geosynthetic materials, there is design software and calculation tools available to engineers, contractors, etc.
Geosynthetics and the Future: What's Next?
Whether construction and site planning, environmental protection, or road infrastructure growth, geosynthetics has a growing future. Societies such as the International Geosynthetics Society (IGS) were created to provide a better understanding of geosynthetic technology, as well as promote its appropriate use in the environment. With advancements in technology, education will be key for engineering bodies to understand more about the capabilities of geosynthetic materials.
Within the field of geotechnical engineering, the civil engineering branch of geosynthetics is advancing quite rapidly. The use of geosynthetics has become not only a more practical application in the last decade, but also an essential material for reaching environmental sustainability. Their use in landslide mitigation, earthquakes, storm repair, construction, and road/highway creation has proven successful. From reducing long term maintenance to better performance, the use of geosynthetics is expected to continuously expand into the future.