GEOTEXTILES & GRIDS GUIDE

A Simplified Lesson In Soil Mechanics

Since geosynthetics help to improve soil characteristics, it’s important to first understand soil mechanics and the three main stresses that can impact a hardscape project: Compression, Tension, and Shear

Here are some definitions and analogies to clarify the meaning of each:

• Compression Two opposing forces that squeeze an object trying to compress it (i.e. standing on a soda can)
• Tension Two opposing forces that stretch an object trying to pull it apart (i.e. tugging on two ends of a rope)
• Shear Two pushing or pulling adjacent forces, acting close together but not directly opposing each other; a shearing load cuts or rips an object by sliding its molecules apart sideways (i.e. pulling on two pieces of wood that have been glued together or cutting a branch with pruning shears)

(TeachEngineering.org)

In the context of a hardscape project, it’s important to simply keep these stresses in mind when choosing a geotextile or grid. Next, we’ll take a look at how these products help to alleviate the three different types of stresses.

1. Separation

Geotextiles help to maintain the integrity of each material used in a project by preventing the bedding material from settling into the subbase and preventing the subbase from mixing with the native soils. This separation of materials will essentially allow each material to perform as it was intended to. The end result: maximized strength and stability.

2. Reinforcement

Acting as a separation barrier between materials used below the surface, geotextiles or grids will reinforce the entire design. These products have a higher tensile strength than soil, therefore, they will add a very high level of durability and help prevent deformations and/or failures from occurring in the future.

3. Filtration

Non-woven geotextiles eliminate the possibility of soil mixing and clogging of drainage systems by serving as a filter. As water moves through the layers below the surface, fine particles and silts are left behind. This will prevent any clogging within the system (as well as future damage that could be costly to fix).

4. Protection

Geotextiles can provide an added level of protection to a hardscaping project. They can be used as a layer of padding (i.e. under a pond liner) and also serve to effectively protect slopes against erosion.

5. Drainage

Lastly, a non-woven geotextile will allow for subsurface water drainage and moisture removal from the system.

Again, depending on the product, the above features will vary. The next section details the most common types of geosynthetics and the benefits they have to offer.

The Difference Between Woven And Non-woven Geotextiles

 Woven geotextiles are manufactured by weaving together narrow strips of film. They perform the functions of separation and reinforcement and are commonly used in applications that require high levels of stabilization or support.

Woven geotextiles are referred to by Tensile Strength, which is the resistance a material has to breaking under tension. Generally speaking, woven fabrics are more plastic-like in feel and appearance. While they are relatively impermeable and don’t offer drainage, they do have a very high load capacity and are commonly used for roads and parking lots.

The primary functions of a non-woven geotextile are separation, filtration, and drainage. The manufacturing process of these fabrics involves needle punching as opposed to weaving. Non-woven fabrics are referred to by weight (i.e. 3.4 oz. per square yard) and are more felt-like in feel and appearance. They offer permeability and are commonly used in applications where drainage and filtration are of key concern (i.e. in ditches or underneath drains). Non-woven geotextiles can also be used for erosion control.

Polyspun geotextiles are non-woven fabrics and their main function is separation. While they are permeable and provide drainage, they will not provide any reinforcement to a project. Polyspun geotextiles are referred to by lifespan and are most commonly used as weed barriers.

Spunbond fabrics are also fairly commonplace within the market. The spunbond process is considered the fastest manufacturing method for non-woven fabrics. During this process, extruded filaments are spun onto a belt and bonded by applying heated rolls. Like polyspun geotextiles, spunbond fabrics will provide drainage but no reinforcement. They are referred to by weight and commonly used as weed barriers or as drainage fabrics.

Properties Of A Geogrid

A geogrid looks and feels much different than a geotextile or fabric. As its name implies, it’s a grid that’s typically made of polyester or polypropylene and feels like plastic. The grid is formed by ribs and apertures. The apertures are the openings that allow soil to strike through.

While geogrids are most commonly seen in large-scale engineering projects such as retaining walls and roads, they can also be used in smaller engineered hardscape projects to help prevent soil and wall movement, which can lead to settling and very expensive repairs down the road. (Geogrids can be used for erosion control and general reinforcement as well.)

There are two types of geogrids available in the market: biaxial or directional. Biaxial grids can be used in any direction and have equal strength in both directions. Directional grids have greater strength along one axis and need to be installed accordingly. With a directional grid, the greatest strength is typically parallel to the roll.

Geogrid placement will depend on many different factors: loading/surcharge, slope, soil type, hydrostatic/hydraulic load, the type of block used, etc. For information on how to utilize a geogrid within a particular design, it’s best to seek the advice of an engineer.