Function and Application

The design and performance of all Geotextiles, irrespective of their composition or type, can be determined by identifying the main functions the Geotextile is required to perform.
In the construction of earth structures, Dird Felt Geotextiles perform five essential functions either individually or in combination, depending on the applications.
These are:
• Separation
• Drainage
• Filtration
• Reinforcement
• Protection
The first step in evaluating Geotextile design and performance is to identify the key functions relative to the application. Table 1 identifies these functions for a variety of typical applications.
The next step is to identify factors that will influence or affect Geotextile performance and determine the properties of the Geotextile required to withstandi these influences.
Finally, a concise specification on the required functional properties of the Geotextile and installation and storage procedure is essential to ensure correct delivery and installation of quality Geotextile on site.
Subgrade Stabilization
Subgrade support problems are most prevalent over wet, saturated, fine grained soils with a high sensitivity to remolding. The major cause of failure is poor drainage and loss of support, due to contamination of the aggregate base materials by the underlying soft subgrade soil during construction.
To prevent contamination, Geotextiles can be placed between the aggregate base course and the subgrade to act as a separator, thus maintaining the designed aggregate thickness of the roadway. The use of Geotextiles such as Dird Felt stable fibre needle punch, nonwoven allows for free drainage of the subgrade. Dird Felt also provides lateral in-plane drainage unlike heat bonded and woven Geotextiles which cannot drain laterally. Improved drainage improves subrade shear strength and its condition over time.
Areas of Application
• Paved and unpaved roads.
• Site access roads, farm and forest tracks.
• Storage areas, container yards.
• Embankments over soft soils.
• Land development for industrial and housing estates.
DIRD FELT Performance Benefits
• Allows more open-graded aggregate to be considered in the design to further enhance drainage.
• Reduces aggregate thickness required to stabilize the
• Reduces the excavation depth of unsuitable sub grade.
• Maintains a more uniform settlement of the subgrade
especially in transition areas.
• Improve subgrade strength and extend the life of the
Important Geotextile Design Criteria
• High construction survivability.
• Effective dynamic filtration/drainage capacity.
• High ultraviolet light resistance.


Typical Geotextile Applications
Advantages of DIRD FELT Geotextiles for Subgrade Separation and Stabilization:
Improved Filtration
To reduce the potential for Geotextile clogging, the Geotextile range of openings must be compared to the grain size of the subgrade to be filtered. Dird Felt has excellent pore size distribution for optimum filtration and separation. In addition, Dird Felt allows lateral in-plane drainage, when poor draining aggregate is used.
Construction Survivability
Research shows that extensive damage of Geotextiles commonly occurs during installation and construction (Koerner and Koerner, 1990). Placement of aggregate and compaction by heavy construction equipment impacts high stress on Geotextiles.
Dird Felt is recognized as one of the toughest and durable Geotextiles available and unlike other Geotextiles, is not easily damaged or punctured.
UV Resistance
All Geotextiles degrade when exposed to sunlight. In many subgrade stabilization applications, construction progress requires Geotextiles to be exposed to sunlight for an extended time. All standard Dird Felt products are produced from polymers specially formulated with Hindered Amine Light Stabilizers (HALS) to provide optimum ultraviolet light resistance. Laboratory testing shows Dird Felt Geotextiles have excellent UV resistance characteristics compared with competitive products (Figure 1).
Cost Savings
Lifective Geotextile design can save between 10% and 50% of the original road cost due to increased construction speed and material savings. In addition to initial construction savings, reduction of maintenance costs in excess of 25% have been reported (IFAI, 1992). However, the major cost savings are achieved through the increased life and utilization of the roadway system.
Reinforced Slopes and Embankments
Slope Reinforcement:
Unreinforced slopes may be unstable at the desired slope angle. Additionally, surface sloughing and erosion can occur at the edges of the slope.
Multiple layers of Geotextiles placed in a slope during construction or reconstruction will reinforce the soil and provide increased slope stability. Soil reinforcement allows for the safe construction of steep slopes, typically on the order of one horizontal to one vertical 1 H : lv). Even vertical structures can be safely constructed. Additionally, Geotextiles placed at the edges of a slope provide increased lateral resistance during compaction, thus allowing an increase in compaction density over that normally achieved. The in-plane drainage capacity of Dird Felt staple fibre needle punched, nonwoven Geotextiles can relieve pore water pressures within the slopes, thereby improving the soil-Geotextile interaction and allow the use of lower- quality, moisture sensitive cohesive soils. Improved drainage also reduces seepage forces and enhances the stability of such structures.
Areas of Application
• Reinforced soil walls and slopes.
• Bridge approach and abutments.
• Road embankments.
• Repair of failed slopes.
DIRD FELT Performance Benefits
• Allows construction of wider roads, without increasing necessary right-of-way, by replacing gentle slopes with
reinforced, steep slopes.
• Reducestheamountoffill material and land required and hence the cost.
• Allows the use of less select in-situ soils as fill.
• Increases the factor of safety of marginally stable slopes.
• Allows the construction of vertical soil structures.
• Allows lateral in-plane drainage.
• Resists construction damage imposed by compaction equipment.


Typical Geotextile Applications
Embankment Foundation Stabilization:
Embankments constructed on soft foundation soils are often difficult to construct. Problems such as local bearing failure caused by heavy construction equipment is common. Embankments constructed on soft foundations have a tendency to spread laterally, causing shear stresses at the base of the embankment, resulting in failure.
Dird Felt geotextiles placed at the subgrade/embankment interface can stabilize the subgrade by maintaining the initial fill thickness through its separation function. This allows unhindered trafficking with heavy construction equipment and more rapid construction rates of subsequent fill layers. The in-plane and cross-plane drainage capacity of Dirdfelt Geotextiles facilitate increased consolidation and shear strength in the subgrade (ResI and Werner, 1986) especially when stage construction techniques are used.
When the overburden pressure, due to the height of the embankment, is sufficient to mobilize lateral strains within the structure, Dird Felt Geotextiles provide additional tensile resistance forces to ensure the stability of the embankment. Depending on the design requirement and other influencing parameters, several layers of Dird Felt Geotextile or a single layer of high tensile strength Dird Felt Geotextile can be laid horizontally near the base of the embankment to reinforce the embankment.
Areas of Application
• Road embankments.
• Seawalls and dykes.
DIRD FELT Performance Benefits
• Increases the design factor of safety.
• Increases the height of embankment.
• Reduces embankment displacements during construction.
• Improves embankment performance by increasing uniformity of post-construction settlement.
• Increases the height of embankment.