Geotextile flow rate measures the ease with which water can flow through such a geotextile fabric under pressure. Measuring the flow rate is important where filtration in the long-term is essential for stability and drainage. Inherent in this test is the difference in behavior between different types of geotextile, non-woven geotextile and woven geotextile. In simple terms, a non-woven type of geotextile will most often provide the highest instantaneous flow rates. A woven fabric shows a more stable flow pattern. The knowledge of how to measure the flow rate plays an important part in the selection of the appropriate material. It helps prevent clogging and drainage failure geotextile fabrics.
What does Geotextile Flow Rate mean? Flow rate is the volume of water passing through a geotextile per unit time and is usually measured at a given hydraulic gradient and head. In effect, it expresses how easy it is for water to travel through the fabric – and shows a range of values. The type of flow rate through or in geotextiles and other materials naturally would bear a relationship to the permeability of a material, although the two terms should not be confused. The flow rate shows broadly how readily the water can travel through the fabric; the greater the flow rate the greater the capacity for drainage / percolation of water.
Higher flow rate values are not necessarily always the best. The choice will depend on the intended use and the type of soil. Generally we can say that fine soil types will require controlled filtration, and that the coarser type soil gauge will require more rapid drainage. In choosing a flow rate for a particular type of project, its import relates directly with construction, site material type, overall project loads, groundwater conditions, and variations. The flow rate is not measured in air, and except in rare cases the test is carried out in a controlled climate to say ASTM standards. The most widely used technique consists essentially of a fixed head of water and a controlled pressure, with some means of measurement connected to an endless tape that will measure breadth wise. The geotextile fabric is a water head that acts as a try-set in place between rigid plates. Water is then applied to each face.
Volume Discharged Over Time
The volume discharged water over time.
Express this as flow rate OR permeability index.
Which means that we can readily see how well each material is performing in relation to one another.
Non woven geotextile more commonly shows a higher initial flow.
Whilst the woven geotextile shows a more stable long term result.
Vertical and In-Plane Flow Testing
There are two directions in which geotextiles are measured for the level of flow as a performance indicator.
Those directions are vertical flow and in-plane flow.
Vertical flow is the amount of water which can pass through the entire thickness.
This is the test which is carried out as part of the majority of filtration design work.
In-plane flow is the amount of water which will flow in the direction of the geotextile surface.
This is of major importance in drainage and landfill applications, for example.
Landfill projects require materials with reasonable in-plane drainage potential.
Road construction has more focus on vertical filtration performance, for example.
What Affects Flow Rate
There are a number of factors which affect the result of flow rate on geotextiles.
Type of material is of course extremely important.
The difference behind performance in the PP geotextile and the PET geotextile.
The structure of the fibres plays a part as to how quickly water will pass through them.
On the whole, a needle punched geotextile is highly porous.
Heat bonded will tend to be more compact.
The presence of soil particles can clog up the spaces within the geotextile material
Which means that long term flow is also a big design consideration.
Why Testing Flow Rate is Important on Projects
Testing flow rate is one of the means by which drainage system failure is avoided by ensuring
That water is effectively able to move away from the soil layers.
In road a protecting the base layer from the damaging affect of water.
With the use of it in retaining walls, reduces hydrostatic pressure build up gear.
Driveway fabrics also depend on stable flow performance.
Not allowing water to compromise subgrade structure.