TRI LAB UPDATES LONG-TERM DESIGN FLOW - PART 1

Will the soil environment cause geocomposite drains
to lose flow capacity over time?

The issues of geotexile intrusion into and chemical precipitate and biological growth clogging within the geocomposite drain will be addressed in Part 2 (next issue).

In-plane Drainage
In-plane drainage is described in terms of flow, q, or transmissivity, 1. Flow and transmissivity are related to each other as follows:

q = 1 i w or 1 = q / (i x w)

Where:

q = flow capacity, m³/s
1 = transmissivity, m³/s/m (m²/s)
i = hydraulic gradient
w = width of drain, m

While flow will vary with hydraulic gradient, transmissivity can be conservatively taken as constant under a given normal load over a range of gradients, if interpolation is used. A product's allowable transmissivity can be described as follows:

1_allow = _____________1ult______________.
               RF_CR x RF_INTR x RF_CC x RF_BC

Where:

1_ult = transmissivity from QC testing
RF_CR = Creep reduction factor (RF)
RF_INTR = Intrusion reduction factor (RF)
RF_CC = Chemical precipitate Clogging RF
RF_BC = Biological growth Clogging RF

Compression Creep of the Drain
An important property of any geocomposite drain is its ability to withstand large compressive loads, such as the weight of many feet of garbage, without being compressed and losing flow capacity. The loss of thickness over time in a drain is called compression creep and occurs to varying degrees in all geocomposite drains. This is why the determination of allowable, or available, flow capacity or transmissivity includes a reduction factor for creep.

The most common geocomposite is a geonet with a geotextile placed over/under it in the field or thermally bonded to it in the factory. Other geocomposites also include a covering filtration geotextile, but the core is a different 3-dimensional shape.

Test the Geonet or the Geocomposite?
Geonets creep differently when a geotextile is bonded, or laminated, to one or both sides to create a geocomposite drain. This phenomenon applies to both short-term compression and long-term creep testing. One reason is that the lamination process slightly melts the geonet and the pressing action tends to flatten the geonet, giving it a slightly different structure. More important, however, is the reinforcing effect of the geotextile. A bonded geotextile provides a kind of reinforcement in the plane of the strands of the geonet. The geotextile provides resistance to the roll-over or collapse of the geonet strands giving the geocomposite more compressive strength than the geonet alone. It's very important that this phenomenon be considered in any compressive creep test of the core.

What is the Effect of the Soil Environment?
Does the soil placed over the geocomposite drain make a difference in the drain's long-term flow capacity? Yes, but not as much difference as compression of the core. A protective cover soil over a geocomposite won=t define the shape of the core as much as it will the final intrusion, or sagging, into the flow paths of the core. The final or time-dependent structure of the drainage core is mostly dependent on the density, core structure, and the laminated geotextile.

The soil environment may also cause chemical precipitation or biological growth to occur on or within the drain, restricting (i.e. clogging) the flow capacity of the drain. These flow restrictions, along with intrusion, are addressed with specific reduction factors which will be addressed in the next issue.

How Long Does Creep Testing Take?
If the product's creep properties have to be determined for decades of performance, how long does the testing take? Only a few days if advanced accelerated creep testing is used. And TRI has pioneered the accelerated creep testing of geosynthetics using a technique called the Stepped Isothermal Method (SIM). This technique "accelerates" the creep by increasing the temperature in steps and then shifts the data using time-temperature superpositioning principles to longer time periods. The SIM technique allows for more than 10,000 hours of compressive creep to be simulated in less than 24 hours.

More on Accelerated Creep Testing via SIM
TRI first applied the stepped isothermal method (SIM) of accelerated creep testing to compressive creep about a year ago. Our interest was propelled by the 1allow concept. The SIM test took approximately 24 hours to complete and showed that the product lost approximately 3-4 percent of its original thickness in the last 99 years. The SIM data was subsequently positively correlated to 10,000 hour conventional creep data.

Accurate determination of the long-term percent reduction in thickness made possible with SIM allows for directly measuring the corresponding flow reduction. With a 24 hour test the Aresidual@ thickness after 10,000 hours or beyond can be determined. Knowing this, a thickness dependent transmissivity test rather than a time dependent transmissivity test can be run.

Several comparison tests have been performed in the TRI lab on geonet composites demonstrating that the same structure results from both short-term compression loading and long-term creep testing. Still, further and on-going testing should be done to verify this for different drainage products.

SIM can be performed on the geocomposite with the geotextile already bonded to the surface. This results in product-specific shift factors that incorporate the geotextile effects right into the long-term testing.

The issues of geotexile intrusion into and chemical precipitate and biological growth clogging within the geocomposite drain will be addressed in Part 2 (next issue).