Creep shear behaviour of Bentofix® GCLs
1. Introduction
2.
Bentofix Product
3. Test Apparatus
4. Testing
5. Observations
6. The
relative displacement over time is summarized below
7. Post
Testing
8. Summary
In April of 1994, NAUE GmbH & Co. KG constructed several large-scale creep shear devices to evaluate the behavior of NS style needlepunched woven / nonwoven Bentofix® GCLs, under simulated in-situ conditions of low normal load applications. The testing program included the measurement of differential creep movement as well as additional post-test shear testing on the subject material.
The material tested was the Bentofix® BFG5000, a "sister product" product within the NS series of Bentofix® GCLs (woven / nonwoven combination). To ensure applicability of this data to all NS series needlepunched Bentofix products, the specimen selected exhibited a peel strength at the lowest end of the strength spectrum.
The Bentofix®
GCL being tested was mounted to a 1 m x 1 m (3 ft x 3 ft) test apparatus
in the following cross section (top to bottom):
· 25 kN/m² (3.6 psi) steel plates
· 30 cm (12 in) of 2 - 8 mm (0.32 - 0.32 in) crushed gravel contained
in a steel box
· Bentofix® GCL with carrier geotextile anchored to the bottom
steel plate
Prior to the initiation of the test, the entire apparatus was gradually tilted to reflect a slope angle of 25° (2.14 : 1), resulting in a constant strain of 14 kN/m² (2 psi), under a confining stress of approx. 30 kN/m² (4.3 psi). To ensure thorough saturation, the sample was hydrated daily.
Testing was initiated on April 17, 1994 and continued until December 1, 1998 for a total lapsed time of over 40,800 hours.
Upon the initial loading
of the specimen with the confining stress, a small amount of movement
was measured in the system, associated with the settling of gravel in
the steel box. While movement occurred, it was not creep or elongation
in the needlepunched GCL. Regardless, it has been included in the final
displacement figures to ensure the most conservative picture. With an
initial value of 2.5 mm (0.10 in) of movement, the final value shortly
before dismantling the box was a total differential movement of 2.9 mm
(0.11 in) during the over 40,000 hour period (includes the shifting).
6.
The relative displacement over time is summarized below
|
10/17/94
|
5/11/95
|
5/23/95
|
8/9/95
|
9/20/95
|
10/26/95
|
7/1/97
|
10/15/98
|
12/1/98
|
|
2.5
mm
|
2.7
mm
|
2.7
mm
|
2.8
mm
|
2.8
mm
|
2.8
mm
|
2.8
mm
|
2.9
mm
|
2.9
mm
|
While relative values for the first 5,000 hours of testing have been approximated up to the 2.5 mm (0.1 in) value, the balance of the creep test is presented below as a function of time, logarithmically plotted with the initial deformation data.
The Bentofix® GCL was tested for its peel and shear strength after dismantling the device to more fully evaluate the specimen. The minimum peel strength was still within certifiable limits for the standard Bentofix NS style material (greater than 60 N/10 cm (15 lbs)). Three direct shear tests performed under full hydration and a normal load of 25 kPa (3.6 psi) showed a maximum shear stress of 64.8 kPa (9.4 psi), 78.4 kPa (11.37 psi) and 84.1 kPa (12.2 psi) after over 40,800 hours of creep testing.
The Bentofix NS style
GCLs, including the BFG5000, NSP 4900-1, NSP 4900-3, X-1000, NSL, and
NSE products are not prone to creep distortion under conditions replicating
low-load/low stress applications described herein. While the creep resistance
of a material is directly related to the needlepunched fibers themselves,
similar testing on other Bentofix® variations reflects the same conclusion,
that the Bentofix® woven / nonwoven style GCLs are resistant to the
long-term affects of constant strain under low normal loads. A second
large-scale creep shear device with a Bentofix® nonwoven / nonwoven
GCL is running under the same testing conditions since Oct 1993 with neither
creep nor displacement being evident.
