Written by Matthew Kuester and Bill Handlos, PE
An accurate understanding of creep resistance is essential to proper material selection when using polymers, and in the case of geocells, this science is being misapplied. The definition of creep deformation is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stress. Creep is something that creates fear and uncertainty with all designers where the possibility of creep factors exist. Yes, creep can occur in almost all materials including plastics, metals and concrete. In cases such as bridge and building design, it is important to properly understand creep factors and account for creep in engineering calculations. Yet, in the case of designing with geocells for load support, creep factors have no relevance.
What Causes Creep?
In order for creep to occur there must be; a constant load applied and a sustained deformation. Creep only applies when there is a sustained load on a material for an extended period. In a case of repeated on and off loading, this would be governed by fatigue and not by creep. The second required factor for creep to occur is an ability to undergo sustained deformation of the material. When a polymer has a load applied, the molecules of the material start to pull apart and stretch which leads to elongation of the material in one direction and typically a thinning of the material’s thickness.
Creep not a factor in Load Support
Now, consider a geocell load support application. The geocell material is expanded out on site and then an infill material is placed into the cells. At this point, there is not an applied load or deformation occurring in the material. Next, the infill material is compacted. This compaction applies a load to the cells, but this load is removed as soon as the compaction equipment is no longer positioned over the cells. In addition, as an individual geocell is loaded it exerts a force (as it starts to bulge slightly) but each of the adjacent cells around it push back on it (passive resistance) and prevent any sustained deformation. Thus, at the time of compaction, there is not a constant load nor is there a sustained deformation. Thus far, the material is successfully installed without any creep effects.
After the geocell load support system has been installed, the two types of live loads that will affect the system are driving loads and stationary (parked) loads. When a vehicle drives over a geocell system the load is applied vertically and as the geocell distributes the load laterally there is a temporary load applied to the geocell material. The load is not a sustained load and therefore would not have a creep effect. In the case of stationary loads, the load is continually applied to the geocell so it meets the first criteria for creep. Due to the pressure from all of the adjacent cells surrounding the loaded cell(s) there is no ability for the cells to move enough to have any appreciable sustained deformation. Therefore, creep cannot effect this scenario.
ASTM D6992 Creep Test Not Applicable
Those who make claims about potential for creep in load support have cited ASTM standard methodology. ASTM standards provide an accepted means for standardizing testing to be able to directly compare products. It is important to review the intention and scope of a test to ensure that it is appropriate and will give relevant results. The Stepped Isothermal Method (SIM) is used to accelerate creep testing. ASTM D6992 uses the SIM method to predict the expected deformation of geosynthetics over time when used for reinforcement applications. This method can be effective yet it is not suitable for polyethylene geocell evaluation. ASTM D6992 5.3 Note 1 states, “Currently, SIM testing has focused mainly on woven and knitted geogrids and woven geotextiles made from polyester, aramid, polyaramid, poly-vinyl alcohol (PVA) and polypropylene yarn and narrow strips.” Additionally, the note continues with a warning against expanded scope of the test saying, “Additional correlation studies on other materials are needed.” So while this test has applicability for geogrids and geotextiles, the test is not intended for evaluating geocells and correlations for polyethylene have not yet been established.
Further, D6992 cannot be considered in isolation. D6992 states, “Results of this method are to be used to augment results of Test Method D5262 and may not be used as the sole basis for determination of long term creep and creep-rupture behavior of geosynthetic material.” This reinforces the importance of reviewing each test standard to ensure that the product is within the scope of the test and that the results are relevant and complete. In the case of geocell evaluation, using ASTM D6992 is inappropriate as it has not been properly correlated to provide accurate evaluation of polyethylene and without ASTM D5262, it provides an incomplete overall evaluation of the product.
HDPE’s Long History of Success and Repeatability
HDPE has been used as the industry standard material for geocells since it was invented over 40 years ago. HDPE is a material that has been extensively researched by independent scientists throughout multiple industries which allows for a complete understanding of its performance capabilities. Using a virgin HDPE material allows for direct verification of resin consistency through laboratory testing to ensure that each manufacturing location and production lot have consistent material performance. This laboratory verification also allows for the comparison of the material to independent scientific results and not just manufacturer’s claims.
A few geocell manufacturers are promoting a Fabricated Inelastic Blend (FIB) to cut manufacturing costs and increase material stiffness utilizing recycled and other unpublished polymer materials. These FIB based materials can vary widely, even for the same product. Due to the vast number of combinations possible with these FIB materials, they pose two key problems when included as a material choice: validation and consistency. Due to the unpublished nature of the blending mixture there is no way to validate this material in comparison with published testing. Any testing of FIB materials must start from the beginning without any experience to rely on for long-term performance. The second concern with FIB materials is controlling consistency of the blend. Because each FIB blend is so variable, there is no way for a 3rd party tester to fully determine consistency of the blend between different manufacturing plants or even between different production lots. This inability to determine consistency creates uncertainty because there is no way to determine if there has been improper blending or changes to material blend.
Manufacturers using FIB materials promote the advantages of increased material stiffness. This stiffness is often a function of multiple generations of recycling. It is important to review the differences between elastic and inelastic materials and how they affect geocell performance. An elastic material is able to undergo a deformation (strain) and then spring back to its original state without permanent (plastic) deformation. Conversely, an inelastic material ends in catastrophic (complete) failure. Many of engineering’s worst failures have come due to catastrophic failures of inelastic materials that were loaded in unexpected ways. This absolute nature of inelastic failure puts projects at great risk because it does not give indication prior to collapse. Conversely, with elastic materials as material limits are reached the material will stretch and yield prior to complete material failure.
True HDPE Performance vs FIB Results
FIB materials bring a new uncertainty to the geocell market. These materials are of unverifiable composition so connecting material to performance is nearly impossible. Ultimately, these FIB materials beg your trust in their performance touting their unnecessary creep resistance. They hide the truth that creep resistance comes at a cost – inelastic material that can fail catastrophically.
After 40 years, HDPE continues to be the industry standard material for geocells. Presto Geosystems’ proudly pioneered the use of HDPE material in its Geoweb products due to the well-understood performance and reliability of that material.
In the 40 years Geoweb has been used for load support projects all around the world, there has never been a failure due to creep effects. While this consistency of performance may sound very impressive, it should not be surprising because creep forces simply do not matter in these applications.