Although geocells may look the same, it’s the material that determines their performance.
Any polymeric material tends to elongate overtime. This phenomenon, known as creep, causes the geocell to lose confinement and dimensional stability overtime, which can cause a structural failure. The question is: how to choose the right geocell which will maintain the required confinement and dimensional stability for the entire project design life?
There are 4 critical factors for geocell long-term durability:
- Dynamic Mechanical Modulus (Elastic Stiffness)
- Permanent Deformation (Creep)
- Cell Tensile Strength
- Environmental Durability
The following section details their test standards and methods used to verify the unique properties of geocells are maintained for long-term engineering purposes under different mechanical stresses.
These test specifications are based on international test standards, and were adapted as general guidelines by the UN and many other international organizations and global engineering firms
When geocells lose confinement and dimensional stability, the structure can fail
1. Dynamic Mechanical Modulus (Elastic Stiffness)
Testing Method: DMA (Dynamic Mechanical Analysis) – ISO 6721-1, ASTM E2254
The DMA defines the elastic behavior and ability of the geocell to store and release dynamic loading, while maintaining its geometry.Polymers tend to lose elastic modulus over time, particularly under dynamic loading. A geocell system must maintain its stiffness and elastic properties without permanent deformation or loss of geometry. This would result in a loss of confinement and could lead to failure.
The Dynamical Mechanical Analysis (DMA) examines the net elastic modulus of polymers. It extracts the polymer stiffness in the elastic mode – the ability to apply loads on the system without permanent deformation. A stable net elastic modulus also ensures elastic behavior at elevated temperatures.
This method is well supported by ASTM and ISO standards, widely available and commonly used in the automotive, electronic, military industries.
2. Permanent Deformation (Creep)
Testing Method: SIM (Stepped Isothermal Method) – ASTM D-6992 (SIM)
This test defines the permanent deformation of the material at the end of project design life under specific designed loading
Each polymeric material creep level is affected by load, temperature and time. The Stepped Isothermal Method (SIM) was developed for the space, military, automotive industries to predict the creep rate of polymers over time. In this test, a specific load is applied on the geocell, based on its expected use (subbase, subgrade, etc.). Then the temperature is used as a time accelerator – by raising the temperature in several steps the test simulates the progression in time. For example, at Step 2 (51°C) 1 test hour = 100,000 hours of use (4166 days or 11 years). The creep reduction factor is extrapolated from the time-temperature data in the test.
This way, the SIM test provides the ability to test, measure and calculate how a material will perform over an extended period, representing 10s of years of service life.
Based on research with leading institutes research, plastic deformation (creep) above 3% in the wall of a geocell will impact confinement: lower the compaction density, decrease service life and increase the chance of failure . Deformation above 3% in asphalt pavements for example will also lead to cracking and failure.
Plastic deformation above 3% will cause failure and end of service life
3. Cell Tensile Strength
Testing Method: Strip Tensile Strength ISO 10319:2015 (Wide-Width)
The test defines the strength to withstand vertical load transferred to hoop (cell wall and weld) tensile forces.
A tensile testing machine is used to determine the geocell tensile strength and seam weld strength. Cell integrity is dependent on perforated strip strength and the weld strength (~90 degrees). The strength of the weld on the strip must be equivalent or higher to the tensile strength of the strip itself.
The test is carried out until the yield point, which indicates the geocell material strength. The higher the strength is, the heavier loads the geocell can withstand. The cell strength (and hoop strength) is critical to the project design and to calculate the factory of safety.
It is important to perform this test on a representative cell (wide-width) and include the entire perforation pattern, to best represent performance of the actual geocell installed in the field.
Strip Tensile Strength & Weld Test (Wide Width Perforated Strip)
4. Environmental Durability Test
Testing Method: Photochemical (UV) & Oxidation Resistance (HPOIT) – Long Term Reliability
Stabilizers that prevent polymer degradation and aging from UV radiation belong to the Hindered Amine Light Stabilizers (HALS) family of heat stabilizers. These are essential additives that allow Neoloy based geocells to survive environmental conditions that include heat and direct and diffused solar radiation for many decades. This includes resistance to oxidation agents as well as resistance to chemical reactions in the soil.
The stability of the geocell material is measured by the High Pressure – Oxidative Induction Time (HPOIT) method (ASTM D5885).
Long Term Durability is measured by HPOIT method (ASTM D5885)
This table summarizes the differences between HDPE & Neoloy geocells:
|SOFT CELLS (HDPE)*||TOUGH CELLS (Neoloy)|
|Elastic Stiffness (DMA test)||X||Low||V||High x2|
|Creep Resistance (SIM Test)||X||Very Low||V||Very High x20|
|Tensile Strength (Tensile test)||X||Low||V||High x7+|
|Environmental Durability (HPOIT test)||X||Low||V||High x5|
*PRS manufactures both HDPE and Neoloy geocells.