Designing geogrid solutions involves more than just simply picking a product. Success is about ensuring that the load, soil property, and physical durability requirements match.
Get it wrong and the entire structure could fail prematurely.
So, prior to picking a geogrid a methodology to arrive at a solution is required.
This article outlines how to provisionally quantify project geogrid needs for discrete soil reinforcement applications including retaining walls, road base, and embankments.
Get to Know the Project Type
Before making any calculations, fully understand the type of engineering application.
Geogrid drawings up a bill of requirements for:
Retaining wall. Earth pressure. Road base. Surcharge.base. Embankment. Earth pressure. Steep slope. Earth pressure Soft soil. Strength.
There’s different stress levels imposed by different ways of construction. To separate out the ‘need’ process asphalt road dressing to a <100mm root cause – walls need higher elevation on tensile strength than lightweight road base separation courses.
Review Soil
Soil property will govern format of geogrid deployed.
This should cover: Type of earth. Angle of friction. Cohesion value. Moisture content. Factors of safety.
Heavy clay results in requirement for stronger product e.g. in biaxial or high tenacity polyester. Well graded gravel may require less.
Understand Design Load and Pressure
The tons that are responsible for generating (design) load, how many tonnes – weight, traffic even, or shelf space and retainment walls.
If heavy loading requirements, so must product have higher tensile strength requirements.Heavy-duty roads may require geogrids such as TGDG120KN or TGDG160KN. Light applications may only need lower strength grades.
Define the Required Safety Factor
Engineering design always works with safety factors. A risk mitigation safety factor, which may depend on, among other things,
project lifespan,
environmental conditions,
soil variability,
construction quality,
etc.
Typical UK safety factors range from about 1.3 to 2.0, with a higher factor leading to a greater required geogrid strength.
This overall step ensures future stability and not just first year performance.
Estimate Tensile Strength Requirement
The tensile strength is the critical parameter in geogrid selection. One overly-simplistic formula for use at this point of the geogrid selection is:
required tensile strength = applied load ÷ allowable strain
But real design also needs to observe:
creep and associated buckling
installation damage
time-related chemical ageing
ultraviolet light
Fibreglass geogrid performs well at asphalt reinforcement where, due to its high modulus and limited deformation, it’s possible to use low weight “low elongation” material. A polyester geogrid (PET) however does a better job with long term soil, roadside and earth fill applications due to lower creep characteristics.
Check Soil – Geogrid Interaction
Good performance depends partly on the soil – geogrid interlocking. Factors include: aperture size viz a viz soil particle size; surface texture of both coated and uncoated; friction factor; pull-out resistance.
Granitic and other granular soils often are better enhanced by using a biaxial geogrid. For fine soils additional separation layers, such as nonwoven fibres, or composite systems may be required.
Decide Layer Spacing and Installation Depth
Geogrid is not just about the strength. Getting proper layout design is also critical. Example decisions are:
vertical spacing of individual layers
number of layers of fiber (reinforcement)
depth of the first layer
total depth of the reinforced zone
For retaining walls it’s not uncommon to see that the layer envelope has to be of closer span down towards the bottom because this is where the pressure forces are greatest. For general road base the geogrid is usually installed at the interface with the subgrade. Good spacing.” reduces deformation transferred to the existing ground beneath and fetters better distribution of the resultant load.
Select the Right Geogrid Type
Proceeding after calculation to choose the right type material. Some of the common types listed are:
Uniaxial geogrid → used for walls and slopes
Biaxial geogrid → used in stabilisation and road base
Triaxial geogrid → used for multi-directional plication of loads
Fiberglass geogrid → used for asphalt reinforcement
HDPE geogrid → soil stabilisation and reinforcement
Polyester geogrid (PET) → embankments etc of long duration
Various coated geogrids, such as bitumen coated geogrid are designed for better bonding with the asphalt matrix.
Verify Long-Term Performance
Short term strength guarantee is not good enough. Long term behaviour of “the total composite” must be checked. Factors look at:
creeping deformation with time
UltraViolet exposure, behaviour in the sun
time-related chemical, physical behaviour in soil
thermal effect with regard to temperature
A quality geogrid of good tenacity such as a polyester geogrid is going to give service over long periods of time under constant load application.