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LINQ TECH NOTE #17

GEOTEXTILES VERSUS GEOGRIDS IN PAVEMENTS

Review of "A Field Evaluation of Geosynthetic Reinforced Haul Roads Over Soft Foundation Soils,"

- Deron Austin, et.al, Geo-'93

At the Geosynthetics 1993 conference held in Vancouver, Canada, a paper titled, "A Field Evaluation of Geosynthetic Reinforced Haul Roads over Soft Foundations," was presented. The paper compares the performance of geotextiles and geogrids as reinforcing/stabilizing elements underneath haul road pavements. The following background information is provided from the study:

• The soil was a soft, expansive clay
• Subgrade CBR used was approximately 1
• Soil properties were obtained at a 1 foot depth
• The base layer was always at least 8 inches

The geotextile (GTF 300 equivalent) was used on a subgrade having a CBR of 0.5%, while the geogrids were used on higher subgrade CBR values. Geogrid (1) was the only geogrid that performed comparable to the woven geotextile. However, the geogrid was placed on a subgrade with a CBR more than 1.5 times that used at the section reinforced with the GTF 300 equivalent. The ultimate wide width tensile strength per ASTM D-4595 of Geogrid (1) was ranked third among the four geogrids used in the research; however, Geogrid (1)'s installation was the first section. It appears that this first section received more compaction effort than the other sections.

The authors indicate in their conclusions, "the performance of polypropylene geogrid reinforced haul roads constructed on soft soils seems to be independent of the tensile strength and the process used to manufacture the products." This supports the taken-for-granted benefit of separation that the geotextile innately provides.

The study also clearly shows that base contamination was significant with geogrid use (in some cases, more than 35% base contamination). It can be concluded that lack of base contamination, i.e. effective separation, with the GTF 300 equivalent was the key for the geotextile's outstanding performance.

This research clearly shows that the geotextile alone is needed for stabilization - not a geogrid/geotextile system.

In the paper, the performance factor results displayed in Table 7 showed Geogrid (1)/Geotextile (2) performed the best when compared to control section (3). However, if all sections are compared fairly to control section (1) or (2), the section reinforced with the GTF 300 equivalent shows better performance. The paper uses three control sections for comparison when only one reference control section should have been used to fairly compare geosynthetics.

It appears that control section (1) has an uncharacteristically high number of passes to failure (refer to the following information),

It follows that all "Performance Factors" calculated using the number of ESAL passes to failure of control section (1) will have artificially low values.

For fair comparison, the "Performance Factors" for each different control section are presented in the following tables.

TABLE 1

Performance Factors/Control Section (1), CBR=0.6

*G = Number of passes to rut depth in geosynthetic test section

C = Number of passes to rut depth in control section

PF = Performance Factor

TABLE 2

Performance Factors/Control Section (2), CBR=1.0

*G = Number of passes to rut depth in geosynthetic test section

C = Number of passes to rut depth in control section

PF = Performance Factor

TABLE 3

Performance Factors/Control Section (3), CBR=0.9

*G = Number of passes to rut depth in geosynthetic test section

C = Number of passes to rut depth in control section

PF = Performance Factor

Taking into account the soil CBR for each section and dividing the performance factor by the CBR, you can see the GTF 300 equivalent has the best short term benefit compared to any grid (See Attachment A).

The "Performance Factor" is an indication of the short term benefits of using any geosynthetic. The long term benefits of separation are only available from the geotextile. From the paper,

"The aggregate subbase layer [in the geogrid sections] was contaminated with the cohesive subgrade soil... excavation of the geotextile sections indicated no contamination of the aggregate subbase. The geotextiles did perform their function as separators and there was no evidence of actual tensile failure."

From a performance standpoint, the GTF 300 equivalent is the winner both short term and long term over every geogrid tested.

Austin, D.M., Coleman D.M., "A Field Evaluation of Geosynthetic - Reinforced Haul Roads Over Soft Soils," Geosynthetics 1993, Vancouver, Canada, pp 65-80.

Attachment A