Technical Notes


Thrace-LINQ TECH NOTE #11
HIGH STRENGTH GEOTEXTILES:
LONG TERM DESIGN ALLOWABLE STRENGTH

Introduction

Geotextile design strengths used in reinforced soil structures is evolving into a material specification defined as the Long Term Design Allowable strength, or LTDA. This value is utilized in design, and is arrived at by determining the ultimate wide strip tensile strength (ASTM D-4595), then reducing this ultimate value to an allowable one by the application of factors of safety (FS).

The following equation summarizes this approach:

Tall = Tult (1 / FScr FSid x FScd)
Where:

Tall = long term design allowable load (lb/in or lb/ft)
Tult = Ultimate wide strip tensile strength (lb/in or lb/ft)
FScr = Partial Factor of Safety for creep potential
FSid = Partial Factor of Safety for installation damage
FScd = Partial Factor of Safety for degradation potential
The following summarizes the methods of determining ultimate strengths, Tult, and discusses Factors of Safety to be used in the above equation (1). Technical references are provided at the end of this technical note.

DETERMINATION OF Tult
"Tensile Properties of Geotextiles by the Wide-Width Strip Method" ASTM D-4595, tests an 8 inch wide (200mm) specimen for strength and deformation (smaller width specimens are sometimes used for very high strength materials). The specimen is mounted in the clamps and load is then applied at a strain rate of 10% per minute, with the load and strain measured. Results are recorded in terms of strength per unit width along with the corresponding elongation. Strain is measured using cross head movement, or with LVDT's. (See Figure 1)

Figure 1


This method is widely accepted for determination of "in-isolation" testing of Geotextiles. It is an unconfined test, where the behavior of the material is measured in air, not confined within the soil backfill. The value of Tult results from this test. Figure 1 shows the stress/strain relationship of Thrace-LINQ's high strength Geotextiles.
TABLE 1
GEOTEXTILE TENSILE STRENGTH PARAMETERS (MARV)

PRODUCT DESCRIPTION ULTIMATE WIDE WIDTH TENSILE STRENGTH ALLOW. CREEP% (FScr) CREEP ALLOW. WIDE WIDTH TENSILE STRENGTH
    WARP X FILL % OF ULTIMATE WARP X FILL
GTF 300 Woven fabric (PP) 180 x 210 lb/in 25% 45 X 53 lb/in
  Single layer 2160 x 2520 lb/ft 4 540 x 630 lb/ft
GTF 375N Woven fabric (PP) 270 x 225 lb/in 25% 68 x 56 lb/in
  Single layer 3240 x 2700 lb/ft 4 816 x 672 lb/ft
GTF 570 Woven fabric (PP) 400 x 400 lb/in 25% 100 x 100 lb/in
  Single layer 4800 x 4800 lb/ft 4 1200 x 1200 lb/ft
GTF 550T Woven Fabric (PET) 500 x 500 lb/in 53% 263 x 263 lb/in
  Single layer 6000 x 6000 lb/ft 1.9 3156 x 3156 lb/ft
GTF 1000T Woven Fabric (PET) 1000 x 800 lb/in 53% 526 x 421 lb/in
  Single layer 12000 x 9600 lb/ft 1.9 6312 x 5052 lb/ft
GTF 1500T Woven Fabric (PET) 1500 x 785 lb/in 53% 789 x 413 lb/in
  Single layer 18000 x 9420 lb/ft 1.9 8468 x 4957 lb/ft
PP = Polypropylene
PET = High Tenacity Polyester

DETERMINATION OF FScr

Creep is defined as the time dependent elongation of a material while under constant load. All polymeric materials are creep sensitive, exhibiting viscoelastic strain behavior when loaded for an extended period of time. When a reinforced structure subjects the geotextile to sustained loading, creep testing, evaluation, and design become critical.

Thrace-LINQ's high strength Geotextiles are composed of two polymeric types; polypropylene (PP), and polyester (PET). Each will be discussed below.

Polypropylene(PP) is a long chain synthetic olefin polymer. It is a thermoplastic material, meaning it can be repeatedly heated to its softening point, shaped, worked or drawn in the case of fibers, then cooled to preserve its shape. The drawn fibers or yarns are used to construct the finished geotextile. PP is widely used in Geotextiles because of its relatively low cost, and because it can easily be formed into a variety of fiber and yarn sizes.

Polypropylene creep susceptibility requires that a factor of safety against creep rupture be utilized in reinforcement design. Factors of safety range from 4 to 5, depending on the reference cited (Ref. 2,3,4,5). This factor of safety translates into a Tall stress of between 20-25% of the ultimate, Tult. Thrace-LINQ's creep testing shows that a FScr = 4 is sufficient (see Table 1 and 2).

Polyester (PET) is a long chain synthetic polymer composed of ester of dihydric alcohol and terephthalic acid. It is also a thermoplastic. PET yarn is composed of many fine filaments bundled together and twisted to form the yarn used to weave the finished product. PET fibers possess high tensile strengths, elastic modulus, and creep resistance.

Polyester (in reinforcement applications high tenacity and high molecular weight PET) exhibits a high resistance to creep strain and rupture. As a result, factors of safety range from 2 to 2.5, depending on the reference used (2,3,4,5). This is approximately 50% of the factor of safety for polypropylene, and results in higher LTDA for polyester constructed geotextiles than for PP. The net result is a Tall stress of between 40 and 50% of Tult. Thrace-LINQ has performed 10,000 creep testing on its polyester materials and has determined that a FScr of 1.9 is acceptable for the PET family of high strength Geotextiles (see Table 1 and 2).

Table 2
FScr Summary
Reference Polymer Condition Factor of Safety
TF27 (8) PP Default 5
  PET default 2
Koerner (2) PP General 4
  PET General 2
Allen (5) PP Noncritical 2.5 - 5
  PET Noncritical 1.7 - 2.5
Christopher (7) PP/PET Absence of data 4
Thrace-LINQ PP General 4
  PET General 1.9


DETERMINATION OF FSid

Installation damage FSid is the loss of strength properties resulting from the act of installing the Geotextile. In some situations, where aggressive backfill and heavy equipment are used in construction, the resultant loss of strength can be significant. Cases have been reported where only 30% of the original strength properties remain after installation (Ref. 2,5). Although this high loss level is unusual, it points out the need to address installation conditions. Thrace-LINQ and Georgia Tech have recently completed a series of laboratory installation damage assessments with various backfills and Thrace-LINQ's high strength polyester geotextiles (Ref.9). Table 3 summarizes this extensive laboratory research effort.

The design engineer can control a number of variables which impact installation survivability conditions. These include:

Backfill - The choice of structural backfill can be guided by the designer, and should be limited to material which is not overlay damaging. Specifications requiring sand and sand and gravel backfill will reduce the installation damage significantly, into a low or moderate condition. A maximum particle size of 3/4 inch (20mm) and a well graded backfill may be specified, and is available in most areas of the country with little or no cost penalty.

Compaction Equipment - High contact pressure compactors, rollers and other earthmoving equipment that will induce high installation stresses should be avoided. Minimum lift thicknesses can be specified to reduce or limit contact stresses. Generally, compacted lift thicknesses of 6 to 12 inches are utilized. Vibratory rollers work well and minimize localized compaction stresses. Limiting the construction equipment weight should not pose an obstacle to the contractor's ability to construct the reinforced fill.

TABLE 3
CONSTRUCTION DAMAGE FACTOR OF SAFETY - FSid
  Gravel Sand Silt Clay
GTF 300 1.4 1.2 1.1 1.1
GTF 375N 1.4 1.2 1.1 1.1
GTF 570 1.4 1.2 1.1 1.1
GTF 550T 1.7 1.4 1.25 1.25
GTF 1000T 1.6 1.4 1.25 1.25
GTF 1500T 1.5 1.3 1.1 1.1


Table 3 suggested FSid by polymer and backfill type based actual Thrace-LINQ laboratory testing (9).

Compactive Effort - Geotextile reinforced fills require compaction which achieves the required design parameters (friction angle, density, etc.) while not inflicting significant damage to the Geotextile. A requirement of 90-95% of Standard Proctor (ASTM D-698-91) density is readily achievable, and should bring the desired fill properties. Exercise care when compacting against connections to facings, or when compacting immediately against temporary forming systems in vertical wall applications.

Field Tests - For critical applications where significant installation damage is anticipated, when a back fill material is questionable and a site specific determination of installation damage is necessary, a test section using proposed installation methods, simulating actual field installation conditions can be performed. While this may not be cost effective on smaller projects, it can be useful on large projects.

DETERMINATION OF FScd

Degradation potential, FScd, is a generalized factor of safety category which encompasses the uncertainty regarding possible chemical, environmental, biological and other degradation forces which may warrant a reduction in strength to accommodate a project uncertainty.

Thrace-LINQ possesses significant information on chemical resistively of the basic polymers. This information is available and can be used to assess potential degradation potential. These are typical, not site specific results, but can serve to guide the designer in determining what factor of safety to apply.

Consideration of UV degradation for exposed Geotextiles in vertical wall applications may require some form of protection to be utilized.

Finally, most naturally occurring site soils are relatively benign, with neutral pH and no chemical, biological or other deleterious activity. However, many sites being developed today do have a history of prior use. If that history indicates that some form of degradation is possible, the allowable strength properties need to be factored to reflect that possibility.

Alternatively, actual exposure testing using EPA 9090 or other exposure testing procedures can be performed. Task Force 27 suggests a minimum factor of safety of 1.1. In the absence of these site conditions a factor of safety of 1.0 - 1.2 may be employed (Ref 2). Thrace-LINQ suggests you use FScd = 1.1 for both polypropylene and polyester geotextiles for a vast majority of conditions.

FINAL NOTES

LTDA's for each of Thrace-LINQ's high strength woven Geotextiles can be firmly established based on thorough research and field experience. Please call Thrace-LINQ at 1-800-543-9966 for further technical assistance.


References:

  1. Bonaparte, R. and Berg, R. "Long Term Allowable Tension for Geosynthetic Reinforcement", Proc. Geosynthetics '87, New Orleans, LA, IFAI Publication, 1987, pp. 181-192.
  2. Koerner, R.M. "Designing with Geosynthetics," 2nd Edition, Prentice Hall 1990.
  3. Fowler, J. and Koerner, R.M. "Stabilization on Very Soft Sites Using Geosynthetics", Proc. Geosynthetics '87, New Orleans, LA, IFAI Publication, 1987 pp. 289-300.
  4. Leshchinsky, D. and Perry E.B. "A Design Procedure for Geotextile Reinforced Walls". Proc. Geosynthetics '87, New Orleans, LA, IFAI Publication 1987, pp. 95-107.
  5. Allen, T.M. "Determination of Long Term Tensile Strength of Geosynthetics: A State of the Art Review", Proc. Geosynthetics '91, Atlanta, GA, IFAI Publication, 1991 pp. 351-379.
  6. Paulson, J.N. "Summary and Evaluation of Construction Related Damage of Geotextiles in Reinforcing Applications", Proc. 4th International Conference on Geotextiles, Geomembranes and Related Products, The Hague, Netherlands, 1990, pp. 615-619.
  7. Christopher, B. FHWA Geotextile Design and Construction Guidelines, FHWA Pub. No. FHWA-HI-90-001, 1990.
  8. "Design Guidelines for the Use of Extensible Reinforcement (Geosynthetic) for Mechanically Stabilized Earth Walls in Permanent Applications", AASHTO-AGC-ARTBA Task Force #27, 1991.
  9. DeBerardino, Stephen J. et al "A Study of Tall for Reinforcement with Respect to Polyester Geotextiles," Proceedings 5th International Conference on Geotextiles, Geomembranes, and Related Products, Singapore, 1994.