ASTM Test Methods for Geotextiles
GEOTEXTILE TEST METHODS
M.A.R.V. (MINIMUM AVERAGE ROLL VALUE)
Until the 1980’s, geotextile values were reported as “typical.” A typical value is an average or mean value, which means that 50% of the results can be expected to exceed the typical value and 50% can be expected to fall below the typical value. ASTM D4759 “Standard Practice for Determining the Specification Conformance of Geosynthetics” requires using M.A.R.V. values. The “minimum average roll value” is a middle ground between the absolute minimum value and the typical value. The MARV is derived statistically as the typical (mean or average) value less two standard deviations. A standard deviation is a measure of the width of the spread of the values, or their variance (dispersion) from the mean. The standard deviation is determined by taking the square root of that variance. The variance takes into account all possible values (not just the extremes which give the range). It is interesting to note that values reported ABOVE the mean count as a negative in the same manner as a value reported below the mean counts as a negative. The following diagram demonstrates:
Weight (Mass per Unit Area) [ASTM D 5261]
The mass per unit area is determined by cutting a minimum of 10 specimens, each at least 100 mm square, and then weighing them on an accurate scale. For civil applications, this property is reported as a “typical” value as opposed to “Minimum Average Roll Value” (M.A.R.V.). However, in the environmental (landfill) industry this is reported and specified as a M.A.R.V. property.
Thickness [ASTM D 5199]
The average thickness of a geotextile is measured using a thickness gauge (electronic micrometer) under a gradually applied, specified pressure, usually by a dead weight mechanism. The ASTM requirement is a pressure of 20 kPa applied through a circular loading tip with a diameter of 6.35 mm.
Grab Strength (Grab Tensile) and Elongation [ASTM D 4632]
This is the standard strength test used in the geotextile industry. It determines the force or load at which the geotextile breaks and how far it stretches (elongates) before it breaks. A geotextile sample is placed in clamps and mechanically pulled at a constant rate until it ruptures (breaks). The sample size is 4” wide x 8” long. The clamps are 1” x 2”. The clamp is buried 1” deep. Therefore, 1” of the sample is left unclamped on each side of the 2” section of the clamp. As such, the force is not applied to the whole sample and the result cannot be reported as “pounds per inch.” Instead it is reported in “pounds of force.” Therefore, it is good for comparing one geotextile to another, but not a reliable test for geotextile strength in a reinforcement design.
Wide Width Strength (WW Tensile) and Elongation [ASTM D 4595]
The wide width tensile test provides a more reliable assessment of geotextile strength. So for critical applications such as reinforcement design, it is the method engineers use to calculate the required tensile strength. Therefore, it is typically only done with woven (reinforcement) geotextiles. 4595 requires the entire width of the sample be clamped. The clamps are 8” x 2”. The geotextile sample is 8” wide x 8” long (minimum). Since the entire width of the sample is held by the clamps, this is a true tensile test. The “pounds of force” is then divided by 8, multiplied by 12, and reported as pounds per inch.
4595 - % Strain
You will often see ASTM 4595 specified in terms of % strain (elongation). 1%, 2%, 5% & 10% are typical. Generally speaking, woven fabrics must elongate 4% to 5% before they engage their full tensile strength. Since fabrics are not fully stretched when they are first installed, if the subgrade softens after construction, the fabric will elongate to match. Soils will typically fail at 1-2% elongation, so in critical applications it is important to know at what strain the geotextile will fail.
Wovens vs Nonwovens - Elongation @ Break
When stress is applied to a needle-punched nonwoven, the fibers can freely move apart and the material can commonly stretch by 40% to 80% before it breaks. In a woven geotextile, the fibers begin to absorb the stress immediately and elongation at break is therefore lower, typically in 5% to 30% range. Thermally bonded nonwoven geotextiles typically have an elongation at break somewhere between that of woven and needle punched nonwoven products. As such, the % elongation specified tells you whether the engineer wants a woven or nonwoven. (>50% nonwoven / <50% woven).
Tensile Creep [ASTM D5262 and D6992]
This test is applicable to geosynthetics (including geogrids) used in steepened slopes and retaining walls. Tensile (tension) creep tests are performed by placing a load on a geotextile sample for up to 10,000 hours (417 days). The samples are gripped across their full width. The creep deformation or elongation (strain) of the sample is monitored over the test period. From these results, the time to rupture at various load levels or the load level that will cause rupture at a given time can be determined. Like 4595, a 8” x 8” sample with an 8” x 2” clamp size is used.
Mullen Burst (Diaphragm Burst) [ASTM D3786]
The Mullen test was devised in 1887 as a measure for the puncture strength of paper and was adapted to textiles. Mullen burst determines how much force is required to rupture the geotextile as it is distended. An inflatable rubber membrane is used to deform the geotextile into the shape of a hemisphere through a 30 mm diameter ring until it bursts. It is literally blown up like a balloon. The resulting value is reported in pounds per square inch (psi). Due to a small sample size and a high variation in the test procedure, the results can vary widely. It is no longer recognized by ASTM as an acceptable geotextile test.
Puncture Strength (Pin Puncture) [ASTM D4833]
This is an index test for puncture resistance of geotextiles. A geotextile sample is clamped without tension between circular plates of a slip-free ring clamp. The ring clamp is secured in a tensile/compression testing machine. A force is exerted by a metal puncture rod attached to a load indicator against the center of the unsupported portion of the geotextile sample until rupture occurs (a hole is poked through). The puncture rod is a solid steel rod with a diameter of 8 mm and a 45 degree bevelled edge. The maximum force recorded is the value reported. It is no longer recognized by AASHTO M288 as an acceptable geotextile test and has been replaced by CBR Puncture.
(Static) CBR Puncture [ASTM D6241]
To eliminate the high degree of variability from the Mullen Burst (3786) and Pin Puncture (4833) test methods, Static (CBR) Puncture Strength (ASTM D 6241) was developed to replace them. CBR stands for California Bearing Ratio, a soil strength test that was adapted for this geotextile test. CBR Puncture is an index of puncture resistance that measures the force required to push a flat ended plunger through a geotextile. A 150 mm geotextile sample is secured between two steel rings. Instead of an 8 mm diameter probe with a beveled edge (Pin Puncture 4833); this test utilizes a 50mm diameter, flat-ended probe (plunger) that is pushed slowly through the geotextile. The relatively large size of the plunger provides a multidirectional force on the geotextile and simulates big stones pressed onto a geotextile laying a relatively soft sub-base.
Trapezoidal Tear [ASTM D4533]
Geotextile samples are cut in the shape of an isosceles trapezoid and then a small cut is made on one side of the trapezoid. The 2 non-parallel sides of the geotextile are gripped in parallel flat faced clamps in a manner which allows the tear to propagate as the jaws move apart and the required strain rate is applied. A continuous tear is propagated in this way and the maximum force recorded. The trapezoid procedure requires the jaw faces to be at least 2 inches x 3 inches.
UV Resistance [ASTM D4355]
UV Resistance is a measure of the potential for the deterioration of tensile strength in the fabric due to exposure to ultraviolet light and water. It is typically expressed @ 500 hours exposure. For some products, such as ground cover, you will see it specified as high as 2,500 hours exposure.
Friction [ASTM D 5321]
This is an adaptation of the direct shear test, in that the fabric is firmly fixed to the top half of the shear box and a standard laboratory soil is used in the bottom half. The force required to cause sliding between the fabric and soil is determined for different normal stresses and the shear strength parameters are obtained. The test is useful for quality control and may be used to compare different geotextiles. However, for reinforced soil applications the proposed fill material should be used in the test.
Apparent Opening Size [ASTM D4751]
A.O.S. is an important parameter in assessing a geotextile's soil filtration capability. Spherical solid glass beads are dry sieved through a geotextile for a specified time and at a specified frequency of vibration. The amount of beads retained by the geotextile sample is then measured. The test is carried out on a range of sizes of glass beads. The apparent opening size is the pore size at which 90% of the glass beads are retained on and within the fabric.
Permittivity [ASTM D 4491]
Permittivity is the mechanism by which water moves through the fabric. The permittivity test measures the quantity of water which can pass through a geotextile perpendicular to the surface of the geotextile. The permittivity may be measured either in a constant head or falling head test, although constant head testing is more common due to the high flow rates through geotextiles which make it is difficult to obtain readings of head change versus time in the falling head test. In the constant head test, a head of 50 mm water is maintained on the geotextile throughout the test. The quantity of flow is measured versus time. In the falling head test, a column of water is allowed to flow through the geotextile and reading of head changes versus time is taken. The flow rate of water through the geotextile needs to be slow enough to obtain accurate readings.
Permeability [ASTM D 4491]
This is derived from the pemittivity test using the nominal thickness of the geotextile. The permittivity is divided by the thickness to determine permeability. It is done to supposedly allow one to compare the geotextile’s permeability to the soil’s permeability. The problem is that geotextiles vary in thickness. Introducing thickness into the equation nullifies a designer's ability to compare geotextiles, because the permeability value is related to geotextile thickness, rather than geotextile cross-plane flow. Not only do geotextile thicknesses vary, but needlepunched nonwoven thicknesses decrease under load. Adding a geotextile's thickness to the equation does not make the geotextile a "soil" or give one a design test value to compare to soil just because both now have cm/sec as their units. Water Flow Rate [ASTM D 4491] The amount of water that travels through the geotextile expressed in gallons per minute per square foot.
Transmissivity [ASTM D 4716]
The volumetric flow rate of water per unit width of a geotextile specimen per unit gradient in a direction parallel to the plane of the specimen. Percent Open Area [CW-02215] The sum of the open area of a sample of a geotextile, divided by the total area of the sample and expressed in percent (Area of Openings/Total AreaX100). A small section of the fabric is held within a slide cover, inserted into a projector and the magnified image traced on to a sheet of paper. Using a planimeter, the magnified open spaces can be measured. The test is primarily applicable to monofilament woven fabrics and provides information on pore size openings which is important in assessing a geotextile's soil filtration capability.