Technical Notes



What are typical properties? What are minimum average roll properties? What does 95% confidence represent? What are certifiable physical properties?

With the increased usage of geosynthetics in the civil engineering community today, the need for proper referencing of physical properties of geotextiles is fast becoming a critical issue. The non-standard ways in which geotextile properties are referenced leads to confusion for all parties including the supplier, designer and contractor. This technical note will explain the terms commonly used in the reporting of geotextile physical properties and provide recommendations for appropriate and consistent property specification.

Typical Properties

All geotextile manufacturing processes result in minor product variations. Similarly, product testing equipment and procedures introduce some degree of variability. These variations result in a range of strength, weight, and other property values. Consequently, a number of samples are normally tested, with the values averaged together.

The plot in attachment A shows the frequency distribution of many test results for a fictitious material "A". If the curve is symmetrical, the highest point on the curve is defined statistically as the average or mean value. This is the "typical" value reported by many manufacturers.

Standard Deviation

For any given material tested, some variation in test results is expected. The measure of the deviation between the average value and the value tested is commonly referred to as the standard deviation. Statistically, standard deviation is defined as follows:
"The square root of the arithmetic mean of the squares of the deviation of each of the class frequencies from the arithmetic mean of the frequency distribution."
The significance of standard deviation lies in the variation in material properties that it can reflect. One standard deviation (s1) from the mean will include 67% of all measured values. This is displayed on the figure. Two standard deviations (s2) include over 95% of all measured values.

Certified Properties

The designer working with any construction material requires specification values he feels confident will be met or exceeded by the materials installed on his project. Consequently, he requires certification that the product delivered to his project site will have property values equal to or greater than those he used in design.

By agreement of the manufacturers belonging to IFAI, certified minimum average roll values (MARV) are equal to the average value less 2 standard deviations (s2). With this approach, assurance is given that over 97% of the product delivered to his site will have material properties greater than those he specified.

Minimum Average Roll Properties

Good quality assurance dictates that all products for use on a project be tested. One sample from each roll is removed and sent to a laboratory. Several specimens are taken from this sample for testing. Minimum average roll value is the minimum value obtained from the average values of the sample rolls.

Roll Samples

1 196 226 209 179 226 211
2 194 224 212 181 216 207
3 191 216 204 184 211 202
4 185 213 224 196 208 199
5 204 217 199 191 205 194
6 196 205 --- 193 203 201
7 197 211 201 184 201 189
8 207 208 208 177 198 ---
ROLL AVG. 196 215 208 185 208 200
The above table shows field sample test results from 6 rolls (8 specimens each) of our fictitious product (A). On any given project, this minimum average roll value must meet or exceed the designers specified value for the product to be acceptable.