Vertical Pullout Test for Measurement of Soil-Geosynthetic Interface Friction Properties
Milind V. Khire, Ph.D., P.E.
Michigan State University
This paper presents a vertical pullout test (VPT) developed by the authors to measure the peak interface friction angle (δ) and adhesion (ca) between soil and planar geosynthetic products (e.g., geomembrane, geotextile). The key advantages of this test are: (1) relatively low capital cost for the equipment (as low as US$150); (2) relatively simple and time-efficient compared to the conventional methods; (3) the setup can be easily adapted by small conventional soil testing laboratories; and (4) the setup can be transported to the field for field-measurement for relatively quick evaluation of soils.
The experimental method developed in this research consists of: (1) measurement or estimation of the angle of internal friction (Φ') for the soil under consideration; (2) conducting vertical pullout tests at pre-defined surcharge loads on a representative sample of the planar geosynthetic product under consideration; and (3) using an analytical method to estimate the interface friction angle and adhesion using the data collected in Steps 1 and 2.
We verified this testing method by comparing the δ and ca obtained from the vertical pullout test to: (1) those measured using a conventional soil direct shear apparatus modified to also measure interface friction between planar geosynthetics and soil according to ASTM5321; and (2) values published in the literature. We tested three coarse-grained soils (coarse, medium, and well-graded sands) and three types of geomembranes (1.5mm thick smooth and textured HDPE and 1 mm thick smooth HDPE). We evaluated three sample sizes for the pullout test of the geomembranes (75mm x 150mm, 150mm x 150mm, and 300mm x 300mm).
The average peak interface friction angle measured from the VPT lied within ± 2° of the angle measured from the conventional direct shear apparatus. The interface friction angles measured using 150mm x 150mm sample for the VPT were more accurate compared to the other two sizes. If you need any further
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