Volume 8 Issue 9 - May 15, 2009
Abrasion Damage of Geogrids Induced by Turbid Flow
Ching-Chuan Huang

Department of Civil Engineering, College of Engineering, National Cheng Kung University

GEOTEXTILES AND GEOMEMBRANES, Vol : 25  Issue: 2  pp: 128-138, 2007

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A circular flow chamber is developed to study the abrasion damage of three woven geogrids.  This chamber creates consistent turbid flow conditions in terms of flow velocities and particle concentrations, and is an effective tool for laboratory studies on abrasion damage of geosynthetic materials induced by turbid flows.  Two types of damages on the strands of woven geogrids are identified : one is the abrasion against the surface of strands in the flow direction; the other one is the cutting of strands normal to the flow direction.  The strength reductions increase with the increase of test duration, while the strength reduction rates decrease as the test duration increases.  An epoxy-coated geogrid (GRID 6) appeared to have higher resistance against abrasion than a polyvinylchloride-coated geogrid (GRID 2) indicating that the use of new coating materials to increase the durability of woven geogrids in turbid flow environments is possible.  This also suggests a need for establishing a standard test method to evaluate the effectiveness and robustness of coating materials.
Fig. 1 Geogrid containers used in a pilot test on seashore protection. (Huang and Liao, 2007)

The use of locally available construction material in infrastructure construction projects is an important part of sustainable development and environment conservation.  The present study was motivated by a damaged site in a pilot test, which uses polymeric geogrid container (as shown in Fig. 1) and gravels available on-site, as a seashore protention measure.  This project presents a state-of-the-art application of polymeric materials (or geosynthetics) in a harsh environment.  In this case, the geogrid is subjected to cyclic movements of gravels on the surface of geogrid container, causing abrasion damage of the geogrid, as shown in Fig. 2.
Fig. 2  Various degrees of damage on the geogrid containers observed a month after completion : (a) an example of moderate damage,  (b) an example of serious damage. (Huang and Liao, 2007)

In order to simulate this type of damage in the laboratory, a circular flow chamber as shown in Fig. 3 is developed.  In contrast to the conventional flume used in hydraulic engineering studies, the circular flow chamber eliminates possible problems associated with producing turbid flow condition, pumping the gravel-suspended fluid, and constructing a high-capacity water storage thank.  Flow velocity measurements (Fig. 4) suggested that uniform flow velocity fields are generated along the inner surface of the chamer, as shown in Figs. 5(a) and 5(b).
Fig. 4 A flow velocity measurement set-up used in the present study. (Huang and Liao, 2007)
Fig. 3 1.5 m-diameter, 1.0 m-high turbid flow chamber developed in the present study. (Huang and Liao, 2007)

Fig. 5 (a)Measured velocity field at 40RPM (Average velocity near the surface of tested material,Vm = 2.2 m/s); (b)Measured velocity field at 50RPM (Average velocity near the surface of tested material, Vm = 2.5 m/s). (Huang and Liao, 2007)

Two types of damage were found for the geogrids tested : 1. abrasive damage of the geogrid strands in the flow direction (as shown in Fig. 6(a)), and 2. damage of the geogrid strand normal to the flow direction (as shown in Fig. 6(b)).  The observed patterns of damage are in good agreement with those observed in the field, suggesting that a laboratory accelerating test is feasible in predicting geogrid damage induced by turbid flows.
Fig. 6 Close views on the damaged geogrid (GRID2) at T = 8 hrs. : (a) a strand in machine direction, (b) strands in cross-machine direction. (Huang and Liao, 2007)

For three types of geogrids tested, extents of damage (expressed by percentage of strength loss, P.S.R.) increase linearly with the flow duration (T) for T ≦ 24 hrs; the value of P.S.R. increase at smaller rates for T > 24 hrs., compared with those for T ≦ 24 hrs. as shown in Fig. 7.  It was also found from Fig. 7 that the geogrid coated with EPOXY (GRID 6) generally has about 20% less P.S.R. comparing with those for the geogrids coated with polyvinylchloride (PVC).  This suggests that the development of abrasion-resistant material, such as the EPOXY used here, as the coating material for geogrids may play a key role in the application of polymeric geogrids in harsh environments.
Fig. 7 P.S.R. vs. T relationships for various types of geogrids under Vm = 2.5 m/s and Pc = 4.6 ± 1 %. (Huang and Liao, 2007)

The present study investigated the effect of particle concentrations to the abrasion damage of geogrids (namely, GRID 2 and GRID 6, as shown in Figs. 8 and 9, respectively).  Under typical particle concentrations of several percents, the extent of damage can be very high within a flow duration of T = 24 hrs., again suggesting the severity of abrasion damage of geogrids in a turbid flow environment.  These test results also suggested that using a higher tensile strength geogrid (with a thicker bundle of polymeric yarns) or using a high abrasion-resisting coating material, such as EPOXY, can be effective measures in reducing the extent of abrasion damage (in terms of P.S.R.) of polymeric geogrids exposed to turbid flow environments.
Fig. 9 Effect of particle concentrations on P.S.R. for GRID 6 under different test durations. (Huang and Liao, 2007)
Fig. 8 Effect of particle concentrations on P.S.R. for GRID 2 under different test durations. (Huang and Liao, 2007)

The above essay is extracted from :
Huang, C.C. and Liao, C.C. (2007) “Abrasion damage of geogrids induced by turbid flow” Geotextiles and Geomembranes, Vol. 25, No. 2, pp. 128-138.
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