The Geosynthetic Institute (GSI) has been asked many times regarding several applications for different styles of geogrids if reinforced levees will induce a seepage path adjacent to and around geosynthetic layers. At this point, GSI has no case histories where it can quantify such behavior around reinforced levees. Geogrids are critical reinforcing elements in existing levees (Figure 1). The significance of this work, with the aid of geosynthetics in the core section, is amplified because of the frenetic pace of heightening levees due to sea level rise (a result of global warming).
Our first foray into quantifying seepage around reinforced levees surrounded work with a modified ASTM D5887 “Standard Test Method for Measurement of Index Flux Through Saturated Geosynthetic Clay Liner Specimens Using a Flexible Wall Permeameter” device. We know from Daniel, Bowders and Gilbert (1997) that hydraulic-conductivity testing using flexible-wall cells is usually preferred over rigid-wall cells due to the potential for sidewall leakage problems with rigid-wall cells. Excessive sidewall leakage may occur, for example, when test soil shrinks during permeation. Also, the use of a rigid-wall cell does not allow for control of the effective stresses that exist in the test specimen. In addition, particles lining up on a rigid boundary may lead to a preferential path, rather than the tortuous one through the center of the soil portion of the specimen.
GSI incorporated geogrids (punched and drawn, strap type and woven coated) into the center of 4-inch (100-mm) cylindrical soil (SC, CL-ML and CH) permeability samples (Figures 2a and 2b). Unfortunately, the answer to the article title is yes, in a few cases of high hydraulic head and light normal pressure. Some geogrids add a seepage path (adjacent to and around the geogrid layers) into the cross section, particularly in fine-grained soils (clays). GSI has tested this using flex-walled permeameters by running side-by-side comparisons between specimens with and without geogrids. Furthermore, seepage is exacerbated at high gradients and low confining pressures once flow gets started.
Unfortunately, this technique suffers from specimen-preparation irregularities in addition to poor repeatability. Specimen preparation criteria are critical for all flow tests, in particular, one with geosynthetics included. It is difficult to get good compactive effort-energy on the soil-geogrid specimens in the cylindrical configuration. Hence, we found disparities in permeability results. In addition, once seepage started, it tended to cause piping with some soil-geogrid combinations. Due to these findings, GSI decided to take a different course of action.
Our second try at quantifying this phenomenon involved ASTM D4716 “Standard Test Method for Determining the (In-plane) Flow Rate per Unit Width and Hydraulic Transmissivity of a Geosynthetic Using a Constant Head” testing. GSI incorporated geogrids into the transmissivity unit with soil as both sub and super stratum.
With this larger 12 × 12 inch (300 × 300 mm) horizontal specimen, we could now easily impart 95% modified Proctor compaction while incorporating the geogrid sandwiched between soil layers (Figures 3a and 3b).
What a difference this setup made! We saw little difference in transmissivity between the soil alone and the soil-geogrid specimen at a gradient of 2 and a normal pressure of 3 psi (21 kPa) throughout a month-long test duration for three soil types.
In addition to the above, GSI built four transmissivity devices for further investigation (Figure 4a). We were uncomfortable with running only a single test for each setup. The quadruplicate devices were set up with a gradient of 2 and a normal pressure of 1 psi (7 kPa). They confirm the results of the official ASTM D4716 transmissivity device (Figure 3b), which was encouraging. It was also interesting to disassemble the device after transmissivity testing. The geogrid in the CL-ML soil appears to be intimately connected (Figure 4b).
This work will have far-reaching implications beyond geogrids in reinforced levees when it is finalized. It might also help in understanding the dynamic response problem of mechanically stabilized earth (MSE) walls, slopes and embankments, as they are affected by earthquake and rainfall simultaneously (Ling et. al. 2010). Hopefully, it will show that the combination of geogrid and soil effectively improves the deformation of MSE structures and the overall stability in seismic construction where water is present. There is always something new to learn when designing with geosynthetics.
Daniel, D. E., Bowders, J. J., and Gilbert, R. B. (1997). “Laboratory hydraulic conductivity testing of GCLs in flexible-wall permeameters: Testing and acceptance criteria for geosynthetic clay liners,” in ASTM STP 1308, ed. by L. Well, ASTM International, West Conshohocken, Pa., 208–26.
Ling, H. I., Yang, S., Leshchinsky, D., Liu, H., and Burke, C. (2010). “Finite-element simulations of full-scale modular-block reinforced soil retaining walls under earthquake loading.” Jour. of Engineering Mechanics, 136(5), 653–61.