Geosynthetics February/March 2022 - 43

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 finegrained
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 soilgeogrid
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 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.
References
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. G
FIGURE 3a Preparing geogrid-soil
transmissivity specimen
FIGURE 3b ASTM D4716 transmissivity
rig used for geogrid-soil testing
FIGURE 4a Low-pressure gradient and
transmissivity rigs set in quadruplicate
FIGURE 4b Disassembled geogrid-soil
transmissivity specimen after testing
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