Geosynthetics April/May 2021 - 9

INSTALLING A GEOMEMBRANE
LOW TO HIGH GRADIENT?
Q: We are a large liner installation firm that
has been asked to install a geomembrane
liner from low gradient to high gradient. Is
this the new normal?
A: This is not normal, and we suggest you
strongly object to this recommendation.
There are many stakeholders in regard to
panel layout (i.e., installer, general contractor,
regulator, design owner, etc.). All have
schedules and are involved with tasks, such
as subgrade approval, access, geometry and
weather. However, the installer is generally
the one holding the short end of the stick
when water gets under a deployed liner
system. In short, rework is not fun. The
geomembrane should be installed from high
to low. Get support from the International
Association of Geosynthetics Installers (IAGI)
on this issue, if needed.

CONCRETE INTERLAYERS
Q: We are interested in using a geotextile
as an " unbonded concrete interlayer " over
existing concrete pavement to float the new
slab over deteriorated pavement, instead of a
thin asphalt layer. Do you have any guidance
for us in this regard?
A: Over the past several years, 16-ounceper-square-yard (542-g/m2) nonwoven
needlepunched geotextiles have been used
successfully as concrete interlayers. The
largest study was conducted on I-35 by the
Minnesota Department of Transportation
(however, Mo., S.D., Okla. and Ga. have also
had success with this technology).
Geotextiles have very good benefit/cost as
compared with hot-mix asphalt layers. In
addition, the geotextile, in this application,
has the following installation advantages:
*	 No asphalt on project with multiple
contractors or large equipment is needed.
*	 Geotextile can be placed in challenging
weather at a much faster rate than asphalt.
*	 White geotextiles reflect solar radiation,
which results in cooler surface
temperature. This is good for workers
and the curing of concrete.
Check out the websites of Propex
GeoSolutions, TenCate Geosynthetics
Americas, AFITEX-Texel Inc., Maccaferri Inc.
and Kaytech Industries Corp. for further details
about geotextiles used in this application.

GRI-GM19 PEEL SPECIFICATION
Q: Our liner system is a double composite and
along a western tie-in. We need to extrusionweld the secondary 60-mil (1.5-mm) HDPE (new
cell) to an existing secondary 40-mil (1-mm)
HDPE (old cell), and the state is continuing to
have issues with the fact that we are using the
40-mil (1-mm) GRI-GM19 peel specification as
a pass/fail. Can you give a recommendation?
A: When completing a tie-in by extrusion
welding the secondary 60-mil (1.5-mm) HDPE
(new cell) to an existing secondary 40-mil (1mm) HDPE (old cell), we recommend using the
40-mil (1-mm) GRI-GM19 peel specification as
a pass/fail criterion. The geometry of the seam
(weakest point is the thinnest geomembrane)
will very quickly lead one to this conclusion after
only a few ASTM D6392 shear and peel tests.

ANCHORS, TIE-RODS AND WHALERS
Q: We are having issues related to obstructions
in the reinforced zone of our mechanically
stabilized earth (MSE) retaining walls. In the
first case, there is a rock outcrop that is limiting
the length of the geogrid reinforcement. In
the second case, we have a drop-down inlet
structure to divert stormwater, which is in the
reinforced zone. Can you make suggestions on
how to handle these situations?
A: Nice to hear from you and please note that
these situations are routinely handled with
anchors, tie-rods and whalers (pipes). Your first
situation has a National Concrete Masonry
Association (NCMA) detail shown in Figure
3. The anchor can be soil nails, grouted earth
anchors or even duckbill tendons (depending
on the site geotechnics), all of which provide a
bolted connection to the whaler. The geogrid
is wrapped around the pipe in the " Z " direction
and then connected to the wall face at
different elevations by conventional means.
Your second situation is handled as shown in
Figure 4. This detail was given to us by a clever
engineer, Dean Sandri (TenCate), after the GRI14 conference on " Hot Topics of Segmental
Retaining Walls " in 2000. It uses similar
technology as previously described. It needs
to be said that this is a detail of last resort. We
strongly recommend that all let-down structures
be moved out of the reinforced zone, as shown
in Figure 5a. One should anticipate some
movement in most MSE walls. This movement
results in leakage of drainage systems in the
reinforced zone. The leakage creates excess pore
water pressure in the reinforced backfill, which is
not good long-term for wall stability. Whenever
possible, drainage needs to be handled outside
the reinforced zone, as shown in Figure 5b. G

**Gravel fill must
be placed in cores
and above lower
portion of
geogrid wrap
prior to returning
to the wall face.
Pull tight and
place next course
above to secure

12 in (300 mm)
Minimum

Loop geogrid and
return to wall face to
secure connection to
spreader pipe
Well-graded
gravel fill 0.25 in
to 1.5 in (5 mm to
38 mm) with less
than 10% fines

Srw
Unit

2 block
maximum
spacing

Earth anchor
or soil nail
Allow for a spreader
pipe to create a
continuous span for
geogrid connection

Geogrid
reinforcement
Provide for a bolted
connection to the
spreader pipe

Exposed vertical shear
face: rock, shotcrete or soil

FIGURE 3 Cross-section (NCMA) detail
for anchoring a geogrid with insufficient
embedment length to a whaler with anchors
GEOGRID LENGTH PER
WALL PROFILE SHEETS

SEGMENTAL
RETAINING
WALL UNIT

DROP
STRUCTURE

A307 ROD
A53 PIPE

GEOGRID

GEOGRID
3½ " (85 mm) Ø A53
PIPE (ANCHOR BAR)

¾ " (20 mm) Ø
A307 NUT AND
FLAT WASHER

¾ " (20 mm) Ø A307
ROD (ANCHOR ROD)
THREADED AT ENDS

FIGURE 4 Plan view of transferring geogrid tensile
face around an obstruction in the reinforced zone
5a
Inlet
and piping
Reinforced
soil zone

5b

Shifted inlet
and piping
Reinforced
soil zone
Back drain

Base drain

FIGURES 5a and 5b (5a) Cross-section
schematic of flawed design of letdown
structure within the reinforced zone of
an MSE wall; (5b) better design
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Geosynthetics April/May 2021

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