Tensar® Biaxial (BX)
Geogrids For Construction
Over Soft Soils
®
Reinforcement, Separation, and Filtration
The Spectra System consists of three primary components:
■
Tensar BX Geogrids — stiff geosynthetic reinforcement
■
Design — developed using SpectraPave2 Software or
Tensar engineering services that include conceptual
designs and design assistance
■
Site Assistance — maximizes time efficiency and
minimizes costs of project construction
Tensar® Biaxial (BX) Geogrids Perform
Better Under Real-World Conditions
Setting the Record Straight — Why Tensar BX Geogrids
are superior to fabrics (geotextiles) when constructing
over soft soils.
Common sense might suggest that fabrics are needed
to separate aggregate fill from soft subgrade. Not
necessarily! That is because the real challenge of
building over soft soils is pressure — pressure that
is mostly induced by heavy concentrated loads.
Unlike geotextiles, Tensar® Biaxial (BX) Geogrids
provide a highly effective means for reducing
pressure below the trafficked surface. BX Geogrid
stiffness and open apertures create a “snowshoe
effect” that evenly and broadly distributes load
while inhibiting particle movement at the interface
between interlocked fill and subgrade. Contrast
this with the less efficient “hammock effect” and
frictional resistance provided by fabrics.
Even so, you might ask, “Isn’t it important to keep
erodible fines in the subgrade from moving up into
the aggregate fill through open apertures?” In a
word, “Yes”. The phenomenon is called piping, and
a simple calculation called “piping ratio” quantifies
this potential under worst-case saturated conditions.*
Snowshoe Effect — Tensar BX Geogrids distribute heavy loads
over soft soils just like a snowshoe supports the weight of a
man over soft snow.
The figures (shown below) describe the mechanics,
and the analysis (shown graphically on the following
pages) illustrate the calculation. Basically, if
the piping ratio is less than 5, the pore spaces
(passageways) of the aggregate fill will be
sufficiently small enough to hold the fine soil
particles of the subgrade in place.* The two cases
highlighted in this brochure illustrate the application
of this principle.
*Note: For silty subgrades (P.I. < 7), “average size ratio”
(D50f /D50s) must also be checked. This value should be
less than 25.
Preventing Piping with Graded (Natural) Filters
Particle Size Ratio
Fundamental to Piping Concept
D15 (filter)
A
B
Spherical particle
“B” will just pass
through pore
space between
three spheres
“A” 6 1/2 times
the diameter
of “B”.
LEGEND
– in-place soil
–
Nominal Boundary
Before Movement
D85 (subgrade)
This size ratio is approximately that of a pea to billard balls.
Subgrade
D85 soil
entrapped
in filter
– soil migrated
into filter, held
by D85 size soil
particles
Water Movement
Reference: Cedergen, Harry R., “Seepage, Drainage, and Flow Nets,” Third Edition. John Wiley & Sons, New York, 1989.
pgs. 153 - 156.
Levee Access Road — Westminster, Louisiana
Tensar BX Geogrid in Action
Filter Analysis — Levee Access Road
A test pit excavated in 2001, again showed no
evidence of subsoil contamination. After more
than 13 years of service, the Tensar BX Geogrid
was still providing effective separation at the soil
interface. In fact, representatives of the West
Jefferson Levee District noted that the site was
continuing to outperform all other comparable
levee structures within the district.
The initial lift of sand received repeated
trafficking by fully loaded dump trucks.
3”
Gravel
#4
Sand
#200
85%
85%
Aggregate Fill (Sand)
Piping Ratio
= D15(F)/D85(S)
= 0.09/0.02
= 4.5
< 5 okay*
15%
15%
D15 = 0.09mm
100
10
1
0.1
Silt & Clay
100
90
Subgrade
(Bayou Mud) 80
PI=83
70
60
50
40
30
D85 = 0.02mm 20
10
0
0.01
0.001
% Finer by Weight
How well does geogrid perform? The levee
foundation and access road for this site in
Louisiana were completed in 1988. Both structures
were built with BX Geogrid. Because of the extremely
soft soil conditions, an initial lift of fine sand was
installed over the geogrid and Bayou mud subgrade.
By applying piping principals, Tensar Earth
Technologies engineers ensured that the distributed
pore spaces within the sand fill would be small
enough to hold the subsoil particles in place (see
“Filter Analysis – Levee Access Road”).
Grain Size (mm)
Bayou mud subgrade at this site is a highly plastic, saturated clay (Plasticity
Index (PI) = 83). Virtually 100% of the soil particles are finer than the
No. 200 sieve (0.075 mm). That’s finer than a person can discern by the
naked eye. Aggregate fill is unprocessed dredge sand from the Mississippi
Delta; virtually 0% of its particles are finer than the No. 200 sieve. Yet the
fine sand is an effective filter for the clay, as evidenced by the piping ratio
calculation and 14 years of successful performance.
* Subgrade PI > 7; therefore, there is no need to check Average Size Ratio (D50f/D50s).
After completion of the sand construction,
for both the access road and the mainline
levee, excavations were made to install
monitoring devices.
The excavation(s) at Westminster Levee
revealed no contamination of black bayou mud
within the light brown sand, all the way down to
the Tensar BX Geogrid.
Filter Analysis — Industrial Plant Site
Manufacturing Plant — Morrow, Georgia
Gravel
#4
Sand
Silt & Clay
100
90
80
70
Piping Ratio
60
= D15(F)/D85(S)
50
= 0.22/0.62
= 0.4
40
< 5 okay*
30
20
D15 = 0.22mm
10
0
0.1
0.01
0.001
Subgrade
(Georgia Red Clay)
PI = ~ 20 - 25
85%
85%
Aggregate Fill
(Graded Aggregate Base)
15%
15%
100
Tensar BX Geogrid in Action
#200
D85 = 0.62mm
10
1
% Finer by Weight
3”
Grain Size (mm)
BX Geogrid performs equally well in more
routine conditions. The storage yard for this
manufacturing plant rests on “Georgia Red
Clay” topped by Tensar BX Geogrid and
standard, well graded DOT-type aggregate base.
Despite its color, the clay soil at this site is quite
similar in terms of particle size (see analysis above)
to soils in many other geographic locations. The
pad supports a high volume of heavy equipment
traffic, which in turn generates significant
subsurface pressures. Examination of several test
pits excavated at the site revealed that the aggregate
fill was continuing to function as a “natural”
filter, and that the geogrid reinforcement was
maintaining a clear separation between the
aggregate and subsoil.
The unpaved Graded
Aggregate Base (GAB)
surface receives
repeated forklift
traffic during dry as
well as wet weather.
“Georgia Red Clay”, like most naturally occurring subgrades, is not 100% clay
sized. In many instances, like this one, there are as many or more sand-sized
particles as clay-sized particles. As such, a DOT-grade base is a very effective
filter, as evidenced by the piping ratio calculation.
* Subgrade PI > 7; therefore, there is no need to check Average Size Ratio (D50f/D50s).
A storage yard for the
plant is created by
applying a single lift of
GAB atop Tensar BX
Geogrid laid over
“Georgia Red Clay”.
An excavation through
the BX Geogrid interface
shows that underlying
red clay particles do
not exist within the
overlying well-graded
aggregate base.
Tensar BX Geogrids are provided by:
To find out how we can help you on your next
project, call 800-TENSAR-1, send an e-mail to
[email protected], or visit www.tensarcorp.com
Tensar Earth Technologies, Inc.
®
5883 Glenridge Drive
Suite 200
Atlanta, Georgia 30328
800-TENSAR-1
www.tensarcorp.com
© 2004, Tensar Earth Technologies, Inc. TENSAR and SPECTRA are registered trademarks. Certain foreign trademark
rights also exist. The manufacture and/or use of Tensar® Geogrids in civil engineering applications is protected under
U.S. patent Nos. 4590029, 4743486, 5156495, and 5419659. Other U.S. and foreign patents may apply or are
pending. The information contained herein has been carefully compiled by Tensar Earth Technologies, Inc. and to
the best of its knowledge accurately represents Tensar product use in the applications which are illustrated. Final
determination of the suitability of any information or material for the use contemplated and its manner of use is the
sole responsibility of the user. Printed in U.S.A
BRO-SEP-3.04