TENAX MS multi-layer geogrids
Cargill Corn Milling, Inc.
The project consists of constructing a single lagoon cell as a liquid
retention pond at Cargill Corn Milling Inc. in Blair, Nebraska.
The lagoon site is located in the alluvial plain 200 meters west
of the Missouri river.
The lagoon has plan dimensions of 180 m by
107 m. The top of the lagoon liner was constructed at an elevation
of 2.4 m below existing grade, with the dikes being built up 0.6
m above existing grade. The lagoon dikes have a maximum 3 to 1 slope.
The lagoon clay liner was at least 0.6 m thick. The liner extended
over the entire lagoon bottom and up to the lagoon sides to at least
0.3 m above the maximum water surface elevation.
the bottom 0.6 m thick lagoon liner resulted in excavations extending
within up to 3 m of groundwater level in relatively wet clays and
fine sands. These soils were anticipated to be inadequate for support
of repetitive loads from heavy-wheeled or tracked excavation and
liner construction equipment.
An effective measure of subgrade stabilisation
was necessary to support proper placement and compaction of the
clay liner materials. The liner soils were required to be compacted
to at least 93% of the material's maximum standard Proctor dry density.
The engineer recommended a geogrid solution to provide adequate
stabilisation and support careful placement of soil liner. Areas
requiring stabilisation were the entire lagoon bottom and the lower
portion of the side slopes.
Two possible geogrid solutions were considered:
- Excavate an additional 0.3 m below the liner base elevations,
import a 0.3 m layer of crushed limestone in conjunction with
a single-layer biaxial geogrid. The crushed limestone was to
be composed of 38 mm minus well graded material containing less
than 8 percent fines.
- A multi-layer biaxial geogrid (TENAX MS 500) in conjunction
with the on-site clay material. The multi-layer geogrid solution
was based on laboratory tests, and project experience with similar
conditions, and more importantly a trial section outlined in
the next section. Eliminating 0.3 m of crushed stone resulted
in a significant cost savings in both material and excavation.
The multi-layer biaxially oriented polypropylene geogrid was
manufactured by a continuous extrusion and orientation processing
with integral junctions.
Field tests were conducted to evaluate the proposed
solution for weak subgrade stabilisation with a multi-layer geogrid.
The testing section was at a depth of 2.4 m below the ground surface.
The geogrid was placed in an area approximately 4.6 m wide and 30
m long. This area was determined to be one of the most unstable
locations at the job site in reference to soil testing. The first
1.8 m of soil was cut by scrapers and the final 0.6 m of excavation
was done using a smooth-edged backhoe bucket. When the test depth
was reached, the soils were very soft, wet and sensitive to disturbance.
Foot traffic on the weak subgrade caused pumping of soil moisture
and left deep footprints. The above photo shows a compactor in the
area that sank to the drum while attempting to compact a 0.3 m lift
of clay liner material placed on the weak subgrade. A 0.3 m layer
of non-processed on-site clay fill material was spread directly
on top of the geogrid. During the installation of the geogrid, a
Caterpillar CP 563 compactor was used to compact the fill soil.
This unit has a single drum in front, driven by rubber tires on
back. The compactor made 6 passes across the test area. A pass consists
of a compactor travel over the area in one direction, the return
to the original location was considered the second pass.
The test revealed at this point we had an 89% compaction with a
water content of 29.3%. After running 4 more passes over the same
area, for a total of ten. The test revealed a 97% compaction with
27% water content. To verify this, a second test was done in an
adjacent location, still in the test area, this test revealed a
98% compaction with 27% water content. Both tests exceeded 93% required
To provide a control for the test, an adjacent unreinforced section
was also tested and subjected to the same test conditions. After
6 passes over the test area, where 0.3 m of fill had been added,
the test showed a compaction of 89% with 27.9% water content, with
4 more passes to take the total to 10, a second test was run, this
test showed 86% compaction with water content of 39.6%. It was noted
that the fill surface in the test area had sunk about 0.15 m from
the original elevation, and the soil was pumping and experiencing
bearing failure under the weight of the compactor.
The use of a TENAX MS multi-layer geogrid in conjunction with a
fine graded soil allowed the construction of a stiff load distribution
platform for increasing the bearing capacity of the embankment foundation.
The "many strands" of the multi-layer geogrid are ideal
to reinforce low shear strength soil thanks to their high soil interlock
and "root system" effect. Trial tests have shown that
compaction, density and increased bearing capacity are easily reached
without overstressing the subgrade and without soil water pumping.