Multi layer geogrids for clay reinforcement, case history, Tenax geosynthetics

problem
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. Construction of 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 stabilization 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 stabilization and support careful placement of soil liner. Areas requiring stabilization were the entire lagoon bottom and the lower portion of the side slopes.

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 stabilization 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 compaction.
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.

conclusions
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.