The three dimensional innovation

Geocells have opened up new avenues of applications, from strengthening geo-systems to erosion protection, writes Shahrokh Bagli

Geotechnology was revolutionised with the appl¡cation of geosynthetics, commencing with the humble non-woven to complex geo-composites. These systems are two-dimensional. Cellular confinement systems, popularly known as geocells, add the third dimension to geosynthetics, which open up new avenues of applications, from strengthening geo-systems to erosion protection.

Geocells are strong, light-weight, three-dimensional systems, fabricated from ultrasonic-welded HDPE strips that are expandable to form honeycomb-like structures (Fig 1). Geocells are filled with compact non-cohesive soils confined within the cellular walls forming a rigid structure.

Generally, the infill is sandy or gravelly material, but can also be concrete depending on the application.

The geocell surface is textured to increase soil-geocell wall friction and punctured for rapid pore-pressure dissipation.

Geocells are used to advantage considering that they are:

1. Prefabricated three-dimensional geosynthetics with significant third dimension properties;
2. Economy in designing as it allows thinner section;
3. Easily transported as flat strips welded width-wise at regular intervals;
4. Easy to install in any weather condition and do not require skilled labour;
5. Savings in terms of labour as skilled labour is not a necessity;
6. Cost-effective solutions for geotechnical issues with economic use of natural resources.
7. Cost-effective, substantially reducing maintenance cost by improving longevity of pavements;
8. Green, fostering carbon sequestration since carbon black is an essential ingredient of the HDPE.
Geocells were first developed and used by the US Army during its Vietnam and Gulf campaigns. Since then, geocells have been used as load-support systems, slope protection, channel lining, and earth retention in the 1980s. Today, the applications broadly include:

1. Load support systems:

a) Increase in load carrying capacity of shallow foundations and grade slabs
b) Reinforcement for embankments on weak ground;
c) Reduction in pavement sections for roads, lay-down areas, etc

2. Gravity retaining walls

3. Erosion control:

a) Embankment/natural slopes;
b) Water channel/pondage linings.

Maintenance of roads and highways is a major issue. When not correctly designed and constructed, life of the roads reduces drastically. Such roads develop pot-holes and uneven riding surfaces. Geocells filled with sand/metal as sub-grade improve strength of the pavement, reduce settlements, and prevent reflective cracks and pot-holes. Geocells reduce the thickness of the pavement sections and significantly reduce maintenance downtime.

Fig 2 sequentially illustrates a case study of a road rehabilitated using geocells. The surface is dressed and compacted. Adjoining sections of geocells are connected using pneumatic staplers. The geocells are spread open over the dressed surface, anchored into position with stakes and in-filled with a front-loader, topping over by 50 mm. The in-fill is compacted by roller compactor. The filled geocells are topped over by surface courses. Geocells filled with cohesionless material form rigid mats capable of distributing imposed loads. The mechanics of geocells as a load carrying system is illustrated in Fig 3.

Confined cohesionless soil within geocells subjected to vertical pressure q0 causes lateral stresses within the confined soil, equivalent to K0 q0 where K0 is the coefficient of earth pressure "at rest". Any lateral deformation of the geocell wall is restricted by adjacent cells, also filled with cohesionless soil, acted upon by similar vertical pressures generating similar lateral stresses. This increases the shear strength of the confined soil, thus creating a stiff mattress, which distributes load over a wider area. The horizontal stress normal to the cell wall increases vertical frictional resistance between infill and geocell wall, diminishing stresses of the applied load on the ground below the geocell.

This phenomenon is used to advantage to transfer heavy vertical loads onto weak soils by spreading load over large areas such as below spread and strip footings, rafts and grade slabs on weak soils.

Retaining walls and toe walls constitute another major application of geocells. Filled with granular infilling, geocells make ideal gravity walls (Fig 4). Multiple layers of geocells filled with granular material are stacked atop each other. The principle of design is similar to conventional gravity walls.

Perforations ensure that hydrostatic pressures behind the wall are dissipated. Consecutive geocell layers are laid to batter to provide a steep angle and increase the usable area on the top of the structure. Suitable vegetation can be cultivated over the horizontal exposed surfaces of the geocells to provide an aesthetically pleasing finish.

Geocell walls have been erected as sidewalls for storm water side-drains along highways in India. Fig 5 shows a trial stretch along the highway. Geocells with soil/concrete infilling provide effective erosion protection. Filled with concrete, geocells can be shaped to form waterways to route storm-water along slopes to prevent formation of gullies which weaken the earth structure (Fig 6). Geocells foster vegetation along slopes, by confining the soil, which further aids in erosion protection and provides a green aesthetically appealing fascia.

Geocells have several applications as structural and protective geosystems, which include: urban/rural roads, highways/expressways, service roads, road shoulders, haul roads for mines, construction sites and oil fields, rail-track stabilisation, foundations on weak soil, pavements in container yards, landfill lining and access roads, tank farms, channel embankments/levees, slope erosion control, and retaining walls.

The author is Chief Technical Officer, Strata Geosystems (India).