Ground Stabilisation, Myth or Fact (Tensar Academy: Livecast)

Soil stabilisation is the process of improving the physical properties of soil to increase its strength and durability. Two main methods used to achieve soil stabilisation are chemical and mechanical stabilisation. Chemical stabilisation involves adding chemical additives like cement and fly ash into the soil, while mechanical stabilisation could be achieved by using stabilisation geogrid to create a confinement zone where interlocking mechanisms take place.

Stabilisation geogrids come in two forms i.e., biaxial, and multiaxial geogrids. Biaxial geogrids have similar strength in both the longitudinal and transverse directions and are commonly used on roads or platform applications to form soil stabilisation. Multiaxial geogrids have the lower strength compared to the biaxial geogrids, but they can achieve better soil stabilisation performance due to their radial confinement characteristics and multi-directional load spread mechanism.

Non-woven geotextiles are commonly used in soft ground construction as separators to prevent aggregate loss into the subgrade and the migration of fine particles into the aggregate pore space. They have no contribution to enhancing soil properties. Stabilisation geogrids, on the other hand, interlock with the soil particles and form a composite layer that improves the mechanical behaviour of unbound granular materials.

The main applications of mechanical soil stabilisation include increasing bearing capacity for platforms, reducing layer thickness, improving trafficking performance for pavement, and controlling differential settlement. Using mechanically stabilised layer also plays a significant role in reducing carbon emissions through material reduction during the construction process.

Mechanically stabilised layer (MSL) can distribute more loads onto weaker subgrade. This is due to the low-stress level in the granular soil at bearing capacity failure, which eases the load distribution process. On the other hand, when MSL is placed on a strong subgrade, the load distribution can be reduced due to high-stress levels in the granular layer. In short, stabilisation geogrid performs better on softer ground.

Geotechnical and pavement design methods differ for different applications of mechanical soil stabilisation. The load spread method was adopted for platform design with an assumption of a 45-degree load spreadsheet. However, this method can be uneconomical and inconsistent in some cases. A new T-value method is recommended, which derives directly from the shear strengths of the 2-layers without the need for empirical-based charts. For unpaved and paved road structures, different design methods such as LR1132 and AASHTO 1993 are used. Using stabilisation geogrid can optimise the design by adding a Mechanical Improvement Factor (MIF) into the layer structure coefficient.

Tensar has developed a new stabilisation geogrid that is introduced with the new design methods, LAAMS method, developed by Dr Andrew Lees based on T-value. LAAMS is applied in unpaved road design whereas T-value is used in working platform application. Tensar also offers a cloud-based software platform called Tensar+ that allows engineers to design and validate various applications while demonstrating the cost, time, and carbon savings of mechanical soil stabilisation.