In Reinforced Soil Its Not Only About Tensile Strength

Geosynthetic reinforcement is used to provide tensile strength to a system that otherwise must rely on compressive shear strength. The reinforcement must sufficiently interact with the surrounding materials in order to impart its stabilizing tensile strength to the system.  Pullout resistance, interface friction, and connection strength characteristics of each unique material combination (i.e. soil-geosynthetic, block-geosynthetic, etc.) must be measured to assure that the chosen geosynthetic doesn’t cause a “slip-up” on the job.

Interacting with Surrounding Materials

Geosynthetic reinforcements (geotextiles and geogrids) must interact with soil and other materials in fills to prevent slippage. This interaction can be in the form of surface friction or a combination of surface friction and bearing resistance.  Slippage can cause failure of a reinforced soil system (walls, slopes or embankments), if it results in a plane of reduced strength. Interaction may be resistance to pullout, interface friction, or the connection of a facing material.

Pullout Resistance

A “block” of reinforced soil is anchored to a stable soil mass beyond the most likely plane of failure by extending the geosynthetic reinforcement into the stable soil mass.  The reinforcement is extended as far as is required to provide sufficient pullout resistance to prevent movement at the likely failure plane.  Pullout resistance is of most concern for the uppermost reinforcement layers. Pullout resistance can be derived from interface friction tests or measured directly from tests that pull the geosynthetic out of representative soil.

Interface Friction

One form of failure within a reinforced soil zone is horizontal sliding of a block of reinforced soil.  Soil-to-soil friction is almost always higher than soil-to-geosynthetic interaction and the critical failure conditions that must be investigated will commonly involve soil-geosynthetic interface friction, also known as direct shear. Direct shear relates to the type of test that is employed to accurately determine the interaction for a given soil and geosynthetic combination.

Connection Strength

Reinforced soils are a cost-effective technique for creating steep to near-vertical structures.  As the structure becomes steeper, its surface becomes more problematic since it becomes more visually important and more difficult to maintain.  Facing systems have emerged to provide good aesthetics with a minimum amount of maintenance.  Masonry blocks, concrete panels, gabions, and other materials for the external face of the reinforced zone have been developed.  In these structures, the geosynthetic extends from the reinforced soil zone to these facing systems, connecting to them and holding them close to the soil surface.  Groundwater seeping through the reinforced soil zone or localized block sliding may exert pressure on the back of the facing units, stressing the connections.  Each facing material has a unique interaction, or connection strength, with each type of geosynthetic.  This connection strength has two parts.  First, connection pullout is determined by measuring the force required to pull the geosynthetic out from between representative facing units. Additionally, connection shear measures the force required to cause one facing unit to slide over the top of another both with and without a geosynthetic “connected”.

Interface Friction / Direct Shear. 

Soil/geosynthetic direct shear resistance is determined by laboratory testing in accordance with ASTM D5321.  TRI has five direct shear apparatus and performs hundreds of soil/geosynthetic and geosynthetic interface tests annually.

Pullout Resistance

The pullout resistance and associated interaction coefficient (Ci) are used in stability analyses to compute the mobilized tensile force at the front and tail of each reinforcement layer in reinforced soil systems.  TRI is installing a state-of-the-art pullout box to fully characterize pullout performance.  Pullout testing is expected to be fully operational by the end of the 1^st Quarter of 2000.  Additionally, as pullout resistance can also be calculated from direct shear data, we will continue to estimate direct shear-based characterization of pullout.

Connection Strength

Coincident with the new pullout apparatus, TRI is installing the most advanced connection strength testing apparatus available.  Computerized controls will assure flexible load application, uniform normal stress distribution, and highly accurate displacement measurements.  Connection testing is also expected to be fully operational by the end of the 1^st Quarter of 2000.