Reinforced soil embankments and toe protection along the Tevere river

Edilgrid® and Filter Unit® for the Tevere river embankments

Edilgrid® and Filter Unit®: geogrids and flexible gabions for reinforcing and controlling erosion of river embankments.

Problem

River embankments are often characterized by high variability in stratigraphy and the mechanical properties of the soil. They also feature partial saturation conditions and boundary hydraulic conditions that vary greatly in space and time, especially during flood events.

In particular, partial soil saturation reduces soil permeability as suction increases. Suction is defined as the difference between air pressure (ua) and pore water pressure (uw). As a result, within an embankment that is fully or almost fully submerged during a flood, high hydraulic gradient and neutral pressure values can develop locally. This leads to a sharp deterioration of the embankment’s stability over time.

This phenomenon occurred along a stretch of the Tevere river embankment, about 10 m high with a slope inclination of about 30°. The embankment consists of silty sands, or sandy silts, arranged in layers of varying thickness between 2 and 5 m.

During the flood of November 2012, the Tevere’s water level rose over about 60 hours. The level went from an ordinary value of 8.52 m above sea level to a maximum of 16.20 m above sea level. This level remained constant for about 24 hours, before returning to the ordinary value in just 12 hours. This sequence proved critical for the embankment, which experienced an instability event. The sliding surface was located at a depth of about 4 m, over a length of 15 m.

Solution

Following this failure, engineers reshaped the embankment’s most superficial section. They used granular materials and six layers of Edilgrid® 35/20, woven polyester geogrids with PVC coating, spaced 0.8 m apart vertically.

At the base of the embankment, above a layer of lean concrete, engineers laid two rows of 4-ton Filter Unit® elements. These elements provide toe erosion control, soil reinforcement and stabilization, as well as filtration.

After completion of the works, a flood occurred with a maximum level similar to that recorded in November 2012. On this occasion, no failure occurred, nor were any significant embankment deformations or surface erosion phenomena recorded.