Geogrids for soil improvement at the Vicenza South BeltWay, Italy

Problem

The new south ring road around the city of Vicenza, Italy, allows drivers to easily commute, and it solves a traffic congestion problem. Construction of the road was planned over highly saturated soft clay with low bearing capacity, meaning that settlement could take place during or after construction, with serious consequences for the life span of the road. The base layer, made up from granular material, can sink into the foundation soil, and horizontal and vertical movement capable of forming ruts can occur at the base.

Solution

To avoid the differential settlements, it is necessary to place one or more layers of reinforcing geogrids, having the ability to confine the aggregate and distribute the load.
Any increase in load may result in an increase of pore water pressure, and in impermeable soils. This water dissipates very slowly, causing instability of the soil and potential slip plane failures.

A vertical drainage system using PVD drains allows for a faster removal of excess pore water, faster consolidation rate, thus decreasing the risk of slip plane failure. The design for the Vicenza road, carried out by highway consultants with project designer SEPI of Italy, required the construction of a geogrid reinforced raft comprising two layers of an integral extruded geogrid and two nonwoven geotextiles (Fig. 1).

The contractor, ATI Astaldi-Mantovani, working on behalf of the client Autostrada BS-VR-VI-PD, proposed the use of TENAX LBO 201 SAMP bioriented geogrids, which met the requirements of the design specification.

A long causeway (the height ranged from 1.5 m to 7 m) was required to connect several bridge abutment structures and pillars. The design allowed for settlement to take place during the construction phase to afford a stable roadway. In all some 800,000m² of TENAX geogrids were installed.

TENAX LBO SAMP geogrids are designed for the stabilisation and reinforcement of soil, and are manufactured from polypropylene (PP). They are produced by a patented extrusion method and successively biaxially drawn to increase their tensile characteristics, and have an elevated tensile modulus and optimal construction damage resistance during installation. Interlocking of the aggregate within the apertures allows for the effective confinement and reinforcement of the soil (Fig. 2).

Conclusions

The design team and the contractor said that including TENAX LBO 201 SAMP geogrids allowed the engineers to:

• increase the bearing capacity of the subgrade;
• reduce the differential settlements at the base of the embankment;
• decrease the required thickness of the imported fill material without reduction in the long term;
• reduce costs due to savings incurred in the reduction of the special fill material;
• reduce construction time due to "the ease of installation of TENAX LBO SAMP geogrids."