Performance of a geogrid reinforced soil wall on PVD drained multilayer soft soils

A geogrid reinforced soil wall was built on Prefabricated Vertical Drain (PVD) improved multilayer soft soils at the end of a road embankment in Shanghai, China. One day after the last loading stage, an arc shaped settlement was observed on the embankment surface with little observable cracks on the side slopes, and sudden increase of pore water pressure was observed in the piezometers installed in the PVD improved zone. A three dimensional finite element analysis was carried out to compare the performance of the wall with PVDs functional and dysfunctional. The comparison supported the assumption that the PVDs may have failed during construction due to bending or kinking of PVDs caused by the large settlement and lateral movement in the ground. The analysis showed that the movement of the wall toe is very sensitive to its location in the cross direction. Therefore attention should be paid while using toe movement as an indication of wall stability. It shows that the failure had mainly taken place in between the shoulders of the embankment rather than the whole wall. For a full slide to develop, pull out or tensile failure may occur in the reinforcements on the side slopes. Large stresses can be generated in the geogrids in the cross embankment direction due to the uneven settlement observed in the ground. Considering this, biaxial geogrid should be considered if the stress is too high and the inclusion of wrap around geogrids on the side slopes may help to increase the integrity and stability of the wall.