A six years survey with vane test and cone penetrometer to test cohesion value of a soft clay soil during road embankment construction

摘要

The new road Douala-Yaounde crosses the Dibamba river after leaving Douala. This mud-flat zone is 2 km long ; it is filled with new geological soils ( lagoon-sea compressible clay). The design was established for a 295 m long bridge with connecting embankments. The achievement of the earthworks comprising dredging, silting methods and vertical drains were conducted phase by phase. Every phase has been controled by tassometer and piezometer and by Cone penetration test (Gouda type) and vane test (Geonor type). During embankment phases, the value of the undrained cohesion (Cu) went down instead of increasing as expected : at 18 m deep Cu value decreased from 35 kN/m~2 to 9 kN/m~2. Quiet surprisingly this hazardous situation did not give way to a failure but design needed to be changed. Usual correlations between undrained cohesion Cu (peak value) and point resistance q_c from Dutch cone penetration test were not proved. The usual ratio (10 to 20 ) is obtained in a different way by comparing point resistance q_c and bearing cohesion value Cp.