Embankments on Soft Soil Design Considering Time Effects on Geosynthetics and on Soil Properties
The design of geosynthetic reinforcements for high embankments on soft subground is traditionally per-formed by the analysis of stability conditions of the structure through methods that consider limit equilibrium principles. The analysis through such methods requires some input parameters; two of them are the object of the paper: the soft soil undrained shear strength and the design (or available) geosynthetic tensile strength. The foundation soft soil, in most cases, is a type of fine and saturated soil. Hence, its shear strength is normally estimated from site tests, and is treated as undrained strength. The geosynthetic design strength, the available strength during the reinforcement service period, is mainly affected by the creep behaviour of the reinforcement material. The work aims to establish analysis criteria to con-sider the variation of these parameters through the time. So, aims to propose a less conservative procedure for designing geosynthetic reinforcements for embankments on soft subground, through the identification of the real critical moment of the structure in terms of stability, along its service life. In general, this moment is taken in project as it was the instant of the end of execution. The proposed procedure considers simple and usual concepts of soil mechanics and geosynthetics science, for an easy and quick analysis using limit equilibrium concepts and tools, allowing a more economic (but not less safe) project.
The parametric analyses performed show an important re-duction in nominal tensile strength required, if the time ef-fects in the soil and geosynthetics are taken into account, even if these analyses adopt simple design considerations.
Table 2 shows that refining the analyses do not means a remarkable change in calculated values but it could be better to work subdividing zone 2.
Table 3 shows that to thin foundation layer, the reduc-tion of the required tensile strength of the geosynthetic is significant: 36% and 54%, to polyester and polypropylene, respectively.
Table 4 shows that even a conservative relationship be-tween Su / σv ´ means a reduction of 21%, for the polyes-ter, and 36%, for the polypropylene, the required tensile strength of the geosynthetic.