Influence of the Loading Duration on Undrained Shear Resistance of Soft Clays Observed during the Construction of a Roadway-Paper

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The paper examines how loading duration and strain rate affect the undrained shear resistance of soft clay, using the Via UHOS roadway in Rio de Janeiro as a case study. The road was built for vehicles transporting exceptionally heavy equipment. Prior research indicates that soft clays gain viscous resistance as strain rate increases, so brief loads may be resisted more effectively than equivalent permanent loads.<br /><br />At section E-694, the original design assumed a four-meter soft-clay layer with undrained shear strength of 12 kPa. After two special vehicles crossed safely under an equivalent load of about 30 kPa, cracks and lateral movement appeared following placement of a gravel layer that added only about 8 kPa. Further investigation found soft clay extending beyond 12 meters. Vane, CPTU, and triaxial tests showed that strength was higher under faster loading.<br /><br />Stability analyses supported the field observations: the original embankment was unstable under the 30 kPa vehicle load when permanent-load strength values were used (factor of safety 0.82), but stable when short-duration strength values were applied (1.03). The embankment was redesigned with reduced height and greater width. Under the maximum vehicle load of 70 kPa, the revised design had factors of safety of 1.06 for permanent loading and 1.41 for short-term loading. The vehicles then completed the route without incident.<br /><br />The authors propose a design method using undisturbed samples and four consolidated-undrained triaxial tests at progressively higher strain rates. Designers would establish separate strength profiles for permanent and short-term loading, analyze both loading scenarios, and use the lower safety factor. Accounting for load duration may enable more economical embankments for roads carrying exceptional loads than conventional methods, which treat all loads as permanent.

Keywords

soft clay

undrained shear resistance

strain rate

loading duration

viscous resistance

stability analysis

embankment design

triaxial tests

special vehicle loads

Rio de Janeiro