Ex-Post Evaluation of Countermeasures Against Residual Settlement of an Ultra-Soft Peaty Ground Due to Test Embankment Loading: A Case Study in Maizuru-Wakasa Expressway in Japan

Abstract

Before the construction of the Maizuru-Wakasa Expressway in Japan, a test embankment was constructed in 2006 on ultra-soft ground containing an approximately 50 m thick, soft peat layer with N-values of 0 to 1, and large-scale settlement over 11 m was observed within 4 years after loading. Numerical analyses were conducted by the authors to predict the future settlement and evaluate the effectiveness of countermeasures. The soil-water coupled finite deformation analysis code GEOASIA was used, employing an elastoplastic constitutive model, the SYS Cam-clay model, to describe the mechanical behavior of the soil skeleton structure. The simulation results were applied to the actual design of repair work performed on the test embankment. Furthermore, in the subsequent embankment construction, the countermeasures were implemented taking future maintenance into consideration. This study conducts ex-post evaluations of the countermeasures against residual settlement based on the valuable monitoring data for 6 years from the start of the expressway operation. It also shares how the design principles for the residual settlement of soft ground have changed throughout Japan’s highway construction history, including in this case.