Use of vertical drains and lime-cement stabilization to improve stability and reduce settlement of a high embankment on very sensitive, soft soil

Norwegian Geotechnical Institute, Trondheim, Norway

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Abstract

This article presents a case-study of the design and construction of a 14 m high embankment for a highway construction project in mid-Norway. The ground consists of up to 30 m soft clay, with high sensitivity over bedrock. The embankment increased stresses in the clay beyond the preconsolidation stress, causing settlements of 1-1.5 meters. To satisfy requirements related to stability and settlements, lime-cement columns and prefabricated vertical drains (PVD) were installed. The aim in the design phase was to limit the extent of lime-cement stabilization and thereby the carbon footprint. The PVDs were designed both to maintain stability and settlement requirements. The increase in shear strength in the clay from consolidation during the fill works, was utilized to document stability both permanent and during the construction phase. During the construction phase, monitoring the pore pressure was crucial to control the filling rate to maintain the assumptions from the design. To verify the design strength, CPTU soundings were performed to investigate strength properties after consolidation. Settlement plates were installed and monitored during construction. By applying a probabilistic prediction fitted to the measured settlements (Asaoka’s method), the future settlements were predicted and shown to be within the given requirements. The measured increase in strength of the clay after consolidation fit well with the design values. The measured settlement during construction were a bit higher than expected. The practical methodology presented in this article can be applied to other embankments constructed on soft soils, especially where stresses under the embankment exceed the preconsolidation stress.