Analysis of Staged Construction of Containment Levees for Sidoarjo Mudflow

Abstract

Mudflows from the ongoing eruption of a mud volcano near Sidoarjo (East Java, Indonesia) have continued for more than 15 years and now cover an area of 8.2 km2 to a depth of more than 15 m. The mudflows are contained within a system of perimeter earthen levees/dykes that were constructed as a series of temporary defenses. The levees have marginal stability and limited freeboard due to the low shear strength and high compressibility of the 25-m-deep soft clay foundations, which have already resulted in ground deformations exceeding 7 m. In this paper, we analyze the deformations and stability of the downstream staged construction at a critical reach in order to (1) understand causes of a recent failure (2018); and (2) assess the current state of the levee following completion of a fifth construction stage (2019) that included a 35-m-wide, 4-m-high stabilization berm with an array of prefabricated vertical drains (PVDs) to accelerate consolidation. Our analyses used large-deformation elastoplastic finite-element analyses to simulate consolidation and lateral spreading of the foundation soils due to levee construction and mudflow loads. Levee stability was evaluated using numerical limit analyses for the deformed geometry and simulating changes in undrained strength with the consolidation state and direction of shearing at each stage of construction. The results were consistent with the magnitudes of the observed levee settlements and mechanism of failure; they also show the potential for an additional >2 m settlement over the next 5 years, even without considering creep of the clay or larger-scale processes controlling subsidence around the volcanic caldera. Future construction stages to contain the expected mudflow will require improvement of the mechanical properties of the alluvial clay to ensure adequate long-term freeboard.