Slope failure in underconsolidated soft soils during the development of a port in Tianjin, China. Part 2: Analytical study

Abstract

This paper presents the analytical study of a major landslide that occurred in a newly dredged slope during port development in the city of Tianjin, northern China. The slope comprised mainly underconsolidated and submerged soft soils. The landslide consisted of a number of individual slides that occurred sequentially and extended retrogressively and laterally into the reclaimed land. The paper proposes a slope stability assessment methodology for the examination of such slope failures, with retrogressive and lateral extension into reclaimed land. Both the total and the effective stress approaches are adopted in the slope stability assessment. Eight different sets of soil shear strength parameters are used in the assessment. They were determined by using different methods in the laboratory and in the field or by back analysis. The assessment results indicate that the total stress approach, together with undrained vane strength values, gives the best result in the stability assessment of the original dredged slope and the subsequent failed slopes. The paper further examines the factors that might have caused or triggered the occurrence of the landslide. Backfilling of a thick general soil layer and a sand cushion for vacuum preloading of the hydraulic fill in the reclaimed land might have rendered the entire slope marginally stable. Lowering of the tidal level might also have had an effect in triggering the landslide. The engineering approach and results presented in this study could be useful in the design and construction of dredged slopes in underconsolidated soft soils.Key words: land reclamation, dredge excavation, slope stability assessment, landslide investigation, marine mud, case studies.