Soft Holocene deposits in coastal lowlands are prone to consolidation-induced subsidence, posing long-term challenges for infrastructure development, flood resilience and sustainable land management. The eastern Izumo Plain, Japan, is underlain by thick unconsolidated Holocene sediments whose geotechnical behavior remains insufficiently documented despite increasing concerns regarding ground deformation in low-lying coastal environments.
This study investigates the geotechnical characteristics and consolidation behavior of Holocene subsurface sediments using borehole-core observations and geotechnical laboratory testing. A 20.7-m-deep borehole core (SJ-1) obtained from the eastern Izumo Plain was examined to characterize the stratigraphy and engineering properties of the Holocene deposits. The sedimentary succession comprises three subsurface units deposited under varying environmental conditions: an upper sand-silt unit, an intermediate silty-clay unit, and a lower clay unit. Depth-dependent index properties, including water content, Atterberg limits, density, and grain-size distribution, and consolidation parameters such as compression index, coefficient of consolidation, and preconsolidation pressure, were evaluated to examine the influence of lithological variability and depositional facies on compressibility and settlement behavior.
Consolidation test results indicate that the lower clay-rich unit exhibits high compressibility, with compression index values generally exceeding 1.0. One-dimensional consolidation theory was applied to estimate primary consolidation settlement under assumed loading conditions of 50, 100, and 150 kPa. Settlement analyses suggest that deformation is concentrated within the lower clay unit, with predicted settlements exceeding 1 m under higher loading conditions. The results further indicate that consolidation in the clay unit may continue over several decades, suggesting prolonged ground deformation and associated impacts on infrastructure.
These findings highlight the importance of geological and geotechnical information when assessing subsidence potential in coastal plains. The study provides a basis for evaluating ground deformation hazards and supports land-use management and infrastructure planning in low-lying coastal environments.
 

Soft Holocene clay,Geotechnical characterization,Consolidation settlement,Land subsidence,Compressibility,Izumo Plain