Grain size distribution and engineering properties of subsurface layers in Niigata City area based on boring database

Abstract

Based on the boring database, soil distribution, N-value and various engineering properties were revealed in alluvial subsurface layers of Niigata City. In northern area of the city, soft fine-grained soils and organic soils are contained in the shallow layer between dune deposits. In the depth of about 10-40 m, coarse-grained soil layers with average N-value over 30 are dominantly. These layers are composed of fine or medium sands with average fines content Fc under 10% in shallow, while Fc tends to increase at the depth of more than 20m. In the southern back mash area, subsurface layers contain fine-grained soils and coarse-grained soils in same degree from shallow layers to deep deposits and average N-values are smaller than in the northern area. As Fc is higher in coarse-grained soil, and as clay content is higher in fine-grained soil, the layer indicates smaller wet density, larger void ratio and natural water content. Fine-grained soils have about 70% average liquid limit and most of them are distributed near the A line of the plasticity chart. Unconfined compression strength and consolidation yield stress increase with depth, and they tend to be overconsolidation in shallow layers, while they are close to normal consolidation in deeper 30m.