Interpretation of density profile of seabed sediment from nuclear density cone penetration test results

Abstract

The nuclear density cone penetration test (ND-CPT) is used to investigate the wet density (ρ) of soft soil deposits under the sea. In the ND-CPT, the density is determined by the count rate ratio (Rρ) of the gamma rays detected by the detector to the gamma rays from the source. Thus, the ND-CPT measures the average wet density of the material within a spheroid volume around the central point of the source and the detector, which is designated as ρc. The measured ρc values are considerably different from the actual ρ of the seabed sediment because the density varies significantly with depth. We designate the layer of seabed sediment in which the density increases rapidly with depth as the “upper layer” and the layer below it as the “lower layer”. A method is proposed to deduce the actual ρ profile from the measured ρc profile. The upper and the lower boundaries of the upper layer of the seabed sediment can be directly determined from the measured ρc profile as well as the distance (2b) between the gamma ray source and the detector. Based on the results of a back analysis of the measured ρc profile at Isahaya Bay, Ariake Sea, Japan, the density distribution in the upper layer of the seabed sediment can be approximated as a square root function of depth. Comparisons of the measured ρc profiles, obtained using the ND-CPT, and the actual ρ profiles, deduced by the proposed method, show that the actual ρ are obviously different from the measured ρc near the upper and the lower boundaries of the upper layer of the seabed sediment. The thickness of the upper layer of the seabed sediment at Isahaya Bay lies in the range of 0.05–0.55m.