Test Simulation And Theoretical Analysis On Sediment Consolidation And Contaminants Transport Caused By Capping

The presence of contaminants in the sediment has caused great threaten to the surrounding environment and human health. Measures to correct this situation are urgently required. Sediment capping is a technique where clean sand or sediment is placed over contaminated sediment to reduce the migration of contaminants to the environment. Compared with the common dredging technology, this technology has many advantages and worthy of being researched and applied in China. However, capping usually neglected the importance of consolidation indecud advective transport of contaminants. The sediment usually has a high water content and compressibility, the placement of a capping layer can cause the consolidation of sediment and discharge of large amount of pore water, accelerate the transport of contaminants into the overlying water. Therefore, studying on the consolidation induced advective transport of contaminants, summary the influence of different thickness of capping layer on the migration of contaminants, propose a simplified analysis method for contaminant transport which consideration the consolidation of the sediment, has an important practical significance for the promotion of capping technology to be widely used in China.The field tests of capping is difficult due to the water depth condition and other field condition limits; even if the contaminants transportation is accelerated by the sediment consolidation, the contaminants usually need at least several months to transport through the capping layer into the overlying water. In this paper, ZJU-400and bench top centrifuge tests were utilized to study on the impact of west lake sediment consolidation induced by capping on chloride migration towards the overlying water and varied capping layer thickness on the migration of contaminant. The tests results showed that relationship between chloride concentration in the overlying water and time had a good correlation with that between sediment settlement induced by consolidation and time. The pore water squeezed by sediment consolidation accelerated the contaminant migration towards the overlying water remarkably. The design of contaminated sediment capping is unconservative if only contaminant diffusion is considered. Tests for caps with varied thicknesses showed that the capping layer retains the migration of contaminant, and thicker capping layer reduces the amount of contaminant moving into the overlying water. Slice and measure the water content of sediment before and after consolidation, the results showed that the water content decreased remarkably after finishing sediment consolidation caused by adding a capping layer, the pore water and contaminants were remarkably squeezed into the overlying water and cause secondary pollution. Thus, the capping layer must be appropriately designed to reduce the effects of consolidation-induced advective transport. This may be accomplished by adding a reactive barrier or geosynthetic barrier layer to the cap design.The centrifuge tests used the modeling of models technique to verify that correct modeling procedures were utilized. In this study, the ZJU-400centrifuge test and bench top centrifuge test were conducted on50g and107g, the two tests use same geotechnical properties materials and model same prototype condition and stess condition, after same prototype time, compared the test results and found that the sediment settlement and chloride concentration in the overlying water of the ZJU-400centrifuge tests had a good correlation with bench top centrifuge tests. The centrifuge tests can be used to model consolidation indecud advective transport of contaminants caused by adding a capping layer.A one-dimensonal transient model for sediment consolidation induced advective transport of contaminants caused by adding a capping layer was presented. Finite element software was used to solve the model, the results showed that the theoretical calculation values of the sediment settlement and chloride concentration in the overlying water had a good correlation with the experimental measuring values. The finite element software can provide a theoretical support for the promotion of capping technology widely used in China.