Study on a contaminated soft soil improved by deep mixing with cement

Abstract

The deep cement mixing (DCM) technique is an in-situ ground improvement method to stabilize and solidify soft grounds. This technique can rapidly intensify the strength and decrease the high water content of marine deposits by binding marine deposits with cement and/or other binder materials, and has been widely adopted in many countries such as Japan, Singapore, and Thailand. The DCM technique has been utilized in the reclamation of Hong Kong, e.g. the Third-runway System Project of the Hong Kong International Airport (HKIA) (Wu et al., 2020). Recent major reclaimed artificial islands in Hong Kong were constructed on Hong Kong marine deposits (HKMD) because of the adoption of the non-dredging method. However, the relationship between the strength and stiffness of cement stabilized HKMD is neither fully understood nor well covered by current practical design guidelines. On the other hand, the dredging of contaminated deposits has been avoided during reclamation as the relocation and disturbance of marine deposits threaten public health and marine ecology. For the Third-runway System Project, approximately 40% of reclamation is located on closed Contaminated Mud Pits (CMP) which are the facilities receiving and collecting contaminated sediments from previous Hong Kong construction projects such as maintenance dredging of the seabed or previous reclamation projects. DCM arises as a viable solution for strengthening soft grounds in non-dredging reclamation projects. Nevertheless, the environmental influence of composited stabilized grounds on surrounding areas remains a concern. The first objective of this study is to investigate and better understand the stress-strain behaviour of the cement stabilized HKMD through laboratory element tests with the influence of different contents of natural seawater and cement by a series of unconfined/confined compression tests. All of the findings are of practical significance for the local ground improvement industry as well as for other coastal cities around the world. According to the experimental results, an attempt was made to predict the unconfined compressive strength (UCS), qu, by use of a simple empirical equation based on water/cement ratio (w/c). The correlation between the strength and secant modulus of improved HKMD was obtained. Importantly, a linear relationship between small-strain (ε<0.1%) stiffness and qu was formulated based on the measurement results from high-accuracy local linear variable differential transformers (LVDTs) and strain gauges. Besides, the effect of w/c on the failure modes of the specimens was revealed. Furthermore, the consolidated undrained (CU) triaxial tests indicated that specimens gained higher peak strength with the increase of confining pressure. The second objective of this study aims to investigate the long-term coupled consolidation and contaminant leaching behaviour of cement-stabilized contaminated soil. Three axisymmetric physical model tests were conducted to investigate the coupled behaviour of the composite ground, which consisted of a central column made of cement-stabilized arsenic (As)-contaminated marine deposits and surrounding untreated marine deposits, during curing, loading, and post-failure stages. The physical behaviours, including water content, settlement, pore water pressure, and total soil pressure were measured under different stages. Simultaneously, the migration of arsenic into seawater, and leaching into the surrounding HKMD was evaluated. The test results revealed the settlement development of the composite ground and the mechanism of load transfer between the DCM column and surrounding soil with the increase of loading. The presence of arsenic decreased the strength and stiffness of the DCM column through the reaction between arsenic and hydration and pozzolanic reaction products. Moreover, Ca-As precipitates reduced the content of inter-aggregate pores and hence the permeability of the DCM column. With the reduction of the cement content in the DCM column, the concentration level of arsenic in the draining-out water of the composite ground under multistage loading increased significantly, while that in the surrounding soil showed no obvious change, indicating that arsenic mainly migrated directly from the DCM column to the free water on the top surface through a preferential flow. The contaminated HKMD could be effectively stabilized and solidified by in-situ remediation using sufficient cement dosage to minimalize secondary pollution to the surrounding environment. Research work presented in this thesis has made contributions to (i) the development and correlation of a simple empirical equation between w/c and UCS on cement stabilized HKMD. The UCS could be predicted based on the designed w/c to facilitate the practical design of DCM; (ii) the development and correlation of the relationship between the secant modulus and strength of cement stabilized HKMD; (iii) understanding the load transfer mechanisms among the DCM column and surrounding soils during loading and post-failure stages; and (iv) the evaluation of the secondary pollution to the surrounding environment of the highly-contaminated soft clay treated by sufficient cement dosage.