A Study of Heaving Material Resulted from Deep Cement Mixing Construction

The deep cement mixing (DCM) method has been used to form foundations for some of the marine structures in Hong Kong. Injection of cementitious slurry into the seabed will inevitably cause the seabed to rise, resulting in a raised soil-and-cement mixture above the top of DCM clusters, which is referred to as heaving material in this paper. The amount and characteristics of heaving material are influenced by several factors such as soil type, improvement depth and area ratio, cement-water ratio, cement injection pressure and workmanship. Due to its weaker strength, heaving material is conventionally dredged to avoid forming a weak layer in the DCM foundation. This paper aims to investigate how to retain heaving material in the DCM foundation system to avoid both causing pollution and incurring additional costs due to dredging. It has four objectives, namely: firstly, to study its formation mechanism; secondly, to investigate its shear strength characteristics, through the results of various lab and in-situ tests; thirdly, to discuss design and construction considerations concerning heaving material; and finally, to discuss the results of a full scale test involving heaving material. It is shown that heaving material may be retained provided it can meet design requirements.