State-of-The-Art Spraying - Case Study Centerpoint of Indonesia Project
CEDA Dredging Days 2019
M. Betman and S. van Ouwerkerk
Abstract: Many reclamation projects in South East Asia face operational construction challenges as they are often built on soft cohesive sediments. In the CenterPoint of Indonesia project in Makassar, Boskalis faced these aspects during the construction of a 75ha reclamation on slightly over-consolidated clays with undrained shear strength varying between 2.5 kPa to 3.5 kPa at seabed level.
To ensure a geotechnical stable construction of the reclamation, a staged reclamation construction process was designed in which the fill was placed in small layers. The chosen construction method was a combination of open sand fill and spraying by means of a spraying pontoon. Main challenge during the spraying process was to gradually load the sub soil in small layers every 24 hours, while avoiding mixing of the sprayed sand with original subsoil, preventing mud waves to form and keeping turbidity induced by the spraying process to a minimum. In order to achieve this Boskalis developed an autonomous spraying pontoon which controls shifting speeds with real time TSHD production data to spray a predefined layer thickness or design. The spray pontoon had the following key features:
- Wireless network connection to TSHD for real time production data (density/velocity);
- Real time data used to autonomously shift pontoon over pre-defined track;
- Adjustable / submergible spraying arms;
- Several spray modes available (pre-defined level, fixed, levelling); and
- 4-point anchor systems
After each sprayed layer a multi-beam survey was performed to validate layer thickness. Potential high spots could be identified and sprayed around in the subsequent layer to even out the sprayed fill. With the precise control of layer thickness and spray location, the spray pontoon was able to place a maximum of 11m of sand on sloping seabed with extremely soft marine clays, creating an underwater slope of 1:10 and challenging geometries with 90 degree corners. After reaching a dry level, wick drains were installed. Using the SHANSEP method, the increase in Undrained Shear Strength was calculated starting at the moment of the first layer being deposited. After enough strength increase, the slopes were back trimmed to final design slope angle of 1:4,5.
Keywords: Spraying, reclamation, state-of-the-art, soft marine clays