Singapore subcontractor L&M Group formed the 3m to 4m thick jet grouted base slab as part of the temporary works for the project.
Its purpose was to hold the diaphragm walls apart during excavation before a permanent reinforced base slab was cast.
Geotechnical contractor Bachy Soletanche had installed 600mm800mm thick diaphragm walls to a depth of approximately 40m.
I-section steel struts were being used at centres of 4m to 5m horizontally and 3m vertically to provide passive resistance as excavation advanced.
Nishimatsu-Lum Chang is understood to have tendered for contract C824 with a 'floating diaphragm walls' design, claimed by many Singapore engineers to be 'unorthodox' given the ground conditions.
Normally diaphragm walls extend well below the excavation depth and would be toed into firmer material than marine clays.
But Nishimatsu-Lum Chang proposed to save costs by taking the walls down no further than the bottom of the base slab, making the base slab the key structural element.
But an engineer close to the project said that as built, the wall design was revised to extend a little deeper, into firmer ground.