The tower is located above a cliff-like bedrock profile varying from about 70m to 130m in depth, which posed uncertainties in the use of traditional pile types. A tower of this height would require large concrete piles to support its enormous weight if the ground immediately under the building could not provide the required bearing capacity. For piles deeper than 90m below ground, there is increasing risk of collapse during excavation and also defective concreting.
In designing the foundations, we talked to good foundation contractors in Hong Kong to understand the difficulty imposed by technology of their equipment and thus the risks they would be taking. At the end we worked with a French contractor who was a specialist in construction diaphragm walls and barrette piles.
After elaborate studies being carried out, we decided to go for shaft-grouted barrette as the foundation system. The barrettes are rectangular reinforced concrete piles. To avoid construction risk at great depth as well as differential settlement, we adopted a friction mechanism for the piles to transfer the tower loads to the ground rather than a bearing mechanism. The piles transfer the loads through friction on their four sides to the adjacent soil instead of their bottom face as they would in the bearing mechanism.
The excavation for each pile employs the diaphragm wall method, which basically uses a machine to grab the soil out of the ground in a bentonite solution. The excavation is not carried down to the bedrock. Upon completion of excavation to the required depth, the pile is concreted while the bentonite solution is simultaneously pumped out to a collection tank. After the concrete sets, the concrete and soil interface of the barrette is injected with pressured grout. This grout creates an approximately 35mm cementitious layer between the barrette and adjacent soil, enhancing the friction between the barrette and adjacent soil and thus the load capacity of the barrette. The foundations consist of 241 barrettes with an average depth of 70m. Two sizes of barrettes are used – 2.8m x 1.0m and 2.8m x 1.5m.