Melbourne Docklands
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1.0 IntroductionThe 220-hectare Melbourne Docklands is one of the largest and most visionary urban renewal projects in Australia. As part of ongoing development at the Docklands precinct, a new mixed-use building comprised of a tower structure of 17-level and a 6-leverl podium structure including above ground level car parking. As proposed, deep foundation is proposed for the tower structure which has a larger column load, while for the surrounded commercial development, shallow foundation is preferred.In previous stages, analysis and interpretation of ground investigation have been performed for designing foundation and a detailed design has been carried out for the shallow foundation. In this stage, this report performed a detail design of deep foundation including two possible pile solutions – rock socketed pile and CFA pile foundation base on ultimate limit state and serviceability requirements for the tower base on column load. As this is a final stage of the whole project, a final recommendation is also made regarding the optimum solutions developed for the entire project.2.0 ObjectivesThe aim of this task is to provide the best solution from different pile systems. Two possible types of pile foundation will be designed, checked and compared in terms of construction methods, ultimate limit state and serviceability requirements. All the detailed calculation and consideration need to be performed. The main items were listed as following:Detailed design of piled foundations considering a minimum of two types of piles for column load P2Determine pile head settlements considering types of piles and ground conditionsUndertake a feasibility study of the proposed solutions leading to preferred foundation option for the project.3.0 Rock Socketed Pile DesignPiles socketed into rock are often used to support heavy loads from buildings and infrastructure. The design of bored piles socketed into rock has developed in the late 1960’s. In this project, by using elastic design approach (Pell, 1999), rock socketed pile was designed based on ultimate limit state and serviceability requirement for the column load of the tower. The pile was designed close to CPT-2. It is noted that the detailed design step required iteration process. Data of bore log and previous tasks was used for calculation of design.3.1 Shaft and base resistancesDesigning for rock socketed piles required the consideration of shaft and base resistance of good quality rock. The method proposed by William and Pell (1980) was used to estimate the ultimate shaft resistance. The ultimate base resistance was determined according to Zhang (2010). These two key parameters as well as elastic modulus were all calculated and presented in Table 1 and the detailed calculations can be found in Appendix A.Table 1 Shaft and base resistances of rock socketed pile, Rock ModulusGeological unitDepthUnit shaft resistance (fsu)Unit base resistance (qmax)Rock modulus (E’)mMPaMPaMPaMelbourne Formation33+0.526.584003.2 Pile Capacity base on Limit State PrinciplesThe reinforced concrete rock socketed pile was in circular shape. After analysing, one single pile will be inadequate in terms of ultimate limit state design since the design column load is quite large, which is 20500kN. Therefore, group piles were designed for the requirements. It was found that two rock socketed piles with 1.6m diameter and 39.8m total length, including 8.84m socket would satisfy all the requirements. Geotechnical requirements and structural requirements have been checked as well. All the above information and relevant design actions were summarized in Table 2 while the detailed calculations and explanations can be found in Appendix A.
Table 2 Summary of important design information of rock socketed pileNo. of pilesPile Diameter (m)Socket length (m)Applied column load (kN)Negative shaft resistance (kN)21.68.84205001890.4Pile Capacity Base on ULS (kN)Design geotechnical strength (kN)Design structural strength (kN)Geotechnical design action (kN)Structural design action (kN)13.818.277.013.829.93.3 Pile settlementThe serviceability criteria ensure that the structure does not deform excessively, in order to maintain an elastic response of the piles the settlement was limited to 1% of the pile diameter. It was found that two rock socketed piles with 1.6m diameter and 39.8m total length which including 8.8m socket would satisfy the serviceability requirements. Elastic shortening of piles has been check as well. The calculated settlements were listed in the Table 2 while the detail calculation can be found in Appendix A.Table 3: The performance of the rock socketed pile design under serviceability criteriaNo. of pilesPile Diameter (m)Socket length (m)Elastic Settlement(mm)Elastic Shortening(mm)Total Settlement(mm)Settlement Limit(mm)21.68.843.473.266.73164.0 Bored/ CFA Pile Design4.1Shaft and base resistances of CFA pileCFA pile is known as a non-displacement pile which does not involve the application of rapid forces onto the piles. As a result, CFA piles could produce lesser vibration and noise level into the ground when comparing with other foundation type. In addition, the soil heave due to driving can be minimized when using the CFA pile.