Melbourne Docklands

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In the project, nine piles in a group are designed to support the 20500KN column load which each pile has the diameter of 1 meter with the length of 31 meter from the top of CIS layer. The pile spacing is designed to be 2.5 meter. For the soil profile, the most critical ground condition is considered. The CPT2 is used for the deep foundation design because of the largest thickness of Coode Island Silt which is 23 meter. Since the high compressibility of CIS, the negative skin friction is taken into account of ultimate and serviceability of pile design. The shaft and base resistance of each geological unit are shown in the Table 4. The detail calculation is shown in the Appendix B.

Table 4 Shaft and base resistances of CFA piles

Layer

Layer

Thickness (m)

Unit Shaft Resistance (kPa)

Unit Base Resistance (kPa)

Soil modulus, Es

(MPa)

CIS

23

16.2

237.3

4

FBS

3

42.3

886.4

20

MSG

5

42.3

10893

85

From the Table 4, it shows that the MSG layer provide the largest base resistance. Therefore, the CFA pile situated in MSG layer is recommended.

4.2 CFA Pile capacity based on limit state principles

Since the ultimate bearing capacity of the pile is equal to the sum of shaft and base resistance, the number of pile to support the design load,P2=20,500 KN can be determined. The Table 5 shows the dimension and the ultimate bearing capacity of the pile.

Table 5 Design parameters of single CFA pile

Pile Diameter (m)

Pile Length (m)

Pile Spacing (m)

Ultimate bearing capacity (KN)

Safety factor

Allowable Load (kN)

1.0

31

2.5

9356.2

2

4678.1

As a single pile is inadequate to carry the design load of 20,500KN, the group piles are required in the design. Nine piles will be used in the design due to the serviceability requirements. The Table 6 shows the required number of piles and the group efficiency of piles.

Table 6 Design parameters of group CFA piles

Number of piles

Column load (KN)

Negative friction force (KN)

Combined factored load (KN)

Group allowable load (KN)

Group efficiency

9

20500

1171.7

28846.7

35737.0

0.8488

4.3 CFA Pile settlement

Apart from the ultimate design for the piles, the pile is also required to satisfy both structural and serviceability requirements. The bending stress in concrete is considered as the structural requirement of the pile. For the serviceability requirements, the total settlement must not exceed 10mm (1% of pile diameter). The structural and serviceability requirement is summarized in the Table 7.

Table 7 Structural and serviceability requirements of CFA pile

Maximum applied bending stress (KPa)

Allowable bending stress (KPa)

Group settlement (mm)

Allowable settlement (mm)

7887.2

40000

9.0

10

5.0 Discussion & Recommendation

In this section, the final recommendation has been made based on both shallow foundation design (P1-3500kN) and deep foundation design (P2-20500kN) for the new mixed-use building, which comprised of a 17-level tower structure and 6-level podium structure.

For shallow foundation part, the square shallow foundation at depth of 2m is designed. The initial design proposed that the size of shallow foundation is 8.8m x 8.8m for the design column load of 3500kN. The corresponding total settlement is found to be 629.6mm. The differential settlement is 311.1mm which is much greater than the allowable limit. In order to reduce the footing size and settlement of the structure, the deep soil mixing is used to improve the soil strength and lower compressibility. By using the deep soil mixing method, the revised footing size is reduced to be 4.5m x 4.5m which is much smaller than the original footing design. In addition, the settlement is reduced due to the lower compressibility of composite soil. As a result, the ground improvement of deep soil mixing is recommended in the Project 2013.

For the deep foundation part, two types of piles were designed by the group, rock socketed piles and continues fight auger piles. Both designs were carried out by group piles, and number of piles for rock socketed piles and CFA piles are 2 and 9 respectively. However, both design satisfied the serviceability limit state check and ultimate state limit check (rock