Shaft grouted friction barrette piles for a super high-rise building

This paper presents experience encountered during the design and construction of a shaft grouted friction barrette pile foundation system for the Kowloon Station Package 7 Development, Mega Tower in the West Kowloon Reclamation Area. Since the footprint of the proposed tower is located above a major fault zone, adoption of conventional end-bearing and frictional piles would have made it very difficult to ensure pile quality and achieve required capacity. Consequently, shaft grouted friction barrettes, were considered the most feasible and viable option although a relatively new foundation form in Hong Kong. To determine their performance and establish construction procedures, a total of 5 numbers of trial piles were carried out on site.

If the proposed frictional pile foundation system were designed in accordance with spacings stipulated in the Hong Kong Building (Construction) Regulation (B(C)R) 26(5), i.e. not less than the length of the pile perimeter measured from the centres of adjacent piles, the total capacity derived from available piles within the circular diaphragm wall would not have been enough to support the proposed Mega Tower. Also, the conventional group reduction factor, i.e. 0.85 (as stipulated in PNAP 66) is considered very conservative for designing a frictional pile group in cohesionless soil with such minimum spacings. Hence to have a more economical design, a group ‘reduction’ factor of 1.0 was proposed. To demonstrate acceptability, a finite element analysis was used to determine the change in lateral effective stress within surrounding soils for both a single pile and for a pile within a group.

The objectives of this paper are twofold. The first objective is to provide an overview of construction procedures and techniques for shaft grouted barrettes, which is not a type of foundation recognised by Buildings Department (BD), and to highlight the key steps affecting pile performance. The second objective is to present the Author’s recommended rational design method for a closely spaced frictional pile foundation.

George Chan, Ove Arup & Partners Hong Kong Ltd
James Lui , Ove Arup & Partners Hong Kong Ltd
Kelvin Lam , Ove Arup & Partners Hong Kong Ltd
K. K. Yin, Ove Arup & Partners Hong Kong Ltd
C. W. Law, Ove Arup & Partners Hong Kong Ltd
Ringo Lau, Bachy Soletanche Group Ltd
Alex Chan, Bachy Soletanche Group Ltd
Ronan Hasle, Intrafor Hong Kong Ltd