Recent field study on the behaviour of jacked piles

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

Construction access constraints and an undulating rock profile, up to 90m deep in places, dictated use of an innovative foundation solution on the Kowloon Canton Railway Corporation’s Light Rail Grade Separation project in Hong Kong. Pre-bored friction mini-piles were adopted to overcome geological and other constraints, which included low headroom due to overhead cables and strict vibration limits imposed by an adjacent Viaduct Structure supporting Light Rail tracks. Bored piles and socketted H-piles on rock are not commonly adopted for depths greater than 60m due to the construction difficulties. Moreover, the expected high vibration levels generated during pile driving ruled out use of percussion piling. Hence Atkins China Ltd working together with Leighton Contractors (Asia) Ltd (LCAL) and Freyssinet Hong Kong Ltd, developed the use of 450mm diameter pre-bored friction piles as a cost effective and practical alternative. In this paper, the design, quality control on site and the results from proof load testing of the trial piles will be discussed. The friction mini-pile foundation system adopted was designed to comply with requirements stipulated in Hong Kong Building’s Ordinance. LCAL commenced foundation works in April 2003 and completed their works in November 2003. C. K. Chan, Atkins China Ltd Alfred H. K. Tsang, Atkins China Ltd Rex N. Chow, Leighton Contractors (Asia) Ltd Joseph Y. C. Tam, Freyssinet Hong Kong Ltd