Prebored friction mini-pile foundation for light rail grade separation

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

In recent years jacked steel H-piles have been introduced into Hong Kong as an alternative to driven piles. In Hong Kong practice, termination of pile driving is often based on dynamic formula and pile tips are normally embedded in a stratum with SPT-N values exceeding 200. On the other hand, installation of jacked piles ceases when the rate of penetration is less than some prescribed termination criterion and jacked piles are normally embedded in layers with SPT-N values of less than 160. As such bearing stratum is less stiff, the load transfer mechanism and capacity of jacked piles could be very different from driven piles and experience from the latter may not be directly applicable. To investigate the behaviour of such piles, a comprehensive study was conducted. In the study, six piles were instrumented with strain gauges and load tested to failure so allowing their load transfer mechanism to be studied. During installation the development of pore pressure around three of the test piles was monitored by electronic piezometers. Different criteria were adopted for determining the termination of jacking. It was found that the test piles passed the same acceptance criteria specified in the building code for driven piles. Shaft friction in the test piles contributed to over 80% of the piles’ ultimate capacity. The pore pressure was found to dissipate quickly after jacked pile installation. P. K. K. Lee, Department of Civil Engineering, The University of Hong Kong L. G. Tham, Department of Civil Engineering, The University of Hong Kong S. T. Chan, Housing Department, the Government of HKSAR F. Yu, Department of Civil Engineering, The University of Hong Kong J. Yang, Department of Civil Engineering, The University of Hong Kong