Maximum calculated settlement of Block E under dead loading from 41 floors is 159 mm

5. Overall tilt of Block E is greater than 1/400 under dead loading from 34 floors.

6. Overall tilt of Block D is greater than 1/200 under dead + live loading from 41 floors.

7. Overall tilt of Block E is greater than 1/200 under dead loading from 41 floors.

8. Maximum calculated settlement of Block D under dead loading from 41 floors is 104 mm.

9. Maximum calculated settlement of Block E under dead loading from 41 floors is 159 mm.

It was also found that several of the piles in Block D would have reached their ultimate capacity under full dead plus live load, and this would be unacceptable in relation to the client’s requirements, which demand a factor of safety in excess of unity on each of the piles within the group. Indeed, the normal design require-ment in Hong Kong (as stipulated by the Hong Kong Buildings Department) is that the factor of safety for each pile within a group should be at least 2. The loads in most of the piles for Block E were com-puted to have reached their ultimate value under full dead plus live loading. The solution for Block E was in fact unstable, indicating that rotational failure of Table 3. Summary of PIGS analysis—Block E.

Foundation Maximum Maximum Minimum factor of tilt settlement settlement

Stage safety (mm) (mm)

0 4.9 1/5519 12 7

1 4.0 1/4290 16 8

2 3.3 1/3223 20 10

Table 4. Summary of computed performance for as-built foundations.

Performance Loading

measure condition Block D Block E Overall factor Dead load at 3.1 2.2

Table 2. Summary of as-designed case, Block D.

Foundation Maximum Maximum Minimum factor of tilt settlement settlement

Stage safety (mm) (mm)

0 4.8 1/3304 16 6

1 4.0 1/2543 20 8

2 3.3 1/1884 26 9

the foundation would have occurred. It was therefore clear that the as-built foundations would have been inadequate to support the completed buildings.

4.9 Assessment of remedial options

PIGS was used to investigate a number of alternative future options, including:

• Construction of 8 new bored piles, socketted 0.8 m into Grade II or better rock, and enlargement of the pile cap.

• Construction of up to 66 new steel H-piles installed in pre-bored holes, each extending 9 m into the Grade II granite. An extended pile cap would be constructed to link the old and new foundation elements.

• Truncation of the buildings so that the existing foundation system would be adequate without any further enhancement.

Both of the first two options would require up to 5 m of excavation and dewatering. Truncation of the buildings was assessed to be technically feasible, but was not considered to be economically feasible by the owner because of the stigma of having two lower-rise buildings (12 or 13 storeys high) within a larger group of much taller buildings. Thus, the focus of the assessment of remedial options was on the use of new remedial piles to enhance the defective existing foun-dation system.

For the PIGS analysis of the remedial pile scheme, the following stages were modelled:

1. Stage 0—Building constructed to 34 storeys and settlement due to investigative core drilling applied to piles;

2. Stage 1—Installation of the remedial piles, and enlargement of the pile cap;

3. Stage 2—Building constructed to 41 storeys (full dead load of 476.1 MN applied);

4. Stage 3—Full live load of 104.6 MN applied.

For Block D, the maximum calculated overall tilt for the proposed remedial scheme with addi-tional bored piles under dead + live loadings from 41 floors was 1/465 and the maximum settlement was 85 mm. The overall foundation system factor of safety was 5.3. Similar values were found for the prebored H-pile option.

For block E, the maximum calculated overall tilt under dead + live loadings from 41 floors was 1/383 and the maximum settlement was 108 mm. The over-all foundation system factor of safety was 3.1.

Based on these results, it was assessed that, pro-vided the remedial works could be executed without disturbance to the existing foundations, this remedial schemes for Blocks D and E would be theoretically adequate in reducing the overall tilts and increasing

the overall foundation system factor of safety to within what may be considered as acceptable levels.

The pile loads within the original bored piles with the remediated foundation scheme were com-puted as part of the PIGS analysis, and it was found that, for each of the blocks, one of the piles would have reached its ultimate capacity. Under the client’s design requirements, this would normally render the design unacceptable unless special dispensation was given for the unusual circumstances of this case. It should however be noted that the axial load on a pile within a group is dependent on the method of analysis used. In both cases, the computed pile loads for the remedial piles were less than the ultimate axial load capacity of the piles.

4.10 Construction risk factors

Despite the findings of the PIGS analysis, it was considered that there were a number of risk factors associated with any of the options involving remedial work to the existing foundations, in particular:

• The fact that such remedial operations had not pre-viously been used for very tall buildings in Hong Kong;

• The remedial operations would in any case have to be carried out to rectify deficiencies in the capacity of the vast majority of piles in the existing founda-tion system;

• The difficulty of access for construction equipment within the confined spaces of the existing founda-tion footprints;

• The difficulty of exercising the essential precise control of construction within the confined spaces of the existing foundation footprints;

• The possible effects on the integrity of the foun-dation system of ground movements arising from drilling for the additional piles;

• The possible effects on the integrity of the foun-dation system of ground movements arising from excavation for the new pile caps;

• The possible effects on the integrity of the foun-dation system of ground movements arising from dewatering for construction of the new foundation elements;

• The proposed strengthening of the existing pile caps may not have been adequate, and in any case, would have been difficult to execute;

• The increased risk of damage to the adjacent school if the construction operations exacerbated the uneven settlement towards this building.

The construction of the proposed remedial meas-ures would have been complex and would have included extensive excavation works in the vicin-ity of the already highly stressed foundation area of both blocks. It was considered that there would be a

potential for additional settlements of the piled foun-dation associated with the drilling of the additional large diameter bored piles, or of the larger number of prebored H-piles, particularly in view of the set-tlements experienced during the investigative coring of the foundations of Blocks D and E. There would also have been a potential for ground movements associated with the excavation, to approximately 4 m below existing ground level, required to expose the existing pile cap and allow connection with the new pile caps.

Numerical analyses were undertaken to assess the likely magnitude of the ground movements from construction, and their effects on the existing piles. A more detailed account of these analyses is given by Poulos (2004). On the basis of these analyses, it was concluded that the risks involved during the construc-tion of the remedial works, and the potential addiconstruc-tional settlements, were unacceptable, particularly given the proximity of the buildings to the nearby school and other existing high-rise residential blocks.

4.11 The final decision

In a decision that was widely publicized in Hong Kong, the decision was taken to demolish both build-ings, and this was carried out in 2001–2.

5 CONCLUSIONS

This paper has presented a versatile analysis of pile group behaviour when subjected to vertical and moment loading. This analysis allows for non-ideal situations in which not all piles in the group are iden-tical, some piles may contain defects, and the group configuration may change during the process of con-struction or loading.

The analysis has been used to examine the effects of the presence of defective piles within the foun-dation systems of the two high-rise buildings that led to their eventual demolition before the comple-tion of construccomple-tion. This was a rather extreme case of the consequences of three major construction imperfections:

Piles with a soft toe;

Piles with variable founding conditions;

Piles of variable length within a group.

The analysis revealed that enhancement of the foun-dation system with either additional prebored H-piles, or with additional large diameter bored piles, would provide a technically satisfactory remedial solution.

However, in this particular case, the risks involved in carrying out the remedial foundation works beneath heavily loaded buildings were considered to be unac-ceptable, particularly in view of the potential for sig-nificant ground movements to be generated during the enhancement works. The concern over these risks was a major factor in the ultimate decision to demol-ish the buildings.

REFERENCES

Kuwabara, F. and Poulos, H.G. 1989. Downdrag forces in group of piles. Jnl. Geot. Eng., ASCE, 115(6): 806–818.

Mandolini, A. and Viggiani, C. (1997). Settlement of piled foundations. Geotechnique, 47(3): 791–816.

Poulos, H.G. (1989). Pile behaviour—Theory and appli-cation. 29th Rankine Lecture, Geotechnique, 39(3):

365–415.

Poulos, H.G. 1990. DEFPIG user’s manual. Centre for Geo-technical Research, University of Sydney, Australia.

Poulos, H.G. 2002. Prediction of Behaviour of Piled Build-ing Foundations due to TunnellBuild-ing Operations. Proc. 3rd Int. Symp. On Geotechnical Aspects of Underground Construction in Soft Ground, Toulouse, Preprint Volume, 4.55–4.61.

Poulos, H.G. 2004. The consequences of defective piles for two high-rise buildings in Hong Kong. Keynote Lecture, Proc. Int. Conf. on Structural and Geotechnical Failures, Singapore, IES.

Poulos, H.G. 2005. Pile behavior—consequences of geolog-ical and construction imperfections. 40th Terzaghi Lec-ture, J. Geot. & Geoenv. Eng., ASCE, 131(5): 538–563.

Poulos, H.G. and Davis, E.H. 1980. Pile Foundation Analy-sis and Design. John Wiley, New York.

Poulos, H.G. and Hewitt, C.M. 1986. Axial interaction between dissimilar piles in a group. Proc. 3rd Int. Conf.

Num. Methods in Offshore Piling, Nantes, 253–270.

Randolph, M.F. and Wroth, C.P. 1978. Analysis Deforma-tion of Vertically Loaded Piles. J. Geot.. Eng., ASCE, 104(GT12): 1465–1488.

Randolph, M.F. 1987. PIGLET, a computer program for the analysis and design of pile groups. Report GEO 87036, University of Western Australia.

Deep Foundations on Bored and Auger Piles – Van Impe & Van Impe (eds)

© 2009 Taylor & Francis Group, London, ISBN 978-0-415-47556-3