SHALLOW FOUNDATION

5.1 Project description

In recent years, there have been more heritage conservation projects. The Urban Renewal Authority (URA) project at Mallory Street, Wan Chai, was a Grade II Historic Building, also known as the “Green House”. It was originally a series of four-storey pre-war tenement buildings with timber flooring and load bearing brick walls. Desk study revealed that the existing site was reclaimed in 1863 and that the existing buildings were built in 1910s. A typical geological section is shown in Figure 23.

Figure 23: Ground profile underneath the “Green House”

The revitalisation project involved the change of usage from residential to cultural/commercial. The increase in imposed load, together with the structural strengthening works, would cause the soil bearing pressure underneath the strip footings to double to nearly 200kPa. Since the footing are resting on reclaimed fill (SPT-N = 4-8), strengthening works have to be carried out.

Conventional foundation strengthening works, such as underpinning using mini-piles, was considered to be too risky as the old brittle structure could be damaged during the invasive pile installation. The low headroom would also impose restrictions on the pile installation.

Ground improvement by grouting was adopted to increase the bearing capacity of the founding stratum of the footings and to reduce the total and differential settlement of existing footings under the additional loading (Wong & Leung 2010). This was the first of its kind for which the approval of the Buildings Department was sought.

5.2 Grouting

In order to have better penetration into the soil, fine milled Portland cement (Micro-fine cement) was chosen for this project. The grain size of this micro-fine cement is 98% passing 20 µm sieve, which is 4.5 times smaller than ordinary Poland Cement. The comparison in grain size of Ordinary Poland Cement and Micro-fine cement is shown in Table 12.

Table 12 : comparison of Ordinary Poland Cement and Micro-fine cement Type of Cement Grain Size Specific Surface Area (m2/kg) Ordinary Poland Cement 98% (<90µm) 320 – 380

Micro-fine cement 98% (<20µm) 450 – 650

The grouting pressure adopted was the effective overburden pressure, σv’, allowing for the effect of bearing

pressure from the existing building. The comparison between designed grouting pressure and actual grouting pressure are shown in Figure 24. It can be seen that the actual grouting pressure was larger than the calculated effective overburden pressure. This was to cater for the head loss during the grouting process and to increase the penetration effectiveness of the grout into the soil underneath the existing footing.

5.3 Preloading

After completion of grouting works, loading tests were carried out concurrently on all strip footings. The arrangement is shown in Figure 25. The loading test was effectively a full scale preloading to the existing footings to verify the effectiveness of the grouting works. Less than 1mm settlement was recorded in the loading test, which confirms the effectiveness of the strengthening works.

Figure 25: The arrangement of preloading

The foundation strengthening works was successfully completed and the revitalised building is now in use. The comparison of the building before and after the revitalisation works is shown in Plate 16.

Before After

Plate 16: Comparison of the building before and after the revitalisation works

Load

Strip Footing Foundation Microfine cement

6 CONCLUSION

Owing to the increasing site constraints, more difficult ground conditions, less public resilience to nuisance caused by constructions and more stringent statutory control, the foundation works have advanced in both design approach and installation techniques. Friction piles, plain or shaft-grouted, or whether it be barrettes, small or large replacement piles, are gathering more popularity. When more test data become available, the friction values can be reviewed and adjusted to achieve more cost efficient design. Jacked piles can provide a relatively quiet environment during its installation process. In light of the higher public demand for a quieter environment, there may be room for further exploring the use of this type of piles.

REFERENCES

Architectural Services Department 2008. Particular Specification for Jacked steel H-piles. Available at http://www.archsd.gov.hk/media/11320/e146.pdf (accessed on 10 May 2014)

Buildings Department. 1990. Practice Note for Authorised Persons and Registered Structural Engineers 141 (1990) Buildings Dept., Hong Kong.

Buildings Department. 2000. Practice Note for Authorised Persons and Registered Structural Engineers 66 (June 2000) Buildings Dept., Hong Kong.

Buildings Department. 2004. Code of Practice for Foundations. Buildings Dept., Hong Kong.

Buildings Department. 2008. The summary of decisions of the structure engineering committee SEC 02/2008. Available at http://www.bd.gov.hk/english/inform/BC/SEC08-02.pdf (accessed on 10 May 2014)

Buildings Department. 2011. The summary of decisions of the structure engineering committee SEC 02/2011. Available at http://www.bd.gov.hk/english/inform/BC/SEC11-02.pdf (accessed on 10 May 2014)

Chan, F. 2005. Termination Criteria for and Behaviour of Jacked Piles in Completely Decomposed Granite, MPhil Thesis, HKUST

Chan, S.T., Wong, F. C.C. & Tsoi, M.W.T. 2002. Trial of jack piling in a Housing Authority Foundation Project. Jack Piling in Hong Kong. Centre for Research & Professional Development: 15-20

Choy, K.K. & Wong, C.M. 2007. Development and Statutory Control of Pile Foundations for Private Buildings in Hong Kong since the Seventies. Proceedings of the 27th Annual Seminar of the Geotech. Div. of the Hong Kong Instn. Engrs, Hong Kong: 119-124.

Environmental Protection Department. 2006. A Concise Guide to the Noise Control Ordinance. Ninth edition. Hong Kong Government. 1975. Building (Construction) Regulations. Laws of Hong Kong, Chapter 123. Hong Kong Government.

Li, V. & Lam, J. 2011. Development of Jacked Piling in Hong Kong, The 2nd VLA Seminar, “New Development in Jacked Piling”: 88-116.

Ng, C.W.W., Rigby, D.B., Li, J.H.M., Yau, T.L.Y., Lee, S.C. & Calton. C. 2000. Side Resistance of Large Diameter Bored Piles Constructed Under Water in Saprolites.

Proceedings of the 19th Annual Seminar of the Geotech. Div. of the Hong Kong Instn. Engrs, Hong Kong: 137-144.

Plumbridge, G.D., Littlechild, B.D., Hill, S.J. & Pratt, M. 2000. Full scale shaft grouted piles and barrettes in Hong Kong – A first. Proceedings of the 19th Annual Seminar of the Geotech. Div. of the Hong Kong Instn. Engrs, Hong Kong: 157-166.

SCMP. 1996. Death site contractors accused of breaches. South China Morning Post, 3rd September, 1996. SCMP. 2008. Residents evacuated as building tilts. South China Morning Post, 10th July 2008.

Sze, J.W.C., Lam, A.K.M., Pappin, J.W. & Chan, K.M. 2007. Design & construction of shaft-grouted friction barrette in Tung Chung Designated Area. Proceedings of the 27th Annual Seminar of the Geotech. Div. of the Hong Kong Instn. Engrs, Hong Kong: 299-304.

Wong, C.M., Lee, C.T.L. & Ting, R.C.M. 2011a. Construction of Hand-dug Caissons for Slope Stabilization near the Peak Lookout. Proceedings of the 31st Annual Seminar of the Geotech. Div. of the Hong Kong Instn. Engrs, Hong Kong: 100-107

Wong, C.M., Lee, C.T.L. & Ting, R.C.M. 2011b. Innovative Materials and Drilling Method adopted for Soil Nailing Works at Po Shan Road. Proceedings of the 31st Annual Seminar of the Geotech. Div. of the Hong Kong Instn. Engrs, Hong Kong: 108-113

Wong, C.M. & Leung B.C.H. 2010. Structural Assessment of Century-old Tenement for Revitalization Purpose. Proceedings of HKIE/IstructE Joint Struct. Div. Annual Seminar 2010.

1 INTRODUCTION

In the conventional design of pile foundations, the piles are designed to share the load at ultimate state and the pile cap would be designed to link the piles together. The contribution of the pile cap to bearing capacity is not included in the design. A piled raft is a raft foundation that has piles to reduce the amount of settlement. The raft foundation and the piles would be designed to act together to ensure the required settlement is not exceeded. A significant part of the bearing capacity comes from the raft and piles. This later method is also known as the Combined Pile-Raft Foundation (Katzenbach and Deepankar, 2013).

The authors will highlight a case where a piled-raft foundation in Bangkok subsoil were optimised to obtain an economical design. The number of piles, size and thickness of the raft were optimised to the required engineering tolerance and factor-of-safety. This paper shows that the usage of advanced 3-D finite element analysis can be routinely applied in the design of Piled-raft foundations. The concept of Piled Raft foundation