Study of field case records

Transcription

1 the neighboring liquefied but not monotonically sheared soil. The buckling pile will also suffer increasing loss of bending stiffness due to plastic yielding, so the restraint necessary to hold it in equilibrium will also increase. This imbalance between increasing bending moment created by displacement of pile cap, deteriorating bending stiffness of the pile and the reducing differential soil support along its length, creates a shallow plastic hinge which then leads to the dynamic collapse of the structure. Study of field case records Fifteen reported cases of pile foundation performance during earthquake-induced liquefaction were studied and analyzed by Bhattacharya (2002, 2003), Bhattacharya et al (2004). Six of the piled foundations were found to survive while the others suffered severe damage. Figure 12 presents the effective length and the minimum radius of gyration for the case histories studied. Table 3 identifies the case history. The effective length is estimated based on the concept described in Figure 7. The effective length depends on the fixity of the pile below and above the liquefiable zone. If the pile is embedded to about 6 times the diameter of the pile in the non-liquefied hard base below the liquefied soil, the base can be assumed to be fixed. On the other hand, if the pile is not sufficiently embedded in the firm ground so that it can rotate then it can be assumed that the pile is pinned at the bottom. Details on the concept of effective length of pile can be seen in Bhattacharya (2003). A line representing L eff /r min = 50 is shown in the plot which distinguishes the piles of poor performance from the piles of good performance. This line is of some significance in structural engineering, as it is often used to distinguish between long and short columns. Columns having a slenderness ratio below 50 are expected to fail in crushing whereas those above 50 are expected to fail in buckling instability. It can be concluded that the case histories support the hypothesis of failure of pile by buckling instability. Figure 12: Plot of effective length (L eff ) and minimum radius of gyration (r min ) Bhattacharya Page 17 of 24

2 Table 3: Study of 15 case histories ID in Fig 12 Case History and Reference Pile section/ type L 0 * (m) L eff (m) r min (m) A 10 storey-hokuriku building, 0.4m dia RCC Hamada(1992) B Landing bridge, Berrill et al (2001) 0.4m square PSC C 14 storey building, Tokimatsu et al (1996) 2.5m dia RCC D Hanshin expressway pier, Ishihara (1997) 1.5m dia RCC E LPG tank 101, Ishihara (1997) 1.1m dia RCC F Kobe Shimim hospital, Soga (1996) 0.66m dia steel tube G N.H.K building, Hamada (1992) 0.35m dia RCC H NFCH building. Hamada (1992) 0.35m dia RCC hollow I Yachiyo Bridge Hamada (1992) 0.3m dia RCC J Gaiko Ware House, Hamada (1992) 0.6m dia PSC hollow K 4 storey fire house, Tokimatsu et al (1996) 0.4m dia PSC L 3 storied building at Kobe university, 0.4m dia PSC Tokimatsu et al (1998) M Elevated port liner railway, Soga (1996) 0.6m dia RCC N LPG tank 106,107 Ishihara (1997) 0.3m dia RCC hollow O Showa bridge, Hamada (1992) 0.6m dia steel tube Analytical studies has been carried out by Bhattacharya (2003), Bhattacharya and Bolton (2004a) to study the effect of buckling of piles. They used Hetenyi s buckling (1946) i.e. Euler s buckling of strut in a resistive medium. The resistive medium was the liquefied soil. A final note on the theory of pile failure The theory of pile failure is a combination of two critical phenomenon and transient flow. The initiation of buckling is guided by the CRITICAL LOAD Euler s elastic buckling load. Whether a structure will become unstable is guided by the critical load. But once it starts buckling, the liquefied soil will offer resistance. The undrained monotonic soil behaviors will dictate the location of the hinge. This can be well modeled by CRITICAL STATE SOIL MECHANICS. The soil behavior suggests that there will be suctions generated in the near field of the soil which would induce transient flow. More research is required to study this complex pile-soil interaction. 5. NEW DESIGN APPROACH TO AVOID BUCKLING OF PILES Through the analysis of reported case histories, geotechnical centrifuge tests and analytical studies, it has been demonstrated that buckling is a possible failure mode of piled foundations in areas of seismic liquefaction. Influences such as lateral loading due to slope movement, inertia effects due to early shaking or out-of-line straightness, will cause lateral deflections which are severely amplified if the axial load is permitted to approach the buckling load. These lateral load effects are, however, secondary to the basic requirements that piles in liquefiable soils must be checked against Euler s buckling. In contrast, all current design methods, such as JRA (1996), NEHRP (2000) or Eurocode 8 (1998), focus on the bending strength of the pile and overlook considerations necessary to avoid buckling in the event of soil liquefaction. In this section a new framework for designing pile foundations in liquefiable deposits is proposed. The principal aim of this framework is to provide a design methodology that takes into consideration all the identified pile failure mechanisms. Details can be seen in Bhattacharya (2003). This section only highlights the basic principles. Bhattacharya Page 18 of 24

3 The design method should safeguard the piles against: 1. Buckling failure due to unsupported pile carrying axial loads in liquefied soil. 2. Formation of a collapse mechanism due to additional lateral spreading forces. 3. Excessive settlement leading to serviceability failure. In design, beam bending and column buckling are approached in two different ways. Bending is a stable mechanism, i.e. if the lateral load is withdrawn; the pile comes back to its initial configuration provided the yield limit of the material has not been exceeded. This failure mode depends on the bending strength (moment for first yield, M Y ; or plastic moment capacity, M P ) of the member under consideration. On the other hand, buckling is an unstable mechanism. It is sudden and occurs when the elastic critical load is reached. It is the most destructive mode of failure and depends on the geometrical properties of the member, i.e. slenderness ratio and not on the yield strength of the material. For example, steel pipe piles having identical length and diameter but having different yield strength [f y of 200MPa, 500MPa, 1000MPa] will buckle at almost the same axial load but can resist different amounts of bending. Bending failure may be avoided by increasing the yield strength of the material, i.e. by using highgrade concrete or additional reinforcements, but it may not suffice to avoid buckling. To avoid buckling, there should be a minimum pile diameter depending on the depth of the liquefiable soil. Euler s buckling of equivalent pinned strut (L eff ) Liquefiable zone(d L ) Effective length (L eff ) (D L ) Length of the pile(l) (D F ) Dense non-liquefiable zone This pile being analysed Point of fixity in non-liquefied zone Figure 13: During full liquefaction, piles are practically unsupported. Pile should have adequate stiffness such that it does not buckle. Bhattacharya Page 19 of 24

4 Lateral spreading starts Non-liquefied crust may be present Liquefiable zone Plastic hinges to be formed for failure Dense non-liquefiable zone Figure 14: In sloping ground, piles will be subjected to lateral spreading loads. Combined action of lateral spreading and axial load should not form a collapse mechanism. Simplified approach to avoid buckling Lateral spreading loads and inertia loads may act in two different planes. Thus the pile not only has axial stress but also may have bending stresses in two axes. The pile represents a most general form of a beamcolumn (column carrying lateral loads) element with bi-axial bending. If the section of the pile is a long column, analysis would become extremely complex and explicit closed-form solution does not exist. The solution of such a problem demand an understanding of the way in which the various structural actions interact with each other i.e. how the axial load influences the amplification of lateral deflection produced by the lateral loads. In the simplest cases i.e. when the section is short column, superposition principle can be applied i.e. direct summation of the load effects. In other cases, careful consideration of the complicated interactions needs to be accounted. Designing such type of member needs a three-dimensional interaction diagram where the axes are: Axial (P), major-axis moment (M x ) and minor-axis moment (M y ).The analysis becomes far more complicated in presence of dynamic loads. The above complicated non-linear process can be avoided by designing the section of the pile as short column i.e. for concrete section - length to least lateral dimension less than 15 (British Code 8110) or a slenderness ratio (effective length to minimum radius of gyration) less than 50. Figure 15 shows one such design graph from Bhattacharya (2003), Bhattacharya and Tokimatsu (2004). The salient features of the design curve are: 1) Figure 15 shows a typical graph showing the minimum diameter of pile necessary to avoid buckling. In this case non-linear analysis can be avoided. The lateral load can now be accounted for in simple bending calculations. 2) The slenderness ratio is kept around 50. The study of case histories (see Figure 12) supports this assumption. 3) Piles are solid concrete section having E (Young s modulus) of MPa) 4) Steel piles are tubular having E of 210GPa. 5) The piles are not in a single row and at least in 2 2 matrix form. Bhattacharya Page 20 of 24

5 6) The thickness of the steel pile is based on API code (American Petroleum Code) i.e. the minimum thickness is 6.35mm + (diameter of the pile/100) based on stress analysis due to pile driving. Diameter of pile (m) Minimum dia of pile from buckling consideration Concrete pile Steel tubular pile Thickness of liquefiable layer (m) Figure 15: Proposed minimum diameter of pile necessary to avoid buckling, Bhattacharya and Tokimatsu (2004). Proposed failure criteria in simplified design approach Bhattacharya and Tokimatsu (2004) proposed the following design criteria for piles: (1) During the entire earthquake, the pile should be in stable equilibrium, the amplitude of vibration should be such that no section of the pile should have an ultimate limiting strain for the material. For example in the case of concrete piles, the ultimate strain in the pile should not exceed At this strain, visible cracks appear in concrete leading to deterioration of bending stiffness. This criterion automatically ensures that no plastic hinge will form and no cracks will open up. Steel tubular piles are ductile i.e. they can withstand large amount of inelastic strain before yield and thus can be a good choice. (2) The settlement of the piled foundation should be within acceptable limits for the structures. However, the settlement should be limited to a maximum of 10% of the pile diameter to avoid base failure (end-bearing failure) based on Fleming et al (1992). Bhattacharya Page 21 of 24

6 6. CONCLUSIONS It has been shown that buckling of a pile under the action of axial load alone due to the diminishing soil stiffness owing to liquefaction is a feasible pile failure mechanism during earthquakes. Lateral loads like inertia, slope movement loads will make the piled foundation unstable at a much lower load. However, these lateral loads are secondary to the basic requirements that piles in liquefiable soil must be checked against Euler s buckling. Recommendations to Practice 1. Codes of practice need to include a criterion to prevent buckling of slender piles in liquefiable soils. The designer should first estimate the equivalent length for Euler s buckling, by considering any restraints offered by the pile cap, or the zone of embedment beneath the liquefiable soil layer. It is then necessary to select a pile section having a margin of factor of safety against buckling under the worst credible loads. 2. Designers should specify fewer, large modulus piles, in order to avoid problems with buckling due to liquefaction. 3. Cellular foundations of contiguous, interlocked sections should also be effective Essential checks that a safe design procedure should ensure A safe design procedure should ensure that the piles have enough strength and stiffness to sustain the following: (1): A collapse mechanism should not form in the piles under the combined action of lateral loads imposed upon by the earthquake and the axial load. Figure 14 shows such a mechanism. At any section of the pile, bending moment should not exceed allowable moment of the pile section. The shear stress load at any section of the pile should not exceed the allowable shear capacity. (2): A pile should have sufficient embedment in the non-liquefiable hard layer below the liquefiable layer to achieve fixity to carry moments induced by the lateral loads. If proper fixity is not achieved, the piled structure may slide due to the kinematic loads. The fixity depth is shown by D F in Figure 13. Typical calculations carried out using the method proposed by Davisson and Robinson (1965) shows that the point of fixity lies between 3 to 6 times the diameters of the pile in the non-liquefiable hard layer. Details can be seen in Bhattacharya (2003). (3): Axial load acting on the pile during full liquefaction without buckling and becoming unstable. It has to sustain the axial load and vibrate back and forth, i.e. must be in stable equilibrium when the surrounding soil has almost zero stiffness owing to liquefaction. As mentioned earlier, lateral loading due to ground movement, inertia, or out-of-straightness, will increase lateral deflections which in turn can cause plastic hinges to form, reducing the buckling load, and promoting more rapid collapse. These lateral load effects are, however, secondary to the basic requirements that piles in liquefiable soils must be checked against Euler s buckling. This implies that there is a requirement of a minimum diameter of pile depending on the likely liquefiable depth. (4): The settlement in the foundation due to the loss of soil support should be within the acceptable limit. The settlement should also not induce end-bearing failure in the pile. Further research needs identified The research presented in this paper has identified the limitations of the existing design methods of piled foundations in liquefaction hazard areas for e.g. Japanese Road Association JRA (1996), Eurocode 8 (1998), Bhattacharya Page 22 of 24

7 and NEHRP (2000). It seems that many of the bridges and buildings designed based on the existing design codes are unsafe. Based on the above fact the following research need is identified. The immediate need is to re-evaluate the safety of the structures designed based on the existing design methods, see Bhattacharya (2004). Structures that are unsafe will need retrofitting to withstand future impacts of earthquakes. Keeping this view in mind, the suggested future research work is outlined below. Identifying the parameters for systematic evaluation of safety of existing structures founded on piles designed based on existing design methodologies. Some of the parameters identified are: Site characterisation i.e. depth of liquefiable soil at the site of the structure, slope of the ground, seasonal variation of ground water table. This would help to identify the nonliquefied crust at the site and expected lateral loads in the pile. Slenderness ratio of piles in liquefiable region. This would check the stability of pile against Euler s buckling. Once unstable structures (for e.g. abutment/piers of bridges, or piled buildings) are identified, strategies for retrofitting have to be researched. This will involve means to improve stability of foundations. ACKNOWLEDGEMENTS The author wishes to thank Cambridge Commonwealth Trust, Nehru Trust for Cambridge University (New Delhi), Overseas Research Award (U.K Government) for funding him to carry out the research. The author wishes to acknowledge the various inputs provided by Dr Gopal Madabhushi, Dr S.K. Haigh, Professor C.R. Calladine, Professor T.D.O Rourke, Mr Allan McRobie (Reader in Structural engineering at University of Cambridge), Professor Ross Boulanger and Dr Kenichi Soga (PhD examiners) in this research. The author also is especially thankful to Professor Kohji Tokimatsu for making arrangements for the fellowship what he is enjoying. He also wishes to acknowledge the technicians of Schofield Centre and the workshop technicians of Cambridge University Engineering Department. On a personal note, the author wishes to thank his elder brother, for his inspiration, who was in Gujarat (Surendranagar) during the 2001 Bhuj earthquake which killed over 20,000 people. REFERENCES 1. Berrill,J.B., Christensen, S.A, Keenan, R. P., Okada, W. and Pettinga, J.R., (2001). Case Studies of Lateral Spreading Forces on a Piled Foundation, Geotechnique 51, No. 6, pp Bhattacharya, S (2004): A method to evaluate the safety of the existing piled foundations against buckling in liquefiable soils, Proc of the 1 st International Conference on Urban Earthquake Engineering, Centre for Urban Earthquake Engineering, Tokyo Institute of Technology, 8-9 th March, Tokyo. 3. Bhattacharya, S, Madabhushi, S.P.G and Bolton, M.D (2004) An alternative mechanism of pile failure in liquefiable deposits during earthquakes, Geotechnique 54, Issue 3 (April), pp Bhattacharya, S and K. Tokimatsu (2004): Essential criteria for design of piled foundations in seismically liquefiable areas, 39 th Japan National Geotechnical Conference. Sponsored by Japanese Geotechnical Society, 7 th to 9 th July, Bhattacharya, S. and Bolton, M.D. (2004a). A fundamental omission in seismic pile design leading to collapse, Proc. 11th Int. Conf. on Soil dynamics and Earthquake Engineering. Berkeley, California, January 7-9, 2004, pp Bhattacharya, S. and Bolton, M.D. (2004b). Errors in design leading to pile failure during seismic liquefaction, Proc. 5th Int. Conf. on Case Histories in Geotechnical Engineering. Eds (Shamsher Prakash) New York, April 13-17, 2004, Paper no 12A Bhattacharya, S and Bolton, M.D (2004c): Pile failure in earthquake liquefaction Theory and Practice, International Conference on Cyclic Behavior of Soil and Liquefaction phenomenon CBS- 04, Bochum, Germany, 31 st March 2 nd April, A.A. Balkema Publisher. 8. Bhattacharya, S and Bolton, M.D (2004d): Buckling of piles during seismic liquefaction, Paper number 95; 13 th World Conference on Earthquake Engineering, Vancouver, Canada. Bhattacharya Page 23 of 24

8 9. Bhattacharya,S. Madabhushi, S.P.G and Bolton, M.D (2003a) Pile instability during earthquake liquefaction, Proc of the 16 th ASCE Engineering Mechanics Conference (EM 2003), Paper no 404, University of Washington, Seattle th July Can be seen at Bhattacharya, S., Madabhushi, S.P.G, Bolton,M.D, S.K.Haigh and Soga, K (2003b) A reconsideration of the safety of the piled bridge foundations in liquefiable soils. Technical report (TR 328) of Cambridge University. ( This is accepted for publication in Soils and Foundations (Japan). 11. Bhattacharya, S. (2003). Pile instability during earthquake liquefaction. PhD thesis: University of Cambridge, U.K. 12. Bhattacharya, S (2002); Analysis of reported case histories of pile foundation performance during earthquakes, 7 th Young Geotechnical Engineers Symposium, th July 2002, Dundee (U.K) 13. Bond, A.J Behaviour of displacement piles in over-consolidated clay PhD thesis, Imperial College (U.K). 14. Eurocode 8 (Part 5): Design provisions for earthquake resistance of structures, foundations, retaining structures and geotechnical aspects, European Committee for Standardization, Brussels. 15. Finn W.D.L and Thavaraj, T (2001)/. Deep foundations in liquefiable soils: Centrifuge tests and method of analysis. Proc. of the 4 th Int. Conf. on recent advances in geotechnical earthquake engineering and soil dynamics. San Diego, California, March 26-31, Fukuoka, M (1966) Damage to Civil Engineering Structures, Soils and Foundations, Volume-6, No-2, pp Hamada,M (1992) Large ground deformations and their effects on lifelines: 1964 Niigata earthquake, 1983 Nihonkai-Chubu earthquake Case Studies of liquefaction and lifelines performance during past earthquake,. Technical Report NCEER , Volume Ishihara,K (1997). Geotechnical aspects of the 1995 Kobe earthquake, Proc. of ICSMFE, Hamburg, pp Indian Road Congress Code (IRC 78, 2000): Standard specification and code of practice for road bridges, Section VII, Foundations and Substructure (Second revision). Published by the Indian Road Congress, Jamnagar House, Shahajahan Road, New Delhi JRA (1996), Specification for Highway Bridges, Part V, Seismic Design, Japanese Road Association. 21. Kawamura, S., Nishizawa, T., and Wada, T., (1985): Damage to piles due to liquefaction found by excavation twenty years after earthquake, Nikkei Architecture, Tokyo, Japan, pp (in Japanese). 22. NEHRP 2000 (National Earthquake Hazard Reduction Program). Commentary for Federal Emergency Management Agency (FEMA, USA 369). Seismic regulation for new buildings. 23. NISEE: National Information Services for Earthquake Engineering, University of California, Berkeley. 24. Soga,K. Geotechnical aspects of Kobe earthquake, Chapter 8 of EEFIT report on Kobe earthquake, Institution of Structural Engineers, UK. 25. Schofield, A.N (1981) Dynamic and earthquake geotechnical centrifuge modelling, Proc. of the Int. Conf on recent advances in geotechnical earthquake engineering and soil dynamics. 26. Timoshenko, S.P and Gere,J.M Theory of elastic stability, McGraw Hill book company, New York, Tokimatsu, K. Mizuno, H and Kakurai, M (1996). Building Damage associated with Geotechnical problems. Special issue of Soils and Foundations, pp Tokimatsu, K., Oh-oka Hirishi, Satake, K., Shamato Y., Asaka Y (1997); Failure and deformation modes of piles due to liquefaction-induced lateral spreading in the 1995 Hyogoken-Nambu earthquake., Journal Struct. Eng. AIJ (Japan) No 495, pp Bhattacharya Page 24 of 24