Piled foundation, failure, liquefaction ... Predominant loads acting on piled foundations during earthquakes and .... Fleming, W.G.K., Weltman, A.J., Randolph.
Essential criteria for design of piled foundations in liquefiable soils by
S. Bhattacharya and K. Tokimatsu
at the 39th Japan National Conference on Geotechnical Engineering, NIIGATA (JAPAN) 7th to 9th July 2004.
Presented
Paper number 904, Page 1805-1806
Proceedings of the 39th Japan National Conference on Geotechnical Engineering, Niigata, 7th-9th July 2004
Essential criteria for design of piled foundations in seismically liquefiable areas Piled foundation,
failure,
liquefaction
Tokyo Tech Tokyo Tech
Member ○ S. Bhattacharya Int.Member K. Tokimatsu
1. Introduction Structural failure by the formation of plastic hinges in piles passing through liquefiable soils has been observed in the majority of the recent strong earthquakes such as 1995 Kobe (JAPAN), 1999 Kocheli (TURKEY) and 2001 Bhuj (INDIA). The failure not only occurred in laterally spreading soils but also was observed in level grounds; see for example Tokimatsu et al (1997). The failure was often accompanied by settling and or tilting of the structure rendering it unusable after the earthquake. This paper proposes criteria for safe design of piled foundations in seismically liquefiable areas. 2. Predominant loads acting on piled foundations during earthquakes and fundamental failure mechanisms During earthquakes, the predominant loads acting on a pile are: (1) Axial load (P) that acts at all times. The axial load may increase due to inertial effect of the superstructure and kinematic effects. (2) Inertial loads due to the superstructure which is oscillating in nature. (3) Loads due to ground movement commonly known as kinematic effects. This load can be of two types, such as transient (during shaking due to the dynamic effects) and residual (after the shaking ceased due to lateral spreading). The fundamental mechanisms that can cause formation of plastic hinges in a pile are: (1): Shear failure in the pile due to the lateral loads such as inertia or kinematic loads or a combination of the above. This is particularly damaging to hollow circular concrete piles (non-ductile) with a low shear capacity. (2): Bending failure due to the combined effect of lateral and axial loads i.e. formation of a collapse mechanism as shown schematically in Figure 1(a). (3): Buckling failure in slender piles due to the effect of axial load and the loss of surrounding confining pressure provided by the soil owing to liquefaction, see Bhattacharya (2003), Bhattacharya and Bolton (2004), Bhattacharya et al (2004). Buckling is sensitive to imperfections such lateral loads, out-of-line straightness. This would imply that in presence of lateral loads, a pile would buckle at a load much lower than the Euler’s Critical Load. A typical configuration is shown in Figure 1(b). The above three forms of failure is stated as “Limit State of Collapse”. It must be mentioned here that each of these types of failure can cause a completely collapse of the foundation. It is worth noting that the pile will also lose its shaft resistance in the liquefiable region due to the loss of effective stress, and thus have to settle for vertical equilibrium. In order to be functional after the earthquake, the settlement of the piled foundation should be within the acceptable limits for the structure. This can be termed as “Serviceability Limit-State”. Lateral spreading starts Euler’s buckling of equivalent pinned strut
Non-liquefied crust maybe present
(Leff)
Liquefiable zone(DL)
Effective length (Leff)
Liquefiable zone Plastic hinges to be formed for failure
(DL)
(DF)
Dense non-liquefiable zone
Dense non-liquefiable zone
Length of the pile(L)
This pile being analysed
Point of fixity in non-liquefied zone
The buckling instability load of a piled structure can be calculated by estimating the buckling load of one pile and multiplying by the number of piles forming the foundation. The buckling load of one pile can be estimated using the π2 Pcr = 2 EI , where EI is the bending Leff stiffness of the pile and Leff is the effective length of the pile based on the fixity of the pile above and below the liquefiable soil, see Bhattacharya et al (2004).
Figure 1: Loads and Collapse mechanisms on a piled foundation; (a): Plastic collapse mechanism due to combined loads; (b): Buckling mechanism.
3. 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 1(a) 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 DF in Figure 1(a). Typical calculations carried out using the method proposed by Davisson and Robertson (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. Essential Criteria for design of piled foundations in seismically liquefiable areas.
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S. Bhattacharya, Tokyo Institute of Technology K. Tokimatsu, Tokyo Institute of Technology
Proceedings of the 39th Japan National Conference on Geotechnical Engineering, Niigata, 7th-9th July 2004
4. Simplified design 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 “beam-column” (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 closedform 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 (Mx) and minor-axis moment (My).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 2 shows the study of 15 reported case histories of pile foundation during earthquakes, after Bhattacharya et al (2004). Six of the piled foundations survived while others suffered severe damage. Essentially, it is assumed that the pile is unsupported in the liquefiable zone. For each of the case histories, the Leff of the pile in the liquefiable region is plotted against the minimum radius of
0.8 0.7 0.6 0.5
Minimum dia of pile from buckling consideration 2.25 2
Good performance
0.4 0.3 0.2 0.1 0
Poor performance
Diameter of pile (m)
In the figure, a line representing a slenderness ratio of 50 could differentiate the good performance piles from the poor performance. Thus the study shows that piles should be designed as short column, i.e. large diameter piles are better.
(rmin) m
I / A where gyration (rmin) of the pile. rmin is introduced to represent piles of any shape (square, tubular, circular) and is given by I is the second moment of area; and A is the cross sectional area of the pile section. For a solid circular section; rmin is 0.25 times the diameter of the pile and for a hollow circular section rmin is 0.35 times the outside diameter of the pile. Leff is dependent on the thickness of the liquefiable zone, depth of embedment and the fixity at the pile head. Concrete pile
1.75
Steel tubular pile
1.5 1.25 1 0.75 0.5 0.25 0
0
10 20 30 40 Effective length (Leff) m
Figure 2: Study of 15 case histories, Bhattacharya et al (2004).
50
4
6
8
10
12
14
16
18
20
Thickness of liquefiable layer (m)
Figure 3: A typical graph showing the diameter of a pile required to avoid buckling.
Figure 3 shows a typical graph showing the minimum diameter of pile necessary to avoid buckling (and carrying out non-linear analysis) depending on the thickness of liquefiable soil. The slenderness ratio is kept around 50. The main assumptions are that the piles are solid concrete section having E (Young’s modulus) of 22.5×103MPa) and for steel E of 210GPa. The piles are not in a single row and at least in 2×2 matrix form. 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. 5. Proposed failure criteria in simplified design approach The proposed design criteria for piles are as follows: (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 0.003. 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). 6. Conclusions For design of piles in seismic liquefaction areas, most of the codes of practice focus on bending strength and do not mention the bending stiffness required to avoid buckling in the event of soil liquefaction. The current design codes needs to address buckling of piles due to the loss of soil support owing to liquefaction. Analytical studies and case histories shows that to avoid buckling instability of piles, it is necessary to keep the slenderness ratio of the piles in the liquefiable zone below 50; i.e. length to diameter ratio of about 15. Simplified design chart to avoid buckling of solid concrete and steel tubular piles passing through liquefiable soils have been proposed in the paper. A pile must also be sufficiently embedded in the non-liquefiable hard layer below the liquefiable soil to ensure fixity and avoid sliding. The settlement of the structure due to loss of shaft resistance of the pile in the liquefiable soils should be within acceptable limits. References Bhattacharya, S and Bolton, M.D (2004): “A fundamental omission in seismic pile design leading to collapse”, Proceedings of the 11th International Conference on soil dynamics and earthquake engineering, Berkeley, 7 - 9th Jan 2004, pp 820-827. Bhattacharya, S., Madabhushi, S.P.G., and Bolton, M.D. (2004): “An alternative mechanism of pile failure in liquefiable deposits during earthquakes”, Geotechnique 54, April issue, No.3. Bhattacharya (2003): “Pile instability during earthquake liquefaction”, PhD thesis, University of Cambridge (U.K). September 2003. Davisson, M.T. and Robinson, K.E (1965): “Bending and buckling of partially embedded pile, Proc. 6th. Int. Conf of Soil Mechanics and Foundation Engineering, Canada, Volume 2, pp- 243-246. Fleming, W.G.K., Weltman, A.J., Randolph. M.F., and Elson, W.K (1992), Piling Engineering, Surrey University Press, John Wiley and Sons. NY. Tokimatsu, K., Oh-oka Hiroshi, Satake, K., Shamoto Y. and 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 95-100.
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