EARTHQUAKE RESISTANT DESIGN OF FOUNDATIONS: DESIGN PRINCIPLES Dr. Ravi Sankar Jakka Asst. Professor (Soil Dynamics) Department of Earthquake Engineering Indian Institute of Technology, Roorkee 1. Introduction Foundation is a substructure built below the super structure. Purpose of the foundation is to transfer the structural loads safely to the underlying soil. Safe and economical design of a foundation under different loading conditions is the role of geotechnical engineer. Earthquake loads are the most complicated and complex. Design of earthquake resistant foundation is highly challenging. Proper design of a foundation against earthquake loading requires through understanding over the behavior of soil, response of structure and interaction of soilstructure under earthquake loading. This material starts with few case studies on foundation failures due earthquake loads to understand the mechanisms of foundation failures. Later, general requirements in the design of an earthquake resistant foundation such as selection of site, selection of appropriate type of foundation, are presented. Codal provisions given in the IS1893 for the design of earthquake resistant foundation are also discussed. 2. Definitions and Terminology Used (IS 6403) Ultimate bearing capacity (q ult or q u ): The intensity of loading at the base of the foundation which would cause shear failure of the soil support. Safe bearing capacity (qsafe or qs): Maximum intensity of loading that the foundation will safely carry without the risk of shear failure of soil irrespective of any settlement that may occur. Safe bearing pressure: The intensity of loading that will cause a permissible settlement or specified settlement of the structure Allowable bearing capacity (qa): The net intensity of loading which the foundation will carry without undergoing settlement in excess of the permissible value for the structure under consideration but not exceeding net safe bearing capacity. 1
3. Case Studies on Earthquake Induced Shallow Foundation Failures Figure 1 Overturning failure of apartment complex buildings during Niigata 1964 earthquake (courtesy of USGS) Figure 2 Foundation failure of a house due to ground slope failure during Loma Prieta quake. Dr RS Jakka 2
4. Modes of Shallow Foundation Failures and their Causative Mechanisms Tilting of foundation(overturning of the structure): Its a clear shear (Bearing Capacity) failure of the supporting soil Due to the action of inertial forces Due to the reduction in strength of supporting soils (liquefaction) Due to the occurrence of sand boils and lateral spreading Ground Instability Caused by slope failures (due to flow slides and lateral spreading) Sliding of foundation Sliding may occur due to the horizontal inertial forces applied by an earthquake Sliding may also occur due to movement (lateral spreading) of under lying soils Settlement of foundations Due to compression/consolidation of liquefied soil upon dissipation of excess PWPs. Due to occurrence of sand boils and lateral spreading 5. Case Studies on Earthquake Induced Deep Foundation Failures Figure 3 Collapse of a pile-supported building due to lateral movement of soil during the Kobe earthquake (Bhattacharya, 2006) Dr RS Jakka 3
Figure 5. Kandla tower after Bhuj earthquake, (Madabhushi et al 2001) 6. Modes of Deep Foundation Failures and their Causative Mechanisms Overturning of the structure due shear failure at pile heads and pile cap Caused by action of inertial forces on the piles Breaking/shear failure of piles in the liquefied soil layer Caused by lateral spreading of liquefied soils, which increases bending stresses (Tokimatsu et al., 1998) Caused by reduction in lateral confinement due to liquefaction of soils, which can lead to buckling of piles (Bhattacharya, 2007) Settlement/failure of foundation may occur Due to reduction in the shear strength of liquefied soils if it is not considered in the initial design Dr RS Jakka 4
7. Design/Construction of a Foundation Figure 6 Various stages in the design/construction of foundations (Gulhati and Datta, 2005) 8. Foundation Design and Parameters Selection of suitable type of foundation Subsurface soil characteristics Magnitude of loads from the superstructure Requirements of super structure Specify dimensions of the selected foundation Shallow foundation - - - - >Strip/Individual/Combined/Raft - - - - > Length, Width, Depth Deep foundation - - - - -> Pile/pier/well - - - - > Length, Diameter, No. of piles Dr RS Jakka 5
9. Selection of Suitable Type of Foundation Shallow foundations should usually be the first choice of the foundation as they are economical no need for special construction techniques no need of special equipment for drilling better quality control Isolated footings are not recommended when excessive settlements and/or large lateral soil movements (lateral spreading) are expected under design earthquake. Mat foundation are preferred On soft or loose soils Stronger than isolated, continuous and tied foundations Earthquake hazards from differential soil movements are minimized by bridging over loose pockets of soil Deep Foundations are preferred When loads from super structure are too high W.T is shallow Restrictions over the open excavation To bypass liquefiable soils 10. Site Selection and Alternative Choices in their Order of Preference 1. Avoid the problematic site if alternative choices are available. Particularly, if site conditions are hazardous, it has to be avoided. (As per IS1893, liquefiable sites are to be avoided/improved) 2. Redesign the foundation if soil conditions are poor (highly compressible under earthquake loading). Go for raft foundation. 3. By pass liquefiable soil by deep foundations, if competent soil(dense sands, stiff clays or rocks) exists at shallow depths (say, 10 to 20m) 4. Improve poor soil by excavation and replacement (GI at Shallow Depths) 5. Treat soil in place (In-situ ground improvement) if poor soil conditions exists up to large depths (say, 20 to 30m) Note: Even while designing deep foundations, poor soil at shallow depths may require ground improvement or else, special considerations are required in the design of deep foundation. Dr RS Jakka 6
10.1. Sites to be Avoided Avoid construction of structures/foundations near vicinity of active faults Site specific studies are to be carried out in high seismicity regions, as local soil and site conditions play important role on site amplifications. Unstable slopes are to be avoided. Ground stability should be assessed before construction under designed seismic action. Loose to medium dense fine sands (SP), located adjacent to deep rivers and located in active seismic regions are to be avoided. Dormant or active mine or cavernous lime stones should be avoided as ground may collapse Flood plains, landfills of hazardous waste sites should also be avoided 11. Relevant Codal Provisions from IS:1893-Part 1(2002) for the Earthquake Resistant Design of Foundations 11.1. Influence of Soil Type on Intensity of Shaking: (i) Influence of soil type on intensity of shaking is accounted in the calculation of horizontal seismic coefficient. Z I * 2 R h A h * Sa g where, Z = Zone Factor (Range: 0.36, 0.24, 0.16 & 0.10) Appropriate zone factor is to be selected based on the location of the project site I = Importance Factor (Range: 1.0 & 1.5) R = Response Reduction Factor (Range: 3.0 to 5.0) Sa = Avg. spectral acceleration coefficient(fig. 2, IS1893), where effect of soil g type is considered (Maximum value 2.5) Dr RS Jakka 7
(ii) Influence of soil type on vertical shaking is also considered indirectly. 2 * 3 v Av h 11.2. Increase in Allowable Bearing Pressures in Soils: IS code permit increase in allowable bearing pressures in soils under earthquake forces, to account the uncertainties in the earthquake loading. When earthquake forces are included, the allowable bearing pressure in soils shall be increased as per Table 1, depending upon type of foundation of the structure and type of soil(is 1893). 11.3. Accounting for Liquefiable Soils: (i) In soil deposits consisting of submerged loose sands and soils falling under classification SP with standard penetration N-values less than 15 in seismic Zones III, IV, V and less than 10 in seismic Zone II, the vibration caused by earthquake may cause liquefaction or excessive total and differential settlements. Such sites should preferably be avoided while locating new settlements or important projects (IS 1893). Desirable N-Values (corrected Values) (ii) (iii) If N-values (corrected values) at the project site are lower than the desired N-values and if there is no option to avoid the site, appropriate site improvement techniques (such as improving compaction or stabilization) should be adopted to achieve suitable N-values. Alternatively, deep pile foundation may be provided and taken to depths well into the layer which is not likely to liquefy (IS 1893). Dr RS Jakka 8
(iv) Marine clays and other sensitive clays are also known to liquefy due to collapse of soil structure and will need special treatment according to site condition(is 1893). Note: As we have seen the failures of pile foundations due lateral spreading and lack of confinement of liquefiable soils, it is very important to design pile foundations to account these liquefaction effects. Otherwise, suitable ground improvement techniques are to be adopted to liquefiable soils even after designing pile foundations. 11.4. Other Guidelines for the Design of Earthquake Resistant Foundations (IS 1893) (i) (ii) (iii) (iv) The allowable bearing pressure shall be determined in accordance with IS 6403 or IS 1888. If any increase in bearing pressure has already been permitted for forces other than seismic forces, the total increase in allowable bearing pressure when seismic force is also included shall not exceed the limits specified above. The piles should be designed for lateral loads neglecting lateral resistance of soil layers liable to liquefy. Isolated R, C.C. footing without tie beams, or unreinforced strip foundation shall not be permitted in soft soils with N<1O. Based on the case studies discussed, the following point is required attention. Note: In addition to neglecting lateral resistance of soil in the design of pile foundation, the loading coming from the lateral spreading of liquefied soil is to be considered. Buckling failure of the pile foundation due to liquefaction of soils, is also required to be examined. 12. References Bhattacharya, S. (2006). Safety assessment of existing piled foundations in liquefiable soils against buckling instability, ISET Journal of Earthquake Technology, Technical Note, Vol. 43, No. 4, December 2006, pp. 133-147. Bhattacharya, S. (2007). A review of methods for pile design in seismically liquefiable soils, Design of Foundations in Seismic Areas: Principles and Applications, Edt. Bhattacharya, NICEE Publication, IIT Kanpur, India, pp.255-295. Gulhati, S. K., and Datta, M. (2005). Geotechnical Engineering, Tata McGraw-Hill Publishing Company Limited, New Delhi. Dr RS Jakka 9
IS 1893 (Part 1) (2002). "Criteria for earthquake resistant design of structures, Bureau of Indian Standards, New Delhi. IS 6403 (1981). "Determination of bearing capacity of shallow foundaitons, Bureau of Indian Standards, New Delhi. Madabhushi, S.P.G., Schofield, A. N., and Lesley, S. (1998). A new stored angular momentum based earthquake actuator, Proceedings of Centrifuge, Tokyo, pp. 111-116. Tokimatsu, K., Oh-oka Hiroshi, Satake, K., Shamoto Y., and Asaka Y. (1998). Effect of lateral ground movements on failure pattern of piles in the 1995 Hyogoken- Namu earthquake, Proceedings of a speciality conference, Geotechnical Earthquake Engineering and SoilDynamics III, ASCE Geotechnical Special publications No 75, pp 1175-1186. Dr RS Jakka 10