nisee

National Information Service for Earthquake Engineering
University of California, Berkeley

      In earthquake-resistant foundation design, the following two main (basic) guidelines should be borne in mind:  first, select a foundation layout and substructure system as simple as possible; and second, tie together the different elements of the substructure [17, 18].  The latter is of utmost importance in the case of a structure built at a site with poor, loose saturated granular soil, where moderate or strong ground-shaking (with effective peak accelerations exceeding about 0.15g) involving several cycles may cause permanent horizontal displacements due to lateral spreading and/or subsidence of the ground.  As illustrated by Slides  J12-J18 these types of soil movement can cause damage to structures.  Other typical examples of the importance of tying together the substructure are illustrated in Slides J42 and J43.

      When the surface soils are very soft and/or can liquefy, piles can be used to advantage.  However, they should be properly designed, keeping in mind the following considerations: first, as discussed above, the pile caps should be tied together with tie beams or a reinforced concrete slab that can work in tension and compression so that the foundation can act as a unit (assuring the integrity of the foundation); second, bearing, rather than friction piles, should be used if the foundation materials might liquefy; third, the piles should be able to carry not only axial but also shear and bending forces (which can be developed due to relative horizontal displacements between different layers in the soil deposit).  Therefore, in the case of concrete piles, these should not only be longitudinally reinforced but also confined by suitable lateral reinforcement, particularly immediately below the pile cap.  Slides J44-J46 illustrate that when properly designed piles are used, it is possible to design and construct buildings even in very soft soil that can liquefy during an earthquake.

      This modern school consists of a series of one- and two-story reinforced concrete buildings built on a soil that liquefied, as it was shown in Slide J18.  Due to the liquefaction the soil settled in some places as much as 0.50 meters (Slide J45).  In spite of this the damage to the buildings was minimal because they were supported on a properly designed pile foundation.

      Dramatic collapse of bridges has been induced by failure of their foundations and/or supports and by the lack of integral action between the substructure and the superstructure [Volume II, Ref. 6 and Ref. 19].  Liquefaction of loose saturated granular foundation soils has been a major source of bridge failure, of which perhaps the Showa Bridge collapse illustrated in Slide J14 has been the most publicized.  Note that the collapse of the Showa Bridge was due to the relative movement of the bridge piers which were supported on steel piles driven through loose sands below the mud line.  The distortion of the pile caused by the loss of lateral support from the liquefied sands induced the relative movement of the piers, causing the simple unconnected spans of the bridge to fall [19].

      There have been numerous other bridge failures at sites where liquefaction did not occur, and examples of this type of damage to freeway bridges are illustrated in Slides J47-J50.

      Due to lack of adequate anchorage detailing between the supporting columns and the piles the reinforcing bars of the columns pulled out of the pile cap or the pile shaft as illustrated in Slide J48 [Volume II of Ref. 6].

The University of California, Berkeley
Copyright 1997, The Regents of the University of California.
Structural Engineering Slide Library, W. G. Godden, Editor
Set J: Earthquake Engineering, V. V. Bertero

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