1853 Soil Structure Interaction to Improve Earthquake Response

Soil-structure interaction modifies the ground-input motion at the foundation. By taking advantage of soil-structure interaction, structures can be protected during earthquakes. Frank Lloyd Wright's 1915 Imperial Tokyo Hotel survived the 1923 Great Kanto Earthquake because he built his foundation in 70 ft of weak clay. During the earthquake, the ground underneath moved violently while his hotel (effectively isolated by this plastic material) remained relatively immobile.

Similarly, a common retrofit technique for bridges is to allow the foundation to rock during large earthquakes. As the foundation rocks, the period is lengthened and damping is increased, all of which lowers the demand on the structure. To insure stability, the foundation may be connected to flexible anchors or an outer perimeter of piles may be placed under (but not connected to) a widened foundation. The piles will provide support for the foundation as it rocks back and forth.

Special foundations are sometimes used to improve seismic response. For instance, a popular bridge foundation in California is the large-diameter drilled pile shaft. These foundations are very flexible, replacing potentially damaging seismic forces with large displacements. Moreover, when the pile shaft is allowed to yield, a large plastic hinge forms, which provides more ductility for the structure.

In contrast to California's efforts to provide more flexibility and ductility in its foundations, Japan has been developing stiffer and more massive foundations for bridges and other structures. Besides the advantage of handling very large forces elastically, many of these new foundations use advanced automation techniques to simplify their construction. The open caisson construction method pushes a precast hollow cylindrical caisson into the ground while excavating the ground beneath with a grab bucket. Additional sections are attached to the top until it bears on good material. The caisson segments can be constructed up to 4 m in diameter and are match-cast for a tight fit. After they are completely assembled,

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FIGURE 18.75 Precast caissons were used for the Chiba City, Japan, monorail. (Drawing Courtesy of Japan's Public Works Research Institute.)

they are filled with soil, and a steel assembly is attached to mount the substructure. This method was used for the Chiba City Monorail System. The open caisson construction method allowed them to build large caissons often within a few feet of an existing building (Figure 18.75).

Another innovative foundation in Japan is the continuous diaphragm wall. This method involves excavating wall-type ditches and casting wall elements in them (Figure 18.76). These walls are connected together with joint elements. Then, an upper slab connects the top of the walls to the pier. The walls typically vary in thickness from 1 to 2.8 m. Continuous diaphragm walls come in a variety of shapes. Circles, rectangles, and grids are all very common. These are used for deep, stiff foundations as well as enormous shafts and storage tanks.

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