粘性土に根入れされた浅い基礎部に作用する地震時土圧 [in Japanese] SEISMIC EARTH PRESSURE ACTING ON SHALLOW EMBEDDED FOOTING IN COHESIVE SOIL [in Japanese]
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This paper presents centrifuge test results of seismic earth pressure and a new theory for active earth pressure acting on a footing embedded in cohesive soil. To determine the effects of clay cohesion on the seismic earth pressure, four dynamic centrifuge tests were performed on a footing model supported by 2 x 2 piles embedded in a clay layer overlying a dense sand deposit, as shown in Fig. 2. The cohesive soils were composed of oil-based clay with cohesion values of 58, 42, 36, and 34 kN/m<sup>2</sup>. Oil-based clay is convenient for use in centrifuge tests because the consolidation process, which requires a long time, need not be performed on the clay. Plates and two-axis load cells were set up at the left and right sides of the footing. The load cells measured the compressive and vertical shear forces acting on the footing. The input motion was “Rinkai92”, which is a synthesized ground motion for the Tokyo Bay area. The maximum input acceleration was approximately 8 m/s<sup>2</sup>. The centrifugal acceleration was 50 G.<br> The hysteresis loop of the relative displacement and earth pressure acting on the right side of the footing are shown in Fig. 5. The earth pressure at the passive side acted on the footing in the form of a compressive force and its magnitude was dependent on the clay's cohesion. The earth pressure at the active side acted on the footing in the form of a tensile force and was nearly independent of the cohesion. The relative displacement required for the earth pressure to reach the active and passive state was approximately 0.4－0.8% and 3－8% of the embedment depth, respectively. The estimated passive and active earth pressures based on the Rankine and Matsunami earth pressure theories are also shown in Fig. 5. Rankine's theory underestimated passive earth pressure and Matsunami's theory overestimated it. On the other hand, both theories apparently overestimated active earth pressure in the form of a tensile force.<br> Shear adhesion ratios (shear adhesion/cohesion) measured by the 2D-load cells at the passive and active sides of the footing are shown in Fig. 6. The measured shear adhesion ratios were smaller than those proposed by Tomlinson (1963). The passive earth pressure estimated by Matsunami's theory that considers the small shear adhesion ratio, agreed with the experimental data. However, the active earth pressure estimated by Matsunami's theory did not agree with the experimental data. This indicates that the conventional earth pressure theories that assume the failure mode as a slip plane cannot explain the measured active earth pressure.<br> Separation of the footing and clay was observed after the tests, as shown in Photo 1 . In this study, a new earth pressure theory considering the separation was proposed. Figure 10 shows a proposed failure mode of the active state. The active earth pressure near the ground surface depends on the tensile adhesion because the ultimate tensile adhesion force between the clay and the footing is smaller than that caused by failure of the clay. However, the active earth pressure at the deeper layer depends on the slip plane. The active earth pressure, considering the separation between the clay and footing, could be estimated by Eq. (2). The boundary depth at which both the active earth pressures are equivalent, could be estimated by Eq. (3). The active earth pressure determined by the proposed method showed fairly good agreement with the experimental data, as shown in Fig. 13.
- Journal of Structural and Construction Engineering (Transactions of AIJ)
Journal of Structural and Construction Engineering (Transactions of AIJ) (743), 81-87, 2018-01
Architectural Institute of Japan