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  PILE FOUNDATIONS UNDER DYNAMIC LOADS A sand mass under vibrations tends to increase in density with a corresponding decrease in voids. In a mass of saturated sand below groundwater level, soils may be subjected to liquefaction resulting in increases in density. The movement of soil grains is associated with the decrease of effective stresses. If the soil is under a certain initial shear stress, the effect of vibrations is felt to a different degree (Prakash, 1981). A pile introduces additional shear stresses in the soil mass. Excessive settlements are likely to occur under vibrations. In order to study the effect of vibrations on piles, Swiger (1948) reported tests on piles in sand. A static load was first applied on a pile. This was then vibrated under this static load. The vibrator consisted of a plate 12 in. (30 cm) in diameter and I in. (2.5 cm) thick that was mounted with an eccentricity of 1 in. The speed of the vibrator could be varied from about 400 evolutions per minute to 3000 revolutions per minute. The pile was vibrated at its natural frequency of 500 revolutions per minute, which had been determined experimentally. The static loads on the pile were 61 and 121 kips. The rate of settlement with the higher static load was several times that with the smaller load. Agarwal (1967) and Prakash and Agarwal (1971) reported tests on vertical model piles embedded in sand at 33 percent relative density. The piles were loaded with a predetermined fraction of upward static pullout resistance. The tank containing piles was subjected to vertical vibrations at 2.3 and 5.2Hz. It was found that the number of cycles of motion needed to pull out the pile a predetermined distance of 0.8 in. (2 cm) decreased with an increase in the static vertical upward load and the vertical peak acceleration. Ghumman (1985) conducted a comprehensive series of model tests on penetration testing of piles under vertical vibrations. A model pile 2.4 in. (6 cm) in 475 Copyright © 1990 John Wiley & Sons Retrieved from:  476 PILE FOUNDATIONS UNDER DYNAMIC LOADS Frame -. 4 channels with guide rails a U Bottom load cell J Pulley Idlers Ek r Wire rope -Accelerometer Clutch Tank Figure 7.1 Ghumman, 1985). A setup for study of penetration of piles under axial vibrations (after diameter and 64in. (160cm) long was subjected to a predetermined static load. The vertical vibrations were.then imparted to the pile by a fully counterbalanced mechanical oscillator, which could be excited to different frequencies (Figurg 7.1). A typical penetration record with time at a frequency of oscillations of 10 Hz s shown in Figure 7.2. A static load of 165 lb (75 kg) had been applied on the pile head and the dynamic force level had been varied from 99 lb (45 kg) n test no. 1.5 to 132 lb (6Okg in test no. 1.6) and 198 lb (Wkg in test no. 1.8). Both the rate of penetration and total penetration increased with dynamic force. The foregoing experimental behavior highlights the importance of vibrations in inducing the settlement of piles. Earthquakes introduce lateral forces on piles. The energy supplied to a structure may be absorbed in the elastic and plastic deformations of both the Copyright © 1990 John Wiley & Sons Retrieved from:  PILE FOUNDATIONS UNDER DYNAMIC LOADS 477 Time secs) 0 49.40 98.80 148.20 197.60 247.0 Figure 7.2 Penetration versus time graph for test no. 1.5, 1.6, and 1.8 (after Ghumman, 1985). superstructure and substructure. Eccentric and inclined loads and moments may be introduced on the pile heads and pile caps. Lateral forces on the superstructure are assumed to be transferred to the ground through the pile cap as lateral loads and moments, and the stability of the piles is checked against these loads. Vertical loads are always present. These may cause buckling of the piles, particularly if freestanding lengths are large, or they may increase the deflections. Therefore, buckling of the piles and the beam- column action become important (Prakash, 1985, 1987). The pile caps of individual columns are interconnected by grade beams. Copyright © 1990 John Wiley & Sons Retrieved from:  scde 0 5 10 15 20Bm 13,750 Fignre 73 Profile of Showa Bridge showing damage to deck slabs due to out-of-phase motions of piers (Fukuoka, 1966 . Copyright © 1990 John Wiley & Sons Retrieved from:

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