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The different soil profile, the SPT—Nvalue with depth and the variation of shear modulus of differentsoil strata are presented in Fig. 2. Some of the measured soil properties of borehole 1 (BH-1) used in this study are presented inTable 1.The piles were constructed in the field by bored cast-in-situmethod. A borehole of diameter 0.1m was made up to the depthequal to the length of the pile by the auger method. After boringthe reinforcement casing was placed into the hole. The boreholewas then filled up to the neck of the longitudinal reinforcement ofthe pile by pouring the concrete into hole. At the time of casting,the longitudinal bar of pile was connected properly with thereinforcement of pile cap, so that the pile and the pile cap act asmonolithic.Then concrete was placed to construct the pile cap.Four nuts were attached to the top of the pile cap at the time ofcasting of pile cap in order to connect the total loading system tothe pile. The diameter of all the piles was 0.1m and dimension ofpile caps was 0.57m 0.57m 0.25m. In total, twelve sets of pilewere constructed. Three sets of single pile of three differentlengths (L=1.0, 1.5 and 2.0m) and nine sets of 2 2 pile groups(s=2d,3d and 4d for each pile length L, where L=1.0, 1.5 and2.0m) were used for the investigation.3. Dynamic pile tests and test resultsForced vibration tests were conducted on the piles in verticaldirection. The mechanical oscillator (Lazan type) was used to induceunidirectional vibrations on pile foundation. The mechanicaloscillator consists of two shafts arranged such that they rotate inopposite direction at the same speed when one of them is driven bya motor. Such an arrangement induces unidirectional sinusoidalvibratory force passing through the center of gravity of the oscillator.Initially the mass on each shaft is balanced when angle y is zero. When angle y is set to a value, mass on the shaft become eccentricand value of me together is given byme ¼ Wge ¼ 0:9sinðy=2ÞgNsec2ð1ÞThe dynamic force (P) in Newton corresponding to this y at anyfrequency can be expressed asP ¼meo2¼ 0:9 sinðy=2Þgo2N ð2ÞwhereWand m are the weight and mass of eccentric rotating part inoscillator, respectively, e is the eccentric distance of the rotatingmasses, g is the acceleration due to gravity and o is the circularfrequency of vibration. The magnitude of the exciting force (P) canbe changed by adjusting the angle of the eccentric mass (y).A number of mild steel ingots or test bodies were placed on thetop of the oscillator to provide desired static weight. The test bodywas comprised of steel ingots each weighing 650N (8 nos) and450N (10 nos). Whole setup was then connected so that it acts asa single unit. The mechanical oscillator was connected by meansof a flexible shaft with a motor and its speed was controlled by aspeed control unit. The vibration measuring equipment consistedof two piezoelectric acceleration pickups and the associatedvibration meter. The pickups were attached vertically on top ofthe steel ingots. The complete experimental setup for verticalvibration of piles is shown in Fig. 3.The steady state dynamic response of the piles under verticalexcitation was measured under different frequencies and excita-tion intensities. Two different static loads (Ws=10 and 12 kNincluding the weight of the pile cap and oscillator) were used. Foreach static load, tests were conducted at four different eccentricmoments (We=0.187, 0.278, 0.366, and 0.450Nm). All the testswere carried out for the following two different embedded depths(h) of pile cap:Case 1: Pile cap embedded into soil (h=0.175m)Case 2: No contact of pile cap with soil (h=0)Frequency versus amplitude data shown by different symbolsof vertical vibration for single pile and a pile group are presented in Figs. 4 and 5, respectively. It can be seen that the observedresponse curves display nonlinearity as the resonant frequenciesdecreases with increasing excitation intensity and also the ampli-tudes are not exactly proportional to the excitation intensity. Theeffects of static loads on the vertical dynamic response of piles arestudied and it is observed that both the natural frequency andresonant amplitude decreases as the static load increases. Thevariations of natural frequency and resonant amplitude wereobserved for different s/d ratio of pile groups and it is found thatthe natural frequency increases and the resonant amplitudedecreases with increasing pile spacing for all L/d ratios. Differentresponse curves are obtained for different L/d ratios and it isobserved that the natural frequency increases and resonant ampli-tude decreases with increasing length of piles. It is also found thatembedded pile cap (Case 1) produced higher stiffness and damp-ing of pile than the no contact condition of pile cap (Case 2).4. Evaluation of nonlinear vertical response