INFLUENCE OF PILE LENGTH ON THE DYNAMIC RESPONSE OF UNDERREAMED PILES

Abstract

Pile foundations are widely used in weak soil deposits for supporting various structures. If the soil in the top layers of the ground does not have adequate bearing capacity then the loads are transferred to the lower strata using deep foundations i.e. piles. If the soil is expansive in nature and hard strata are absent then under-reamed piles are used. Under-reamed piles are bored cast-in-situ concrete piles having bulbs towards their lower end. In addition to static loads, pile-supported foundations and structures may be subjected to dynamic loads such as in machine-induced vibrations, moving traffic, ocean waves and earthquakes. In recent years, with developments in offshores structures, nuclear power industry etc., the dynamic behaviour of pile foundation has received renewed attention. In thesis the dynamic response of an under-reamed pile, having a single bulb, has been investigated. The experimental programme consisted of casting of insitu under-reamed piles in a silty sand deposit having a length to diameter ratio of 15, 20 and 25. This resulted in pile lengths of 3m, 4m and Sm for a pile diameter of 200mm. The piles had a single bulb located at 430mm above the pile bottom. The pile cap consisted of a 700mmx700mmx300mm (height) cement concrete block. Foundation bolts were cast into this pile cap to acconimodate a motor-oscillator assembly. The pile was excited to different force levels using the motor - oscillator assembly. In the test programme, the piles were excited in the vertical and horizontal direction to study the pile response under vertical and horizontal vibrations. Induced accelerations were monitored using two accelerometers for vertical vibrations, placed on top of the block and three accelerometers for horizontal vibrations Frequency versus displacement plots were made to obtain resonant frequency for each eccentricity setting of the mechanical oscillator. It was observed that the natural frequency decrease with an increase in the eccentricity of the mechanical oscillator i.e. exciting force level. It was also observed that for the same eccentricity value for different piles, the natural frequency as well as the observed maximum amplitude of vibrations are almost same in case of vertical vibration a. This may be attributed to the fact that the piles are not excited to its full length. It is observed that the natural frequency decreases and resonant amplitude of pile increases with the length of pile in case of horizontal vibration. This difference is, perhaps, due to the variation of soil profile surrounding the pile. Using the experimental data, stiffness and damping of the soil-pile system were computed..