RESPONSE OF A PILE DRIVEN BY LONGITUDINAL VIBRATIONS

Abstract

The vibratory method of pile driving has recently been developed and has proved to be quite successful under certain conducive conditions. The phenomenon of response of a pile driven by low frequency longitudinal vibrations is complex as the number of physical quantities involved are large. In the absence of any meaningful constitutive relations of the soil response during pile penetration under vibrations, theore tical analysis of the problem is difficult and has not been attempted fruitfully so far. Studies based on prototype tests are neither economical nor practicable. Thus it is essential to develope a working laboratory model of a pile driver for a comprehensive study of the phenomenon. After having put in consistent and time consuming efforts, a unique laboratory set-up along with related instrumentation has been raised. This laboratory pile driver ensures vertical penetration of a pile into the soil medium under axial vibrations. The model has the flexibility of changing the vibratory parameters over a certain range to study the phenomenon of vibro-pile driving. The set-up enables driving of a pipe pile (dia =6.1 cm; length = 160 cm) in a medium dense sand (D = 65%) over the range of the vibratory parameters namely static force or surcharge 50 to 125 kg, oscillator output force 30 to 150 kg, and frequency, 10 to 40 cps. Provision to vary the static force has been achieved through development of a counter balancing system. The response of pile during penetration is studied by using recording equipment in conjunction with suitably designed transducers and rotating drum system. Penetration phenomenon in a medium dense sand has been studied. The pile characteristics and soil condition have been maintained constant from test to test. The variables considered for investigations are (i) frequency (ii) oscillator output force (iii) static force (iv) oscillating mass (v) ultimate depth of pile penetration (vi) sudden depth of pile penetration (vii) time-dependent depth of pile penetration (viii) dynamic point and skin resis tances at ultimate depth of penetration (ix) static ultimate, skin and point resistances corresponding to ultimate depth of penetration. Based on the test data obtained by varying the vibra tory parameters over the limited range and its analysis, the major conclusions arrived at are (i) the critical frequency of the vibratory driver-pile-soil system can be obtained from the expression fcri7|^= 1.1828 (ii) the prediction equations for the response parameters (ultimate, sudden and (iv) (v) time dependent depths of penetration, dynamic point and skin resistances, static ultimate, skin and point resistances) evolved on the basis of dimensional analysis involve only the vibratory parameters (frequency, oscillator output force, static force and oscillating mass), acceleration due to gravity and diameter of pile (iii) the nature of pile pene tration v/s time relation is exponential (iv) for a given combination of the vibratory parameters, pile driving should be done at a frequency within a range 0.78 to 1.25 times the critical frequency of the system to get 80 to 100% of the maximum possible depth of pile penetration (v) vibro driven pile in a medium dense sand developes total static skin resistance of the order of 70 to 75% of the total static ultimate resistance.