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Experimental data on the processes of vibro-driving and vibro-extracting were obtained for the first time by D. D. Barkan, who established the possibility of a significant decrease in the constant force required to drive a pile or extract it, in the case when the pile makes longitudinal vibrations.
The practice of applying the vibromethod provides convincing examples of the reduction of the mentioned forces. So, for example, in order to overcome the resistance of the soil to the immersion of a shell pile, static forces of thousands or tens of thousands of kilonewtons would be required; however, thanks to the alternating forces excited by a vibrator mounted on it, the shell pile is immersed under the action of gravity. It is also known that in order to extract a casing pipe from a deep well, it is required to apply a static force of several thousand kilonewtons to it, while with intense vibration, it is enough to apply a static force slightly exceeding the gravity of the extracted system to extract this pipe.
D. D. Barkan explained the effect of vibration exposure by a sharp change in the mechanical properties of the soil under the influence of vibrations in the zone adjacent to the pile being loaded or extracted.
This point of view for a number of soil conditions has been confirmed both by natural observations and special experimental studies.
Thus, the experience of vibrational driving of piles into water-saturated sands showed that, due to the vibrational impact in the area surrounding the pile being driven, the sand liquefies and a sharp decrease in the forces of resistance to sinking occurs. Vibratory driving in water-saturated sands is usually very effective.
Experimental studies of the effect of vibration on the change in the internal friction of sand gave the following results.
At a constant amplitude, the coefficient of internal friction of sand gradually decreases with increasing vibration frequency and then drops sharply in a certain critical frequency range. A further increase in frequency has almost no effect on the coefficient of internal friction.
At a constant frequency, with increasing amplitude, the coefficient of internal friction gradually decreases, tending to a certain limit, after which an increase in amplitude does not lead to its further decrease. The effect of vibration exposure increases in proportion to the diameter of the sand grains and significantly depends on its moisture content.
Studies of the shear of plates on sand under the action of vibrational and static loads (N. N. Maslov, 1959) showed that the change in the shear resistance of sandy soils that perceive vibration effects is mainly determined by a change in the stress state of the soil. This change can occur due to both the transition of sand to a liquefied state, and the imposition of additional stresses on the stressed state (in static conditions) arising from vibrational effects (P. L. Ivanov, 1964).
Experiments with a vibrating steel skid dragged along the surface of the sand have established that the friction forces of the skid on the sand are immediately completely restored after the end of vibrating (Yu. V. Izbash, 1959.)
The liquefaction of clay soils is associated with the thixotropic properties of clays. It has been established that the zone in which thixotropy manifests itself when vibrodrilling clay soils is small. its thickness does not exceed a few millimeters (B.M. Rebrik, 1966).
The shear resistance of clays decreases with increasing frequency and amplitude of oscillations, but to a lesser extent than that of sands.
Experiments with a vibrating steel skid dragged along the surface of a clayey soil showed that in clayey soils, under the influence of the skid vibrations, the near-wall layer liquefies. As a result, the forces of external friction after the cessation of vibration are lower than those observed before the start of vibration. During the “relaxation” of the soil, these forces increase again. E. M. Perlei (1964) found that the shear resistance of a vibrating surface along the ground in low-moisture sands corresponds to the hypothesis of dry friction, and in water-saturated sands and in plastic sandy loams, the type of friction approaches viscous. Under the influence of vibrations, the resistance to piling in water-saturated soils sharply decreases. At the same time, in low-moisture soils, the resistance during vibrational pile driving changes insignificantly.
The nature of the decrease in resistance during vibration driving of a pile in sandy and clayey soils is different. When the sands are liquefied, contact between the particles is lost, the soil passes into a fluid state, and then becomes compacted. The nature of thixotropic changes in clayey soils during vibro-immersion is more complex, since the softening of water-colloidal films weakens the bonds between soil particles and reduces the pile sinking resistance.
Significant influence on the nature of changes in soil properties have the magnitude of the acting dynamic forces. With weak dynamic impacts, the vibration effect is determined mainly by the change in the stress state of the soil near the vibrating pile. At the same time, under strong dynamic impacts, the soil structure is destroyed, the latter acquires the properties of a viscous medium, which leads to a significant decrease in the pile sinking resistance.
The physical picture of the vibration immersion process can be significantly different. So, for example, when immersed in weak water-saturated soils, the resulting alternating hydrodynamic pressure under the tip of the pile leads to deconsolidation of soil particles and their subsequent liquefaction. Driving resistance is reduced both along the lateral and frontal surfaces. The pile is lowered under the action of the gravity of the submerged system.
At the same time, during vibration driving into low-moisture soils, impacts of the pile toe against the soil, which in this case is compacted and protrudes to the sides, are of decisive importance for the efficiency of the process.
Both for sandy and clayey soils, when vibrators operate at frequencies of about 100 Hz and transverse accelerations up to 1.5 g, the main reason for the decrease in skin friction is transverse deformations during the propagation of longitudinal elastic waves along the submerged element. Since the physical and mechanical properties of low-moisture sandy and dense clay soils change little under the influence of vibrations, a number of researchers began to look for the reason for the usefulness of vibration, not by changing soil properties during vibration immersion, but in the efficiency of overcoming soil resistance by vibration method.
Yu. I. Neimark and I. I. Blekhman took the second path almost simultaneously. Yu. I. Neimark (1952) explained the decrease in the lateral resistance of the soil to the extraction of a vibrating body from it, without resorting to the hypothesis of a change in the properties of the soil, and showed that the average force of extracting the vibrating body becomes much less than the static resistance force (dry friction force). In this case, the value of the average force almost linearly depends on the speed of the vibrating body (the so-called phenomenon of linearization or “transition” of dry friction into viscous.)
The effectiveness of the vibration impact (without taking into account changes in the properties of the contacting bodies) was most clearly explained in the work of I. I. Blekhman and G. Yu. Dzhanelidze (1958). When determining the effective friction coefficient (the concept of which was introduced by the authors,) in contrast to the classical friction coefficient, along with a constant shear force, a periodic driving force is introduced. When using the effective friction coefficient in the calculations, the value of the friction force decreases significantly at a constant value of the true friction coefficient, i.e., without taking into account changes in soil properties.
Due to the complexity of the phenomena accompanying vibrational immersion under various conditions, even when conducting subtle physical experiments, it is difficult to assess to what extent the vibration effect is determined by the application of additional periodic forces to the immersed body and to what extent by the change in soil properties, since these processes are inseparable. Such a scene can be made by analyzing the energy consumption during vibrational immersion.
It is known that the energy required to move a vibrating body must be greater than the energy required when moving a body without vibration (I. I. Blekhman, G. Yu. Dzhanelidze, 1964). If, however, when moving a vibrating body, the energy consumption decreases, then this means that the physical properties of the contact zone of the contacting bodies change, i.e., the true coefficients of friction.
So, for example, in low-moisture sands, the energy spent on vibro-driven piles is much greater than the energy that would be required to statically jack piles at the same speed.
Another result is obtained by vibrating the pile into water-saturated sands. In this case, the energy consumption during vibration driving of the pile is significantly reduced in comparison with the energy of static jacking, which indicates a sharp decrease in the forces of resistance to sinking, i.e., a change in soil properties.
Thus, at present, the energy criterion is the most reliable indicator by which it is possible to judge the change in the properties of the soil during vibrational action on it.
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