Vibration method of bored pile installation is more efficient compared to other methods of sinking elements with relatively small cross section area. Operating principle of vibratory driver is based on sharp fall in coefficient of friction between soil and surface of sunk (extracted) element under the effect of arising oscillations. Vibration method is therefore most commonly used for driving of metal sheet piles, casings and tubular piles and for drilling activities.
Operation of a vibro-driver is based on sharp drop of coefficient of friction between a soil and a surface of the immersed (taken) element under the influence of the arising fluctuations.
For the device of bored piles the casing with the lost tip of a flat or cone-shaped form (figure 1) plunges into a soil due to the vibration influence created by vibro-driver which is rigidly fixed at the top of casing. After penetration, a reinforcing cage is inserted in the casing and the concrete mixture is pumped; when raising a tube under pressure of concrete the lost tip separates from a tube, and the formed With this technique it is possible to construct bored piles with a diameter from 273 mm to 2000 mm, up to 25 m in depth.
After the rig is installed at the point of pile construction and lost tip is mounted on the casing with a waterproofing layer pad following steps are taken (figure 2):
- Penetration to design level by driving the casing with vibration;
- Visual check of pipe body for absence of groundwater and installation of reinforcement cage into the casing prior to filling it with concrete;
- Filling the casing with concrete through the upper casing edge using bucket or concrete pump ( tremic pipes are used if required). When the casing is extracted the lost pile tip becomes detached from the pipe under concrete pressure. As the casing is extracted with vibration the concrete mix inside the borehole shaft becomes compacted;
- Formation of pile cap. Moving the drilling rig to the next point of pile installation.
At immersion of casing with vibration it is necessary to control its vertical position continuously. Extraction of casing is made in the vibration mode, and the speed of rise is limited only to absorber loading capacity.
The effect of vibration influence in soils is explained by sharp change of mechanical properties of a soil under the influence of vibrations in the zone adjoining to the immersed or taken bored pile.
At vibroimmersion there is a sealing of a soil due to its replacement in the parties in volume of a pile that as a result brings an increase of the bearing ability of the bored pile. The zone of offset of soil particles of at vibroimmersion in loq wet soils makes 2,5 — 5 radiuses of the bored pile, and in the water-saturated — 4 — 5 radiuses.
At immersion of piles in water-saturated sand, owing to vibration influence in the zone surrounding the immersed pile there is a fluidifying of sand and sharp drop of forces of resistance to immersion which upon termination of vibrating are almost completely restored. At the same time in low-damp soils resistance at vibration immersion of a pile changes slightly that can cause difficulties with a driving.
Fluidifying of clay soils is cased by thixotrop properties of clays. The zone in which thixotropy when vibrodrilling clay soils is shown, is small, thickness doesn’t exceed several millimeters. Resistance of clays to shift falls with increase in frequency and amplitude of fluctuations, but to a lesser extent, than at sand.The installation of bored piles using vibration is most effective in sandy grounds with degree of moistness 0,5 < G < 1 and clay soils – with an indicator of a consistence 0,25
At vibration immersion of elements in a soil by means of introduction of additional sign-variable forces and torques it is possible to lower significantly a constant component of force, necessary for effective immersion, that gives the chance by means of vibration cars of rather small weight to immerse elements in a soil, resistance to which introduction many times over surpasses the gravity of the vibrating system. At vibration immersion or extraction when dynamic impact on a pile is carried out by the vibroactivator which is rigidly connected to it, efficiency of process is defined mainly by the annex to a pile of significant periodic forces which together with constant forces (gravity of system, inertialess pressing, effort of extraction) provide conveyance of a pile in a soil.
To construct bored piles with vibration method Delta Drilling Company uses rigs of the following brands:
– BAUER RTG RG16T with the resonance free MR 105V vibrator
– BAUER RTG RG19T with the resonance free MR 125V vibrator (figure 1)
– BAUER RTG RG21T with the resonance free MR 150V vibrator
– ABI 12/14 with the VRZ 700GL vibrator
– ABI 12/15 with the MRZV 600 vibrator
– ABI 14/17L with the MRZV 925/18VS vibrator
– ABI 10000 with the VRZ 400 vibrator
The vibrators with having the frequency of fluctuations of 40-50 Hz exceeding own frequency of fluctuation of buildings and constructions allow to work successfully in the constrained building near buildings and constructions as provide resonance free modes of behavior, a low gear of vibration in environment and the depressed noise.
At vibration immersion the mass of the immersed element, as a rule, shouldn’t exceed 3-5 t as for the most effective immersion by this method in the conditions of considerable front resistance the relation of mass of the immersed element and shock part of a vibrohammer has to come nearer to unit. Application of vibrohammers with a mass of the shock part exceeding 3-5 t is limited to the durability of the gear which is sharply decreasing with increase in weight.
– high speed of performance of work;
– profitability;
– dump soil is absent;
– possibility of performance of work in dense city building when using resonance free vibro-drivers
