Technological features of arrangement Continuous Flight Auger piles (CFA) in clay soils
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Abstract
Recently bored piles are widespread. Bored piles are produced by drilling borehole with its subsequent concreting and reinforcement. The main complication in the manufacture of these piles is to ensure the stability of the borehole walls while filling them with concrete. Therefore, to prevent the collapse of soil, casings that deepened during drilling borehole with auger are used. But the use of casing complicates and slows the arrangement of bored piles.
Purpose of the work is to analyse the options on the arrangement of СFA piles and assess their impact on the quality of piles shaft. To achieve this goal using the method of statistical data (eg. torque auger, concrete mix feed pressure, speed of elevation the screw) obtained from the drilling machine for the production of СFA piles.
The main key parameters controlled at the arrangement of СFA piles are raising auger speed and the supply pressure of the concrete mixture. These two parameters form the pile body. So if the first case raising the auger speed will be low and the supply pressure of concrete will be large - it turns piles larger diameter compared to the project size, leading to cost overruns of concrete and appearing of negative friction effect. In the second case, by contrast, the rate of uplift increases and pressure decreases - then we get smaller pile diameter, which can cause the collapse of the borehole walls and reduce the protective level along the pile length to the reinforcing frame and therefore СFA piles project load carrying capacity will not be provided. If the above two named cases occur regularly along the length of the piles, the piles are excluded from the work of the foundation slab.
It is shown that in the group of piles medium supply pressure of concrete mix while constructing into semi-solid clay is less than the standard value 0,1MPa and concrete overrun does not exceed 12%. Maximum cost of concrete overruns is 16%, which is less than the allowable standards - 26%. This fact is expedient to consider during designing and constructing piles.
Conducted control of Continuous Flight Auger piles in clay soils (P <0,1MPa and k <= 1,16) confirmed the continuity of the shaft and geometric design size.
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