ESTIMATION OF EFFICIENCY OF LOCAL BORED PILES IN THE BASE OF HIGH-RISE BUILDING RAFT FOUNDATIONS

Сurrently in high-rise building to avoid intolerable relative settlement of separate constructions raft foundations are used. In connection with excessive loads base settlement often exceeds assumed value. In that case pile foundations are used. Often pile foundations are applied in comparative favorable ground conditions at the surface of the base. In such cases increase the bearing capacity of foundations and reduce construction cost are questions of first-rate importance for designers. One of the ways to increase the bearing capacity of a piled foundation is taking into account soil resistance in the base of the raft. Piled raft is similar to shallow foundation and can transfer significant part of the load into the base. It allows reducing number of pile in the foundation or shortening their length. As a result building terms and foundation costs reduce considerably. However nowadays there is no reliable and suitable in design practice methods of calculation pile foundations with bearing rafts. In order to devise such methods we have analyzed the results of piled foundation tests fulfilled by various authors up to date and have carried out series of field investigations. The review of the latest sources of research and publications was carried out. The most important research results carried out by Prof. V. N. Golubkov, Prof. A. A. Bartolomey, Prof. R. Katzenbach et al. were considered. The part of the load that the raft carries grows up when the pile length is reduced and the space between piles is increased. Some test results carried out by different authors confirm it. It has been found experimentally that the soil under the raft in the pile space does not settle with a pile group and in spite of traditional design approaches can resist a considerable part of the external load. Design and construction operations features in construction of piled foundations, piled-raft foundations and raft-piled foundations are described. It has been found that bearing capacity of soil in the base of raft foundations is large enough and only excessive deformations should be checked to avoid serviceability limit state. In case of calculated value of deformations exceed serviceability criteria local bored piles is the most effective method to reduce it. Piles are implemented under load-bearing structures of the building frame. To decrease pile-raft interaction length of the piles should be more than active zone of foundation bed under the raft. Pile space should be large enough to allow the soil compacting at the surface with vibroroller. The efficiency of local bored piles in the base of high-rise building raft foundations was estimated be means of finite element method. Application of local bored piles carried 30% of total force reduce reinforcement of the foundation raft by 20-35%. Settlement of raft-piled foundation cut by half in comparison with shallow raft foundation. In a few instances cost of raft-piled foundation can be less than cost of raft foundation