Combined Pile-Raft Foundations (CPRF) and Slab-Pile Foundation (SPF) – Prospects for Development
Today combined pile-raft foundations (CPRF) get fairly widespread, which proves that most specialist try to use the joint work piles and cap plates. In other words they try to take into account the loadbearing capacity of cap plates on the ground.
On the basis of engineering and geological research the developing modern computational methods make it possible to calculate the load that piles and cap plates bear. However, the accuracy of the load distribution, as well as the bearing capacity of piles and cap plates will depend on the range of the characteristics that will be derived from the results of research and design parameters adopted by the existing regulations.
The results of load re-distribution that are obtained this way can be quite widely used for pre-design studies and even for the development of documentation at the designing stage. Nevertheless, due to the lack of other opportunities CPRF calculations have been often carried out on the basis of these data both in Russia and abroad in the projects that are realized by famous scientific and engineering firms, especially in high-rise structures. This indicates rather large and still unused CPRF resources.
Earlier based on the lack of knowledge and some uncertainty in terms of the distribution of the perceived load was proposed to limit the load of the cap plate to no more than 15% (SR 2004), which seemed to be quite realistic for the following reasons.
During detail design the main criterion for the pile raft foundation is the bearing capacity of piles on the ground that is determined on the basis of the results of static imposed load tests with the pile setting that is equal to 0,2 of the average allowed setting of the building; it is usually 10–18 cm, or 2–3,8 cm. The cap plate will have the same unessential setting.
But according to the test results with these minimal allowed pile settings the share of the possible load of the cap plate can hardly be realized. The partial safety factor for grounds of 1,2 that is applied to the bearing capacity of the pile, further reduces the computational load of the pile and minimizes its setting. If we take into account the reduced load that is perceived by the cap plate and is related to loosening of the surface layer of the ground under the plate during excavation and pile installation, in the result the real CPRF usually turns to be a simple pile foundation.
In Set of Rules 24.13330.2011 with the updated version of Construction Rules and Regulations 2.02.03-85 “Pile Foundations” they eliminated the 15%- index. In paragraphs 7.4.10-7.4.16 that refer to CPRF they dwell on this combined structure in general without any certain specifications. This precautiousness is obviously justified, as we have little experience in pilot construction and we do not have a sufficient number of various calculating patterns to make important decisions to reduce the bearing capacity of pile foundation by means of transferring part of the load to the cap plate.
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Text: Iosif Ladyzhenskiy, Ph.D. in Engineering Science;
Alexey Sergienko, Chief Engineer of Research,
Planning & Survey and Design Technological
Institute of Foundations and Underground Structures (NIIOSP) n.a. N.M. Gersevanov