Vertical Load Transfer Mechanism And Application Of Large Diameter Rock Socked Pile And Piled Raft Foundation With Sand And Pebble Cover

The large diameter rock-socketed piles(LDRSPs) not only have stronger bearing capacity with less settlement, but also have some other advantages: good seismic behavior and large diameter, etc., thus LDRSPs are used as the first priority foundation in high-rise building, heavy-weight factory and bridge, etc. However, some factors have influence on bearing behavior of LDRSPs ,such as thickness and modulus of overburden, ratio of length to diameter, rock modulus of pile tip , ratio of rock-socketed depth to diameter, contact interface conditions between pile and rock and construction, etc., together with restriction of test condition and test means at present, complete systematic data of static load test of LDRSPs was seldom seen, which has limited understanding of bearing behavior of LDRSPs. In this paper, under the support of Shanxi research project (Numbers 2007031172) "construction technology research of rock-socketed piles in the special geological conditions" and other project, this thesis studies the load transfer mechanism and settlement characteristics of LDRSPs and piled raft foundation with a overburden of sandy pebble soil, and discusses the failure mode of rock-socked pile, by using of finite element numerical simulation method. Achievements:1)through measuring the internal forces of LDRSP's shaft with bearing capacity up to 24000kN (pile diameter 1.8m) and 16000kN (pile diameter 1.2m) shaft during the vertical tests in situ, where is a sand pebble soil in Taiyuan. Some valuable rules of load transfer of LDRSP are revealed and summaried, which provides a fact hand information for the its research and design in the some engineering geological condition in Taiyuan.2)The soil pressure under the cap and internal force along pile shaft for LDRSP raft foundation under the working condition were measured in the same site, and the useful and reliable data are obtained, which are refered for the research and design of LDRSP raft foundation in the same engineering geological condition in the area.3)Applying finite element numerical simulation method, many factors, which influenced the settlement and bearing behavior of LDRSPs, are analyzed and discussed systematically ,such as thickness and modulus of overburden, rock modulus of pile tip ,ratio of length to diameter, ratio of rock-socketed depth to diameter, rock-socked and non-socketed pile tip, rock stratotype, soft substratum of pile tip, etc. The laws of pile-soil interaction of LDRSPs are put forward.4)Applying finite element numerical simulation method, integral interaction model of superstructure-piled raft foundation-subsoil are established, many factors, which influenced the settlement and bearing behavior of piled raft foundation are analyzed,including thickness and modulus of overburden,rock-socketed or non-socketed tip, rock-socketed depth, load sharing ratio of bearing columns of superstructure, etc. the Interaction laws between superstructure and piled raft foundation are put forward in sandy pebble soil cover.5)According to field measurement and numerical analysis, the total resistance of rock-socketed segment (including the side resistance of rock-socketed segment and tip resistance) of rock-socketed piles is put forward as the tip resistance of rock-socketed pile, taking into account lateral resistance of rock-socketed segment and tip resistance of the rock- socketed pile is either unnecessary strict division, or difficult to accurately distinguish in the actual situation. This could make the concept of rock-socketed piles more explicit and more useful in the project.6) Four failure modes of rock-socketed piles are proposed based on collating, summarizing and analyzing the existing rock-socketed pile test results. and previous studies, ie. the piles yield failure mode, the pile-rock interface failure mode, and the failure mode of the rock shear of rock-socketed segment and buckling of pile tip rock and the general shear failure mode of the pile tip rock among them. the failure mode of the rock shear of rock-socketed segment and buckling of pile tip rock is the first time put forward.7)Based on the hypothesis of the pile-rock interface failure mode, the failure mode of the rock shear of rock-socketed segment and buckling of pile tip rock and the general shear failure mode of the pile tip rock, the formulas of ultimate bearing capacity of rock-socked piles are proposed. by using cavity expansion theory and the Hoek-Brown criterion,8)Considering the imperfect of the site and indoor testing means of bearing capacity of rock-socked piles, the ideas of section static load test in site of the vertical load for rock-socked pile and indoor pile-rock interface friction determination method are first proposed, based on which, the formulas are presented to evaluate the bearing capacity of rock-socked pile.