| [Abstract] Since bored piles came into being in our county in 1960s, they have been extensively applied in various construction works of railways, highways, port, petroleum and buildings etc., and have developed into a leading type of piles for architectural engineering, especially for high-rise and super high-rise buildings in our country. However, there exist following problems about the slurry protective bored piles due to characteristics of construction technology:(1) After finishing boreholes, the bottom settlings are difficult to be removed thoroughly and the thickness of the residual settlings often exceeds the specified standard, thereby resulting in pile tip resistance reduction and subsidence increase. (2) The slurry consistence has to be increased in order to enhance wall protection effect during construction. Due to over long pouring time of bored pile concrete and inappropriate control of slurry consistence and weight etc., the slurry cake on pile body becomes over thick, and further the pile side resistance is reduced. The later grouting technology is none other but a new type technology developed for solving the problems on bottom settlings and slurry cake in slurry protective bored piles. The technology is also the popularization and application of the conventional pervious grouting theory, split grouting theory, compact grouting theory and electrochemical grouting theory in bored pile treatment. The paper has discussed the mechanism of action of the later grouting technology and grouting effect on soil body in the following several aspects: (1) Filled cementation effect: impervious grouting is realized in pebble, gravel and sand, and the soil body pores are partially filled by grout and particulates are cemented. (2) The soil body of single medium is split by reticular stone and reinforced into composite soil body, and the strength and deformation behavior of composite soil body are greatly improved due to constrained reinforcement effect of reticular structure, showing "reinforcement effect".(3) The pile bottom settlings and pile side slurry cakes generate physiochemical reaction with the injected slurry and then are solidified, thus remarkably increasing unit pile tip and side resistance and showing "solidification effect".(4) Compact grouting also occurs during grouting at pile side and bottom under certain conditions, and split grouting is also accompanied by soil body compaction; therefore, there exist different degrees of compaction effects on the soil body at pile side and bottom. Just because the later grouting technology has the mechanism of action of the above four effects, it has an effect of reinforcement on the side and tip of slurry protective bored piles. The paper has deeply discussed the mechanism of reinforcement effect of the later grouting technology on the pile side and tip resistance. The enhancement of single pile bearing capacity by later grouting is dependent upon design of later grouting construction process parameters, especially design of slurry selection, grouting time, grouting quantity and permissible grouting pressure. Based on the discussion, the paper presents: (1) During later grouting construction, it is advisable to adopt ordinary Portland cement; the cement fineness shall meet the requirement that the cement screened by 70# screen (4900 meshes per cm2) cannot exceed 15%; the specific surface area cannot be less than 2400cm2/g; the cement grade shall not be lower than P.O32.5 in general; it is advisable to adopt a water cement ratio of 0.5~0.6 to obtain optimal effect. (2) Later grouting shall be carried out after three days. (3) The slurry injection quantity shall be determined according to formation conditions and in combination with site slurry injection tests. (4) The slurry injection pressure shall be generally selected to realize normal slurry injection on the premise of not resulting in surface uplift and over large rise of pile foundation and shall be determined through site slurry injection tests. Due...
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