In situtests on improvement of collapsible loess with large thickness by down-hole dynamic compaction pile

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Abstract
This paper presents three in situ tests about down-hole dynamic compaction pile of 35 m, down-hole dynamic compaction pile of 50 m and pipe sinking compaction pile. To develop a method on improvement of collapsible loess foundation with large thickness and investigate improvement effect and quality control of the method. The tests results showed that (1) down-hole dynamic compaction pile can improve collapsible loess at a thickness of 50 m under reasonable design and required construction quality, improvement effect can be guaranteed; (2) for three tests, collapsible coefficients of soil between piles are all less than .015 in range of improvement depth, the collapsibility elimination effect has a downtrend along depth, whereas the compacting range and extent have no obvious difference, the depth is not a major factor influencing compacting effect; (3)the hammer intersection angle can influence improvement effect of down-hole dynamic compaction pile (4) a calculation formula of tamping number is given, which can simplify pre-tamping work because of acquiring tamping number only through field measurement to meet quality requirement. The tests results confirm a useful method for improvement project of collapsible loess foundation with large thickness, and to make those areas become improved and usable engineering construction site.

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