Research Of The Applying About UHTCC Composite Material In Capped-pile Supported Embankment

Pile-caped embankment is a new reinforcement method which is widely approved for deep soft foundation in recent years of China, it’s composed by embankment filling, caped-pile(such as a prefabricated reinforced concrete pile with a cap) and the foundation soil. By the use of a cap on the pile, the soil arch effect make most of load cost by the soil above transferred to the pile and then the deeper soil layer with better bearing capacity, so the total settlement of the composite foundation could be reduced evidently. Domestic and foreign scholars accustomed to use the stress concentration ratio and load share ratio to describe the soil arching effect, here the load share ratio of pile is the ratio of load undertaken by the pile with the total loads of the foundation in the treatment scope of a caped-pile, a bigger pile load ratio means a larger loads on a pile, which decides the quality of the soil arching. Many research findings revealed that enlarge the size of pile cap could evidently rise the load ratio applied on the pile.In the study previously, the cap always be considered as a rigid board but ignored the punching-shear failure occurred on it, also by the limit capacity of the traditional reinforced concrete structure on its tensile and shearing behavior, which caused the size of the cap could not be enlarged arbitrarily, the soil arching action is limited in a small scope. By reasons above, the paper is aimed to introduce a new material named UHTCC to be used in the concrete structure, which would make the cap designed bigger under a same bearing properties, compared with the traditional one, the new structure induced a larger load ratio of pile and a better quality of the soil arching effect.The ultra-high toughness cementitious composite (abbreviated as UHTCC) is a new concrete material commonly used in civil engineering in recent years, because of the excellent performances in resistance to tension, compression and bending, we could expect a good prospects of its approving in a punching structure as a pile cap.Because of the coupling of the bending and shearing action happened on the cap, the mechanism of the punching failure is very composite, in order to solve the problem, the paper use a composite model considering bending and punching behavior to simply analyze the punching mechanism, and take a strength criterion actually including the bending damage when we calculate the limit punching loads using plastic limit analysis method. The accurate calculated results, identical to the experimental results, are suitable for engineering applications.Based on a field test of the Jing-Hu high speed railway, we set a finite element model accords with the real structure firstly, and analyze the behavior on the displacement and stress distribution on pile, embankment and grid, the also the soil arching action caused by the cap enlarged. By the orthogonal experiment analysis under different groups of cap dimension and pile spacing, we give a optimal design of the combined value for UHTCC material, which could provide a theoretical basis for its practical engineering application.The conclusion from analysis above are shown as below:(1) Under the same load condition, the punching capacity of slab using UHTCC is1.2times as the traditional concrete one. From this calculate result, the cap dimension and the soil arching coefficient could be optimized.(2) The result from the finite element calculation of a single pile shows that the traditional concrete cap and UHTCC cap could take the soil loads transferred on it under a small cap, but if we enlarge the cap size, the traditional one would show a punching failure without considering the enhancement of steel bar, but the cap with UHTCC still can play its normal functions. Because the stress and strain on the two cases of grids is still very small, the improvement effect of the grid is not obvious under the model.(3) The orthogonal experiment analysis for UHTCC pile-caped embankment shows that:under a same cap size, a larger pile spacing leads to a larger soil settlement between the pile, but reveals a little change on the settlement of the pile; if enlarge the cap dimension with the pile spacing unchanged, the settlement on the pile and soil could be reduced by a same ratio, the stress on soil will be larger and the soil arching effect could be increased. From a range analysis we can provide the optimal design of the combined value for UHTCC pile as4.5-5.5D pile spacing and1.5-2.0m pile cap.