A study of vertical bearing mechanism of pile foundation in volcanic ash soil

Pile foundations for bridge piers and abutments constructed in volcanic ash ground are mainly designed based on specifications for sandy soil ground due to the similar density and internal fiction angle of volcanic ash as those of normal sandy soil. However, mechanical characteristics of volcanic ash, such as friability against confining pressure, differ strongly from those of normal sandy soil. To confirm the vertical bearing capacity and deformation characteristics of piles in volcanic ash ground, vertical loading tests using actual piles for bridge foundations and static cone penetration tests were conducted at 14 construction sites. As the results, it was found that the skin friction f of piles in pyroclastic fall deposits (fa) of volcanic ash soil developed in a manner similar to that of normal sandy soil, but that in pyroclastic flow deposits (fl) of volcanic ash soil tended to be less compared with that of normal sandy soil, and also a correlation was confirmed between the cone tip resistance qtand skin friction f of piles.