At 5:30 a.m. in June 2009, a 13-story frame-structure residential building about to be completed at the intersection of Luoyang Road and Lianhua South Road in Minhang District, Shanghai, collapsed. National media reported it in detail one after another, stating that the cause of the collapse was that the piled soil on this side of the building was too high, and there was an underground garage on the south side of the building; the lateral pressure from the soil pile on the north side caused the building to topple southward. This explanation makes some sense from the perspective of structural mechanics, but the lateral pressure of that pile of soil on the north side alone could hardly cause the overall collapse of a 13-story frame-structure residential building. The real cause of the collapse was uneven settlement of the building foundation, larger on the south side and smaller on the north side, which caused the overall collapse of the building. Why would uneven settlement larger on the south and smaller on the north occur? The analyzed causes are as follows: First, the building was designed with precast concrete jacked pipe piles. During pile jacking, the pile top elevation was not carefully controlled, so the heads of the precast concrete pipe piles did not effectively enter the bearing stratum but floated at some height above the bearing stratum, laying hidden dangers for future pile settlement. Second, the groundwater level on the south side of the building changed, increasing the moisture content of the soil around the precast concrete pipe piles on the south side and reducing the pile side friction. Third, the building was about to be completed, and the structural load had approached its maximum value and had exceeded the reduced pile side friction. In an instant, the precast concrete pipe piles underwent relatively large uneven settlement, larger on the south and smaller on the north, ultimately causing the 13-story frame-structure residential building to collapse entirely toward the south. 

　　Based on the above discussion, the problem that the floating of precast concrete jacked piles prevents the pile heads from being seated in the bearing stratum must be taken very seriously.