Method for improving bearing capacity of existing pile foundation

The invention discloses a method for improving the bearing capacity of an existing pile foundation, which comprises the following steps of: drilling a hole in the existing pile foundation, forming a grouting hole, cleaning the hole after the hole is drilled to a designed depth, pumping a casing material to the bottom of the grouting hole through a drill rod after the hole cleaning is completed, then putting a sleeve valve pipe into the grouting hole, connecting a grouting pressure pipe and a grouting inner pipe, and sealing the grouting pressure pipe and the grouting inner pipe. The grouting inner pipe is placed in the sleeve valve pipe, grouting work is carried out in the grouting inner pipe through the grouting pressure pipe, grouting is carried out through a segmented retreating method, in the grouting process, after grouting of each segment is completed, the grouting inner pipe is moved upwards by the length of one step pitch, and open-loop grouting is carried out step by step till grouting of all hole se