SUPPORT LAYER CONFIRMATION METHOD AND FOUNDATION IMPROVEMENT METHOD

PROBLEM TO BE SOLVED: To confirm accurately the position of the support layer and to bring securely a ground improvement body and the support layer into close contact with each other when constructing a ground improvement body by a deep mixing treatment method.SOLUTION: The method includes: a step of setting a drill rod 20 of a high pressure injection device 10 on a pile core to start drilling; steps (a, b) of performing hole drilling at a normal penetration pressure up to a position higher than a predicted depth of a support layer predicted to be the depth of the support layer beforehand, and setting the depth as a support layer determination start depth; a step (c) of detecting a supporting layer confirming drilling speed which is a relationship between the drilling length and the drilling time by drilling while rotating the drill rod 20 at a constant rotation speed and penetrating at a constant penetration speed with the penetration pressure of the drill rod 20 being a support layer determination penetration pressure smaller than the normal penetration pressure; and a step (d) of determining that the drill rod 20 has reached the support layer when the drilling speed of the support layer confirmation drilling speed continues to be later than when the support layer determination is started.SELECTED DRAWING: Figure 1 【課題】深層混合処理工法により地盤改良体を造成する際に、支持層の位置を正確に確認して、地盤改良体と支持層とを確実に密着させる。【解決手段】高圧噴射装置１０の削孔ロッド２０を杭芯にセットして削孔を開始する工程と、予め支持層深度であると予測した支持層予測深度よりも上方まで通常の貫入圧力で削孔を行って、当該深度を支持層判定開始深度とする工程（ａ、ｂ）と、削孔ロッド２０の貫入圧力を通常の貫入圧力よりも小さい支持層判定貫入圧力として、削孔ロッド２０を一定の回転速度で回転させるとともに一定の貫入速度で貫入させながら削孔を行うことにより、削孔長と削孔時間との関係である支持層確認削孔速度を検知する工程（ｃ）と、支持層確認削孔速度が支持層判定開始時よりも遅くなった状態が継続した場合に、削孔ロッド２０が支持層に到達したと判定する工程（ｄ）とを含む。【選択図】図１