PROBLEM TO BE SOLVED: To provide a method and a device for detecting looseness in a natural ground on a working face side of a shield machine for detecting looseness that induces collapse instead of detecting a collapse, thereby calling for attention before a collapse occurs.SOLUTION: A collapsing surface of a natural ground is forecasted by clarifying wet density and moisture density (moisture content) of soil with wet density and moisture density (moisture content) detection means for the soil installed in a front part of a shield machine. The wet density detection means and the moisture density (moisture content) detection means for the soil has a radiation source rod that radiates gamma rays and neutron rays from the front part of the shield machine, and the gamma rays and the neutron rays are scattered repeatedly while advancing in the soil. The wet density and the moisture content are derived from a radiation dose that reaches the detector.SELECTED DRAWING: Figure 1
