High energy accelerator has always been fluctuated by the impact of earthquakes or storm rainfalls, against an ideal operation of it. This is a study about the stability of the ring that is belonging to High Energy Accelerator Research Organization located on the Tsukuba upland, during the rainfall events. Tensiometers and TDR moisture content gauges are set in the mound which cover the acceleration tunnel. It is considered that the moisture content of the mound is highly related to the stable operation of the tunnel, so that, a pore air escape route and a drainage pipe are dug in the mound, in the mid observation period. It is made clear from the observation that the control of the pore air leads the stability of the moisture content of the soil (success to keep the low water content of the mound), and we could estimate the fluctuation of the ring after a (heavy) rainfall.
