A parametric analysis of slope failure with numerical method was presented in this paper. Before the parametric simulation, a procedure of analysis was examined. In consequence, the identifying method of parameters except the aimed parameter was shown and cumulative rainfall was selected as the evaluation value in the analysis. With this method, sensitivity evaluations of some parameters are invesitigated. As a result, parameters which accelerated landslide are, steeper, longer, shallower soil slope and convex slope figure. And the parameters on physical and mechanical properties, smaller hydraulic conductivity, smaller cohesion and internal firiction angle accelerate landslide. In addition, parametric values, which concerned with the seepage and made the initial wet condition of slope, promoted the failure.

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It's important to know safety factor F_s in order to design countermeasure of landslides. To calculate F_s, the cohesion “c” and the frictional angle “φ” have to be given. And, in general, these in-situ factors vary from point to point, in other words, they has space deviation from the average value. Needless to say, this deviation sometimes gives a considerable error in the calculation of the safety factor F_s.
Since it's needed to estimate this deviation in this sense, the method that is based on the mathematical theory “the reliability analysis” and can evaluate it, is gained here in this study. And an example analysis case using this method is given.

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In 1783, the great eruption of Mt. Asama buried the villages of Kambara and Koyado, which lay at the northern foot of the mountain, with earth and sand and there was a major mudflow along the Agatsuma and Tone rivers further down. It was a major disaster which caused more than 1, 000 deaths.
We have clarified the nature of the flow and the deposition of its sediments from a survey on the deposits left there at the time, soil tests, paleomagnetic measurement of essential blocks and analysis of old records. We have estimated the velocity of the mudflow and its discharge at the main affected places by hydraulic calculation based on the results of the survey.
The particle size distribution of the 1783 eruption mudflow depopsits remaining along the Agatsuma and Tone Rivers roughly ranges from 0.3mm to 1.0mm for 50% particle size. The distribution hardly changes with the descent of the mudflow and closely resembles the mudflow deposition on the slope at the northern foot of Mt. Asama. The mudflow that originated on that slope thus maintained a state of equilibrium with little change in comparison as it moved down the Agatsuma and Tone Rivers.
Measurements of the paleomagnetism of huge essential blocks deposited along these rivers showed that direction of their geomagnetism generally agreed with that at the time of the 1783 eruption. It is inferred that they were deposited at more than the Curie temperature, so they might have been effused in that eruption.
Hydraulic calculation of the mudflow by the Manning formula produced a water level of 25m-100m, a flow velocity of 9m/sec-24m/sec and a discharge of 110, 000m³/sec-330, 000m³/sec for the section Mihara to Shibukawa city and it follows that the mudflow took about 80 minutes to reach Shibukawa which is about 70km from the crater. This result generally agrees with what is indicated in the old records. Water levels below Shibukawa City are difficult to estimate for many reasons, including subsequent repairs and development, and the fact that the mudflow distributed there. However, if the water level and the average flow velocity are roughly estimated at 5-10m and 3m/sec respectively from the topography of the present river channels, the flow time was generally as indicated in old cords.

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