This study examined the amount of moisture contained in the unsaturated weathered shale soil slopes with emphasis on soil moisture distributed in different time and in different locations and thereby identified the correlation between time factors and hydrological behaviors during the rainfall period. A natural slope covered with miscellaneous woods located in Jiaxian District, Kaohsiung City, was selected as the experiment site, automatic surveillance method was adopted, soil moisture sensors were buried in various spots with different altitudes and different depths in order to measure soil moisture over a long period of time, rainfall data was measured in situ, and nine rainfalls were chosen for quantitative analysis.
After the correlation between the time factors and rainfall characteristics was analyzed, this study concluded that, as far as the typical weathered soil slopes with different rainfall types and different altitudes are concerned, rainfall types influenced the average rainfall intensity as well as the accumulated rainfall. When the rainfall has a low degree of intensity, the arrival time of water front in all depths (Ts) and the arrival time of unsaturated soil’s shearing force at least value (Te) slow down. After various altitudes were analyzed, the research results indicated that the soil moisture content in shallow layer (1 meter or less below the ground) varies more than the soil moisture content in all other layers. Apparently, the distribution of soil particles influences the soil moisture content the most, particularly the coarse-grained soil. Secondly, the moisture content increase rate (Rm) of antecedent soil moisture content changes in accordance with the distribution of rainfall intensity; the distribution of rainfall types and rainfall intensity substantially influence the time of destruction to the slopes. Therefore, the accumulated rainfall alone is not sufficient for the weather authority to issue alert. Thirdly, after typical slopes were compared with trench slopes regarding their distributions of moisture content increase rates, the research results indicated that the typical slopes did not reveal any significant difference associated with the altitudes; trench slopes collected rainwater and underground water. Thus, trench slopes were shaped differently in different latitudes.