10:45 AM - 12:15 PM
[AHW19-P04] Measurement of pore-air pressure change in the bedrock layer in small headwater catchment
Keywords:Pore-air pressure, Pore-air entrapment, Bedrock layer, Headwater catchment
In the field observations, to examine the subsurface water behaviour in the unsaturated soil layer, it has been common to measure the soil water potential (pressure head) using tensiometer. Despite the fact that the pore pressure (P) measured using a tensiometer is the sum of the pore-water pressure (Pw) and the pore-air pressure (Pair), Pair has been neglected. Whereas the Pair behavior and the formation of the pore-air entrapment during the rainfall has been reported by some laboratory experiments and simulation studies. Pore-air entrapment will occur when the pore-air in the unsaturated soil layer; entrapped by infiltrating rain water and the underlying groundwater table or low permeability layer. Compressed entrapped Pair may have a possibility to effect on the increase of stream discharge, however not been clearly understood in the field yet.
Since it is known that ridging groundwater level rise occur at the lower part of the slope during the rainfall event, Pair rise at the lower part of the slope may related to such groundwater level rise in the bedrock layer. And therefore, when assuming the behaviour of pore-air pressure in the slope scale including the bedrock layer with large fluctuations in the groundwater level, it may be necessary to take into account of pore-air in the weathered bedrock layer and bedrock layer with fractures. In this study the authors focused on the bedrock layer. The aim of the present study is to measure the Pair in the bedrock layer and confirm the pore-air entrapment.
Observation of Pair in the bedrock layer was conducted by measuring the inner pressure of the borehole which is deep enough penetrating to the bedrock layer. Together with measuring the atmospheric pressure (Patm), pore-air entrapment and its compression was considered to be detected as the positive pressure difference between Pair and Patm (ΔP = Pair – Patm). Observation was conducted in a small headwater catchment; a sub-catchment of Hitachi-Ohta Experimental Watershed (HA, area: 8.4 × 10-3 km2) located in Ibaraki Prefecture, Japan. The inner pressure of borehole was measured at 6 boreholes (B1(3) at lower part of the slope, B2(5) and B2(20) at middle part of the slope, and B3(5), B3(15) and B3(30) at the upper part of the slope, where the number in parentheses indicate borehole depth. The observation period was from January to October 2022.
As a result, positive ΔP was observed corresponding to the rainstorm events. The number of pore-air entrapment events detected were 11 with the ΔP ranging from 1.9 to 15.6 kPa. Focusing on the largest pore-air entrapment event occurred on 25 September with total rainfall of 58.7 mm and maximum rainfall intensity of 8.0 mm/h, the response of ΔP was compared to the estimated ΔP due to compressive effect of air volume change in the borehole corresponding to rising groundwater level. Estimated ΔP was calculated using Boyle's law, under assumption that temperature changes over short time steps (5 min) could be negligible. The predicted ΔP was almost the same with the observation which indicates that compression of entrapped pore-air in the bedrock layer will induced by increase in groundwater level and that effect of rain water infiltration is negligible. Such ΔP rise was found at the deeper borehole (B2(20) and B3(30)) and was not found in the shallow boreholes (B2(5), B3(5), and B3(15)), except at B1(3) at lower part of the slope. These results are indicating the occurrence of high entrapped Pair at the deep part of the slope. Continuous research and accumulation of such data will enhance the discussion of rainfall-runoff processes and landslides in the future.
Since it is known that ridging groundwater level rise occur at the lower part of the slope during the rainfall event, Pair rise at the lower part of the slope may related to such groundwater level rise in the bedrock layer. And therefore, when assuming the behaviour of pore-air pressure in the slope scale including the bedrock layer with large fluctuations in the groundwater level, it may be necessary to take into account of pore-air in the weathered bedrock layer and bedrock layer with fractures. In this study the authors focused on the bedrock layer. The aim of the present study is to measure the Pair in the bedrock layer and confirm the pore-air entrapment.
Observation of Pair in the bedrock layer was conducted by measuring the inner pressure of the borehole which is deep enough penetrating to the bedrock layer. Together with measuring the atmospheric pressure (Patm), pore-air entrapment and its compression was considered to be detected as the positive pressure difference between Pair and Patm (ΔP = Pair – Patm). Observation was conducted in a small headwater catchment; a sub-catchment of Hitachi-Ohta Experimental Watershed (HA, area: 8.4 × 10-3 km2) located in Ibaraki Prefecture, Japan. The inner pressure of borehole was measured at 6 boreholes (B1(3) at lower part of the slope, B2(5) and B2(20) at middle part of the slope, and B3(5), B3(15) and B3(30) at the upper part of the slope, where the number in parentheses indicate borehole depth. The observation period was from January to October 2022.
As a result, positive ΔP was observed corresponding to the rainstorm events. The number of pore-air entrapment events detected were 11 with the ΔP ranging from 1.9 to 15.6 kPa. Focusing on the largest pore-air entrapment event occurred on 25 September with total rainfall of 58.7 mm and maximum rainfall intensity of 8.0 mm/h, the response of ΔP was compared to the estimated ΔP due to compressive effect of air volume change in the borehole corresponding to rising groundwater level. Estimated ΔP was calculated using Boyle's law, under assumption that temperature changes over short time steps (5 min) could be negligible. The predicted ΔP was almost the same with the observation which indicates that compression of entrapped pore-air in the bedrock layer will induced by increase in groundwater level and that effect of rain water infiltration is negligible. Such ΔP rise was found at the deeper borehole (B2(20) and B3(30)) and was not found in the shallow boreholes (B2(5), B3(5), and B3(15)), except at B1(3) at lower part of the slope. These results are indicating the occurrence of high entrapped Pair at the deep part of the slope. Continuous research and accumulation of such data will enhance the discussion of rainfall-runoff processes and landslides in the future.