Discussion of Response of a residual soil slope to rainfall 1

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1 979 DISCUSSION / DISCUSSION Discussion of Response of a residual soil slope to rainfall 1 Ana C. Londono Received 22 January Accepted 15 June Published on the NRC Research Press Web site at on 22 September A.C. Londono. Department of Geology, University of Cincinnati, P.O. Box , Cincinnati, OH , USA ( londonaa@ .uc.edu). 1 Appears in Canadian Geotechnical Journal, 42: A.C. Londono. Bank stability resulting from rapid flood recession along the Licking River, Kentucky. Unpublished MS thesis, Department of Geology, University of Cincinnati, Cincinnati, Ohio. 120 pp. Londono 983 The advance of the wetting front in unsaturated soils has motivated continuous laboratory and field research. Rahardjo et al. (2005) presented an interesting field study of the response of residual slopes to rainfall, monitoring the runoff generation and rapid advance of infiltration within soils. A separate study by A.C. Londono 2 in 2004 of the instability of the lower Licking River banks (Fig. 1) found similar results for riverbanks under flooding conditions. The research focused on (a) the mechanical characteristics of the material forming the banks, (b) the processes responsible for the instability of the banks, (c) a sensitivity analysis of soil parameters in the stability computations, and (d) the role that pore-water pressure played in affecting the strength of the cohesive materials. The following discussion will report only the pore-water pressure behavior portion of the study. Floodplains have been centers of human occupation since the dawn of civilization because of their fertility and the accessibility to water for agriculture and industry. There are, however, problems with floodplains: they flood, and their banks are often unstable and prone to erosion. The banks of the Licking River in northern Kentucky display continued instability, making them ideally suited for analyzing the processes initiating mass movement and ways they may be mitigated. In 1997, a major flood occurred in portions of southern Ohio, Indiana, and northern Kentucky. Floodplains in these areas were covered by 10 m (32.8 ft) of water and remained inundated for 2 weeks. During this period, water percolated into the silty-clay soil, changing its hydraulic conditions from partially saturated to saturated with positive pore pressure, thereby decreasing the effective stress. When the flood receded, the banks were fully saturated under undrained conditions without lateral confinement. Elevated pore pressures combined with the loss of soil strength triggered several rotational slumps, causing damage to adjacent property. The resulting slope failures motivated the study of slope stability conducted by A.C. Londono 2 in Recent studies (Simon et al. 2000; Darby et al. 2000; Simon et al. 2002) have also correlated bank instability with increased pore-water pressure resulting from flooding events. During periods of flooding or high water level, soils become saturated. When the river level drops with sufficient rapidity, the soil remains saturated and significant seepage forces develop, thereby increasing lateral shear stresses, which may cause the banks to fail. In natural conditions, the soil is subjected to changes in moisture content along its profile owing to seasonal variations. The uppermost layer of the profile is affected by daily evaporation and incipient infiltration during low-intensity rainfalls. As depth increases, the changes in moisture are controlled by the rate of infiltration and by the fluctuations in the position of the water table, thus enabling rapid changes in the soil from unsaturated to saturated conditions. Different approaches can be taken to approximate the infiltration of water into the soil. Field experiments with simulated rainfall conducted by Londono (1995) considered infiltration as a function of rainfall intensity and slope by variation of the resistivity. That study found the wetting front rapidly advanced into the soil (Fig. 2), increasing its moisture content hence decreasing its resistivity. The same values of specific resistance were found in the deepest sections of the soil after a rainfall event. This was interpreted in two ways: (i) all infiltrated water was incorporated into the soil in the upper levels, or (ii) the moisture content did not change because the soil had reached saturation. Rahardjo et al. (2005) found that the recession limb of the rainfall hydrograph is influenced by slope topography and rainfall characteristics. They suggested that the content of water in the soil was greater at the toe of the slope than at the crest. This influence of slope topography was also found by Londono (1995) to be an important factor in the advance of the wetting front. Tests of simulated rainfall in slopes of similar soil characteristics and different inclination indicated that there is a gradual decrease in the depth the infiltration reaches with increase in slope inclination. During dry periods, soils are partially saturated and have relatively high shear strength, which allows them to remain stable. During wet periods, however, soils lose strength. Although many studies have correlated landsliding with highintensity rain events, several studies have found just one Can. Geotech. J. 43: (2006) doi: /t06-084

2 980 Can. Geotech. J. Vol. 43, 2006 Fig. 1. Study site at Licking River, Kentucky. Fig. 2. Advance of wetting front through a soil derived from colluvium at different times (readings every 5 min; Londono 1995). Fig. 3. Initial and final conditions for specific resistance within a soil after a 5 day rainy period (Londono 1995). high-intensity rain may not cause instability because soils respond differently, mechanically and hydraulically, when they are dry or wet. Londono (1995) found that after a simulated rainy period of 5 days (about 20 min of rainfall per day), it was possible to identify the cumulative changes in moisture content in the soil (Fig. 3), highlighting the importance of antecedent rainfall on those increases. The importance of antecedent events as they affect slope instability has been explored by Reid et al. (1988), Ng and Shi (1998), and Rahardjo et al. (2001). Rahardjo et al. (2005) found that runoff, peak runoff rate, and infiltration rates appear faster after simulated rainfalls, supporting the idea that antecedent rainfalls play an important role in the behavior of soil slopes. Ng and Shi (1998) found that the factor of safety of a slope depends on the rainfall duration and intensity, as well as on the initial hydrological conditions of the site (location

3 Londono 981 Fig. 4. Matric suction measurements along the eastern bank of the Licking River at the city of Wilder, Kentucky. of the water table). If the groundwater table is well below the failure surface, even after a high-intensity rainfall, the slopes can remain stable. In their study in which they modeled the response of an unsaturated slope to rainfall events, Ng and Shi (1998) found that prolonged rainfall does not cause significant changes in groundwater level; instead it changes the pore pressure distribution, thereby lowering the factor of safety. They also found the factor of safety to decrease in short duration storms when the slopes have been subjected to antecedent rainfall equivalent to the rain of critical duration. Chowdhury and Flentje (2002) found that landslides triggered by high-intensity and short-duration rainfall do not occur unless the soil antecedent moisture content is already high. They also found that pre-existing landslides may be reactivated after periods of intense and prolonged rainfall. Reid et al. (1988), in the study of a landslide in the Santa Cruz Mountains of central California, found that rainfallinduced positive pore pressures developed in the slope after a storm event may be accentuated by the presence of a perched water table, changing the behavior of the soil. Rahardjo et al. (2001) modeled a storm that triggered landslides in Singapore and concluded that antecedent rainfall was significant in triggering the landslides. They demonstrated that isolated rainfall events of similar magnitude as the event that caused the failures were not responsible for the failure of the slope. Their explanation is that antecedent rainfall causes the matric suction to decrease in the slope, consequently the hydraulic conductivity of the material increases, making the soil more permeable to infiltration. This decreases the shear strength of the soils and thus the factor of safety. After the storm, water continues percolating within the soil, which gradually loses moisture, causing the factor of safety to begin increasing. Rahardjo et al. (2005), in their study of residual soil response to rainfall, found that matric suction responds rapidly to rainfall events, lowering its magnitude and gradually recovering after rainfall stops; also, that runoff rate depends on rainfall intensity. They indicated that as soil increases its moisture content, less time is required for the runoff to occur, indicating that antecedent rainfall plays an important role in modifying the soil characteristics. Similar results were found by A.C. Londono 2 in a bank stability study in the northern Kentucky area with matric suction being recorded continuously for 1 year. For this study, four tensiometers and one piezometer were installed to evaluate the influence of water-level fluctuation on the stability of the eastern banks of the Licking River. These instruments were connected to a datalogger that monitored the changes in matric suction related to variations in moisture content every 10 min. The tensiometers were placed at depths of 1.2, 1.8, 2.4, and 3 m (4, 6, 8, and 10 ft), and a piezometer was set at a depth of 3.7 m (12 ft). The alluvial deposits tested were chiefly silty clays varying in color from olive-brown to greenish-gray. The recording period demonstrated that matric suction is particularly sensitive to rainfall, infiltration and rapid fall from flood stage. As water percolates through the soil or as the water table rises, matric suction is gradually lost (Fig. 4), and after the rainfall or floods have passed, it starts recovering. During periods of scattered rain, water percolates into the ground increasing the soil moisture content. The free water remaining flows through the soil pores, moving within the soil profile without saturating the soil, thus matric suction is still present in the soil. During the study period, several floods were observed and measurements of the groundwater table were taken after the flood recession. In the majority of the cases, the water table was m below the ground surface, demonstrating a lag time between percolation and the rise of the water table.

4 982 Can. Geotech. J. Vol. 43, 2006 Fig. 5. Flood occurred on 15 November 2003; river elevation 144 m asl. Fig. 6. Seepage developed after two consecutive flood events. Two other flood events were observed in the area separated by 8 days. During the recession following the first event (144 m asl water stage, 15 November 2003, Fig. 5), the groundwater table was below the ground surface, resulting in partial saturation of the soil in the upper metre of the profile. After the recession of the first flood, the river stage rose again resulting in a water table elevation of 146 m asl (23 November 2003, Fig. 6). Following the recession of that second flood event, the groundwater table was observed to be at the surface of the slope (seepage conditions, Fig. 6). These seepage conditions encountered in the slope after two consecutive flood events underscore the importance of antecedent events in the behavior of water within the soils. After one flood event that covered the slope for a couple of days, the soils did not reach full saturation. Therefore, positive pore pressures were not developed during that period.

5 Londono 983 This first event prepared the slope for saturation. Hence, after the second flood event, full saturation was achieved in the slope causing positive pore pressures to develop. In the case of failure along the riverbanks, it was observed that rainfall prior to the flood events modified the degree of saturation of the materials, increasing the hydraulic conductivity of the soil, thus facilitating the percolation of water within the soil. As the banks were flooded, the soil had enough time to become saturated, and after the water receded, undrained conditions developed accompanied by loss of confinement of the slope. Seepage forces may have developed under those conditions triggering the slope failures. Qualitative studies on the advance of the wetting front and the rapid response of soils to moisture increase have been made by several authors. Rahardjo et al. (2005) made an important contribution not only because they document the response of soil to infiltration and time for runoff to occur, but also because they demonstrate for the conditions of their slope that rainfall contributes from 40% to 100% of its total to infiltration and that a 10 mm amount of rainfall is the threshold for runoff to occur. These numerical values for percent of rainfall contribution to infiltration and the threshold values for runoff to occur are of great value in the assessment of slope stability problems. The results of A.C. Londono s study indicate that the wetting front travels rapidly through the soils and its advance depends on topography. Further, it was found that runoff occurs faster when the slopes have been previously wetted by rainfall; or in the case of slopes under influence of flooding, subsequent floods allowed full saturation of the slopes. These two cases demonstrate the importance of antecedent events on the water behavior within the slopes. A reduction of matric suction in a soil is produced by infiltration during and after a rainfall, by percolation during floods, and by changes in water table elevation. The results of this study support the findings of Rahardjo et al. (2005) in which matric suction responds rapidly to changes in moisture content of the soil, lowering its magnitude during a rainfall event and gradually recovering after the event passed. Acknowledgments The author gratefully acknowledges the financial support from the Geology Department at the University of Cincinnati (Cincinnati, Ohio) and H.C. Nutting Company (Cincinnati, Ohio) for their contribution on borehole drilling, sample extraction, and laboratory analysis. I wish to extend my gratitude to Terry Vince, Administrator of the City of Wilder, Kentucky, for allowing the installation of monitoring equipment of the Licking River bank and to Dr. David B. Nash and Dr. Mark T. Bowers for their guidance on the research and comments on this manuscript. References Chowdhury, R., and Flentje, P Uncertainties in rainfallinduced landslide hazard. Quarterly Journal of Engineering Geology and Hydrogeology, 35: Darby, S., Gessler, D., and Thorne, C Computer program for stability analysis of steep cohesive riverbanks. Earth Surface Processes and Landforms, technical communication, 25: Londono, A Variación del régimen de infiltración en un suelo en función de la intensidad de lluvia y pendiente del terreno. Tesis de grado, Universidad Nacional de Colombia, Medellín. p Ng, C., and Shi, Q Influence of rainfall intensity and duration on slope stability in unsaturated soils. Quarterly Journal of Engineering Geology, 31: Rahardjo, H., Li, X., Toll, D., and Leong, E The effect of antecedent rainfall on slope stability. Journal of Geotechnical and Geological Engineering, 19: Rahardjo, H., Lee, T., Leong, E., and Rezaur, R Response of residual soil slope to rainfall. Canadian Geotechnical Journal, 42(2): Reid, M., Nielsen, H., and Dreiss, S Hydrology factors triggering a shallow hillslope failure. Bulletin of the Association of Engineering Geologists, 25(3): Simon, A., Curini, A., Darby, S., and Landgendoen, E Bank and near-bank processes in an incised channel. Geomorphology, 35: Simon, A., Thomas, R., Curini, A., and Shields, D Case study: channel stability of the Missouri River, Eastern Montana. Journal of Hydraulic Engineering, 128(10):

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