2. Problem background

In highly populated and mountainous countries, quantifying landslide hazard and other risks associated with heavy rainfall is important, because landslides cause damage to property and loss of human lives. Due to high levels of impact on mass movements, this has generated a great interest in the study of related phenomena in an attempt to understand physical aspects [1, 3–6] and economy-related issues [7–10].

The behavior of natural slopes can be associated in a broad sense as a combination of often different and complex hydro-mechanical processes. These kinds of physical processes depend on the slope geometry, the nature, and the structure and hydro-mechanical properties of the soils. Also, the boundary conditions and the initial state of the slope must be considered. It is important to take into account that any action causing a change in the boundary conditions or even in the loading conditions is prone to trigger both strains and displacement; climatic factors can be considered among the main triggering causes of such displacements that may generate mass movement. Any modification of these factors influences the water content and the pore water pressure regime in the slope, hence the stress state, and the available strengths, possibly generating slope failure [11].

infrastructure, hampering the economic development of the countries. For this reason, landslide risk assessment has become more applied in recent years. The rainfall-induced landslides occur almost every year in all mountainous regions, and globally, 14% of economic losses and 0.53% of deaths from disasters caused by natural phenomena are attributed to landslides [1]. Landslide hazard assessment and the capacity to predict these phenomena has been a topic of great interest within scientific community to try to prevent human and economic losses aforementioned, by implementation of early warning systems (EWSs), and they have been applied to reduce the risk from natural hazards through monitoring devices designed to minimize the impact imposed by a threat [2]. In regard to the magnitude of the landslide issue, numerous studies have been developed in recent years, and they have increased the understanding of the causes involved in these morphodynamic processes. However, because of the complexity related to landslide occurrence, there is still great uncertainty in predicting their

occurrence, considering three relevant aspects: magnitude, place, and periodicity.

relation to the potential damage.

164 Engineering and Mathematical Topics in Rainfall

2. Problem background

[1, 3–6] and economy-related issues [7–10].

The landslides often occur in environmental settings that are harsh for the conventional methods of landslide monitoring. For this reason, it considers pertinent and usefully combines physical models with ground-based monitoring technique on a GIS environment, since the aim is to minimize human losses and damage to infrastructure by a natural phenomenon such as landslides, preferably in urban zones, since the degree of urbanization is very important in

This chapter presents an assessment of the effect of rainfall infiltration on unsaturated soils on slope stability. Initially, a theoretical approach of the problem is presented, and a model of probabilistic analysis is described. Subsequently, an application of the model is carried out in an eastern area of Medellin, Colombia. The probability of saturation and the landslide hazard are determined by simulating the effect of a rainfall event registered in November 2010 that caused severe damages in the city. Finally, the effect of infiltration under a static scenario is evaluated. Soil parameters for these evaluations are determined by field and laboratory tests.

In highly populated and mountainous countries, quantifying landslide hazard and other risks associated with heavy rainfall is important, because landslides cause damage to property and loss of human lives. Due to high levels of impact on mass movements, this has generated a great interest in the study of related phenomena in an attempt to understand physical aspects

The behavior of natural slopes can be associated in a broad sense as a combination of often different and complex hydro-mechanical processes. These kinds of physical processes depend on the slope geometry, the nature, and the structure and hydro-mechanical properties of the soils. Also, the boundary conditions and the initial state of the slope must be considered. It is important to take into account that any action causing a change in the boundary conditions or even in the loading conditions is prone to trigger both strains and displacement; climatic factors can be considered among the main triggering causes of such displacements that may The hydro-mechanical and hydraulic conditions of the terrain and the state of saturation of the soil are determinant on the conditions of stability of slopes. The rainfalls have a double effect: reduce the soil cohesion and increase the pore pressure. This influence of the rainfall on slope stability depends of the duration and intensity of the rainfall. Usually, just a single factor becomes the triggering element, generating an almost immediate response, which is to mobilize slope materials. This triggering factor is generally rainfall or earthquakes. In tropical regions covered by residual soils and subjected to tropical rainfall regimes, a high percentage of these landslides are triggered by heavy, frequent, or prolonged rainfall [6, 12–15]. The role of rainfall infiltration on triggering landslides in tropical regions is being a challenge for geotechnical and geologists engineering [16].

Due to geological, geotechnical, and geomorphological uncertainties, it is usually difficult to predict where and when a landslide may occur. Nevertheless, it is generally recognized that changes in the water content of the soil imply changes like increase in pore pressure, decrease the effective stress of the soils, and, thus, reduce the shear strength [16]. Thus, understanding the physical conditions (i.e., if they are saturated or not) within variably saturated slopes when failures occur is needed for accurate assessment and prediction [17]. Many authors have been working worldwide on the subject of the rainfall-induced landslides, especially in countries like Italy, Switzerland, Spain, Taiwan, Singapore, and China. To do this, they have been proposed in the literature, empirical rainfall thresholds, and physically based models. Even in countries affected by the effects of intertropical convergence zone, area with high rainfall periods, such as Colombia and Brazil, have been joining forces for the generation of computer applications that allow the evaluation of slope stability based on empirical and physically based methods.

Understanding the processes that trigger a landslide is crucial to any successful landslide assessment and zonation. This topic is an active field of research worldwide, and it is required to find out the critical factors that trigger landslides. Research is required to establish the spatial and temporal prediction of hazardous zones and estimation on the probability and magnitude of future landslide. According to this, it considers the following research questions: Are the engineers relating the effect of infiltration on the instability of slope processes correctly? Can physically based models be used as a significant tool to evaluate the effect of infiltration process in slope stability? Proposed models reproduce the real phenomenon in an adequate form? The limitations of some techniques of evaluation and monitoring of landslides triggered by rainfall can be compensated or minimized with the advantages of other techniques used for the same purpose, so to be articulated to establish an integral proposal on this subject?

Some of the answers to the questions aforementioned are well known, also processes involved. However, some mismatches between theory and experiment exist yet. Hence, further investigations are required to understand better, if these gaps correspond to lacks in the theoretical background of the phenomena involved or to the experimental errors in field measurements. Hence, the current approach involves several fronts to solve the complexity and uncertainty of the addressed problem:


These approaches clarify the effects of rainfall and its consequent infiltration in slope stability of unsaturated deposits of tropical mountainous regions.
