Abstract Landslides can substantially impact the fluvial systems, which is why the continuous mapping of their extent, evolution and stability assessment is crucial. However, in such environments, material identification (e.g. colluvium) and subsurface characterization by the methods used for geologic mapping and geotechnical investigation is often a challenging task. Thus, these classical invasive methods may benefit from geophysical techniques to enable and enhance our understanding of the subsurface in these areas. To examine such integrated approach, Multi-Channel Analysis of Surface Waves (MASW) combined with Electrical Resistivity Tomography (ERT) were applied on a geomorphologically active fluvial valley in Sobradinho (the Federal District of Brazil). The subsurface materials showed a specific range of resistivity values as dry soil, saprolite, and landslide slip surface. The 1D shear wave velocity (Vs) model showed an increasing trend of Vs with depth at a location away from the landslide mass, while the longitudinal profile (over the landslide) showed an anomalous change in Vs (~ 250 to 400 m/sec). Based on the existing information about the landslide, the ERT appeared to be an effective method over MASW. This study shows how the integration of geophysical data with the geological and geotechnical investigation helps to obtain a more realistic or unambiguous model of the subsurface.

RESUMEN Los deslizamientos de tierra inducidos por lluvia representan un riesgo significativo para las comunidades y la infraestructura. Para mejorar la predicción de tales eventos, es imperativo investigar adecuadamente la dinámica generada por la lluvia que conduce a la fluidización, así como, cualquier deslizamiento interno que esté asociado y que se desarrolle a lo largo de los planos de falla dentro de los taludes arcillosos. Por lo tanto, para medir la variación estacional de la elipticidad de la onda Rayleigh, como un indicador de la deformación interna y de la transición del estado del material, el presente estudio adoptó la Relación Espectral Horizontal a Vertical (REHV) basada en el ruido ambiental. La técnica fue aplicada en un área de deslizamiento existente, donde se esperaba que ocurrieran variaciones en la rigidez del suelo y deslizamientos internos debidos a la lluvia. Para mejorar la interpretación de los resultados de la REHV y, por tanto, la predicción del deslizamiento generado por lluvia, la REHV fue calibrada en campo en una prueba de carga en pilote. La prueba en el permitió una comparación de los datos sísmicos generados por el movimiento del suelo a lo largo de los planos de falla. Las curvas REHV de esta prueba de campo mostraron dos picos de frecuencia sin cambios en la de la resonancia. En comparación con los datos obtenidos del deslizamiento de tierra, las curvas REHV resultantes mostraron tres patrones de frecuencia: ubicuo (2Hz), deslizamiento de tierra (4-8Hz) y plano (sin pico). Debido a que la recopilación de datos fue relativamente corta, las curvas REHV no mostraron ninguna respuesta a las variaciones estacionales inducidas por el deslizamiento de tierra. Sin embargo, la obtención de curvas REHV en un periodo de tiempo es una técnica prometedora que puede complementar otros métodos geofísicos para mejorar el monitoreo de deslizamientos de tierra.

ABSTRACT Rainfall-induced landslides pose a significant risk to communities and infrastructures. To improve the prediction of such events, it is imperative to adequately investigate the rainfall-dependent dynamics (leading to fluidization) and any associated internal sliding along shear planes within clayey slopes. Therefore, the present study adopted ambient noise analysis based on the Horizontal to Vertical Spectral Ratio (HVSR) method, to measure the seasonal variation of Rayleigh wave ellipticity as an indicator for the internal deformation and transition in the material state. The methodology was applied to an existing landslide, where variations in soil stiffness and internal sliding were expected to occur in response to rainfall. To improve the interpretation of the HVSR results (and hence the prediction of landslide' reactivation by rainfall), HVSR measurements were also conducted on a field-scale pile load test. The pile test allowed a comparison of the seismic data generated by the soil movement along shear planes. The HVSR curves of this field test showed two frequency peaks with no changes in the resonance. In comparison with the data obtained from the landslide, the resultant HVSR curves showed three frequency patterns: ubiquitous (2Hz), landslide (4-8Hz), and flat (no peak). However, the HVSR curves did not show any response to the expected seasonally induced variations in the landslide mass because of the relatively short data acquisition. Nevertheless, time-lapse HVSR is a promising technique that can complement other geophysical methods for improving landslide monitoring.

RESUMO Simuladores de chuva (SC) são desenvolvidos para estudar processos de infiltração, geração de escoamento superficial e erosão. Vários projetos de SC estão disponíveis, mas atenção limitada é dada aos efeitos de escala do equipamento, condições iniciais e de fronteira no escoamento medido. Este artigo apresenta uma avaliação baseada em um modelo de SC usando uma solução de elementos finitos para a equação de Richard e uma nova condição de fronteira para a superfície do solo, desenvolvida para acomodar o comportamento não saturado do solo. Foram consideradas as propriedades hidráulicas de dois solos tropicais, com múltiplas combinações de conteúdo inicial de água, dimensões da amostra e ângulos de inclinação. Os exercícios numéricos indicam que solos com menores valores de entrada de ar exigem um estágio de equilíbrio para o estabelecimento de condições iniciais. Protocolos de ensaio com tempos de equilíbrio de 48 horas são recomendados. Foi demonstrado que o fluxo de umidade produzido pela gravidade, ao inclinar a amostra, tem o potencial de afetar as condições de superfície e, consequentemente, o escoamento superficial. Especificações de ensaio para minimizar esses efeitos são apresentados. O mecanismo de escoamento superficial no SC apresentou três estágios, todos com claro significado físico. No terceiro estágio pode ocorrer uma indesejável consequência do efeito do fundo impermeável do SC. Foi demonstrado que o estabelecimento da espessura mínima da amostra que evita efeitos de contorno tem grande importância para os resultados dos testes.

ABSTRACT Runoff apparatuses (RA) are developed to study infiltration, runoff generation, and erosion processes. Several RA designs are available, but limited attention has been given to the effects of the equipment scale, initial, and boundary conditions on measured runoff. This paper presents a model-based evaluation of RAs using a finite element solution for Richard’s equation and a novel ground surface boundary condition designed to accommodate unsaturated soil behavior. The hydraulic properties of two tropical soils were considered, with multiple combinations of initial water contents, specimen dimensions, and sloping angle. The numerical exercises indicate that soils with lower air-entry values require an equilibrium stage for the establishment of initial conditions. Testing protocols with equilibrium times of 48 hours are recommended. Moisture flow produced by gravity when sloping the specimen was shown to potentially affect surface conditions and, consequently, runoff. Testing specifications to minimize the effects of specimen sloping are presented. The runoff mechanism in an RA was shown to have up to three stages, all with clear physical meaning. The third stage is an undesirable consequence of the influence of the RA’s impervious bottom. The establishment of the minimum specimen thickness that prevents boundary effects was shown to have major importance to testing results.

Abstract Reactivation of a landslide is usually accompanied by microseismic signals emitted from the deforming soil mass. The reproduction of similar signals in a physical model test conducted under control conditions can allow researchers to explore and test such complicated signals to improve the prediction of full-scale failure. The present study investigates the similarity between the slidequakes (microseismicity) naturally emitted from an existing colluvial landslide (Sobradinho, Brazil) in response to rainfalls and the emissions generated by a pullout test of an expander body (EB) pile in tropical soil under controlled conditions. The microseismic signals emitted from both experimental sites (i.e. the landslide and the EB pile test) were recorded and compared. Data were acquired by mini-arrays of four short-period seismometers. For the signal nomenclature, a typological scheme was adopted, in which sonograms/spectral contents of the signals were used. As a result, short duration microseismic signals were observed during the pullout test. In contrast, at the Sobradinho landslide, the testing detected signals of different characteristics whose source mechanisms have remained ambiguous, mainly because of the short duration of the data campaigns. However, at the landslide, propagating events were observed that might be attributed to the energies generated by the river bedload during the heavy rains. The present study offers some insight into the pre-collapse dynamic behavior of unstable slopes in clayey formations.

Abstract The application of geophysical monitoring technologies may offer an opportunity to understand the dynamic of slopes in response to factors triggering their instability. In this study, Ambient Noise Interferometry was used as a monitoring approach on a man-made reduced-scale vertical slope and on a natural-scale landslide in Sobradinho (Brazil), under the influence of mechanical stress and rainfall, respectively. For both experiments, we adopted similar data acquisition system and processing workflow. After preprocessing of ambient seismic noise, the time-lapse changes were determined in terms of relative velocity changes using the moving window cross spectral technique. For the vertical slope, terrestrial laser scanning was also performed to detect crack or fissure generation. The prototype experiment results showed a decreasing trend of relative velocity changes and reached a minimum value of -0.6% at the end of the experiment. No change was detected on the digital elevation model that was computed from terrestrial laser scanning images, due to the absence of centimeter scale superficial fissures. At natural scale (Sobradinho landslide), no significant variation in relative velocity changes was detected for the rainy and non-rainy days, mainly because of the inadequate change in the degree of saturation, which was found within a relatively short period of data acquisition.

Resumen En este artículo se describe una metodología basada en el método de estimación puntual de Rosenblueth para el análisis del empuje activo desarrollado en un muro de retención con relleno cohesivo-friccionante bajo condiciones de carga sísmica. El principio básico de esta metodología es usar dos estimaciones puntales, i.e., la desviación estándar y el valor medio, para examinar una variable en el análisis de seguridad. Es posible mostrar que aumentando el valor del coeficiente de aceleración sísmica horizontal, el factor de seguridad por volteo decrece y la probabilidad de falla aumenta, especialmente para coeficientes mayores que 0.2. Por otro lado, es observado que el valor medio del factor de seguridad crece cuando aumenta el coeficiente de aceleración sísmica vertical, sin embargo la probabilidad de falla se mantiene prácticamente igual para el valor del factor de seguridad considerado como crítico (1.15).

Abstract This paper describes a methodology based on the Rosenblueth point estimate method to analyze the active earth pressure developed on retaining walls for c-φ soil backfill under seismic loading conditions. The fundamentals of this methodology are to use two point estimates to examine a variable in the safety analysis: the mean value and the standard deviation. When the horizontal seismic acceleration coefficient increases, the overturning factor of safety decreases, whereas the probability of failure increases, particularly for coefficients greater than 0.2. In addition, the mean factor of safety increases when the vertical seismic acceleration coefficient increases, but the probability of failure remains practically identical for the considered critical factor of safety (1.15).