Monitoring and behavior of unsaturated volcanic pyroclastic in the Metropolitan Area of San Salvador, El Salvador.

José Alexander Chávez,Reynaldo López Landaverde,Václav Tejnecký,José Landaverde

doi:10.1186/s40064-016-2149-x

Abstract

Field monitoring and laboratory results are presented for an unsaturated volcanic pyroclastic. The pyroclastic belongs to the latest plinian eruption of the Ilopango Caldera in the Metropolitan Area of San Salvador, and is constantly affected by intense erosion, collapse, slab failure, sand/silt/debris flowslide and debris avalanche during the rainy season or earthquakes. Being the flowslides more common but with smaller volume. During the research, preliminary results of rain threshold were obtained of flowslides, this was recorded with the TMS3 (a moisture sensor device using time domain transmission) installed in some slopes. TMS3 has been used before in biology, ecology and soil sciences, and for the first time was used for engineering geology in this research. This device uses electromagnetic waves to obtain moisture content of the soil and a calibration curve is necessary. With the behavior observed during this project is possible to conclude that not only climatic factors as rain quantity, temperature and evaporation are important into landslide susceptibility but also information of suction–moisture content, seepage, topography, weathering, ground deformation, vibrations, cracks, vegetation/roots and the presence of crust covering the surface are necessary to research in each site. Results of the field monitoring indicates that the presence of biological soil crusts a complex mosaic of soil, green algae, lichens, mosses, micro-fungi, cyanobacteria and other bacteria covering the slopes surface can protect somehow the steep slopes reducing the runoff process and mass wasting processes. The results obtained during the assessment will help explaining the mass wasting problems occurring in some pyroclastic soils and its possible use in mitigation works and early warning system.

Highlights

Since Terzaghi in 1936 explained the behavior of saturated soils and helped to underline the principles of the soil mechanics, different authors (Fredlund 1997) have tried to explain the behavior of the “problematic” unsaturated soils (Fredlund et al 2012) that didn’t fit the behavior explained by the Terzaghi’s equation
The pyroclastics are classified as a sandy silt or silty sand, based on the Unified Soil Classification System (USCS), Table 1 Equations obtained from field and lab calibration of different Tierra Blanca Joven (TBJ) units including the R-squared statistic
The urban areas resting on top of unsaturated pyroclastic soils (e.g. TBJ) or close to the streams are prone to mass wasting processes and collapse

Summary

Introduction

Since Terzaghi in 1936 explained the behavior of saturated soils and helped to underline the principles of the soil mechanics, different authors (Fredlund 1997) have tried to explain the behavior of the “problematic” unsaturated soils (Fredlund et al 2012) that didn’t fit the behavior explained by the Terzaghi’s equation. These types of soils comprise soils above the water table (sands, silts and clays and even gravels) including man-made fills, colluvium, residual soils, air transported soils and gas generating soils (Fredlund et al 2012). This situation makes that the slopes of the pyroclastics deposits are almost vertical and temporally stable; but will collapse when saturated

Methods

Results

Discussion

Conclusion