Estimating soil suction from electrical resistivity

E Piegari,R Di Maio

doi:10.5194/nhess-13-2369-2013

E Piegari, R Di Maio

Open Access

https://doi.org/10.5194/nhess-13-2369-2013

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Abstract

Abstract. Soil suction and resistivity strongly depend on the degree of soil saturation and, therefore, both are used for estimating water content variations. The main difference between them is that soil suction is measured using tensiometers, which give point information, while resistivity is obtained by tomography surveys, which provide distributions of resistivity values in large volumes, although with less accuracy. In this paper, we have related soil suction to electrical resistivity with the aim of obtaining information about soil suction changes in large volumes, and not only for small areas around soil suction probes. We derived analytical relationships between soil matric suction and electrical resistivity by combining the empirical laws of van Genuchten and Archie. The obtained relationships were used to evaluate maps of soil suction values in different ashy layers originating in the explosive activity of the Mt Somma-Vesuvius volcano (southern Italy). Our findings provided a further example of the high potential of geophysical methods in contributing to more effective monitoring of soil stress conditions; this is of primary importance in areas where rainfall-induced landslides occur periodically.

Highlights

Soil suction plays a crucial role in affecting slope equilibrium conditions
It has been found that suction values are quite low in the pyroclastic soils affected by the flow slides that occurred in May 1998 on the Sarno mountains (Evangelista et al, 2001); the maximum values – at each site and at any investigated depth – did not exceed 65 kPa and minimum values of about 1–2 kPa were recorded in the period from January to May (Cascini and Sorbino, 2002)
Starting from in situ resistivity data obtained by electrical resistivity tomography (ERT) surveys in an area susceptible to debris-flow phenomena, we calibrated these measurements through laboratory experiments performed on samples collected from different depths of the selected cover

Summary

Introduction

Soil suction plays a crucial role in affecting slope equilibrium conditions. In particular, if the thickness of the unsaturated zone is large and the slope inclination is greater than the soil’s internal friction angle, stability is mainly controlled by the apparent cohesion due to suction, which contributes to the shear strength of unsaturated soils. De Vita et al (2012) have performed geophysical and geotechnical laboratory analyses on pairs of pyroclastic samples collected from the same sites in an area susceptible to debris-flow landslides, and have studied the correlation between electrical resistivity and matric suction in these soils. Starting from in situ resistivity data obtained by electrical resistivity tomography (ERT) surveys in an area susceptible to debris-flow phenomena, we calibrated these measurements through laboratory experiments performed on samples collected from different depths of the selected cover. From the fit of the data, we obtained values for the empirical parameters of Archie’s law (Archie, 1942) These parameters were used to invert the Archie relationship and to introduce the dependence on resistivity into the van Genuchten expression (van Genuchten, 1980), which relates water retention data to soil suction. We show an application of the proposed procedure in a test area of the Campania Region susceptible to debris-flow phenomena

Geological setting

Analysis of electrical resistivity tomography data

Analysis of laboratory resistivity data

B Bb Bbbasal

Soil suction maps

Conclusions