Guidance for better mapping the horizontal variability of the subsoil. The goal of this project was to develop statistical methods to incorporate geophysics into the characterization of subsoil heterogeneity. The aspects of heterogeneity that have been addressed are the geometry of soil layers and the variability of material properties at small and large scale.
Figure 1. Geophysical survey in dikes and associated instrumentation (Photos by Juan Chavez Olalla).
Motivation and practical challenge
Traditional site investigation methods measure the horizontal variability of the subsoil often insufficiently. As a result, geophysical methods are becoming increasingly popular for engineering applications such as dikes and roads because they map the subsoil in a horizontally continuous manner. However, the operational effort required by many geophysical methods does not always pay off. Expectations are, in some cases, beyond the physical limits of the methods. The practical challenge is, therefore, to find the scale of heterogeneity that geophysical methods can resolve which at the same time gives valuable information for geotechnical calculations.
I formulate methods to answer the question: how to incorporate geophysical data for better mapping the subsoil variability? Specifically, I work on uncertainties related to the geometry of soil layers and material properties.
Figure 2. Combination of geophysical data and point data measurements to improve the schematization of the subsoil below the dikes (source: left-figure adapted from van Beek (2015), right-figure based on scheme prepared by Richard Marijnissen and bottom figure provided by Juan Chavez Olalla).
I look at the type of subsoil variability that plays a role in failure mechanisms of dikes (Figure 2, top-left). For example, in clay-over-sand dikes, the thickness of the clay layer on the landward side provides resistance against piping (Figure 2, top-right). Dikes are longitudinal structures, so it is challenging to map variability with point data, such as cone penetration tests (CPTs). One component of my research is to study the geometrical variability of layers with geophysical methods (Figure 2, bottom). I pay special attention to electromagnetic methods that quickly cover large distances. The innovative aspect is the statistical combination of geophysical data and point data. Previous approaches use only point data, so they do not explicitly consider the geometrical variability between data points. Another component of this research is related to more elaborate geophysical methods, such as seismic exploration, which require large operational efforts. I study the cases where these methods could bring useful information for geological schematisation.
Relevant for whom and where?
The output of this research is relevant for advisors who assess dike safety where horizontal variability of geological layers is uncertain.
A test site in Montfoort is studied in collaboration with the group of Physical geography from Utrecht University.
Progress and practical application
Preliminary surveys in test sites show that geological architecture is captured in geophysical data. The level of detail with which it is captured is smaller than that of cone penetration tests. However, the horizontal coverage is larger. By studying the patterns in geophysical data, it is possible to describe more extensively geological architecture. An approach to combine geological knowledge, point data and geophysics is formulated in this research. Part of this approach is aimed at retrieving geometrical variability from tomograms. Another part of this approach is aimed at correlating geophysical and geotechnical properties. For example, electrical resistivity is highly correlated to the cone resistance of a cone penetration test. For details on the results, see the related outputs.
Recommendations for practice
- Use a sequential approach to geophysical investigation from coarse detail (cheap and fast) to fine detail (expensive and slow).
- Interpret geophysical data within a bigger geological context.
- Interpret geophysical data in a consistent and reproducible manner.
- Define the target geological features to be mapped with geophysics before surveying.
- Establish quantitative relationships between geophysical and geotechnical properties.
Last modified: 30/12/2021
Juan Chavez Olalla
Delft University of Technology
Geophysical tomography as a tool to estimate the geometry of soil layers: Relevance for the reliability assessment of dikes
A combined approach to estimate the geometry of soil layers is presented. The approach combines local point data, i.e. data obtained from a CPT or a borehole log, and geophysical tomography in a universal cokriging framework.
Bevat: Publication open access journal
Main research outputs are under preparation, currently the following abstract is available:
- Chavez-Olalla, J. (2020) Layer interpolation with tomographic aid. Athens, Third European Regional Conference of IAEG, 20-24 September 2020.