Learning from Temporal and Spatial Geotechnical and Geophysical Data

September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.

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Learning from Temporal and Spatial Geotechnical and

Geophysical Data

M.A. Mooney & C. Bocovich

Colorado School of Mines

J. B. Rittgers

US Bureau of Reclamation

Keywords: internal erosion, geophysical, monitoring.

This presentation describes our efforts to better characterize internal erosion on field sites using spatial and temporal geotechnical and geophysical data. The presentation first generalizes the topic, describing spatial geotechnical and geophysical measurements and their relationship to parameters of interest, introduces the use of temporal data and what unique value it may provide, and then relates geophysical measurements to geotechnical parameters of interest. The presentation then describes the application on a seaside levee in Colijnsplaat, NL, where the levee experiences diurnal tidal fluctuations from the North Sea and numerous sand boils are observed to activate along the downstream ditch during high tide. Along the Colijnsplaat levee section, geophysical measurements of many types were recorded during various stages of tidal loading, and time-varying pore water pressure data were also recorded with piezometers installed within the levee at various offsets from the shoreline. All of these data were incorporated to investigate what can be learned about the state of internal erosion, and progression, in an active levee.

The occurrence and failure due to internal erosion is dependent on spatially variable and highly uncertain soil parameters, including the location and dimensions (geometry), hydraulic conductivity, unit weight, and grain size distribution of soil layers. The uncertainty of soil parameters propagates through numerical and semi empirical methods of predicting piping occurrence and failure such as the Army Corps of Engineers blanket equations (USACE 2000) to predict clay uplift and sand heave and Sellmeijer’s Equation (Sellmeijer et al. 2011) to predict piping progression. These uncertain parameters are difficult to measure directly due to spatial variability, difficulty to take undisturbed soil samples, in-situ conditions, and the extent of levee embankments. However, these uncertain parameters effect geotechnical and geophysical measurements such as pore water pressure as well as active and passive seismic and electrical resistivity. Recording these types of data in both space and time in theory allows for reduced uncertainty of soil layer geometry and porosity distributions (from passive and active seismic and resistivity imaging), and hydraulic conductivity (from better geometric information and pore pressure behavior).

To demonstrate the applicability of temporal and spatial geotechnical and geophysical measurements toward better understanding the state of internal erosion, a case study of a levee southeast of Colijnsplaat, NL is presented. Temporal pore water pressure measurements were taken between September 2010 and June 2011, and geophysical datasets were recorded during several levels of tidal loading during the fall of 2014. Passive seismic data collection and analysis is presented in Planes et al. 2017, and locations of measurements are presented in Figure 1.

This case study presents how temporal and spatial pore pressure, active and passive seismic data, and resistivity data are used to constrain uncertain soil parameters that influence the occurrence of and failure due to internal erosion. Specifically, soil layer geometry and spatial variations in porosity and hydraulic conductivity along the levee are constrained using time-lapse changes in active and passive seismic and resistivity data. Hydraulic conductivity can be constrained with Bayesian updating using time lag and magnitude shifts in the temporal fluctuations of observed pore water pressure created by the tidal loading as demonstrated in Bocovich et al. 2017. Constraining these parameters with the collected

M.A. Mooney & C. Bocovich

J. B. Rittgers

September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.

51

field data and Bayesian updating allows us to update the probability of failure and to improve our overall understanding of piping progression along this embankment.

Figure 1. Location of geotechnical and geophysical measurements along Colijnsplaat, NL.

Bocovich, C., Kanning, W., Mooney, M. (2017). Multiple pore pressure measurements to reduce uncertainties in piping risk assessment of levees. Proc. 6th Intl. Symp. on Geotechnical Safety and Risk.

Planès, T., Rittgers, J.B., Mooney, M.A., Kanning, W., Draganov, D. (2017). Monitoring the tidal response of a sea levee with ambient seismic noise. Journal of Applied Geophysics, 255-263

Sellmeijer, H., de la Cruz, J.L. van Beek, V.M., Knoeff, J.G. (2011). Fine-tuning of the backward erosion piping model through small-scale, medium-scale and IJkdijk experiments. European Journal of Enviornmental and Civil Engineering.

USACE (U.S. Army Corps of Engineers). (2000). Engineering and Design – Design and construction of levees. EM 1110-2-1913.