Pontus Sjödahl

2.5D resistivity modeling of embankment dams to assess influence from geometry and material properties

Repeated resistivity measurement is a potentially powerful method for monitoring development of internal erosion and anomalous seepage in earth embankment dams. This study is part of a project to improve current longterm monitoring routines and data interpretation and increasing the understanding when interpreting existing data. This is accomplished by modeling various occurrences typical of embankment structures using properties from two rockfill embankment dams with central till cores in the north of Sweden. The study evaluates the influence from 3D effects created by specific dam geometry and effects of water level fluctuations in the reservoir. Moreover, a comparison between different layout locations is carried out, and detectability of internal erosion scenarios is estimated through modeling of simulated damage situations. Software was especially developed to model apparent resistivity for geometries and material distributions for embankment dams. The model shows that the 3D effect from the embankment geometry is clearly significant when measuring along dam crests. For dams constructed with a conductive core of fine-grained soil and high-resistive rockfill, the effect becomes greatly enhanced. Also, water level fluctuations have a clear effect on apparent resistivities. Only small differences were found between the investigated arrays. A layout along the top of the crest is optimal for monitoring on existing dams, where intrusive investigations are normally avoided, because it is important to pass the current through the conductive core, which is often the main target of investigation. The investigation technique has proven beneficial for improving monitoring routines and increasing the understanding of results from the ongoing monitoring programs. Although the technique and software are developed for dam modeling, it could be used for estimation of 3D influence on any elongated structure with a 2D cross section.

Embankment dam seepage evaluation from resistivity monitoring data

Methods for monitoring seepage are important for the safety of embankment dams. Increased seepage may be associated with internal erosion in the dam and internal erosion is one of the main reasons for dam failures. Internal erosion progresses inside the dam and is difficult to detect by conventional methods. Therefore there is a need for new or improved methods. The resistivity method is a non-destructive method that may accomplish this task. It has been tried in an on-going research programme in Sweden. Daily resistivity measurements are carried out on permanent installations on two Swedish embankment dams. In this paper the installations on the Sadva embankment dam are described and selected parts of the results are presented. In addition, a method for evaluating the seepage from resistivity monitoring data is theoretically described and tested for four selected areas in the foundation of the Sadva dam. Seasonal resistivity variations are apparent in the reservoir as well as inside the dam. Most parts of the dam have a homogeneous resistivity distribution with consistent variations. The overall status of the dam is satisfactory. However part of the foundation demonstrates a slightly different behaviour pattern with regard to the seasonal variation. The four selected areas represent localities with low, intermediate and high variations in seasonal resistivity. The areas are compared qualitatively and thereby permeable zones within the dam may be identified. Quantitative assessment of the seepage flow is also carried out as an initial test of the described method. It is concluded that the experiences from the Sadva dam are valuable with regard to the use of the resistivity method on embankment dams. Resistivity monitoring data may be used to qualitatively assess the seepage through the dam. For quantitative assessment, the method is promising and the data from the Sadva dam constitute an interesting initial approach. However, many assumptions and simplifications are made and more work on refining the method is needed.

Embankment Dam Seepage Evaluation from Resistivity Monitoring Data

Methods for monitoring seepage are important for dam safety of embankment dams. Increased seepage may be associated with internal erosion in the dam, and internal erosion is one of the main reasons for dam failures. Internal erosion progresses inside the dam, and is difficult to detect by conventional methods. Therefore, there is a need for new or improved methods. The resistivity method is a non-destructive method that may accomplish this task. A method for evaluating the seepage from resistivity monitoring data is theoretically described and tested for four selected areas in the foundation of the Sadva dam. Seasonal resistivity variations are apparent in the reservoir as well as inside the dam. The four selected areas represent areas with low, via intermediate to high variations in the seasonal resistivity variation. The areas are compared qualitatively and thereby permeable zones within the dam may be identified. Quantitative assessment of the seepage flow is also carried out as an initial test of the described method. It is concluded that the experiences from the Sadva dam are valuable for the application of the resistivity method on embankment dams. The presented method is a promising first step for quantitative assessment of seepage.