Kansas Geological Survey, Open-file Report 2007-17
by
Richard D. Miller, Julian Ivanov, and Shelby L. Walters
KGS Open-file Report 2007-17
for
Joseph B. Dunbar
U.S. Army Corps of Engineers--WES
3909 Halls Ferry Road
Vicksburg, MS 39180
June 2007
Seismic research detailed in this report is part of an exhaustive and comprehensive study by the U.S. Army Corps of Engineers formulated to evaluate the integrity of levees along the Rio Grande River. This particular part of that applied research project focused on adapting a variety of seismic methods to characterize the physical condition of and changes in levee cores and foundations that might be expected as a result of prolonged flood river levels at or near the design limits of the levees. Most levees along the Rio Grande River were originally constructed in the 1930s and 1940s as part of the International Boundary and Water Commission (IBWC) program in south New Mexico. These series of investigations (Weslaco, Texas, and Las Cruces, New Mexico) are the first explicitly designed to evaluate the sensitivity of seismic methods to detect water infiltration and any changes in core strength during a mock flood event at a real levee site.
Seismic methods have proven marginally successful at identifying anomalies in levees previously on a few occasions. Most of these studies have focused on direct wave analysis, targeting areas with reduced seismic velocities. Lower seismic velocities are usually indicative of less strength or softer materials. Therefore, anomalously low velocities for a particular levee could be an early indicator of failure potential. Based on currently available scientific literature, seismic testing at each of the five sites in Texas and three in New Mexico as part of this IBWC research program has been more extensive than at any previous earthen structure study. The testing included compressional first-arrival analysis (classic refraction, turning-ray tomography), multi-channel surface-wave analysis, and vibration harmonics analysis. Tests were conducted on the levee crest with expanded studies at sites identified as good candidates for ponding experiments.
Three sites with different surface characteristics along a mile-plus-long stretch of levee west of Las Cruces were selected and subjected to continuous split-spoon sampling, surface electrical and EM interrogation, and a variety of seismic imaging. From these initial studies and surface evaluations a single site was selected that possessed seismic characteristics suggested to represent the structurally weakest of the sites, which, compounded with rodent burrows evident in the levee shell, makes it the best choice. Preliminary seismic investigations indicated the selected ponding site possessed slightly lower shear velocities and distinctly interpretable fundamental-mode surface-wave energy within the higher frequency portion of the spectra. This site was selected for the ponding experiment based on several characteristics indicative of structural weakness and therefore the greatest failure potential during a simulated full-board event.
A simulated flood event was orchestrated coincident with a series of near-surface geophysical surveys. These geophysical surveys were time-lapse with the hope that both absolute and relative changes and measurements would be consistent with structural and/or material properties indicative of levee strength. Ideally these changes could be directly related to levee fitness and establish areas susceptible to levee breech. In general, both compressional- and shear-wave velocity estimates for this fine-sand levee near Las Cruces were indicative of increased pore fluid resulting from the increased head associated with the pond. Even though no failure occurred, the observed increased Vp and decreased Vs values, and thus decreased levee strength, could eventually result in levee failure.
Two trips to the levee sites resulted in seismic data sets with uniquely different characteristics. Comparing the Vp and Vs estimates between the two trips shows that, at the time of trip 2, there is a general decrease of Vs and at least 15% increase of Vp within the levee (top 10 ft) compared to trip 1. A Vp velocity increase between 15 and 20% is evident for most of the top 10 ft, with the exception of the 2110-2160 ft offset range, where the velocity decrease is more than 40%. It is hypothesized that Vp velocity increase is due to higher moisture content in the fine-sand levee shell, a suggestion that is consistent with the rainy period experienced in this area after the first trip.
The Vp/Vs ratio (Poisson's ratio) is higher during trip 2 in comparison to trip 1. High Vp/Vs ratio can produce larger amplitude higher modes and weaker fundamental mode as observed during the second trip. This observation is important to the overall understanding and significance of results from MASW surveys. It is relatively easy to estimate the fundamental mode of the surface wave from data acquired during the first trip, while it is very challenging to evaluate it during the second trip.
The baseline compressional-wave survey during the ponding experiment revealed a high-velocity Vp anomaly between locations 2120 and 2160 ft, an area characterized by a large number of animal burrows. This anomaly extended to 10 ft of depth near the start of the flooding experiment and after the first 4 hours could still only be observed within the very top 3-4 ft of the levee. Refraction-tomography Vp results for the rest of the line suggest compressional-wave velocity does not appear sensitive to any changes in seismic characteristics of affected levee material that can be attributed to the ponding. It is also possible that, once saturated, the clay particles within levee surface or skin sealed and resisted saturation, and therefore no material changes actually took place within the principal imaged zone of the levee.
A baseline MASW survey was designed to establish an initial 2-D Vs section and to be used as a reference for subsequent time-lapse surveys. Two anomalous high-velocity zones can be observed within the levee body at a depth of about 6 ft between locations 2095 and 2103 ft, and 2145 and 2165 ft. At time slices 2 and 3, representing 4 and 8 hours after the start of the water fill, the shear-wave properties change and the high-velocity anomalies begin to diminish. As the flooding simulation continues at time slices 3, 4, and 5, the Vs values within the levee continue to decrease to about 10-15% of their original values. At the same time, a new low-velocity zone begins to form between locations 2098 and 2110 ft at depths greater than 11 ft below the crown of the levee.
This trend continues during time slices 6 and 7, and by time slices 8 and 9 the low-velocity anomaly is well established and clearly represents a change in the seismic characteristics of the levee and its foundation. Most likely this anomaly relates to ground water movement under the levee. Such water movement could result in piping making this section of levee susceptible to reduced strength and therefore performance that might not meet design specifications. These anomalies could be catalysts for seepage and therefore threaten levee core integrity, indicative of zones susceptible to failure in extreme cases where the levee is exposed to prolonged high-water conditions. Detection of such anomalies meets one of the objectives of the survey, which is to detect old streambeds running under the levees that could provide conduits for increased ground-water flow, thereby indicative of weak spots more prone to failure during flooding.
Time-lapse shear-wave profiles calculated from surface-wave energy using the MASW method appear to have changed with increased water depth during the ponding experiment. We assumed these seismic changes are in response to increased saturation below the levee as water infiltrated the foundation during the flood simulation. No noticeable changes occurred in the compressional-wave properties within the levee except for a small volume speculated to be compromised by animal burrows. Vs was the measured material property most significantly affected from increased saturation driven by the hydraulic head provided by the water in the pond.
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Kansas Geological Survey, Geophysics
Placed online Feb. 10, 2009
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