Acoustic Techniques for Soil Characterization and Levee and Dam Assessment
Zhiqu Lu - zhiqulu@olemiss.edu
James M. Sabatier - sabatier@olemiss.edu
National Center for Physical Acoustics, The University of Mississippi
1 Coliseum Dr. University, MS 38677, USA
Popular version of paper 1pPA1
Presented Monday afternoon, May 18, 2009
157th ASA Meeting, Portland, OR
Soils are granular or particular materials that consist of solid particles, water, and air. The mechanical behaviors of soils are determined by external and inter-particle forces, interconnected porosity, and multiphase conditions. For a partially-saturated soil, inter-particle forces are formed through a combination of capillary effects and an absorptive layer of fluids surrounding the soil particles. These internal forces create a negative pressure, called water potential or soil suction, applied to the soil skeleton that increases the stiffness and strength of soils. Soil moisture content and water potential are two soil parameters, defined as the amount and energy state of water in the soil. They have significant influences on soil mechanical and hydraulic properties such as shear strength, compactibility, penetrability, trafficability, permeability, and infiltration and drainage rate of soils.
Acoustic waves traveling through soils interact with soil particles and interstitial fluids. As a result, the soil texture and structure affect acoustic responses that, in turn, are sensitive to the variations of soil properties. Therefore, acoustic parameters can be used to evaluate and monitor changing soil properties (porosity, water content, and water potential) and conditions (pressure and temperature). Acoustic velocity or sound speed is widely used for soil characterization. It can be determined by measuring the time delay required for an acoustic wave traveling through a known distance between two acoustic transducers.
Studies show that sound speeds are sensitive to the external pressure [1-3]. A recent longterm field soil survey [4] revealed that a power law relationship exists between sound speed and water potential. Water potential has been proved to have the dominant influence on the sound speed, whereas moisture content and soil temperature have relatively minor impacts when the soil is under low pressure.
The sensitivity of sound speed to both external pressure and water potential can be explained using Terzaghi’s effective stress principle. According to this principle, soil mechanical and acoustic behaviors are governed by the effective stress, which is defined as the difference between the total pressure and pore water pressure when a soil is fully- saturated. For a partially-saturated soil, the effective stress can be modified as the difference between the total pressure and soil suction to the first approximation. While pore water pressure is a positive value, soil suction is a negative one, and its presence actually enhances the effective stress.
For moist conditions, when more water gets into a soil, the soil becomes softer due to the reduction of soil suction, resulting in decreased strength and sound speed. When the soil becomes very dry, soil suction can reach a value high enough to pull adjacent soil patches apart and thus create cracks. Drier soils present higher values in strength and sound speed, except for an extreme case in which soils are completely dry. In this case, soils lose all trapped water as well as soil suction and turn into powders that can be easily blown away. During a dry season, the difference in soil strengths induced by soil suction contrasts between two layers of soils may trigger a landslide when a dry upper layer of soil overlies on a wet, inclined soil substrate.
The above mechanical and acoustic properties of soils can be utilized for levee and dam embankment integrity assessment. For example, the variation in water level in a reservoir that changes the loading pressure on the embankment can be monitored using in-situ sound speed measurements. Increased seepage rate and/or the presence of internal cracks or channels (piping) due to degradation or weakening of embankments may introduce more water into the bodies of levees and dams. Two consequences may occur as water content increases: rising pore water pressure for fully-saturated soils and declining water potential for partially-saturated soils, both leading to decreased sound speed. In fact, sound speed, in terms of shear wave velocity, is related to stiffness, an important engineering parameter of soils. The deterioration of the integrity of embankments can be indicated or identified by the appearance of relatively low sound speed zones.
Many acoustic techniques, mostly developed in geotechnical and civil engineering, can be used for levee and dam studies. These techniques include: cross-hole, down-hole, and surface methods. The cross-hole and down-hole approaches are invasive techniques that require drilling boreholes in a tested site. They can be used for real-time monitoring of the conditions of a levee or dam. The surface methods include reflection and refraction surveys as well as surface wave methods. These surface methods measure sound speed at different depths in a non-invasive manner, i.e. all the measurements are achieved on the surface of the ground. Therefore, they are rapid, cost-effective, and environmentally friendly.
Recently, a surface wave method, called the multi-channel analysis of surface waves (MASW) method, has been developed [5-6] as a promising tool for levee and dam assessment [7]. The MASW uses Rayleigh waves to explore the soil surface and soil vertical profile in terms of shear wave velocity. A general configuration of the method is to vibrate the ground to generate surface waves and detect the surface vibrations by a number of geophones placed on the ground.
An innovative MASW method was recently developed at NCPA [8]. Unlike the conventional technique, which uses geophones as sensors, the new MASW method employs a laser Doppler vibrometer (LDV) to measure surface vibration. This allows exploration of the soil surface and subsurface a few meters below surface with high spatial resolution. This non-invasive and non-contact technique can be further developed using a multiple-beam LDV and mobile platform system for surface mapping and 2-D soil profile imaging. This will allow the improved MASW method to be used for levee and dam embankment integrity assessment.
In conclusion, sound speed is sensitive to external pressure and soil suction or water potential in terms of effective stress and can be used for soil characterization. The weakening of levees or dams can be detected as low sound speed zones. The MASW method is a rapid, non-invasive, and automatic imaging technique that is a promising tool for levee and dam integrity assessment.
[1] T. Bourbié, O. Coussy, and B. Zinszner, Acoustics of porous media, Gulf Publishing Company, Book Division, Houston Texas, p. 335, 1987.
[2] J.C. Santamarina, K.A. Klein, and M.A. Fam, Soils and waves- particulate materials behavior, characterization and process monitoring, John Wiley & Sons, LTD, Chichester, p. 488, 2001.
[3] Z. Lu, C.J. Hickey, and J.M. Sabatier, “Effects of compaction on the acoustic velocity in soils”, Soil Science Society of America Journal, Vol. 68, 7-16, 2004.
[4] Z. Lu and J.M. Sabatier,“Effects of soil water potential and moisture content on sound speed”, Soil Science Society of America Journal, 2009 (in press).
[5] C.B. Park, R.D. Miller, and J. Xia, “Multichannel analysis of surface waves”, Geophysics, Vol. 64, 800-808, 1999.
[6] C. B. Park, R.D. Miller, J. Xia, and J. Ivanov, “Multichannel analysis of surface waves (MASW) – active and passive methods”, The Leading Edge, January 2007, 60-64, 2007.
[7] J.W. Lane Jr., J. Ivanov, F.D. Day-Lewis, D. Clemens, R. Patev, and R.D. Miller, “Levee evolution using MASW: Preliminary findings from the Citrus Lakefront levee, New Orleans, Louisiana”, Symposium on the Application of Geophysics to Engineering and environmental Problems, Apr. 2008, Philadelphia, PA, Proceedings: Denver, Colorado, Environmental and Engineering Geophysical Society, p. 10. 2008.
[8] Z. Lu and J.M. Sabatier, “Multichannel analysis of surface waves method for shallow soil profile exploration”, 157th ASA Meeting, Portland, OR, May 2009.