Pore Structure Variation of Porous Media under Vibrations
Ming Xiao1, M. ASCE, Lakshmi N. Reddi2. M. ASCE
1Assistant Professor, Department of Civil and Geomatics Engineering and Construction, M/S EE94. California State University, Fresno, California 93740. Phone: (559) 278-7588. Email: [email protected] 2Professor and Chair, Department of Civil and Environmental Engineering, University of Central Florida, Orlando, FL 32816. ABSTRACT: Variation of pore structures of soils due to seismic activities affects density and fluid distribution in the pores, which in turn could affect the strength and liquefaction potential of the porous media. This paper, based on experimental investigation, studied the effect of vibrations on pore sizes of porous media. A monolayer of glass beads of different sizes under full saturation condition was used to simulate porous media, and the glass beads were subjected to the vibrations provided by a small-scale shaking table. A microscopic camera, which was positioned above the glass beads and connected to a computer, captured the pore variations during the vibration at 1sec interval. Then, graphical software was employed to analyze the changes of pore size distributions before, during, and after the vibrations. The experimental study revealed that the pore size distributions of saturated and densely packed glass beads changed little before and after the vibration. During the vibration, however, the number of bigger pores decreased and the number of smaller pores increased. It may be concluded from this preliminary experimental study that although the pore structure of dense subsoil may remain relatively unchanged before and after seismic activity, the soil could experience significant change of pore structure during the vibration, which could affect the soil’s density, strength, and liquefaction potential. INTRODUCTION Seismic induced soil failure is a major geohazard. Earthquakes could result in strength reduction, liquefaction, and large ground deformation of subsoil. For instance, Makdisi and Seed (1977) found that the dynamic yield strength can reduce to 80% of the static undrained strength for clayey soils, dry or partially saturated cohesionless soils, or dense saturated cohesionless soils. Also, pore water pressure was shown to increase with the amplitude of shear strain in cyclic shear tests (Ohara and Matsuda, 1988; Dobry, 1989). Pore pressure increase in saturated sand or silt due to seismic loading can result in the loss of bearing capacity and consequently
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catastrophic failure of a structure. Soil behaviors during vibrations have been widely studied using macro-scale characteristics, such as porosity, density, deformation, and strength. The soil behaviors under vibrations in pore scale, on the other hand, have been less explored, probably owing to the complex geometry, tortuosity, and irregularity of interconnected pores. Understanding the pore structure variations before, during, and after seismic loading in micro-scale could facilitate the understanding of the effect of seismicity on the density and the pore water pressure in the sample and field scales.
Conceptualization of pore geometry was proposed by many researchers. An example of an idealized pore structure was shown in Figure 1 (Payatakes et al., 1980). A pore formed by the surrounding grains is comprised of a pore body (measured by pore diameter, a) and the constraints (defined as pore throats and measured by pore throat diameter, d). The wall profile of a pore is assumed to be a sinusoidal function, and h is the pore length. Idealization of pore geometry was used in pore network models to study pore fluid transport and fate in porous media. Xiao et al. (2006) conducted experimental and model study on the effect of vibrations on the fate of discontinuous pore fluids and found the size distribution of discontinuous pore fluid after vibrations was less uniform than that before the vibration. In this study, pore geometry variation and the size distribution of the pores of a saturated porous medium under vibrations was explored experimentally. The purpose of this study is to provide a fundamental understanding of the effect of vibrations on the pore structure of porous media. VIBRATION TESTS To measure the pore structure variation under idealized conditions of pore geometry, spherical glass beads were used as the porous media. Three sizes of glass beads were used: 3mm, 4mm, and 5mm. They were mixed with equal mass and tightly packed in a transparent cell made of transparent Plexiglas. In order to clearly monitor the possible pore geometry change under vibrations, a monolayer of glass beads was used. The glass beads sample was saturated with water in the cell, and the cell was capped to prevent spill of glass beads and water in the cell during vibrations. The cell
(a) 2D illustration of a pore in porous medium
(b) 2D conceptual model of an idealized pore (From Payatakes et al., 1980)
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Figure 3. Glass beads and pore structures was fastened on the top of a small- scale shaking table (Model: VP51D1; Frequency: 60Hz or 3600 vibrations/min; Amplitude: 0.33mm; Manufacture: FMC Technologies), as shown in Figure 2. A portable digital microscope (Proscope®) was mounted on the top of the cell to record the pore geometry variation before, during, and after the vibrations. The microscope can take 1.3 Mega-pixels image at 12 frames per second for 30 min. The microscope was connected to a computer so that the images could be instantly displayed and stored for later analysis. The saturated glass beads were shaken for 30sec on the shaking table using the vibration characteristics mentioned above. The microscope was set to capture one image per second. During the vibration, it was observed that the individual pores dilated, shrank, merged, or broke up into smaller pores. Two representative images of the pore structures of the monolayer glass beads before and during the vibration are shown in Figure 3. In order to quantify the pore geometry change before, during, and after the vibration, a graphical software, Sigma Scan®, was used to measure the pore sizes. For an
(a) Experimental Setup
(b) Microscope and glass beads in a cell
(a) Before vibration (b) At 10 seconds of vibration
Figure 2. Experimental setup of the vibration tests
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idealized pore (Figure 1), the pore length and the pore body diameter can be determined by the pore throat diameter (Payatakes et al, 1980). Thus, in this study, the individual pore geometry was represented using pore throat diameter, as identified in Figure 4. It is noted that the Proscope® software can only take square images that are inscribed in the circular cell. Therefore, the two photographs in Figure 3 showed the major area of the glass beads.
Figure 4. Identification of pore throats, as shown by the arrows.
TEST RESULTS AND DISCUSSION Four images of the pore configurations of the glass beads were selected for quantitative analysis: one image taken immediately before the vibration, one at 10 seconds of the vibration, one at 20 seconds of the vibration, and the last image taken immediately after the vibration (at the end of the 30-second period). Each pore throat of the pores in the four images was visually identified (as demonstrated in Figure 4) and measured using the graphical software. The pore structure of the entire glass beads sample was presented using pore throat distribution (PTD), as illustrated in Figure 5. The PTD is expressed using pore throat diameter and the percentage of the number of pore throats of each size. The results in Figure 5 showed that the PTDs before and after the vibration had the similar size range of pore throats, and the numbers of different pore throats after the vibration were slightly more uniform than those before the vibration. Figure 5 also prominently revealed significant variation of PTDs during the vibration, as compared to the PTDs before and after the vibration. At the moments of 10sec and 20sec of the vibration, less data points in the bigger size range were present, indicating reduced number of larger pores in the porous media. Meanwhile, the results showed that more pores in smaller size range were created as a result of the vibration. Duplicate tests were conducted and the same analysis was performed. The PTD variation demonstrated the same trend as described above.
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0 500 1000 1500 2000 2500
Pore Throats (micron)
num
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before vibration
at 10sec of vibration
at 20 sec of vibration
after vibration (30sec)
Glass beads combination:
3mm, 4mm, 5mm
Water saturated
Figure 5. Effect of vibrations on pore throat distributions The effect of vibrations on soil pore structure may be extended to subsoil performances in the field, using the capillarity theory as expressed in Equation (1).
rp
!" cos2 #= (1)
The capillary pressure (p) is related to surface tension (σ), contact angle (α), and pore radius (r). Capillary pressure increases with smaller pore size. In order to drain the pore water in the reduced pores caused by seismic activity, a larger hydraulic gradient is required to overcome the increased capillary pressure. Reduced drainage capability may contribute to the difficulty of reducing pore water pressure and increasing effective stress, thus causing reduced strength. As initial steps in studying the pore structure variations due to vibrations were taken, limitations of this study should be noted. The conclusions are based on the study that used spherical glass beads. In addition, seismic characteristics of the small-scale shaking table are different from those of actual earthquakes (for example, the earthquake frequency is usually 0.2~2 Hz, lower than that of the shaking table). Further studies on real soil samples under real or simulated ground shakings may be needed to verify the conclusions of this study. In addition, the image analysis employed in this study can only measure the pore sizes in the plan view, and it only works for uniform or narrow-size-range porous media. This is a limitation of most image analysis of soil pore structure. In this study, narrow size range (3, 4, and 5mm) of glass beads was used. In a monolayer of different sizes of grains, large pores may occur on top of the small glass beads. Three-dimensional pore size variation was not investigated in this study.
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CONCLUSIONS This paper presented a simple experimental study on the pore structure variations due to vibration. The study shows that vibrations can cause larger pores to reduce in size; meanwhile, more pores of smaller sizes can be created during vibration. It may be concluded from this preliminary experimental study that although the pore structure of dense subsoil may remain relatively unchanged before and after seismic activity, the soil could experience significant change in pore structure during the vibration, which could affect the soil’s density, strength, and liquefaction potential. ACKNOWLEDGMENTS This research is a portion of a project funded by the Advanced Life Support Program of NASA John Space Center (Project Number: NAG9-1399). The support from this agency is gratefully acknowledged. The authors thank the anonymous reviewers for their constructive comments that help to improve the paper. REFERENCES Dobry, R. (1989). “Some basic aspects of soil liquefaction during earthquakes.”
Earthquake hazards and the design of constructed facilities in the Eastern United states. Annals of the New York Academy of Sciences. 558: 172-182.
Makdisi, F.I. and Seed, H.B. (1977). “A simplified procedure for estimating earthquake-induced deformation in dams and embankments.” Report UCB/EERC-7719, Earthquake Engineering Research Center, University of California, Berkeley, CA.
Ohara, S. and Matsuda, H. (1988). “Study on the settlement of saturated clay layer induced by cyclic shear.” Soils and Foundations, 28: 103-113.
Payatakes, A.C., Ng, K.M., and R.W.Flumerfelt (1980). “Oil ganglion dynamics during immiscible displacement: model formulation.” AIChe Journal, 26(3):430-442.
Xiao, M., Reddi, L.N., and Steinberg, S. (2006). “Effect of vibrations on pore fluid distribution in porous media.” Transport in Porous Media, 62(2):187-204.
Copyright ASCE 2008 Geotechnical Earthquake and Engineering and Soil Dynamics IV Congress 2008 Geotechnical Earthquake Engineering and Soil Dynamics IV
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