sand aging field study

Sand Aging Field Study

R.A. Green1, M. ASCE, P.E., R.D. Hryciw2, M. ASCE, D.A. Saftner3, S.M. ASCE,

C.D.P. Baxter4, M. ASCE, P.E., Y. Jung3, and T. Jirathanathaworn3

1Associate Professor, Department of Civil and Environmental Engineering, University of Michigan, 2372 G.G. Brown, Ann Arbor, MI 48109-2125; [email protected] 2Professor, Department of Civil and Environmental Engineering, University of Michigan, 2366 G.G. Brown, Ann Arbor, MI 48109-2125; [email protected] 3Graduate Student Research Assistant, Department of Civil and Environmental Engineering, University of Michigan, 2340 G.G. Brown, Ann Arbor, MI 48109-2125; [email protected]; [email protected]; [email protected] 4Associate Professor, Departments of Ocean/Civil and Environmental Engineering, University of Rhode Island, Narragansett, RI 02882; [email protected] ABSTRACT: Aging effects in sand, such as increases in penetration resistance with time after deposition, densification, and/or liquefaction, are known to occur in situ, but the causes of these effects are not fully understood. Nonetheless, these effects have important ramifications in earthquake engineering. First, the lack of understanding of the phenomenon is an impediment to quality assurance/quality control (QA/QC) for ground densification projects aimed at mitigating the damaging effects of liquefaction. This can be understood by considering that most liquefaction evaluation procedures correlate liquefaction susceptibility to in situ indices, such as penetration resistance (SPT and CPT) and small strain shear wave velocity (Vs), all of which are influenced by aging. Consequently, it is unclear as to how long after ground densification QA/QC in situ tests should be performed to ensure that the densification was sufficient to mitigate liquefaction susceptibility. Presented herein is an overview of an ongoing sand aging field study where liquefaction is being induced by explosives, vibrocompaction (using a vibroflot), and a NEES vibroseis in a heavily instrumented sand deposit. The state and properties of the sand are being monitored as a function of time after the disruption of the soil structure. INTRODUCTION Presented herein is an overview of an ongoing field study, the objective of which is to develop a better understanding of the mechanisms and engineering implications of time-dependent changes, commonly referred to as "aging," in the state and properties of recently deposited, liquefied, and/or densified sands. The research study is collaborative and synergistic in nature and involves researchers from the University of

Geotechnical Earthquake Engineering and Soil Dynamics IV GSP 181 © 2008 ASCE

Copyright ASCE 2008 Geotechnical Earthquake and Engineering and Soil Dynamics IV Congress 2008 Geotechnical Earthquake Engineering and Soil Dynamics IV

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Michigan (UM) and the University of Rhode Island (URI). Additionally, Nicholson Construction is a corporate partner in the project, donating equipment time and use, and Dr. James K. Mitchell (Virginia Tech) is a project consultant.

Numerous field case histories in the literature attest to the existence of the aging phenomenon, which manifests itself through increases in in situ test indices such as CPT and SPT penetration resistances and small strain shear wave velocity (Vs) after deposition, densification, and/or liquefaction (e.g., Mitchell and Solymar, 1984; Dumas and Beaton, 1988; Jefferies et al., 1988; Schmertmann, 1991; Charlie et al., 1992; Ng et al., 1996; Stokoe and Santamarina, 2000; Howie et al., 2000, 2001; Amini et al., 2002; Ashford et al., 2004a,b). Several differing hypotheses have been proposed for the underlying mechanisms for the aging phenomenon, including secondary compression and microstructural changes (Mesri et al., 1990; Schmertmann, 1991; Bowman and Soga, 2003), chemistry (Mitchell and Solymar, 1984; Joshi et al., 1995), blast gas dissipation (Dowding and Hryciw, 1986), and biological activity (Martin et al., 1996). However, laboratory investigations have been inconclusive in determining the controlling mechanism (Baxter and Mitchell, 2004), and no study has been able to replicate in the laboratory the large increases in penetration resistance and shear wave velocity observed in situ. Impeding the understanding of the mechanisms underlying the aging phenomenon is that most published field case histories lack sufficient detail about spatial variability of soil properties, soil and pore water chemistry, and long-term property changes (> 1 yr). Nevertheless, field case histories provide ample empirical evidence of the phenomenon, and under controlled conditions, field investigations represent the best opportunity to study the aging process and to develop and test a quality assurance/quality control (QA/QC) metric for remedially densified sand.

In the remaining portions of this paper, an overview of previous studies examining the sand aging phenomenon is given first. This information was used to design the current field study, an overview of which is then presented. PREVIOUS SAND AGING STUDIES

Published data showing the existence of sand aging come primarily from remedial

ground densification case histories (explosive compaction, vibrocompaction, and deep dynamic compaction), supplemented in part by data from laboratory investigations. Because liquefaction is typically induced as the first step in the remedial ground densification process, it is logically surmised that aging also occurs in sand deposits following liquefaction by earthquake shaking (e.g., Arango and Migues, 1996; Lewis et al., 1999; Arango et al., 2000; Olson et al., 2001; Gassman et al., 2004). Extensive summaries of aging investigations are given by Mitchell et al. (1997) and Baxter (1999). The following brief review is excerpted from C. Baxter's contribution to Mitchell et al. (1997).

Field Investigations

The Jebba Dam project on the Niger River, Nigeria, was the first well documented field study where aging effects in sands were both significant and widespread



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(Mitchell and Solymar, 1984). The project involved the treatment of foundation soils beneath a 42 m high dam and seepage blanket. The foundation soils consisted of deep deposits of alluvial medium to coarse silica sand. In some areas the depth to bedrock was greater than 70 m. Due to the large depths of loose sand requiring improvement, densification was performed in two stages. The upper 25 m of sand (and a 5 to 10 m thick sand pad placed by hydraulic filling of the river) was densified using vibrocompaction, while the deposits between 25 to 40 m were densified by blasting.

Following the blasting operations, it was observed that the sand exhibited both sensitivity (i.e., strength loss on disturbance) and aging effects after redeposition and/or densification. This phenomenon occurred throughout the site. Initially after improvement, there was a decrease in penetration resistance, despite the fact that surface settlements ranging from 0.3 to 1.1 m were measured. With time (measured up to 124 days after improvement), however, the cone penetration resistance was found to increase to approximately 150-200 % of the pre-densification values.

Aging effects were also observed after placement of hydraulic fill working platforms and after densification by vibrocompaction. In the case of vibrocompaction, however, there was considerable variability in the degree of aging throughout the site. Because of the greater increase in density and lateral stress caused by vibrocompaction than by blast-densification, no initial decrease in the penetration resistance (sensitivity) was observed following compaction.

A more recent example of aging effects in sand was reported by Ng et al. (1996) during construction of the Chek Lap Kok airport in Hong Kong. Vibrocompaction was performed in specific areas to improve the penetration resistance of the hydraulically placed sand. Cone penetration testing was performed at one location at different times, up to 47 days after improvement. A clear increase in penetration resistance was observed. As with the Jebba Dam project, the time-dependent increase in cone penetration resistance after vibrocompaction occurred with no detectable increase in density (i.e., surface settlement). Laboratory Investigations

Although most of the examples of aging in sands reported in the literature are from field studies, some observations have been made in the laboratory as well. Afifi and Richart (1973) showed that when sand specimens are maintained under a constant confining pressure, the shear modulus determined at small strains increased with time of confinement. Thomann and Hryciw (1992) furthermore showed that a moderate shear straining following an initial aging period will decrease the small strain shear modulus, (i.e. negate the aging effects) only to be regained during a subsequent aging period. Daramola (1980) investigated the effects of aging on both the stiffness and shear strength of Ham River sand. Four consolidated drained triaxial tests were performed on samples with the same relative density and confining pressure (400 kPa), but consolidated for different periods of time (0, 10, 30, and 152 days). The test results showed that the stiffness increased and the strain to failure decreased with increasing time of consolidation. The samples consolidated for long periods of time also exhibited greater volumetric expansion (dilatancy) for given values of axial strain. Daramola concluded that a 50 % increase in modulus occurred for each log cycle of



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time under stress. Despite the increase in modulus and dilatancy, however, there was no increase in peak shear strength with time of aging. More recent studies by Howie et al. (2001) and Bowman and Soga (2003) present results showing similar trends.

A laboratory study on the effect of time on penetration resistance was performed by Joshi et al. (1995). The influences of both sand type and pore fluid composition on the magnitude of aging effects were investigated. Two different sands were tested: a river sand and Beaufort Sea sand. Three different pore fluids were used: air, distilled water, and sea water. Specimens were prepared by pluviating the sand through either air or water (depending on the pore fluid to be used) into fixed wall cells and vibrated under a static vertical stress of 100 kPa until a desired density was achieved. This method resulted in very dense samples, ranging from 87 to 100 % relative density.

After loading, the specimens were aged for two years, and values of penetration resistance were obtained at various times in each specimen using a series of four, 1 cm diameter penetrometers so that redundant data could be obtained at each time. Aging effects were observed in all cases, but the effects were greater for submerged sand than for dry specimens. Scanning electron micrographs of the aged specimens in distilled water and sea water showed the presence of precipitates on and in between sand grains. An energy dispersive x-ray analyzer was used to determine the composition of the precipitate. For the river sand in distilled water, the precipitates contained calcium and possibly silica. For the river sand in sea water, the precipitates contained sodium, silica, calcium, and chlorine.

Baxter and Mitchell (2004) present the results of an extensive laboratory investigation that examined the influence of different combinations of relative density, temperature, and pore fluid on the aging effects of two different sands: Evanston Beach sand and Density sand. Evanston Beach sand is a tan, sub-angular, poorly graded fine sand and was selected because it had been used in a previous aging study by Dowding and Hryciw (1986). Density sand is a white, rounded, poorly graded, fine to medium sand used for sand-cone tests, and was chosen because its chemical composition (almost pure quartz) and grain shape are markedly different than those of Evanston Beach sand. The results from this study showed increases in the small strain shear modulus throughout most of the tests, and chemical analyses suggested that precipitation of carbonate and silica occurred in two tests. However, despite these changes, there was no corresponding increase in the mini-cone penetration resistance with time in any of the tests. The dichotomy between their laboratory results and the field observations of others lead to the following statement: "…some condition in natural deposits is not replicated in small-scale laboratory testing. Possible conditions that may be different in the field include the introduction of air and gas into the soil during ground improvement, heterogeneity of the deposit, energy imparted by ground improvement, and biological activity."

Hypothesized Aging Mechanisms

Historically, the most widespread theory used to explain aging effects in sand has involved interparticle bonding. Terzaghi originally referred to a "bond strength" in connection with the presence of a quasi-preconsolidation pressure in the field (Schmertmann, 1991). Generally, this bonding mechanism has been thought of as



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cementation, which would increase the cohesion of a soil without affecting its friction angle. The cementing agent has commonly been thought to be silica-acid gel, which has an amorphous structure and would form a precipitate at particle contacts (Mitchell and Solymar, 1984). There is little direct evidence that cementation of sand particles by silica precipitation occurs after ground improvement. However, in a laboratory-scale blasting study, Hryciw (1986) demonstrated that explosive generated gases produce a significant fluctuation in pore fluid pH, which enhances dissolution and precipitation of silica and thus possibly assists the early diagenesis of sands. The strongest evidence to date of a chemical mechanism being responsible for aging effects is the work done by Joshi et al. (1995) presented earlier. A precipitate was observed on aged specimens of sand. In addition, the composition of the precipitate in the samples of river sand was found to contain calcium (the much more soluble fraction of the sand), possibly silica, and sodium and chlorine when sea water was the pore fluid.

Mesri et al. (1990) proposed that aging effects in sands are mechanical in nature and are due entirely to an increased frictional resistance which develops during secondary compression. However, this increased resistance does not occur solely from the change in density that occurs during drained secondary compression. Rather, it is due to a continued rearrangement of particles resulting in increased macro-interlocking of particles and increased micro-interlocking of surface roughness. These mechanisms are postulated to cause an increase in both stiffness and horizontal effective stress. Although no direct evidence was presented supporting their hypothesis, Mesri et al. (1990) used the triaxial test data from Daramola (1980) to argue against a chemical mechanism responsible for aging effects in sands. Specimens of Ham River sand that underwent drained secondary compression for up to 152 days had an increased modulus up to approximately 3 % strain. According to Mesri et al. (1990), this large strain would destroy any cementation and another less brittle mechanism must be responsible for the increase in stiffness.

Schmertmann (1991) also hypothesized that aging effects in sands are caused by mechanical effects. Like Mesri et al. (1990), increased interlocking between particles with time was proposed by Schmertmann as a significant factor. He also suggested that small particle movements during secondary compression would lead to internal stress arching and a more stable arrangement of particles. Bowman and Soga (2003) recently measured such particle movements in samples of dense sand. They showed that microstructural changes during secondary compression, including particle rotation and variations in local void ratio distributions, leads to the development of load chains and increased dilatancy. CURRENT SAND AGING FIELD STUDY

As evident from previous studies, the underlying mechanisms of sand aging remain

uncertain. The present study builds on the previous research and involves a field investigation where liquefaction/densification is being induced by explosives, a vibroflot, and a NEES vibroseis in a heavily instrumented sand profile and the state and properties of the sand are being monitored as a function of time after disruption of the soil structure. Explosives and vibrocompaction were selected because they are



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ground densification methods in which the aging phenomenon has been observed in the past. However, both methods introduce foreign elements into the soil, as explosives generate gases and vibroflotation introduces heavily aerated water. The dissipation of the blast gases and air from the ground water with time after treatment may be one of the contributors to aging in sand. Also, as noted above, the blast gases change the pH of the pore fluid, which may contribute to the aging effects. To determine whether the dissipation of gases from pore water and the change in pH of pore water influences aging, liquefaction will be induced in the third region using a vibroseis from the University of Texas (i.e., NEES equipment). The vibroseis only introduces seismic waves into the ground and have been known to induce liquefaction, even unintentionally, in deposits of loose, saturated sand (e.g., Hryciw et al., 1990).

The site where the field aging study is being carried out is an active sand quarry owned by Mulzer Crushed Stone, Inc. and is located in southwestern Indiana. The authors characterized the field study site using vision cone penetration tests (VisCPT) (Hryciw and Shin, 2004; Jung et al., 2008), Marchetti dilatometer tests (DMT), seismic cone penetration tests (SCPT), and grain size analysis. The site profile consists of a clay cap of approximately 2 m overlying a thick deposit of sand. The water table at the time of the site investigation was approximately 1.75 m below the ground surface. Figure 1 shows a representative CPT sounding from the site. As may be observed from this figure, there is a loose sand layer from 2.5 to 3.2 m below the ground surface. Immediately below the loose sand layer is a dense sand layer, which will allow the influence of density on the aging phenomenon to be investigated.

-50 0 50 100 150 200

Pore Pressure, Pw (kPa)0 5 10 15

Friction Ratio, fs/qc (%)

0

1

2

3

4

5

6

0 5 10 15 20

Tip Resistance, qc (MPa)

Dep

th, z

(m)

Dr = 30%40%

50%60%

70%

Loose sand

Clay

Dense sand

Static pore pressure

FIG. 1. CPT logs from the field test site. Superimposed on the tip resistance log are contours of equivalent relative density per Jamiolkowski et al. (1985).

The sand at the test site is "poorly graded" (SP), with a coefficient of uniformity of

2.05 and a coefficient of gradation of 0.78. Figure 2 shows the grain size distribution



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for the sand. As observed, the grain size curve falls close to the "coarse boundary" that defines "most liquefiable soil." The VisCPT recorded movement of sand (or boiling) as the CPT probe advanced through the loose sand layer, clearly indicating that this soil is liquefiable.

0102030405060708090

100

0.0010.010.1110Diameter (mm)

% P

assi

ng b

y w

t.Mulzer Quarry SoilMost Liquefiable SoilPotentially Liquefiable Soil

FIG. 2. Grain size distribution of sand from the loose sand layer. Superimposed on the plot are ranges of the grain size distributions of "Most Liquefiable Soil" and "Potentially Liquefiable Soil," per Tsuchida (1970).

At the site, liquefaction/densification will be induced by the three methods (i.e.,

explosives, vibrocompaction, and vibroseis) in three, non-overlapping regions. It is estimated that each region should have a minimum 15 m radius to ensure the energy imparted to induce liquefaction in one region does not influence an adjacent region, as well as to ensure that elevated pore water pressures do not migrate among adjacent regions. A plan view of the site test layout is shown in Figure 3, wherein the compaction/blast/shake points (labeled as energy points in Figures 3 and 5) are arranged in a diamond pattern with approximately 2.5 m between the points. The purpose for using multiple points is to replicate actual conditions for remedial densification projects. For the explosives test region, the four charges will be detonated sequentially, with a 0.5 sec delay between detonations 1 & 2 and 3 & 4, with a 4 hr delay between detonations 2 & 3. The selected delays are to ensure that the soil is subjected to multiple "hits," similar to actual explosive densification projects.

Within each of the regions, pore pressure transducers, accelerometers, and settlement tubes will be installed. The purpose of the pore pressure transducers is to confirm that liquefaction has been induced (i.e., excess pore pressure equal to the initial effective overburden pressure), and to allow the temporal and spatial monitoring of the excess pore pressure generation and dissipation. The accelerometers will allow the induced strains to be computed (e.g., Rathje et al., 2002, 2004). Also, using a similar approach to that of Zeghal and Elgamal (1994) or Davis and Berrill (1998, 2000) the accelerometers at different depths can be used to calculate the energy dissipated in the soil during the liquefaction process, which will allow an assessment of whether the amount of energy imparted in the soil influences the magnitude of aging effects (e.g., Baxter and Mitchell, 2004). Settlement tubes will provide settlement data as a function of depth and distance from the energy source.



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Additionally, a light detection and ranging (LIDAR) system will be used to survey the ground surface before and at various times after the disruption of the soil structure, which will provide temporal and spatial ground surface settlement information.

explosivestest region

vibrocompactiontest region

vibroseistest region

15 m 15 m

15 m

energy points

a)

2.5 m 2.5 m

2.5 m

FIG. 3. Plan view of the site test layout: three non-overlapping regions will be used, one for each method by which liquefaction will be induced. The "energy points" are the location where liquefaction will be induced via vibro-compaction, explosive compaction, or using the vibroseis.

Two data acquisition systems will be used in the field study, with different types of

data being collected by each. The first acquisition system is a series of wireless sensors that are being integrated with the MEMS accelerometers and pore pressure transducers. The wireless sensors were developed at the University of Michigan by Professor Jerome Lynch (Figure 4).

Sensors

Geophones

Accelerometers

Shaker

Data acquisition computer

FIG. 4. Wireless sensing unit prototype developed at the University of Michigan by Prof. J.P. Lynch. Right: Sensor without battery and external container (Dimensions of the fully assembled unit are approximately 8 × 15 × 3 cm.). Left: Laboratory calibration of sensors with geophones and accelerometers.

The second data acquisition system being used is a tethered monitoring system by

Olson Instruments, Inc. (i.e., the Freedom Data Acquisition System PC). This system



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uses SHM Version 2.1 software and has 16 channels. Presently, the authors are performing comparisons of data acquired by the wireless and Olson systems.

A layout of the post-liquefaction in situ test plan is shown in Figure 5. The purpose for this plan is to monitor for aging effects in each of the three regions. The use of different techniques for inducing liquefaction/densification will allow chemical versus mechanical mechanisms to be discerned, to some extent. For example, if larger aging effects manifest in the region in which explosive compaction is performed relative to the other two regions, one possible explanation may the change in the pH of the groundwater due to the blast gases. However, conclusive discernment of the chemical versus mechanical aging mechanisms will be made from the synergistic laboratory study using soil and groundwater from the field site. In large part, the laboratory study will be driven by the field observations. The laboratory will provide a more controlled environment wherein the boundary conditions can be systematically varied so that the influence of each on aging can be ascertained, with the field investigation providing a comparative baseline of the cumulative influence of the various testing conditions and parameters.

Using the test plan shown in Figure 5, the in situ indices will be measured on a regular basis for the first four weeks after liquefaction/densification. However, the time interval between the radial sets of soundings will increase as the changes in the values of the in situ indices with time decrease. Monitoring of the aging effects will continue for the duration of the project.

2.0 m

5.0 m

pre-liquefaction

1 day10 days

100 days

pre-liquefaction

1 day 10 days

100 days

Energy points

VisCPT, uSCPT, and DMT soundings

Potential VisCPT, uSCPT, and DMT soundings

FIG. 5. Idealized plan view of in situ sounding locations for monitoring post-liquefaction/densification aging effects. In this figure, the four "energy points" indicate the locations of the vibrocompaction points, the explosives, or the vibroseis. At a given time after liquefaction/densification, in situ soundings will be performed at the prescribed radial distances from the center of the energy points.

As currently scheduled, explosive densification will be performed in Spring, 2008. This method of inducing liquefaction is being performed first because it will be the most demanding on the instrumentation and will serve to debug the test setup (and will be repeated if needed as a result of any experienced equipment failure). Vibrocompaction and vibroseis will be performed during the Summer or Fall of 2008.



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CONCLUSIONS

The sand aging phenomenon posses an impediment to remedial ground densification programs, as it is uncertain how long after ground densification in situ index tests should be performed to ensure the soil is sufficiently densified. An ongoing field study and synergistic laboratory study is being conducted to improve the understanding of the mechanisms underlying the aging phenomenon and their quantitative influences on engineering properties. An increased understanding of sand aging will lead to greater efficiency in site improvement projects aimed at mitigating the risk of seismically induced liquefaction. ACKNOWLEDGEMENTS

Support for the research presented herein came in part from the NSF grants CMMI 0530378 and CMMI 0636710. Mulzer Crushed Stone, Inc. is donating the use of its quarry and the expertise of its employees, and Nicholson Construction is donating the vibroflot equipment time and the expertise of its employees. Drs. Richard Woods, Jerome P. Lynch, and Kyle Rollins provided valuable assistance in the selection of field instrumentation and data acquisition systems. Mr. Jan Pantolin, Mr. Ayman Ibrahim, and Capt. Theresa White, USAF, provided valuable assistance in performing the preliminary site characterization. This support and assistance is gratefully acknowledged REFERENCES Afifi, S.S. and Richart, F.E., Jr. (1973). "Stress-History Effects on Shear Modulus of

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