Erosion Mechanisms of Organic Soil and Bioabatementof Piping Erosion of Sand

Benjamin T. Adams, A.M.ASCE1; Ming Xiao, M.ASCE2; and Alice Wright3

Abstract: This paper presents the findings of a preliminary research program that aimed to investigate the mechanisms governing the progres-sion of piping erosion in organic soils and attempted to use the findings to reduce the severity of piping erosion of sand. The hypothesis was thatthe presence of organics within mineral soils results in a reduction in piping erosion progression. Erosion behaviors of the soils were quantifiedusing a simple erosion test with a preformed hole to simulate an initial piping channel. The researchwas split into five test phases. In Phase 1, theinfluence of grain-size distribution was eliminated to develop a preliminary understanding of the role that organic matter plays in the erosionprocess. The results indicated that organic matter likely contributed to a reduction in piping erosion. The second phase excluded both grain-sizedistribution and individual particle shape as variables in the erosion process. The results further confirmed the connection between organic mat-ter content and erosion resistance. The third phase revealed the positive correlation between the reduced piping erosion and increased organicmatter content. The fourth phase investigated the quantitative relationship between certain biologically derived substances (polysaccharidesand glomalin) and piping erosion reduction. The positive correlations that were preliminarily derived from this phase indicated that these sub-stances likely play a major role in reducing piping erosion. The final phase comprised of introducing organic matter into mineral soil and quan-tifying the subsequent changes in geomechanic properties. The results suggested that the introduction of organic materials into mineral soildecreases strength, increases consolidation settlement, and reduces permeability. DOI: 10.1061/(ASCE)GT.1943-5606.0000863. © 2013American Society of Civil Engineers.

CE Database subject headings: Soil erosion; Sand (soil type); Biological processes.

Author keywords: Erosion; Piping; Sand; Organic soils; Biological properties; Biological treatment; Geomechanics.

Introduction

As water flows within a soil mass, such as an earth-fill dam or levee,a form of internal erosion known as piping erosion can occur. Thisform of erosion can be highly destructive, and breaches of earthenlevees and dams have been attributed to piping-erosion inducedfailures. In a survey of 11,192 dams, Foster et al. (2000) concludedthat approximately 46% of the dam failures among the entire surveysample could be attributed to internal erosion. In reanalyzing thissurvey, Richards and Reddy (2008) determined that approximately31% of all dam failures resulted from the piping mode of failure.Traditional mechanistic remediation methods include filters, cutoffwalls, impermeable blankets and berms, and combinations of toetrenches, drains, and pressure relief wells (U.S. Army Corps ofEngineers 2000).Additionally, recent studies focusing onmicrobial-induced carbonate precipitation (MICP) demonstrated the ability ofcertain microorganisms to facilitate soil cementing through naturalmetabolic reactions (Mortensen et al. 2011; Hamdan et al. 2011).

However, like other more traditional methods for reducing thelikelihood of piping erosion, these techniques are most appropriatefor in situ applications. MICP relies on various enhancements ofthe soil ecosystem in which the microorganisms live and results inin situ soil cementing and stabilization. The ability to increasethe soil’s natural resistance to erosion during construction of theembankment could serve as a complement to, or in some appli-cations, as a replacement of, the current remediation methods.Recent studies have recognized the soil aggregating and stabilizingproperties of certain organically derived substances (Brady andWeil 2002; Comis 2002). Results from laboratory-scale tests byXiao et al. (2010) and Xiao and Gomez (2009) provided furtherevidence of the ability of organic materials to reduce the severity ofsurface erosion on sloped embankments. The purpose of this re-searchwas to develop an understanding of some of the fundamentalmechanisms governing erosion in organic soils, with the ultimategoal of developing improved design methods for resisting erosionin water-retaining embankments through a biologically derivederosion reduction technique,which is referred tohere as bioabatement.

Materials and Experimental Methods

The research included five test phases. In Phase 1, the influence ofgrain-size distribution (GSD) was eliminated to develop a pre-liminary understanding of the role that organic matter plays in theerosion process. In Phase 2, both theGSD and individual grain shapewere excluded as variables in the erosion process to more preciselyreveal any connection between organic matter content and erosionresistance. In Phase 3, a potential correlation between increasedorganic matter content and reduced piping erosion was established.The fourth phase investigated the quantitative relationship between

1Staff Engineer, Grundfos Corp., 5900 E. Shields Ave., Fresno, CA93727; formerly, M.S. Student, Dept. of Civil and Geomatics Engineering,California State Univ., Fresno, CA 93740.

2Associate Professor, Dept. of Civil and Geomatics Engineering, Cal-ifornia State Univ., Fresno, CA 93740 (corresponding author). E-mail:mxiao@csufresno.edu

3Professor, Biology Dept., California State Univ., Fresno, CA 93740.Note. This manuscript was submitted on February 9, 2012; approved on

November 14, 2012; published online on November 15, 2012. Discussionperiod open until January 1, 2014; separate discussions must be submittedfor individual papers. This paper is part of the Journal of Geotechnicaland Geoenvironmental Engineering, Vol. 139, No. 8, August 1, 2013.©ASCE, ISSN 1090-0241/2013/8-1360–1368/$25.00.

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certain biologically derived substances (polysaccharides and glo-malin) and piping erosion reduction. Thefinal phase quantified someof the changes in a mineral soil’s geomechanic properties resultingfrom the introduction of organic matter.

Materials

Four types of base soils were used in this experimental program. Thefirst soil was a naturally occurring fibrous peat that was sampledfrom the banks of a river near the town of Kerman in the San JoaquinValley of central California. The soil’s location and compositionsuggested that it was likely a fibric histosol resulting from the partialdecomposition of the abundant tule-reed beds in the region (Bradyand Weil 2002). The Kerman peat (KP) was light brown in color,contained visible organic fibers and large particles, and was non-plastic. The peat was sampled and immediately placed in airtightcontainers at room temperature (20–22�C) for storage and testing.

The second soil was a commercially produced cocompost. Thecocompost constituted equal mass proportions of green waste andbiosolids containing large, relatively fresh organic particles.

The third soil was a fine-grained silty sand (FGS), originallysampled directly from a construction site stockpile. This type of fine-grained cohesionless soil is known to be highly susceptible to pipingerosion (Terzaghi et al. 1996;Wan and Fell 2004a, b). The silty sandwas regraded to match the GSD of the KP using the followingprocedure: (1) a sample of theKPwas separated using six sieve sizes,and the corresponding passing percentages were calculated, (2) theFGS was separated using the same six sieves, and (3) the passingpercentages of the KP served as the basis of the mixing ratio for theportions of the FGS collected on the six sieves, resulting in similarGSDs for both soils. Fig. 1 shows the GSDs of the KP, cocompost,and FGS.

The fourth soil was Turface, a manufactured aggregate product.Turface, containing negligible organic matter, was used here to spe-cifically replicate the distinctive shape and textural characteristics ofthe organic soils. Turface is created by heat-treating montmorilloniteclay into a ceramic material that is then milled and processed intovarious grain sizes that resemble those of natural soil. The productis generally applied as an athletic field conditioner to help controldrainage. The flakey shape and rough surface texture of the in-dividual particles made Turface a suitable candidate for replicatingthe unique shape characteristics exhibited by organic soil particles,as indicated by the 5003magnification scanning electron micro-photographs (SEMs) in Figs. 2(a–c). As a reference, the SEM of theFGS is shown in Fig. 2(d).

Erosion Test Methodology

The experimental methods were designed to test a soil’s overallresistance to erosion along a preexisting piping channel. The erosiontest setup shown in Fig. 3 was used to conduct the erosion testing.The test setup borrowed the concept of the constant-head holeerosion test (HET), which was originally developed by Wan andFell (2004a, b). Recent work by researchers at the U.S. Bureau ofReclamation (Wahl et al. 2008, 2009; Marot et al. 2011) notablyimproved the HET methodology. In this research, the erosion testprocedure deviated from the HET methodology: a single hydraulicgradient was used for all erosion tests, and the variation of theerosion rate with seepage was monitored to consider the time factorin the erosion resistance.

The test was conducted on specimens 70mm in diameter and 140mm in length. Clear acrylic tubing and end caps were used for thespecimen mold and allowed for easy observation of the specimen

Fig. 1. Grain-size distributions of the Kerman peat, cocompost, andfine-grained silty sand

Fig. 2. Scanning electron microphotographs of the four base soils usedin this study: (a) Kerman peat; (b) cocompost; (c) Turface; (d) fine-grained silty sand

Fig. 3. Erosion test setup

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during all phases of testing. The end caps were designed in sucha way that eroded soil particles were able to exit from the specimenunhindered from the specimen for collection within buckets directlybeneath the specimen.

The specimen was created by compacting soil directly within themold to its maximum dry density in thin, uniform layers at the op-timum moisture content. The maximum dry density and optimummoisture content were obtained using the Harvard miniature com-paction apparatus. In the Harvard miniature compaction test, each ofthe three uniform layers of the specimens received 25 tamps from the89-N spring. A concentrically located hole was preformed duringspecimen compaction using a metal rod with a diameter of 6.5 mm.This hole simulated an initial piping channel and spanned the entirelength of the specimen. A specially designed tamper covering nearlythe entire cross-sectional area of the specimen was used to achieveconsistent densification within each layer. The tubular handle of thetamper was hollow and closely encased the metal rod that was usedto form the initial piping hole. This ensured that the soil adjacent tothe piping hole was not over- or undercompacted.

Deionized water was introduced at the top of the cylinder viaa constant-head reservoir and then passed through a 20-mm-thicklayer of uniform glass beads before reaching the specimen. Theglass beads, measuring 6 mm in diameter, were used to evenlydistribute the influent before it entered the simulated piping channel.A metal mesh with 4.75-mm-square openings separated the glassbeads and the soil specimen, preventing the glass beads from fallingthrough the hole during the erosion test. Effluent with eroded soilparticles was incrementally collected in buckets directly beneath thespecimen as it exited the lower end cap.With the downstream side ofthe specimen open to the atmosphere, a constant hydraulic gradientof four was established for the erosion tests. The hydraulic gradientwas selected based on previous research work. The slot erosion testsconducted by Wan and Fell (2004b) used a constant hydraulicgradient of 2.5; the piping erosion tests by Indraratna et al. (2008)used the hydraulic gradients from 0.5 to 7.0. The ASTM pinholetest specification [ASTMD4647-06e1 (ASTM2011b)] recommendeda hydraulic gradient range from 1.3 to 26.7.

Each specimen was tested for durations of 60min or until failure,whichever occurred first. Failure was defined as enlargement of theinitial piping hole to the perimeter of the mold. The enlargement ofthe piping hole was visually monitored through the transparentmold. When the piping hole enlarged to the perimeter of the mold,the erosion test was terminated. The total dry mass of eroded soilparticles was measured for each effluent collection increment. Thisvalue was determined through decanting and drying of the collectedeffluent with eroded material.

Phase 1: Biological Effect of Organic Matter Content onPiping Erosion Progression Excluding the Influence ofGrain-Size Distribution

Both physical and biological characteristics of a soil can affectpiping erosion progression, and GSD can play a major role in de-termining erosion behavior (Terzaghi et al. 1996). Therefore, in thefirst phase of the study, the effect of GSD on piping erosion pro-gression was eliminated. The purpose was to qualitatively revealthe effect of organic matter content on erosion resistance. This wasaccomplished by testing three different soils that exhibited similarGSDs but varied in organic matter content (0–22.6%).

The first soil was the KP, the second soil was an FGS with thesame GSD of the KP, and the third soil, here denoted as FGS-5%ORG, was a composite soil created in the laboratory by mixingthe FGS and theKP that exhibited the sameGSD. The two soils weremixed at a mass ratio of 22% KP to 78% FGS, creating an organic

sandy soil that contained 5% organic matter content by mass. Be-cause the FGS and the KP with the same gradation were the onlycomponents used in creating the composite mixture, all three soilsexhibited similar GSDs. Table 1 provides a summary of the physicalcharacteristics of the three soils. The erosion test results for the threesoil types are subsequently presented.

Phase 2: Biological Effect of Organic Matter Content onPiping Erosion Progression Excluding the Influences ofGrain-Size Distribution and Particle Shape

In addition to the gradation, individual grain shapes and texturesmay also affect piping erosion progression. The unique shape andtextural characteristics of the organic constituents of fibrous peatshave been well documented in the technical literature (Mesri andAjlouni 2007). In this research, the flakey, highly perforated andcellular structure of the biologically derived organic material was instark contrast to the smooth and rounded nature of themineral grainsof the FGS, as demonstrated by the SEMs in Fig. 2.

In Phase 1 of this research, organic matter (KP) was added to theFGS and erosion reduction was observed, as subsequently shown.The erosion reduction may be because of the combined effects oforganic matter constituents and the unique shape of peat particles.Aside from grain sizes, the interactions of soil grains may be in-fluenced by the shapes of the grains, thus affecting the mobilizationof soil particles on the wall of a piping channel. Therefore, to moreprecisely reveal the biological effect of organic soil on piping ero-sion progression, the two major physical characteristics of soils(grain sizes and shapes) must be excluded. Accordingly, the secondphase aimed to compare the erosion rates of an organic soil and amineral soil (FGS) (with essentially zero organicmatter content), eachexhibiting the same particle shape and texture.

To eliminate the influence of grain size, the GSD of the Turfacewas adjusted to match that of the organic soils (Fig. 1), using thesame GSD adjustment technique utilized in Phase 1. In this phase,two types of organic soil were used to create two independentcomparison groups (Table 2). One organic soil was the KP used inPhase 1 and the otherwas cocompost. Both soilsweremixedwith theFGS at mass ratios of 20% organic soil to 80% mineral soil (FGS).The sand-peatmixturewas referred to as FGS-20%KP, and the sand-cocompost mixture was referred to as FGS-20%cocomp. The GSDof the Turface, as explained previously, was matched to that of theKP and the cocompost, respectively, and mixed with the FGS at thesame 20:80 mass ratio. The resulting mixtures were referred to asFGS-20%Turface(KP GSD) and FGS-20%Turface(cocomp GSD).Turface(KP GSD) was the Turface material with the same GSD asthat of the KP, and Turface(cocomp GSD) was the Turface materialwith the same GSD as that of the cocompost. A summary of the twosoil comparison groups is provided in Table 2. Table 3 contains thephysical characteristics of the four soil types.

In summary, the two soils in each comparison groupwere createdsuch that they exhibited the same GSD and a similar particle shape,but different organicmatter content. The erosion tests were conducted

Table 1. Fine-Grained Silty Sand, Kerman Peat, and FGS-5%ORGCharacteristics in Phase 1

Soil propertyFine-grainedsilty sand

Kermanpeat

FGS-5%ORG

Organic matter content(percentage)

0.0 22.6 5.0

Maximum dry density (g/cm3) 1.94 0.95 1.59Optimum moisture content(percentage)

9.5 46.0 16.0

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on the four soils after being compacted to their maximum dry den-sities at optimum moisture content within the specimen mold. Thetwo soils within each comparison group were analyzed for theirerosion resistance.

Phase 3: Effect of Varied Organic Matter Content onPiping Erosion Progression

With a qualitative understanding of the biologically derived effecton piping erosion resistance, the third phase of the study aimed toquantitatively reveal the influence of varied organic matter contentson piping erosion progression. The cocompost and FGSwere mixedto create four mixtures of 5, 10, 15, and 20% cocompost by mass.The physical characteristics of each soil are provided in Table 4. TheGSDs of the four FGS-cocompost mixtures were almost identical.Therefore, their graphical inclusion was omitted from the paper.Because eachmixturewas created from the same two base soils, theyalso possessed similar particle shapes. Therefore, the effects of GSDand particle shape can be discarded in this phase to exclusivelyreveal the effect of varied organic matter contents on the erosionprogression. The FGSwasfirst tested as a control sample. The pipingerosion progressions of the four soil mixtures were quantified usingthe erosion tests.

The FGS used in this phase was not regraded, whereas the FGSused in Phases 1 and 2 was regraded to obtain a certain GSD.Therefore, the FGS in both Phases 1 and 2were slightly different insize distribution from the FGS in Phase 3. Consequently, theircompaction results were slightly different, as shown in Tables 1and 4.

Phase 4: Analysis of Biologically Derived SubstancesAffecting Piping Erosion Resistance

The fourth testing phase aimed to quantify two biologically derivedsubstances, polysaccharides and glomalin, to discover their potentialrelationships with and effects on the observed erosion reduction. As

components of what are known as biofilms, glomalin and poly-saccharides have been shown to increase soil stability by acting assoil-aggregating organic glues (Brady andWeil 2002; Comis 2002).Measurements were made on the four base materials utilized in thefirst three phases of testing: FGS, KP, cocompost, and Turface(KPGSD). Quantification of the two biological characteristics allowedfor a base-level analysis of the possible correlations between theobserved reduction in erosion and the measured concentrations ofpolysaccharides and glomalin.

Polysaccharide concentrations were determined via optical-density bioassays utilizing sucrose as the comparison standard.For the glomalin measurement, the Bradford method was used toextract the glomalin and determine its concentration (Janos et al.2008).

Phase 5: Effects of Piping Erosion BioabatementMeasures on Soil Geomechanical Properties

Because the bioabatement methods involved the direct incor-poration of biologically derived organic material into a mineral soil(FGS), the potentially detrimental side effects caused by the pres-ence of the organic constituents required investigation. Therefore,the three most significant engineering properties that relate to leveestability, namely settlement, strength, and permeability, were ana-lyzed. Two soils, the FGS and the FGSmixed with 20% cocompost,as used in Phase 2, were subjected to consolidation using incrementalloading [as per ASTM D2435-11 (ASTM 2011d)], consolidated-undrained triaxial compression [as per ASTM D4767-11 (ASTM2011a)], and flexible wall permeability tests [as per ASTMD5084-10(ASTM 2011c)]. For the settlement testing, 24-h incrementalloading was used to study both total and time rate of consolidation.A pressure range of 15:4e247:0 kN=m2 was implemented, with

Table 2. Summary of Soil Comparison Groups in Phase 2

Comparisongroup Soil name Description

1 FGS-20%KP Fine-grained silty sand mixedwith 20% Kerman peat

FGS-20%Turface(KP GSD)

Fine-grained silty sand mixedwith 20% Turface with the samegradation as Kerman peat

2 FGS-20%cocomp Fine-grained silty sand mixedwith 20% cocompost

FGS-20%Turface(cocomp GSD)

Fine-grained silty sand mixedwith 20% Turface with the samegradation as cocompost

Table 3. Characteristics of Four Soil Mixtures Used in Phase 2

Soil property

FGS-20%Turface

(KP GSD)

FGS-20%Turface(cocompGSD)

FGS-20%KP

FGS-20%cocomp

Organic mattercontent (percentage)

0.0 0.0 4.5 8.5

Maximum drydensity (g/cm3)

1.60 1.55 1.46 1.60

Optimum moisturecontent (percentage)

20.7 21.1 18.7 20.8

Table 4. Characteristics of Fine-Grained Silty Sand, Cocompost, and Fine-Grained Silty Sand and Cocompost Mixtures of 5, 10, 15, and 20%Cocompost by Mass in Phase 3

Soil propertyFine-grainedsilty sand 5% 10% 15% 20% Cocompost

Organic mattercontent (percentage)

0.0 2.1 4.3 6.4 8.5 42.6

Maximum dry density(g/cm3)

1.88 1.77 1.67 1.58 1.46 0.79

Optimum moisturecontent (percentage)

12.2 13.7 17.0 20.1 20.8 51.0

Fig. 4. Fine-grained silty sand, FGS-5%ORG, and Kerman peat ero-sion rates

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time-deformation measurements taken at pressures of 30.9 and247:0 kN=m2. Standard triaxial consolidated undrained com-pression andflexible wall permeability testswere performed on soilspecimens 71 mm in diameter and 142 mm in length. An effectiveconfining pressure of 55:2 kN=m2 was achieved by providinga total confining pressure of 110:4 kN=m2 and a pore pressure of55:2 kN=m2, simulating possible field conditions within a levee orearth-fill dam.

Results and Discussion

The results from the five phases of testing are presented in the fol-lowing sections. The graphs containing erosion rates were generatedusing the dry mass of eroded soils collected during each test. Thisrate represented the percent erosion per liter of effluent collectedduring each effluent collection increment, where percent erosionwas calculated as grams of dry eroded material removed from thespecimen during each increment divided by the specimen’s initialdry mass. Each erosion rate value was plotted against the corre-sponding cumulative seepage volume.

In the graphs displaying erosion rate versus cumulative seepagevolume, the first point may not be reliable. When forming the pipingchannel during specimen compaction, the removal of the metal rodthat was used to create the piping hole could cause some soil par-ticles to dislodge from the wall of the hole. The assembly of thecylinder and the end caps could also cause a slight disturbance ofthe specimen. These loose soil particles were easily washed from thespecimen upon the initiation of the test and were collected in the firsteffluent collection bucket resulting in a potentially exaggeratederosion rate.

Phase 1 Results: Biological Effect of Organic MatterContent on Piping Erosion Progression Excluding theInfluence of Grain-Size Distribution

The erosion test parameters for Phase 1 are summarized in Table 5.Fig. 4 shows that the erosion rates of both the KP and the FGS-5%ORG followed a decreasing trend toward a relatively stable value.Because of the high erosion potential of the FGS and the sub-sequently low number of data points for this test, a clear trend is

difficult to distinguish. Most important to recognize, however, is themuch higher erosion potential of this nonorganic soil with respect tothe two organic soils, indicating the possible effect of organic mattercontent on a soil’s piping erosion resistance.

Posterosion-test observation of the specimens validated thequantitative analysis, and at the end of each test, the condition of thepiping hole was examined. As shown in Fig. 5(c), no measurabledifference was observed in the preformed piping hole after the KPspecimen experienced a full hour of eroding flows. Fig. 5(a) showsthat after experiencing only three minutes of the same flow con-ditions, the initial hole in the FGS specimen had eroded to theperimeter of the mold. A drastic variation in erosion potential wasobserved between the two soils. The FGS-5%ORG specimen isshown in Fig. 5(b). Like the KP specimen, this soil was tested fora duration of 60 min and showed a level of piping-hole enlargementand soil erosion only slightly greater than that of the KP specimen.Closer examination of this specimen’s posterosion piping holeshowed that the coarse-grained sand particles (lighter in color)remained on the wall of the piping hole, indicating the possiblebinding of organic particles with the mineral particles.

Phase 2 Results: Biological Effect of Organic MatterContent on Piping Erosion Progression Excluding theInfluences of Grain-Size Distribution and Particle Shape

The erosion test parameters for the Phase 2 tests are summarized inTable 6. In this phase of study, two groups of tests were conducted,and comparisons were made within each group (Table 2). A com-parison of the FGS-20%KP and the FGS-20%Turface(KP GSD)erosion rate curves in Fig. 6 reveals the drastic difference in erosionpotential between the two soils, despite their similarities in GSD andparticle shape.A comparison of erosion rates of the FGS-20%cocompand the FGS-20%Turface(cocomp GSD) (Fig. 6) reveals the samefinding, which is the two soils with similar GSDs and particle shapesexhibited very different erosion behaviors. These results furtherillustrate the potential relationship between organic matter contentand erosion resistance.

When compared with the FGS-20%Turface(KP GSD), the initialerosion rate of the FGS-20%Turface(cocomp GSD) was relativelylow. Toward the end of the test, however, the erosion rate increasedsignificantly to reach the same level as that of the FGS-20%Turface(KPGSD). These two soils also exhibited considerable variability intest duration—themixture containing Turface with the sameGSD asthe KP only sustained 6 min of seepage before reaching the pre-defined failure point (enlargement of the preformed piping hole tothe perimeter of the specimenmold), whereas themixture containingTurface with the same GSD as the cocompost sustained 42 minof seepage before reaching the failure point. The reasons for thisdifference are not fully understood, but it is possible that the var-iation in the GSDs of the two soils (Fig. 1) plays a role.

Table 5. Erosion Test Parameters for Kerman Peat, Fine-Grained SiltySand, and FGS-5%ORG in Phase 1

Test parameterFine-grainedsilty sand

FGS-5%ORG

Kermanpeat

Duration of test (min) 3 60 60Duration of eachincrement (min)

0.5 5.0 5.0

Fig. 5. Posterosion-test conditions: (a) fine-grained silty sand; (b) FGS-5%ORG; (c) Kerman peat specimens

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Phase3Results: EffectofVariedOrganicMatterContenton Piping Erosion Progression

Table 7 summarizes the erosion test parameters for Phase 3, andFig. 7 graphically presents the erosion rates. The FGS and the 5%mixture failed at 6 and 15 min, respectively, where the pipingchannels enlarged to the perimeter of the mold at the top portion ofthe specimens. The other three mixtures and the cocompost weretested for 60 min, and the piping channels did not enlarge to theperimeter of the specimen. Each soil exhibited a relatively constanterosion rate throughout the duration of the test, and the erosion ratesfor the cocompost and the 20% mixture were several orders ofmagnitude lower than those exhibited by theFGS and the 5%mixture,once again suggesting a positive correlation between organic mattercontent in the soil and the reduction in erosion progression.

Fig. 8 shows a side-by-side comparison of posterosion piping-hole castings from the FGS, the 20% mixture, and the cocompost.The castings were made using a silicon rubber fluid (OOMOO 25Silicone Rubber, Smooth-On Inc., Easton, PA) that was slowlyinjected into the posterosion piping hole. The fluid occupied allvoids in the piping channel and solidified in 75 min at roomtemperature. It had negligible shrinkage and good tear strength.

The solidified silicon rubber castings could be easily detached fromthe soil and accurately represented the shape of the posterosionpiping hole. In the figure, the vertically oriented shaft extending theentire length of the photograph is the metal rod used to form theinitial piping hole during the specimen compaction. To the left ofthe rod the cylindrical erosion-test mold with the specimen isshown to provide a physical reference with respect to the erosionhole castings. The piping hole in the FGS enlarged to the perimeterof the cylinder at the top portion of the specimen, whereas thecocompost and the 20% mixture showed only a slight enlargementin hole size.

Phase 4 Results: Analysis of Biological SubstancesAffecting Piping Erosion Resistance

Table 8 summarizes the results of the biocharacteristics quantifi-cation of the four base soils. The polysaccharide and glomalincontents are reported inmilligrams of each substance per gram of thesoil sample, and each test was performed in duplicate to confirm theresults. In Figs. 9 and 10, the average erosion rates of the four soilsare plotted versus the soils’ two biological substance concentrations.The average erosion rate was determined by averaging the erosionrates of each time increment of effluent collection during the erosiontest. For each of the base soils, higher concentrations of the biological

Table 6. Erosion Test Parameters for Fine-Grained Silty Sand Mixed with20% byMass Turface(KP GSD), Turface(cocomp GSD), Kerman Peat, andCocompost in Phase 2

Test parameter

FGS-20%Turface

(KP GSD)

FGS-20%Turface

(cocomp GSD) FGS-20%KPFGS-20%cocomp

Duration oftest (min)

6 42 60 60

Duration of eachincrement (min)

1 6 6 10

Fig. 6.Erosion rates offine-grained silty sandmixedwith 20%bymassTurface(KP GSD), Turface(cocompGSD), Kerman peat, and cocompost

Table 7. Erosion Test Parameters for Fine-Grained Silty Sand,Cocompost, and Fine-Grained Silty Sand and Cocompost Mixtures of 5,10, 15, and 20% Cocompost by Mass in Phase 3

Test parameterFine-grainedsilty sand 5% 10% 15% 20% Cocompost

Duration oftest (min)

6 15 60 60 60 60

Duration of eachincrement (min)

0.75 3.00 5.00 6.00 10.00 10.00

Fig. 7. Erosion rates of fine-grained silty sand, cocompost, and fine-grained silty sand and cocompost mixtures of 5, 10, 15, and 20%cocompost by mass, respectively

Fig. 8. Physical observation and comparison of posterosion piping-hole conditions of the fine-grained silty sand, themixture of fine-grainedsilty sand and 20% cocompost, and the cocompost

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constituents generally correlatewith lower rates of erosion. Interestingto note, however, is the variation in erosion rate between the KP andthe cocompost, despite their nearly identical concentrations of pol-ysaccharides (Fig. 9). This can perhaps be explained by observing theglomalin concentrations in Fig. 10. The cocompost, which containedapproximately 60%more glomalin than the KP, had significantly lesserosion. The overall correlation between erosion reduction and bi-ological constituent concentration supports the hypothesis that thepresence of biofilm substances can reduce a soil’s susceptibility toerosion along a preexisting piping channel. The test program was notable to exclude other possible biological mechanisms. Therefore,based on this research, it is concluded that there may be a positiverelationship between erosion resistance and the polysaccharides andglomalin contents. Further research is needed to verify that lowerosion is in fact a result of the high concentration of polysaccharidesand glomalin.

Phase5Results:EffectsofPipingErosionBioabatementMeasures on Soil Geomechanic Properties

The consolidation curves for the FGS and the FGS mixed with 20%cocompost bymass are shown in Figs. 11 and 12, respectively. Withthe addition of 20% cocompost to the FGS, the compression indexincreased by approximately 233%. An increase in total settlement ofapproximately 140% was also observed. The rate and magnitude oftime-dependent settlement appeared to experience an increase aswell, as shown by the time-deformation curves in Figs. 13 and 14 fornormal stresses of 30.9 and 247:0 kN=m2, respectively. Secondaryconsolidation of the 20% mixture was not revealed in Fig. 14, even

Table 8. Biocharacterization Summary for Fine-Grained Silty Sand,Turface(KP GSD), Kerman Peat, and Cocompost in Phase 4

Soil typePolysaccharidecontent (mg/g)

Glomalincontent(mg/g)

Fine-grained sandy silt 3:03 1026 Not detectedTurface(KP GSD) 5:63 1026 Not detectedKerman peat 6:13 1024 0.515Cocompost 5:63 1024 0.819

Fig. 9. Average erosion rate versus polysaccharide content for fine-grained silty sand, Turface(KP GSD), Kerman peat, and cocompost

Fig. 10. Average erosion rate versus glomalin content for fine-grainedsilty sand, Turface(KP GSD), Kerman peat, and cocompost

Fig. 11. Consolidation curve for fine-grained silty sand

Fig. 12. Consolidation curve for fine-grained silty sand mixed with20% cocompost by mass

Fig. 13. Time-deformation curves for fine-grained silty sand and fine-grained silty sandmixed with 20% cocompost by mass for normal stressof 31:1 kN=m2

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after 7 days of consolidation at a pressure of 247:0 kN=m2. Moretime may be needed to observe the possibly large secondary con-solidation, as is normally observed in organic soils (Mesri andAjlouni 2007).

The maximum deviator stress sustained by the 20% mixtureduring consolidated undrained triaxial compression was only aboutone third that of the FGS, as shown in Fig. 15. Also, significantlygreater deformation was needed to fully mobilize the compressivestrength of the 20% mixture compared with the FGS, indicatinghigher plasticity of the mixture. Permeability reduction by addingthe compost into the sand was significant; seepage velocity droppedby two orders of magnitude approaching the low values typicallyexhibited by clays. The permeability values, along with a summaryof the consolidation and strength characteristics of the two soils, areprovided in Table 9. The significant degradation of these importantgeomechanic properties may prohibit the use of this bioabatementmethod in the field. However, potential alternatives using the bi-ological mechanisms explored in this research may warrant furtherinvestigation.

Conclusions and Future Research

Conclusions Drawn from Experimental Observations

The objectives of this research were to develop a better under-standing of the mechanisms governing the progression of piping

erosion in organic soils and to investigate the possibility of applyingerosion bioabatement techniques. Based on the materials’ physicaland biological characterization and the erosions tests, the followingpreliminary findings were drawn:• Inclusion of organic matter in a mineral soil can decrease the

soil’s susceptibility to the progression of piping erosion, andincreased soil organic matter content correlates with increasedresistance to piping erosion.

• There could be positive correlations between erosion resistanceand the polysaccharide and glomalin content in a soil. Althoughthis paper quantitatively established the relationships betweenerosion resistance and the polysaccharides and glomalin con-tents, this paperwas not able to exclude other possible physical orbiological mechanisms. To fully substantiate these correlations,materials with highly concentrated polysaccharides or glomalin,instead of the peat or compost used in this research, should besystematically tested.

• The introduction of organic material significantly altered impor-tant geomechanic properties; both total and time rate of consol-idation increased, consolidated undrained compressive strengthdecreased, and permeability was reduced.

Research Limitations

This research has the following limitations: (1) the long-term effectsof incorporating organic materials into mineral soils were not ad-dressed, and an explicit examination of the role of particle shape inpiping erosion progression has not been completed. (2) Althoughthis paper demonstrated the quantitative relationship between thebiological substances and erosion reduction and identified the bi-ological substances that could cause the reduction in erosion, thispaper did not offer explanation of the mechanic bondingmechanismcaused by the biological substances. (3) The concept of bioabatementof piping erosion was to add a small proportion of highly erosion-resistant organic materials to erodible mineral soils. However, thisresearch showed that mixing organic materials with erodible mineralsoils might affect the soil’s geomechanic properties. Consideringthese limitations, the proposed continuation of this research is pre-sented in the following section.

Future Research: Proposed Synthetic Abatement UsingConcentrated Polysaccharides and Glomalin

Further research is needed to identify a soil additive that does notdegrade the geomechanic properties of the soil, but can still providethe benefits of reducing the progression of piping erosion. Furtherstudies involving the use of synthetic polysaccharides, glomalin, orother currently unidentified substances as possible alternatives toincorporating raw organic materials, such as peat and compost, arerecommended. A complementary investigation into the long-termeffects of using organic materials as soil additives should also be

Fig. 14. Time-deformation curves for fine-grained silty sand and fine-grained silty sandmixed with 20% cocompost by mass for normal stressof 247:0 kN=m2

Fig. 15. Triaxial consolidated undrained compression test results forfine-grained silty sand and fine-grained silty sand mixed with 20%cocompost by mass

Table 9. Consolidation, Strength, and Permeability Summary for Fine-Grained Sandy Silt and Fine-Grained Sandy Silt Mixed with 20%Cocompost by Mass in Phase 5

Soil characteristics Fine-grained sandy silt 20%

Compression index 0.03 0.10Undrained peakcompressive stress at55:2 kN=m2 effective stress(kN/m2)

687.1 229.9

Coefficient of permeability,flexible wall (cm/s)

2:313 1024 1:993 1026

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conducted. Increased standing time and stabilization of the organicmixtures over time may result in less impact on the soils’ geo-mechanical properties. On amore fundamental scale, the precise roleof the individual particle shapes on the process of piping erosiondeserves additional investigation. It is likely that the unique shapesand textures exhibited by biologically derived constituents affect theprogression of piping erosion. Completion of these research sug-gestions would form a more comprehensive understanding oferosion progression and facilitate the development of a more ef-fective abatement technique.

Acknowledgments

The authors appreciate Dennis Margosan, plant pathologist at theUSDA-Agricultural Research Service (Parliar, CA), for taking thescanning electron microphotographs of the soils. The authors alsosincerely thank the anonymous reviewers for providing insightfulcomments that greatly improved this paper.

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