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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.
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 soils 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:firstname.lastname@example.org
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 1090-0241/2013/8-13601368/$25.00.
1360 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / AUGUST 2013
J. Geotech. Geoenviron. Eng. 2013.139:1360-1368.
certain biologically derived substances (polysaccharides and glo-malin) and piping erosion reduction. Thefinal phase quantified someof the changes in a mineral soils geomechanic properties resultingfrom the introduction of organic matter.
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 soils 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 (2022C) 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(ac). As a reference, the SEM of theFGS is shown in Fig. 2(d).
Erosion Test Methodology
The experimental methods were designed to test a soils 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 ra