INTERACTION OF FOUNDRY SANDS WITH GEOSYNTHETICS

By Marc J. Goodhue,1 Tuncer B. Edil,2 and Craig H. Benson3

ABSTRACT: Excess foundry sands from gray-iron casting are a mixture of sand, bentonite, and additives thatcan have properties desirable for structural fills and hydraulic barriers, depending on their bentonite content. Tofacilitate beneficial reuse of foundry sands, typical strength parameters need to be available so that designerscan make comparisons with designs employing virgin earthen materials. To provide typical design parameters,a testing program was conducted to characterize the strength of foundry sands and their interaction with geo-synthetics. Small-scale direct shear tests, large-scale multistage interface shear tests, and pullout tests wereconducted using foundry sands with bentonite contents representing the range normally found in the castingindustry and three geosynthetics (geotextile, geogrid, and geomembrane). The results indicate that foundry sandscan be used effectively in geotechnical construction. Friction angles of the as-compacted foundry sands generallyranged between 397 and 437, and the as-compacted cohesions ranged between 17 and 28 kPa. Drained frictionangles were similar to as-compacted friction angles except at high bentonite content. Typical interface frictionangles ranged between 257 and 357, with efficiencies ranging between 0.5 and 0.9. Interaction coefficients fromthe pullout tests ranged between 0.2 and 1.7.

INTRODUCTION

Excess foundry sands from gray-iron foundries are uniformsands mixed with bentonite, coal dust, and other additives thatare used for making molds (Javed and Lovell 1994a; Abichouet al. 1998a). Past regulations required that excess foundrysands be disposed in engineered landfills, but new legislationin many states permits their beneficial reuse provided leachatefrom the sand meets specific water quality criteria [e.g., Sec-tion NR 538, Wisconsin Administrative Code (‘‘Beneficial’’1998)]. Most foundry sands from iron foundries in Wisconsinmeet the water quality criteria and can be used as fill abovethe water table. Previous research has shown that some foundrysands can have properties desirable for structural fills—namely, low compressibility and high shear strength—and lowswell potential (Javed and Lovell 1994b; Mast 1997; Abichouet al. 1998a; Kleven et al. 2000). Foundry sands with higherbentonite content have also been shown to have low hydraulicconductivity, suggesting that waste foundry sands can be re-used in barrier layers in landfill final covers (Freber 1996;Abichou et al. 1998b).

One of the primary impediments to beneficial reuse of in-dustrial by-products such as foundry sands is a lack of engi-neering data that designers can use to evaluate the efficacy andeconomy of using a by-product in place of a natural soil (Ediland Benson 1998). The engineering properties and behaviorof soils can be readily estimated from the literature for use inpreliminary design. In contrast, there is a dearth of similarinformation for industrial by-products and there are insuffi-cient data to confirm that industrial by-products, which appearsimilar to soils, also have comparable engineering properties.With emphasis now being placed on engineering for sustain-able development, there is a pressing need to provide this prac-tical information to designers.

Fulfilling this need is the primary purpose of this paper. Theobjective is to provide practical information, regarding theshear strength of foundry sands and their interaction with geo-

1Volunteer, U.S. Peace Corps, Cochabamba, Bolivia. E-mail:marcgoodhue@hotmail.com

2Prof., Dept. of Civ. and Envir. Engrg., Univ. of Wisconsin, Madison,WI 53706. E-mail: edil@engr.wisc.edu

3Prof., Dept. of Civ. and Envir. Engrg., Univ. of Wisconsin, Madison,WI 53706. E-mail: benson@engr.wisc.edu

Note. Discussion open until September 1, 2001. To extend the closingdate one month, a written request must be filed with the ASCE Managerof Journals. The manuscript for this paper was submitted for review andpossible publication on May 19, 1999; revised December 19, 2000. Thispaper is part of the Journal of Geotechnical and GeoenvironmentalEngineering, Vol. 127, No. 4, April, 2001. qASCE, ISSN 1090-0241/01/0004-0353–0362/$8.00 1 $.50 per page. Paper No. 21048.

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synthetics, that can be used by designers when evaluating thepotential for reuse of foundry sands in geotechnical construc-tion. The focus is on foundry sands rather than geosyntheticbehavior. Applications where foundry sands can be used inconjunction with geosynthetics as an inexpensive constructionalternative include structural fills over soft soils where geo-synthetics are used for base reinforcement, backfill for retain-ing walls and mechanically stabilized walls, and landfill coverswhere foundry sands with higher bentonite content may beused as hydraulic barriers in conjunction with geomembranes(Abichou et al. 1998b).

BACKGROUND

Foundry Sand Generation and Constituents

Excess foundry sands are generated as a by-product of metalcasting. Molds for metal casting are often made from a mixtureof clean, uniformly graded silica sand, a binding agent (eitherclay or a chemical), water, and additives such as coal dust,cereal, fuel oil, or wood flour (Javed and Lovell 1994a). Thismixture is referred to as foundry sand. Excess foundry sand isgenerated because varying amounts of these additives mustcontinually be reintroduced to the foundry sand to maintain itsdesired properties, resulting in a larger volume of sand than isneeded for the foundry process (Abichou et al. 1998b).

In this study, clay-bound foundry sands (as opposed tochemically bound sands) from iron foundries were used andare referred to herein simply as ‘‘foundry sands.’’ Clay-boundsands are also referred to as ‘‘green sands’’ in the foundryindustry. Green foundry sand usually includes 85–95% of silicasand by weight. The clay binder, around 5–15% of the mix,is usually sodium or calcium bentonite, or occasionally kao-linite. About 2–7% water is added to ‘‘activate’’ the clay por-tion, giving the mixture cohesion and plasticity. The organicportion usually makes up 2–8% of the mix and is added toimprove the surface finish of the cast metal (Kleven et al.2000).

Geotechnical Properties of Foundry Sands

Javed and Lovell (1994b) conducted a variety of laboratorytests on seven clay-bound foundry sands to determine theirsuitability as highway construction materials. They found thatthe plasticity index ranged from nonplastic to 12 and that thefriction angles of two dry sands were 377 and 417 at relativedensities of 90 and 98%, respectively. Constrained moduli forthe sand ranged between 1,500 and 2,000 kPa for verticalstresses between 0 and 1,000 kPa. Javed and Lovell (1994b)

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concluded that foundry sands have shear strengths comparableto those of natural sands but are more compressible than cleansands.

Mast (1997) evaluated the performance of an embankmentconstructed with foundry sand. The 275-m-long and 9-m-highembankment consisted of three sections constructed with clay,foundry sand, and clean sand. The foundry sand had 9% clay-sized particles (<0.005 mm), a plasticity index of 6, and afriction angle of 387 at the as-received water content and in adense state. The embankment was instrumented with settle-ment plates, piezometers, vertical and horizontal inclinome-ters, and earth pressure cells.

Settlement of the foundry sand was <4 mm, which wascomparable to that of the clean sand. Lateral movements were<6 mm. Standard penetration testing gave N-values rangingfrom 33 to 54, indicating that the foundry sand had relativelyhigh strength. Mast (1997) concluded that the foundry sandhad acceptable shear strength and compressibility for use asan embankment material, although values for compressibilitywere not reported. Mast cautioned that some construction is-sues must be addressed, including removal of large objects(e.g., core butts, metal, and slag), dust control, and erosioncontrol.

Kleven et al. (2000) conducted tests on 11 clay-boundfoundry sands to evaluate their use as a highway subbasematerial. Atterberg limits, particle-size distribution, bentonitecontent, compaction characteristics, unconfined compression,California Bearing Ratio, swell potential, and resilient modu-lus were measured. The plasticity index ranged from nonplas-tic to 8, and the bentonite content ranged from 4.7 to 10.5%.The unconfined compressive strength ranged from 71 to 190kPa for foundry sands with plasticity and appeared to be in-dependent of fines content, clay content, or Atterberg limits.Swell was found to be negligible for all the foundry sands,even for those with the highest bentonite content. Kleven etal. (2000) concluded that foundry sands are suitable for reuseas highway subbase material.

Abichou et al. (1998b, 2000) measured the hydraulic con-ductivity of the foundry sands tested by Kleven et al. (2000).They found that the hydraulic conductivity of foundry sandscompacted at standard Proctor optimum water content andmaximum dry unit weight ranged from 5.3 3 1025 to 8.5 31029 cm/s, with lower hydraulic conductivities being obtainedat higher bentonite content and plasticity index. For bentonitecontents >6%, hydraulic conductivity <1 3 1027 cm/s wasobtained. Thus, foundry sands can be suitable for reuse ashydraulic barriers in landfills.

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FIG. 1. Particle-Size Distribution Curves

Leaching Potential

Lovejoy et al. (1996) conducted laboratory and field teststo determine the leaching characteristics of excess foundrysands. Toxicity characteristic leaching procedure tests wereperformed on three foundry sands. None of the sands werefound to be hazardous by the definition in the Resource Con-servation and Recovery Act. Embankments constructed withfoundry sand and natural soil were instrumented with lysim-eters for collecting leachate and monitoring wells for collect-ing ground water. There was no statistically significant differ-ence between concentrations of contaminants of concern inwater collected from both lysimeters. In addition, the moni-toring well data showed no evidence of ground-water contam-ination. Lovejoy et al. (1996) concluded that foundry sandscan be environmentally suitable materials for structural fillsand can be a cost-effective source of fill material.

MATERIALS AND METHODS

Foundry Sands

The four foundry sands tested in this study were obtainedfrom four foundries in Wisconsin and Illinois. The sands haddifferent percentages of bentonite, representative of the rangeof bentonite contents found in foundry sands. Abichou et al.(1998a) reported that the bentonite content of most foundrysands is between 0 and 16%. The particle-size distributions(ASTM D 422) are shown in Fig. 1. Index properties of thesands are shown in Table 1. The test for Atterberg limits andstandard Proctor compaction followed the procedures of

TABLE 1. Index Properties of Sands

Sand(1)

Specificgravitya

(2)

Averageroundnessb

(3)

Atterberg Limitsc

PL(4)

LL(5)

PI(6)

Bentonited

(%)(7)

Classificatione

USCS(8)

AASHTO(9)

Standard Proctorf

gd,max

(kN/m3)(10)

wopt

(%)(11)

A 2.62 0.69 NP NP NP 2.3 SP-SM A-3(0) 18.4 9.6B 2.73 0.55 NP NP NP 4.7 SP-SM A-3(0) 17.3 12.3C 2.52 0.61 20 18 2 7.5 SW-SM A-2-4(0) 18.2 13.0D 2.51 NTg 27 20 7 13.0 SC A-2-4(0) 16.9 15.0Base 2.66 0.63 — — — 0 SP A-3(0) 17.2h —Portage 2.68 0.85 — — — 0 SP A-3(0) 17.7 —aASTM D 854-92e1.bKrumbein (1941) chart.cASTM D 4318 (with hydration period of at least 1 week).dASTM C 837-81.eASTM D 2487.fASTM D 698 (with hydration period of at least 1 week).gNT = not tested.hMaximum dry unit weight determined by vibration following ASTM D 4253.

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ASTM D 4318 and ASTM D 698, respectively, except thehydration period was at least 1 week to allow for completehydration of the bentonite (Kleven et al. 2000).

The foundry sands classify as silty or clayey sands and havethe designations SP-SM, SW-SM, or SC in the Unified SoilClassification System (USCS). Their plasticity index rangesfrom nonplastic to 7. Water leach tests (ASTM D 3987) con-ducted on each foundry sand showed that the sands are clas-sified as Category 2 by-products in the Wisconsin Administra-tive Code, Section NR 538. Category 2 by-products can beused in all fill applications above the ground-water table with-out installation of monitoring systems for water quality. Com-parative tests with local fill soils showed that leachate fromthe sands was not significantly different from leachate fromsoils. Thus, chemical incompatibility between the foundrysands and geosynthetics is not anticipated.

Reference Sands

Two sands were used as reference materials: Portage andbase sand. Portage sand is a medium, uniformly graded quartzsand with a USCS classification of SP, maximum dry unitweight of 17.7 kN/m3, minimum dry unit weight of 14.4 kN/m3, and average roundness of 0.77. Base sand is fine, uni-formly graded quartz sand that classifies as SP and is used byseveral foundries as their base for system sand. Base sand hasa maximum dry unit weight of 17.2 kN/m3, minimum dry unitweight of 14.7 kN/m3, and average roundness of 0.63. Particle-size distribution curves for these sands are also shown in Fig.1.

Comparison of the particle-size distributions of the base andfoundry sands (Fig. 1) shows that the primary difference be-tween the sands is the quantity and type of fines in the mixture.The exception is Foundry Sand A, which contained small frag-ments consisting of crushed remnant core butts. These angularremnants were composed of chemically bonded uniform finesand.

Geosynthetics

A woven geotextile, geogrid, and textured geomembranewere used. The geosynthetics were chosen because of theiravailability and were intended to represent typical geosyn-thetics that would likely be used with foundry sands in rein-forced earthen structures or in landfill covers. The geotextileis a woven, slit-film polypropylene geotextile commonly usedin reinforcement applications. The geogrid is a woven poly-ester geogrid with PVC coating and an aperture size of 81.3mm in the machine direction and 12.7 mm in the cross-machine direction. The geomembrane is 1.5-mm high-densitypolyethylene textured on both sides. Tests conducted perASTM D 4595 on the geotextile and geogrid yielded maxi-mum wide-width tensile strengths of 51.1 and 57.8 kN/m, re-spectively, in the machine direction.

Direct Shear Tests on Sands

Shear tests on the sands were initially attempted in the large-scale direct shear machine used for interface testing (see sub-sequent discussion on interface testing), but reliable data couldnot be obtained because the machine did not have the capacityto test soil specimens. Consequently, small-scale direct sheartests were conducted on the sands with a Wykeham-FarranceEngineering Ltd., Slough, England, direct shear machine(Model No. 25301) equipped with a square shear box 64 mmwide and 32 mm deep. The tests were conducted followingmethods described in ASTM D 3080. A load cell measuredshear force and linear variable differential transducers(LVDTs) measured horizontal and vertical displacements. Data

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were collected using a personal computer (PC) equipped witha data acquisition card.

Small-scale direct shear tests were conducted on the tworeference sands and the four foundry sands. The foundry sandswere tested both as-compacted and drained. Friction angle andcohesion from the as-compacted sands are denoted as fa andca. For the drained tests, the designations f9 and c9 are usedfor the effective stress friction angle and cohesion. Three nor-mal stresses were used for each soil: 11, 34, and 56 kPa.

Reference sands were placed dry in the shear box at 90%relative density, and foundry sands were compacted at opti-mum water content to 100% of the maximum dry unit weightas determined by standard Proctor (ASTM D 698). A shearingrate of 0.3 mm/min was used for the as-compacted tests,whereas the shearing rate for the drained tests was determinedfrom consolidation data per ASTM D 3080. Shearing rates forthe drained tests were consistent with values anticipated fordouble-drained conditions based on hydraulic conductivities ofthe sands measured by Abichou et al. (1998b).

Interface Direct Shear Tests

Interface shear testing was conducted using an interface di-rect shear machine, as shown schematically in Fig. 2. Themachine, which was manufactured to comply with the require-ments in ASTM D 5321, consists of an upper box 300 mmlong, 300 mm wide, and 76 mm deep, and a lower box 356mm long, 300 mm wide, and 76 mm deep. The longer lowerbox permits 50 mm of travel without requiring an area cor-rection for the stress calculations.

The soil being tested was used initially as the substrate inthe lower box. Results of these tests indicated that the shearstress increased with increasing displacement, rather thanreaching a peak or residual value. Examination of these spec-imens showed that soil in the upper box indented into soil inthe lower box, resulting in passive resistance. Takasumi et al.(1991) reported similar behavior in large-scale interface testsconducted using a soil substrate. Thus, for subsequent tests, arigid substrate, which consisted of a porous stone preparedwith a mixture of sand and epoxy, was used in the lower box.The stone provided a rigid base for the geosynthetics, provideddrainage, and had frictional and hydraulic characteristics sim-ilar to the soils being tested.

Normal stress was applied by a square loading platen loadedby an air bladder. The bladder is confined above by a steelplate that is connected by a rod to the outer frame (Fig. 2).Vertical load was measured by a load cell. The shear box iscontained by an outer box, which allows soaked tests to beperformed. LVDTs measured horizontal and vertical displace-ments. Data were collected using a PC with a data acquisitioncard.

Multistage testing was used to accrue timesavings and elim-inate variability in test specimens. In multistage shearing, onlyone test setup is used instead of a new specimen for eachnormal stress (Bemben and Shulze 1993; Stark and Eid 1994).The interface friction angle d obtained from multistage testsis smaller than the peak friction angle obtained from a tradi-tional test (Bemben and Shulze 1993, 1995). In this study, theshear strength obtained from multistage shearing was essen-tially the same as that from the new specimen setups at adisplacement of about 5 mm, as shown in Fig. 3 for testsconducted with Foundry Sand B and the geotextile.

The percent reduction in tan d from peak, using the multi-stage setups with the geotextile, was 2% for Foundry Sand B,4% for Foundry Sand C, and 18% for Foundry Sand D basedon the shear stress at 8 mm of displacement, beyond whichthe shear stress changed a negligible amount (<5%). In a fieldsituation, displacements of 8 mm are likely to occur duringconstruction. Thus, the friction angle corresponding to this dis-placement should reasonably represent the field condition.

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FIG. 2. Schematic of Interface Direct Shear Setup

FIG. 3. Shear Stress-Displacement Curves from Multistageand New Specimen Test Setups Using Foundry Sand B and Geo-textile

From a practical perspective, these interface friction angles canbe considered as large-displacement friction angles.

The reference sand (base sand) was placed dry by pouringa known mass of soil into the upper box and tamping it to thecorrect volume to achieve a relative density of 90%. Foundrysands were compacted to maximum standard Proctor dry unitweight at optimum water content. Maximum standard Proctordry unit weight was achieved by compacting a measured massof soil at optimum water content into the known volume ofthe upper box. Multistage interface tests were run for threefoundry sands (B, C, and D) and the two reference sands withthe geogrid, geomembrane, and geotextile. The foundry sandswere tested as-compacted and drained.

For tests conducted with the reference sand and the as-com-pacted foundry sands, the displacement rate of 1 mm/min rec-ommended in ASTM D 5321 was used. For soaked tests, thespecimens were consolidated before shearing after beingsoaked until a tensiometer indicated that the pore-water pres-sure was zero. Consolidation data were used to determine the

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FIG. 4. Schematic of Pullout Box

displacement rate required to prevent generation of excesspore-water pressures per ASTM D 3080. The displacementranged from 0.01 to 0.03 mm/min.

Pullout Tests

Pullout testing was performed on the geotextile and geogridin the pullout box described by Tatlisoz et al. (1998), whichis 1.52 m long, 0.61 m wide, and 0.41 m deep (Fig. 4). Twoside-mounted hydraulic pistons driven by an electrically pow-ered hydraulic pump apply the pullout force. Normal stressis applied by a rubber air bladder that is confined by a rein-forced steel cover bolted to the top of the box. An earth pres-sure cell was used to verify the normal stress applied to thesoil (Goodhue 1998).

Front displacement of the geosynthetic is measured usingan LVDT mounted on the head (Fig. 4), which monitors dis-placement of the clamp. Displacements of the geosynthetic are

INEERING / APRIL 2001

measured at 60, 360, 720, and 1,300 mm from the front byfour LVDTs mounted on a rack at the back of the box. TheseLVDTs are attached to ‘‘telltale’’ wires that enter the boxthrough a slot in the back and are protected by aluminumtubes. During pullout, the wires are pulled along with the mov-ing geosynthetic.

A load cell mounted on the head (Fig. 4) measures pulloutload. The geosynthetic clamp consists of two 13-mm-thicksteel plates bolted together. To reduce the amount of soil nec-essary for testing, a false bottom, which consisted of 19-mm-thick plywood screwed to 100 3 100 3 600 mm woodensupports, was placed in the box. A piece of smooth polypro-pylene geomembrane was placed on top of the plywood toprevent soil moisture from moving into the wood. To reducesidewall friction, plastic was attached to the sidewalls of thebox and a nonwoven geotextile was placed next to the plastic.Preliminary tests conducted without the false bottom yieldedessentially identical results as those obtained using the falsebottom (Goodhue 1998). Also, examination of the foundrysand during disassembly suggested that the shear zone adja-cent to the geosynthetics was thin (several millimeters), whichwas probably due to the relatively fine particles in foundrysands.

All pullout tests were conducted in accordance with theGeosynthetic Research Institute (GRI) Test Methods GG6 andGT6. The test program included three foundry sands and thebase sand with the geogrid and geotextile. For the tests withbase sand, the dry sand was weighed, shoveled into the box,and tamped to achieve the desired density. Foundry sandswere first passed through the No. 4 sieve and then wetted tooptimum water content based on standard Proctor. The moistfoundry sand was then sealed with plastic and allowed to sit1 week to allow for hydration and equilibration. Kleven et al.(2000) showed that 1 week is necessary to rehydrate the drybentonite in foundry sands. The foundry sands were then com-pacted to 92% of standard Proctor maximum dry unit weightgd,max, except for Foundry Sand D, for which only 85% ofgd,max could be achieved.

A square impact compactor was used to compact foundrysands in the box. Soil below the geosynthetic was compactedin three 30-mm-thick lifts, with the top lift being leveled tothe appropriate height for the geosynthetic. Although this bot-tom layer of soil is relatively thin, it was adequately thick forthese tests because of the relatively thin shear zone that de-veloped. The geosynthetic (1.31 3 0.41 m) was placed on topof the compacted foundry sand, the telltale wires were con-nected, and then the upper soil was compacted on top of thegeosynthetic in three 47-mm-thick lifts. A 50-mm gap was leftbetween the top of the soil and the top of the box for theinflatable bladder.

The pullout tests used a displacement rate of 1 mm/min,which is recommended in GRI Test Methods GG5 and GG6.All tests were run to 100 mm of displacement or until geo-synthetic failure occurred. Pullout tests were also attempted ina soaked and drained condition. However, these tests wereunsuccessful because of equipment problems.

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FIG. 5. Failure Envelopes for Small-Scale Direct Shear Testson As-Compacted Foundry Sands and Reference Sands

RESULTS OF SMALL-SCALE DIRECT SHEAR TESTS

Failure Envelopes

Failure envelopes obtained by least-squares regression forPortage sand and base sand are shown in Fig. 5, which in-cludes a summary table of the strength parameters. The peakfriction angle (f9 = fa) is 427 for both reference sands, whichis reasonable for dense sands (Mitchell 1993). A cohesion in-tercept of 6–8 kPa is also evident for the reference sands,which most likely corresponds to machine friction.

Failure envelopes for the four foundry sands prepared atoptimum water content and tested as compacted are alsoshown in Fig. 5. The strength parameters for both as-com-pacted and drained tests are summarized in Tables 2 and 3.Friction angles in the as-compacted condition fa for FoundrySands B, C, and D range from 397 to 437, which is similar tothe values of 377 and 417 reported by Javed and Lovell (1994a)for two dry foundry sands at relative densities of 90 and 98%.These friction angles are similar to the friction angles of thereference sands (427). Foundry Sand A, however, had an as-compacted friction angle of 507. This was likely due to inclu-sion of the angular core remnants and slag.

The drained friction angles f9 ranged from 31 to 457 (Table2). For Sands A and C, the drained friction angles were similarto the as-compacted friction angles. For Foundry Sand D, thedrained friction angle was 117 lower than the as-compactedfriction angle, whereas for Foundry Sand B it was 57 higher.Despite retesting in both as-compacted and drained states, con-clusive reasons for these differences in friction angles couldnot be determined. As will be shown subsequently in discus-sion of Fig. 6, only the drained friction angle for Foundry SandB deviates significantly from the general trend.

Cohesion of the as-compacted foundry sands ca ranged be-

TABLE 2. Results of Direct Shear and Interface Direct Shear Tests [Interface Friction Angle (degrees) and Efficiencya]

Soil Data

Foundrysand(1)

Fines(%)(2)

fa

(degrees)(3)

f9(degrees)

(4)

Geogrid

d(&)/Ef

Drained(5)

d(&)/Ef

As-compacted(6)

Geomembrane

d(&)/Ef

Drained(7)

d(&)/Ef

As-compacted(8)

Geotextile

d(&)/Ef

Drained(9)

d(&)/Ef

As-Compacted(10)

Base 1.1 42 42 — 32/0.7 — 30/0.6 — 28/0.6A 2.4 50 45 — — — — — —B 9.2 39 44 31/0.6 30/0.7 32/0.7 35/0.9 32/0.7 31/0.7C 11 43 43 32/0.6 31/0.6 24/0.5 28/0.6 29/0.6 30/0.6D 19 42 31 26/0.8 25/0.5 32/1 33/0.7 29/0.9 29/0.6

aSoaked efficiencies are calculated with f9 and c9 and as-compacted efficiencies are calculated with fa and ca.

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TABLE 3. Results of Direct Shear and Interface Direct Shear Tests [Interface Adhesion (kPa) and Efficiencya]

Soil Data

Foundrysand(1)

Fines(%)(2)

ca

(kPa)(3)

c9(kPa)

(4)

Geogrid

cad/Ec

Drained(5)

cad/Ec

As-compacted(6)

Geomembrane

cad/Ec

Drained(7)

cad/Ec

As-compacted(8)

Geotextile

cad/Ec

Drained(9)

cad/Ec

As-Compacted(10)

Base 1.1 8 8 — 3/0.3 — 8/0.9 — 8/1A 2.4 28 10 — — — — — —B 9.2 17 8 10/1.3 17/1 6/0.9 4/0.3 5/0.6 8/0.5C 11 19 6 5/0.9 14/0.7 9/1.5 10/0.5 2/0.3 8/0.4D 19 28 10 4/0.4 22/0.8 4/0.4 8/0.3 2/0.2 12/0.4

aSoaked efficiencies are calculated with f9 and c9 and as-compacted efficiencies are calculated with fa and ca.

FIG. 6. Effect of Fines on As-Compacted and Effective Stress(Drained) Friction Angles for Base Sand and Foundry Sands

tween 17 and 28 kPa (Table 3), which is higher than the co-hesion of the reference sands (6–8 kPa) and the cohesion re-ported by Javed and Lovell (1994a) (<2 kPa) for two dryfoundry sands. Matric suction is believed to be responsible forthe higher cohesion of the as-compacted foundry sands.Drained tests conducted on the same compacted sands aftersoaking resulted in effective cohesions c9 ranging between 6and 10 kPa (Table 3), which are approximately the same asthe machine friction (;7 kPa) obtained by testing the refer-ence sands (Fig. 5).

The shear strength equation for unsaturated soil proposedby Fredlund et al. (1978) is useful for comparing the strengthcontributed by matric suction c in the foundry sands to similarstrength contributions in the literature for soils. Their shearstrength equation is

bt = c9 1 (s 2 u )tan f9 1 c tan f (1)a

where s = total stress; and ua = pore air pressure (normallyzero, as was assumed in this study). The parameter tan fb

describes the increase in shear strength Dt per increment ofsuction Dc. In effect, tan fb accounts for intergranular frictiontan f9 and the reduced cross-sectional area of the pore water

358 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

in the unsaturated soil and thus is bounded above by tan f9(Fredlund et al. 1995; Vanapalli et al. 1996b).

The relative magnitudes of tan fb and tan f9 for the foundrysands can be described by the parameter b

btan f c 2 c9ab = ' (2)

tan f9 c tan f9

To determine c, tensiometers were installed in specimens ofeach compacted foundry sand. The measured suctions are sum-marized in Table 4 along with b computed using c9 and f9from the drained tests. The b values range between 0.4 and0.8, which is comparable to the range of b (0.3–1.0) reportedin Fredlund and Rahardjo (1993) for soils having similar tex-ture as foundry sands and by Vanapalli et al. (1996a) for com-pacted clay (b ; 0.9 for s 2 ua = 25 kPa and c = 25 kPa).This analysis suggests that the as-compacted cohesion offoundry sands does arise from suction in the compacted spec-imens and becomes negligible when the sand is soaked.

Effect of Bentonite Content or Fines Content

Because bentonite usually makes up the clay fraction ofmost foundry sands, a relationship between shear strength andclay content (or bentonite content) was investigated. However,fines content (percent particles by weight <0.075 mm) wasultimately used rather than bentonite content or clay contentbecause it is easier to measure. Fines content and bentonitecontent of foundry sands are also correlated (Abichou et al.1998a).

Friction angle (fa and f9) versus fines content is shown inFig. 6 for the base sand and the four foundry sands. In general,the friction angle for foundry sands is nearly independent offines content and is comparable to the friction angle for basesand, regardless of whether it is obtained from as-compactedtests or drained tests. However, at high bentonite or fines con-tent, the drained friction angle may decrease. Lupini et al.(1981) obtained fairly constant residual friction coefficients forsand-bentonite mixtures having 0–15% bentonite content, butthey observed a decrease in friction coefficient for higher ben-tonite contents.

As-compacted cohesion ca increases with fines content (asshown in Fig. 7), as does matric suction, whereas the drainedcohesion is independent of fines content and comparable tomachine friction. The reduction in pore size that occurs as the

TABLE 4. Matric Suctions and Their Contribution to Shear Strength of Foundry Sands

Soil(1)

Matric suctionc

(kPa)(2)

Effective frictionangle f9(degrees)

(3)

c tan f9(kPa)

(4)

As-compacted adhesionca

(kPa)(5)

Effective cohesionc9

(kPa)(6)

b(7)

Foundry Sand A 23 45 23 28 10 0.8Foundry Sand B 25 44 24 17 8 0.4Foundry Sand C 32 43 30 19 6 0.4Foundry Sand D 38 31 23 28 10 0.8

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FIG. 7. Effect of Fines Content on Cohesion and Matric Suc-tion

fines content increases and the affinity that bentonite has forwater are responsible for the trend between matric suction andfines content and the trend between as-compacted cohesionand fines content. The exception to the trend is Foundry SandA, which has higher as-compacted cohesion (28 kPa) than ex-pected for sand with low bentonite content (2.3%). FoundrySand A had higher fa and f9 than the other sands, whichmay have resulted in higher cohesion in the as-compacted con-dition.

In summary, the drained strength of foundry sand is com-parable to that of base sand and is not significantly affectedby the bentonite content, at least for bentonite contents <13%.The base sand and foundry sand have similar c9 and f9. Incontrast, the total stress undrained strength of as-compactedfoundry sand increases as the bentonite content increases. Thisincrease in strength is attributed to matric suction and is man-ifested as an as-compacted cohesion ca that is greater than theeffective cohesion c9. The as-compacted friction angle fa,however, does not show any dependency on bentonite contentand is comparable to the effective friction angle f9.

RESULTS OF LARGE-SCALE INTERFACESHEAR TESTS

Reference Sands

Results of the interface direct shear tests are summarized inTables 2 and 3. Of the tests conducted with base sand, theinterface of base sand and geogrid had the highest frictionangle (d = 327), followed by the interface of base sand andtextured geomembrane (d = 307), and then the interface of basesand and geotextile (d = 287). All the interface friction anglesare smaller than the internal friction angle for the base sand(427). Frictional efficiency (Ef = tan d/tan f9) ranged from 0.6to 0.7, indicating that the base sand has good frictional inter-action with all the geosynthetics. The small adhesion interceptscad obtained for the sand interfaces are believed to be due tomachine friction.

As-Compacted Foundry Sands

Interface friction angles ranged from 257 to 357 for the threefoundry sands tested as-compacted with the three geosynthet-ics (Table 2). Frictional efficiency for the foundry sands (Eg =tan d/tan f9) ranged from 0.5 to 0.9. The textured geomem-brane gave the highest interface friction angle for Sands B and

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D, which is reasonable because the geomembrane had theroughest surface of the three geosynthetics. The interface be-tween Foundry Sand C and the geomembrane, however, hadthe lowest friction angle of the three geosynthetic interfacestested with Foundry Sand C. Interface friction angles for thegeotextile and geogrid with Foundry Sands B and D were typ-ically 27–57 lower than those obtained for the geomembrane,whereas Foundry Sand C interfaces with geotextile and geo-grid yielded friction angles that were 27–37 higher than thoseobtained with the geomembrane.

Interface adhesion for as-compacted foundry sands rangedbetween 4 and 22 kPa, with adhesion efficiencies (Ec = cad/ca)between 0.3 and 1.0 (Table 3). The largest adhesions wereobtained with the geogrid (14–22 kPa) and are attributed tocohesion between the as-compacted foundry sand (protrudingthrough apertures in the geogrid) and the lower porous stone.The geomembrane and geotextile, which permitted no contactbetween the sand and the lower porous stone, had similar ad-hesion (4–12) with the foundry sands. These adhesions were27–74% lower than the adhesions obtained with the geogridand are in the range of the expected machine friction.

Soaked Foundry Sands

Interface friction angles ranged from 247 to 327 for the ninecombinations of foundry sands and geosynthetics that weretested under soaked and drained conditions (Table 2). The in-terface friction angles d are similar to those obtained in theas-compacted state, differing by 47 at most and typically by17 or less. The similarity in the interface friction angles iscomparable to the similarity of the internal friction angles re-ported earlier.

Adhesion cad for the soaked interfaces ranged from 2 to 9kPa (Table 3). Adhesions obtained for Foundry Sands C andD under drained conditions were as little as 20% of the ad-hesions obtained in the as-compacted tests and were similar tothe adhesions obtained with base sand (3–8 kPa). The excep-tion was Foundry Sand B, which yielded similar adhesions inthe soaked and as-compacted states for all interfaces exceptthe geogrid. The low bentonite content (2.4%) of FoundrySand B is likely responsible for the similarity in the adhesionsobtained for the geotextile and the geomembrane. The loweradhesions obtained for all the foundry sands in the soakedinterface tests with the geogrid, which are at a level compa-rable to machine friction, also support the supposition that suc-tions contributed to the large adhesions obtained for the geo-grid in the as-compacted condition.

Effect of Fines Content on Interface Shear Strength

The effect of fines content on interface shear strength ispresented in Fig. 8, which shows interface friction angle d[Fig. 8(a)] and adhesion cad [Fig. 8(b)] versus fines content.The interface friction angle is unaffected by fines content un-der either as-compacted or drained conditions. A similar con-dition was obtained from the soil-only direct shear tests, whichare also shown in Fig. 8(a).

The effect of increasing fines on adhesion depends on thedrainage condition and the type of geosynthetic, as shown inFig. 8(b). The data in Fig. 8(b) are grouped into soaked (i.e.,drained) tests and as-compacted tests to emphasize the contrastbetween interface adhesion in the drained and as-compactedconditions. For the soaked interfaces, adhesion decreasesslightly with increasing fines content but is comparable to themachine friction. In contrast, for the as-compacted interfaces,there is an increase in adhesion with increasing fines content.For the geomembrane and geotextile interfaces, the as-com-pacted adhesion is slightly greater than the machine frictionfor fines content >6.5%. For the geogrid, the as-compacted

NICAL AND GEOENVIRONMENTAL ENGINEERING / APRIL 2001 / 359

FIG. 8. Effect of Fines Content on: (a) Interface Friction Angle;(b) Interface Adhesion (GG: geogrid, GM: geomembrane, GT:geotextile)

adhesion is greater than the machine friction for all thefoundry sands (i.e., fines content $6.5%).

The as-compacted geogrid interface also exhibits thegreatest increase in adhesion with increasing fines—near thatof the soil alone. This trend is also consistent with the previousdiscussion of the contribution of suction to adhesion in theinterfaces of geogrid and foundry sand. That is, suction causesan increase in adhesion at the interface between two porousmaterials but not at the interface between soil and a geosyn-thetic. The open apertures in the geogrid permitted soil-to-soilcontact and cohesion to exist at the contact because of suctionsin the soil. This cohesion is reflected as adhesion in the geo-grid-soil interface, unlike the other geosynthetics.

RESULTS OF PULLOUT TESTS

Pullout Force-Displacement Curves

Pullout tests were conducted with the geogrid and geotextileusing Foundry Sands A, C, and D. Foundry Sand B was notused because of lack of availability. These sands were selectedto represent low (2.4%), medium (7.5%), and high (13.0%)bentonite contents. Typical graphs of pullout force per unitwidth versus displacement are shown in Fig. 9 for the geogridin Foundry Sand A and the geotextile in Foundry Sand C. Ingeneral, the pullout force increases rapidly at low displace-ments (<20 mm) and then either increases steadily or becomesconstant at higher displacements, without displaying a peak.At low normal stresses, the pullout force becomes constantbecause the entire geosynthetic is displaced; thus, shear

360 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENG

FIG. 9. Typical Graphs of Pullout Force per Unit Width versusDisplacement for: (a) Sand A-Geogrid; (b) Sand C-Geotextile

strength is mobilized completely along the entire length of thegeosynthetic. At higher normal stresses, the pullout force con-tinually increases because of progressive failure caused bynonuniform displacement along the geosynthetic. At a normalstress of 50 kPa, both the geogrid and the geotextile failedwhile the pullout force was still increasing. Failure of the geo-grid occurred in all sands at a normal stress of 50 kPa. Failureof the geotextile occurred in all sands except Foundry Sand Aat a normal stress of 50 kPa.

Interaction Coefficients

The pullout test data were used to calculate the interactioncoefficient Ci. For cohesionless soils, the interaction coefficientis (GRI GT5 and GT6)

PC = (3)i 2WL(s tan f)n

where P = maximum pullout resistance; W = width of thereinforcement; L = length of the embedded portion of the re-inforcement; sn = normal total stress applied to the soil; andf = total stress friction angle of the soil. For cohesive soils,the interaction coefficient is (Bergado and Chai 1994; Tatlisozet al. 1998)

PC = (4)i 2WL(s tan f 1 c)n

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FIG. 10. Relationship between Interaction Coefficient andNormalized Mobilized Length (Lm /L) (for All Tests L = 1.31 m)

TABLE 5. Interaction Coefficients

Soil(1)

Fines(%)(2)

Geogrid

10kPa(3)

30 kPa(4)

50 kPa(5)

Geotextile

10kPa(6)

30kPa(7)

50kPa(8)

(a) Calculated with Cohesion

Foundry Sand A 6.5 0.33 0.31a 0.28a 0.24 0.23 0.20Foundry Sand C 11.3 0.48 0.45 0.40b 0.38 0.33 0.26a

Foundry Sand D 19.2 0.40 0.48 0.43a 0.28 0.28 0.22a

(b) Calculated without Cohesion

Base sand 1.1 1.4 1.0 0.8 1.4 0.54 0.25a

Foundry Sand A 6.5 1.1 0.56a 0.42a 0.80 0.41 0.30Foundry Sand C 11.3 1.5 0.98 0.71b 1.2 0.56 0.36a

Foundry Sand D 19.2 1.7 0.98 0.70a 1.2 0.58 0.35a

aGeosynthetic failure.bEstimated for 50 kPa by extrapolating data.

where c = total stress cohesion of the soil. The maximumpullout force at the maximum displacement of 100 mm or atgeosynthetic breakage was used when calculating the inter-action coefficients. The f and c used to calculate the Ci valueswere obtained from the maximum displacement of the shearbox; i.e., the Ci values were computed in the context of largedisplacements. Calculations were made with and without soilcohesion [(1) and (2)] for each foundry sand. For the 50-kPanormal stress, the peak pullout resistance used to compute theinteraction coefficients corresponded to failure of the geosyn-thetic specimen. Thus, the interaction coefficients at 50-kPanormal stress represent a lower bound on Ci.

The interaction coefficients are summarized in Table 5. Aninteraction coefficient between 0.5 and 1.0 indicates a goodbond between the soil and reinforcement. Poor bonds or break-age of the geosynthetic correspond to Ci < 0.5 (Tatlisoz et al.1998). Increasing the normal stress generally causes a decreasein interaction coefficient as a result of increased progressivefailure.

In all cases, the Ci values for the geogrid were higher thanthose for the geotextile. The higher Ci values obtained for thegeogrid are believed to be due to mobilization of shearstrength along a greater length of the specimen than occurredfor the geotextile in addition to the passive resistance againsttransverse ribs of the geogrid. These two factors apparentlywere large enough to more than offset the reduced contactareas of the geogrid relative to the geotextile. Mobilized length

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TABLE 6. Recommended Parameters for Preliminary Designwith Foundry Sands

Parameter(1)

Value(2)

Internal friction angle f9 a 407Cohesion c9 a 0Frictional efficiency

Geotextile 0.6Geogrid 0.6Geomembrane (textured) 0.7

Interface adhesion cad 0Geogrid interaction coefficientsb

Normal stress <20 kPa 1.0Normal stress >20 kPa 0.5–0.8c

Geotextile interaction coefficientsb

Normal stress <20 kPa 1.0Normal stress >20 kPa 0.3–0.5c

aDrained strength parameters.bIgnore soil cohesion when using interaction coefficients.cUse values at higher end of range for sands with higher bentonite

content.

Lm , defined as the length of geosynthetic that moved duringthe pullout test, was determined from displacements monitoredby the telltales. For each normal stress and for each sand, Lm

for the geogrid was >Lm for the geotextile. Greater Ci is ob-tained with greater Lm, as shown in Fig. 10 for Ci calculatedwithout cohesion.

The Ci calculated with cohesion are substantially lower(one-third to one-half) than the Ci calculated without cohesionand fall below values typically reported for interaction coef-ficients (Table 5). These Ci are low because cohesion contrib-utes 28–77% of the shear strength of foundry sands in theranges of stresses that were used, but only a fraction of thecohesion is transferred to the geosynthetic as adhesion, espe-cially for the geotextile. Given that cohesion and adhesion willdiminish under drained conditions, the Ci values calculatedwithout cohesion are more useful for designs where thedrained strength (i.e., c9 ; 0 for foundry sands) will control.

RECOMMENDATIONS FOR DESIGNERSCONSIDERING FOUNDRY SANDS

Results of the laboratory tests conducted in this study pro-vide designers with the data to conduct preliminary designcalculations for evaluating the efficacy of reusing foundrysands in geotechnical construction. Recommended design pa-rameters are summarized in Table 6. These design parametersare intended for preliminary design calculations. Final designshould be conducted using measured properties for the foundrysand and the geosynthetic to be used. Field conditions shouldbe simulated as closely as possible when conducting tests forfinal design.

The presence of bentonite in foundry sands also warrantsconsideration regarding potential interactions with water. Al-though swelling of foundry sands has been found to be neg-ligible, swell testing should be conducted to ascertain the swellpotential in cases where swelling might have a detrimentalimpact. The impacts of drainage should also be considered,because foundry sands have lower hydraulic conductivity (par-ticularly those with higher bentonite content) than clean sandsand thus are not freely draining.

Shear Strength

Foundry sands friction angles typically range from 397 to437 when sheared rapidly in an as-compacted state and gen-erally are similar to that of base sand (427). The as-compactedcohesion ranges between 17 and 28 kPa and generally in-

NICAL AND GEOENVIRONMENTAL ENGINEERING / APRIL 2001 / 361

creases with increasing fines (or bentonite) content. Underdrained conditions, the cohesion is comparable to that of thebase sand and the machine friction. In many cases, the drainedstrength will ultimately control. Thus, for preliminary designwith foundry sand, reasonable estimates of the strength param-eters are f9 = 407 and c9 = 0.

Interface Shear Strength

The frictional efficiency for foundry sands and geosynthet-ics typically ranges from 0.5 to 0.9. A frictional efficiency of0.7 can be used for foundry sands in contact with texturedgeomembranes. For woven geotextiles and geogrids, a fric-tional efficiency of 0.6 can be used. Adhesion can be ignoredgiven that it will be small under drained conditions.

Pullout Capacity

For normal stresses <20 kPa, an interaction coefficient Ci

of 1 can be used for woven geotextiles and geogrids. At highernormal stresses, Ci ranging between 0.5 and 0.8 can be usedfor geogrids and values between 0.3 and 0.5 can be used forwoven geotextiles, with higher Ci being used for foundry sandshaving higher bentonite content. Cohesion of the soil shouldbe ignored when using these interaction coefficients. Theseinteraction coefficients are similar to those recommended byMirafi, Inc., Charlotte, N.C. (Simac 1990) and Keystone Re-taining Wall Systems, Inc., Minneapolis (Race 1995), for soilshaving texture similar to foundry sands.

SUMMARY

The objective of this study was to provide the shear strengthand pullout data necessary for evaluating the reuse of foundrysands as embankment fill, retaining wall backfill, and in land-fill covers constructed with or without geosynthetics. Directshear tests, interface shear tests, and pullout tests were con-ducted to meet this objective. The shear tests were conductedunder as-compacted and drained conditions. Recommendeddesign parameters were selected based on general charac-teristics of the data. These parameters are summarized inTable 6.

The recommended parameters provide a starting point fordesign and are useful for evaluating the feasibility and efficacyof reusing foundry sand in geotechnical construction. How-ever, good practice dictates that final design calculations bebased on testes conducted with the foundry sand and geosyn-thetics to be used on a project. These tests should be conductedin a manner that simulates the field condition as closely aspossible.

ACKNOWLEDGMENTS

The Wisconsin’s Recycling Market Development Board provided sup-port for this project. The geogrid was donated by TC Mirafi, Pendergrass,Ga., and the geomembrane by GSE, Inc. AFK Corp., Brillon Iron Works,Wagner Casting, and Waupaca Foundry, Inc. donated the foundry sands.Badger Mining Co. donated the base sand. Robert Swan of GeoSyntecConsultants design and manufactured the pullout box and interface directshear machine. Comments provided by the anonymous reviewers of thismanuscript are appreciated.

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