LIME-INDUCED HEAVE IN SULFATE-BEARING CLAY SOILS

By Dal Hunter1

ABSTRACT: Expansive reactions between lime and sulfate-bearing clay soils have attracted little attention until relatively recently. Lime treat­ment of Stewart Avenue in Las Vegas, Nevada, has induced heave in excess of 12 in. Heaved areas are found to contain abundant thaumasite, a complex calcium-silicate-hydroxide-sulfate-carbonate-hydrate min­eral. Thaumasite forms a solid solution series with ettringite, a calcium-aluminum-hydroxide-sulfate-hydrate mineral. In the presence of alumi­num, ettringite forms first and is replaced by thaumasite only at temperatures below 15°C. The mechanism of heave is a complex function of available water, the percentage of soil clay, and ion mobility. Only the long-term pozzolanic chemistry of normal lime-soil reactions is disrupted. Cation exchange, agglomeration, and carbonation are unaf­fected. With the present state of knowledge, lime-induced heave is difficult to predict for all but most obvious conditions.

INTRODUCTION

Within a period of two years following lime stabilization, Stewart Avenue and Owens Street in Las Vegas, Nevada, were heaved by adverse chemical reactions between lime and salts in the native soils. In 1985, the more severely damaged portions of Stewart Avenue were reconstructed. Localized areas of distress were removed and replaced. Remaining portions of the pavement were bounded by subdrains to minimize infil­tration of water into undamaged lime subbase. Total cost of rehabilitation exceeded $2,700,000—more than the cost of the original 1976 improve­ments.

CASE HISTORY OF STEWART AVENUE AND OWENS STREET, LAS VEGAS

Site Conditions: Stewart Avenue Stewart Avenue is a major east-west trending roadway through down­

town Las Vegas. The study area extends approximately 2.75 miles between 28th Street and Nellis Boulevard (Fig. 1). In 1976, it was widened from two to four lanes by complete reconstruction.

Between 28th Street and Pecos Road, the 1976 structural section was 4 in. of asphaltic concrete, 5 in. of aggregate base, and a 12-in. thickness of lime-treated native soil (subbase). From Pecos Road to Nellis Boulevard, the aggregate base section was increased to 8 in. Project specifications called for mixing 4.5 weight percent quicklime into moisture conditioned native soil. The treated material was cured a minimum of 16 hr prior to compaction.

Evidence of distress appeared within six months following construction. By the end of two years, damage was severe. Distress consisted of heaved

•Engrg. Geochemist, SEA, Inc., 950 Industrial Way, Sparks, NE 89431. Note. Discussion open until July 1, 1988. To extend the closing date one month,

a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on October 24, 1986. This paper is part of the Journal ofGeotechniealEngineering, Vol. 114, No. 2, February, 1988. ©ASCE, ISSN 0733-9410/88/0002-0150/$1.00 + $.15 per page. Paper No. 22168.

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4 .N

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PROJECT LIMITS 0'

BHIHGTOH g j

-PROJECT LIMITS-CHSBLESTOH

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FIG. 1. Location Map for Stewart Avenue and Owens Street, Las Vegas, Nevada

asphaltic pavement, concrete and asphalt medians, as well as occasional curbs and gutters. The worst damage was between Mojave Boulevard and Pecos Road. In this area, a ridge in the south parking lane paralleled the roadway. This ridge rose as much as 12 in. above the adjacent pavement and ranged from 1-2 ft in width. Often, the adjacent pavement had also been heaved so that the maximum uplift was hard to gauge. Ridges generally displayed large fractures in the asphalt, typically 1-6 in. in width (Fig. 2).

Remaining asphaltic pavement and medians between Mojave and Pecos showed extensive damage with the pavement exhibiting a generally uplifted and undulating surface. Concrete median slabs were often dis­placed 3-6 in. (Fig. 3). From 28th Street to Mojave and from Pecos to Sandhill, the distress was very similar in form but of a lower magnitude. Damage was predominantly in the median and on the south side of the street. Between Sandhill Road and Lamb Boulevard, heave was generally confined to isolated areas on the south side of Stewart or in the median. East of Lamb, damage was even more localized but occurred on both sides of the street and in the median. The majority of the roadway east of Lamb Boulevard was undamaged.

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FIG. 2. View West along South Site of FIG. 3. View North from South Side of Stewart Avenue, just East of Mojave Stewart Avenue, just West of Mojave Boulevard; Heave in This Area Exceeds Boulevard; Concrete Median Has Been 12 in.; Note 6-in. Wide Crack in Pave- Heaved over 6 in. ment at Top of Heaved Ridge

Site Conditions: Owens Street Owens Street was reconstructed between Eastern Avenue and Pecos

Road in 1978 (Fig. 1, shown earlier). This segment incorporated an 8-in. lime-treated base directly overlain by 10 in. of asphaltic concrete. Aggre­gate base from the older roadway was mixed into the native soil with 4.5 weight percent quicklime.

Distress along Owens Street is similar in form to that of Stewart Avenue but of a considerably lesser magnitude. Maximum heave, again along linear ridges, is 6-8 in. The ridges are less continuous than at Stewart Avenue and occur equally on both sides of the street. The majority of the roadway is undamaged or only slightly damaged, and damage is often limited to cracking of the pavement. Damage to the median is common.

Geology and Soil Conditions The Stewart and Owens alignments lie in an area of thick basin-fill

sediments. These materials consist of sandy silts and sandy clays originally deposited in marshy or playa environments (Bell 1982; Mifflin and Wheat 1979). The deposits were derived from a variety of source rocks dominated by carbonates. Some of the source areas contain abundant bedded gypsum (Dinger 1977).

Soils underlying the two roadways are poorly to slightly stratified and, on a gross scale, fairly homogeneous. On a much finer scale, they are complexly interbedded and contain areas with significant differences in engineering and chemical properties. The upper 8 feet typically consist of

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moist, slightly stiff to stiff, brown to tan, sandy silty clay with 70-90% low plastic fines.

Soil Mineralogy Mineralogy of the native soils can be divided into primary minerals, clay

minerals, and evaporites. Primary minerals are dominated by quartz with lesser muscovite and feldspar. Sepiolite and montmorillonite are the most abundant of the clays. Mixed-layer kaolinite-montmorillonite and kaolinite may also be present. Calcite, closely followed by gypsum, is the dominant evaporite. Thenardite (Na2S04) and arcanite (K2S04) were positively identified in the salty efflorescence commonly seen at the soil surface.

Field Methodology Thirty-four test pits were excavated along Stewart Avenue and six along

Owens Street. Test pits were excavated through the asphalt in severely damaged, moderately damaged, and undamaged areas of travel lanes, turning lanes, shoulders, medians, gutters, and curbs. Control trenches were placed in unpaved areas adjacent to badly damaged portions of roadways as well as adjacent to undamaged sidewalks. A typical test-pit profile is presented as Fig. 4 for Stewart Avenue.

Representative samples for mechanical and chemical testing were col­lected from lime-treated subbase and native soils. In situ soil temperatures were measured to accuracies of plus or minus 0.5°C at various levels within selected test pits.

Field Observations Effects of lime treatment on the native soil varied drastically between

two extremes. In severely damaged areas, subbase consisted of soft, light gray, mineral aggregate with mechanical properties of a plastic silt. In undamaged areas, the subbase was an extremely hard, cemented, medium brown material that could not be considered soil. All gradations between the two extremes were observed. Often, extremely hard material graded to a soft, wet subbase in the length of a single trench. Vertical gradation was also observed.

Undamaged subbase generally contained less than 5% white mineral aggregates. These minerals were clearly of secondary origin. Maximum size of mineral aggregates was approximately 3/4-in. Damaged subbase showed very little of the light brown coloration typical of native soils or the undamaged subbase. In the most severe cases, the subbase consisted of 30-60% light-colored secondary minerals, grown subsequent to lime treat­ment. Maximum size for mineral aggregates was, again, approximately 3/4-in.

Contacts of undamaged subbase with the overlying aggregate base and the underlying native soils were straight and parallel. The layer was of uniform thickness, typically 12 in. on Stewart Avenue and 8 in. on Owens Street. These thicknesses are in good agreement with the design.

The damaged subbase was distinctly lens shaped, thickened in the most distressed areas and thinning progressively with lower degrees of damage (Fig. 5). The maximum measured thickness was 2.25 ft on Stewart Avenue—more than twice that of the design. The lower contact was slightly convex downward, indicating some expansion into soft native

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I I I I

FIG. 5. Photograph of Typical Exploration Trench on Stewart Avenue; Note Lense Shape of Subbase; Photograph Is from Same Location as Fig. 2; Thermometer Dials in Trench Wall Are 1.5 in. in Diameter

soils. The upper contact was sharply convex, indicating that the majority of the expansion was upward.

Areas of major damage were almost always found adjacent to an obvious source of water, including permeable utility trench backfill, yard drainage, areas of poor surface drainage, and construction joints, such as that occurring between the concrete median and asphaltic pavement. In most instances, the worst damage was found adjacent to or overlying utility trench backfill.

Laboratory Analysis

Soil Testing Samples of aggregate base, lime-treated subbase, and native soil from

each test pit on Stewart Avenue and Owens Street were analyzed in the laboratory to determine their grain-size distribution and plasticity. A moisture-density curve was also generated for most samples of lime subbase and native soils. Volume change tests were conducted on re­molded samples of lime-treated subbase and native soil. Tests were run at various moisture contents in order to evaluate the relationship of moisture to expansive pressure. Test results for native soils are summarized in Table 1.

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TABLE 1. Summary of Soil Testing Results For Native Soils

Statistic (1)

Maximum value Minimum value Average Standard

deviation Number of tests

performed

Plastic index (%) (2)

43 0

18 10.3

34

Cation exchange capacity

(Meq/100gm) (3)

19 7

13.4 3.5

11

Volume change at 10%

moisture (%)a

(4)

8.4 0.1 4.2 2.7

30

Volume change at 15%

moisture (%)a

(5)

7.4 0.0 2.7 2.2

21

a90% relative compaction (ASTM D1557) with 100 psf surcharge.

Mitchell (1986) provided various test results for untreated soils, treated soils from failed zones, and treated soils from unfailed zones. His results are in excellent agreement with extensive testing performed during this study.

Chemical Testing Samples of damaged and undamaged, lime-treated subbase and native

soils were analyzed for Ca2+ , Mg2+ , Na+ , K+ , SO2." , COf" , CI - , and HCO^~. Selected samples were also analyzed for Mn2+ , Al3+ , and Si02 . In order to dissolve most of the soluble ions, native soils were extracted at a water to soil ratio of 50:1. Even at this dilution, not all calcium carbonate dissolved.

Representative samples of damaged subbase, undamaged subbase, and native soil were analyzed by X-ray diffraction to determine their miner­alogy. Selected samples were also examined with a Scanning Electron Microscope (SEM). The SEM uses energy dispersion to identify the constituent elements of a compound. This is particularly useful for differentiating minerals with similar X-ray patterns.

Results of Investigation The investigation demonstrated that heave has been induced by chemi­

cal reactions between lime and the native soils. The results of mechanical analysis of native soils (Table 1) show that the clays are slightly to moderately expansive and not capable of swelling 100%. The in-place dry density of damaged subbase was found to be as low as 54 lb/cu ft, a clear indication of where the heave occurred.

X-ray and SEM techniques identified an abundance of the mineral thaumasite in damaged subbase. In the worst cases, thaumasite may constitute 20-40% by volume of the total mass. Thaumasite, a rare calcium-silicate-hydroxide-sulfate-carbonate-hydrate, is not present in the native soils and does not occur naturally in sedimentary environments. Other secondary minerals, including calcite (CaC03), gypsum (CaS04 • 2H20) and

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polyhalite [K2MgCa2(S04)4 • 2H20] were also produced by the lime reactions.

Lime-treated subgrade soils were found to be chemically expansive in the laboratory. Samples placed in sulfate solutions with a 100 lb/sq ft surcharge showed a constant volume increase of approximately 0.1% per day. Expansion reached a 12% volume change, which was double that of the untreated soil at the same level of compaction. Tests were terminated prematurely due to a poor understanding of the geochemical mechanism at the time. Additional experiments are in progress to further evaluate the potential maximum volume change.

ADDITIONAL CASES OF LIME-INDUCED HEAVE

Although the Stewart/Owens cases are the most severe examples known, other cases of lime-induced heave have been documented. A lime-treated school parking lot, in the east Las Vegas area, has suffered extreme heave subsequent to the Stewart Avenue distress. A lime-treated parking lot in Witchita, Kansas, is also reported to have failed through growth of ettringite sometime after 1980 (Mitchell 1986).

Personal communications with various western state highway depart­ments uncovered additional cases of lime-induced heave in Texas and Utah. Neither was well documented.

PREVIOUS LABORATORY RESEARCH ON LIME-CLAY-SULFATE REACTIONS

Lambe et al. (1960), Ladd et al. (1960), and Hollis and Fawcett (1966) concluded that small amounts of sulfate produced a significant increase in strength for certain soil types. Evidence for detrimental effects of higher sulfate concentrations on cement and lime-treated soils was presented by Mehra et al. (1955); Sherwood (1958, 1962); Lambe et al. (I960); and Cordon (1962). A report published in 1972 by the California Division of Highways stated: "Sulfates can be detrimental to lime treated soils because they enhance swelling and may induce disintegration when the mixture is saturated." In a major laboratory study of lime stabilization, these California researchers demonstrated that the potential for swell rose with sulfate content of the treated soil. Maximum volume increases were on the order of 4%, which is attributed to growth of ettringite.

NORMAL LIME-SOIL REACTIONS

In addition to increasing bearing strength, lime decreases the plastic index of soils and greatly reduces the shrink-swell characteristics of highly plastic clays. It also increases the permeability by giving the clay silt-like mechanical properties and reduces the maximum dry density. Normally lime stabilization proceeds through a combination of four mechanisms (Thompson 1966): (1) Cation exchange; (2) flocculation/agglomeration; (3) carbonation reactions; and (4) pozzolanic reactions. The first two increase soil workability and result from changes in electrical charges of the clay minerals. The second two are cementation reactions that produce the increase in bearing strength. The mechanism of lime-induced heave affects only the pozzolanic reactions. Pozzolanic reactions consist of a large group

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of cementing reactions involving lime with silica or lime with silica and alumina.

Clay minerals become unstable and begin to deteriorate above a pH of 10.5 (Eades and Grim 1950; Davidson et al. 1965). Once a soil is sufficiently saturated with lime, the pH of the soil-water system stabilizes at approxi­mately 12.3. Dissolution of clay provides the siliceous and aluminous pozzolans for reaction with calcium (Thompson 1966).

MECHANISM OF LIME-INDUCED HEAVE

Lime-induced heave is a relatively unusual phenomenon. This is largely due to the complex mechanism involved in growing disruptive volumes of hydrous calcium-hydroxide-sulfate minerals. The presence of sulfate in a lime-treated soil affects only the long-term pozzolanic reactions. Cation exchange, flocculation/aglomeration, and carbonation occur quickly and produce the expected results. Unfortunately, this means that there is no immediate indication of a soil's potential to heave.

Thaumasite-Ettringite Solid Solution Series Thaumasite {Ca6 [Si(OH)6]2 • (C03)2 • (S04)2 • 24H20} forms a solid

solution series with the slightly more common mineral, ettringite {Ca6[Al(OH)6]2 • (S04)3 • 26H2Oj (Kollman et al. 1977; Kollman 1978; Kollman and Strubel 1981). In theory, this means that any composition between the two end members is possible. Indeed, minerals of intermedi­ate composition have been known for years. However, it is only recently that the solid solution series has been demonstrated (Gouda et al. 1975; Kollman 1978; Kollman and Strubel 1981).

In the laboratory, thaumasite can be synthesized from a mixture of gypsum (CaS04 • 2HzO), calcite (CaC03), and lime (CaO) with any compound releasing silica at high pH. When clay minerals provide the silica, alumina is also available, and an interesting phenomenon is ob­served. Ettringite always forms first and converts slowly to thaumasite, only at temperatures below 15°C. Above 15°C, ettringite appears to remain stable (Kollman et al. 1977; Kollman 1978). In the Las Vegas soils, temperatures of 13°C were measured in the trench walls in November 1982. The mineral identified at both Stewart Avenue and Owens Street was very nearly pure thaumasite. The implication from experimental research (Sherwood 1962; "Highway research report" 1974; Kollman et al. 1977; Kollman 1978) is that the original lime-induced mineral precipitated was ettringite. It is not clear whether distress was produced by ettringite, which later converted to thaumasite, or if the ettringite changed to thaumasite during the first winter after construction and continued thaumasite growth caused the heave.

Soil Chemistry Chemistry of native soil was extensively analyzed to determine if

lime-induced heave was controlled by variations in soluble ions. Fig. 6 shows a trilinear diagram of native soil chemistry from both roadways. The soluble ions are dominated by calcium and sulfate with variable ratios of sodium plus potassium and total carbonate. The percentages of magnesium and chloride are very uniform throughout the soils. The significant factor demonstrated by Fig. 6 is the broad range of chemistry in the native soils.

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.., Co" Cl-"" CATIONS PERCENT OF TOTAL ANIOW

MILLIEOUIVALENT8 PER LITER

o DISTRISSID AREAS OF STEWART AVE. e HON DISTRESSED AREAS OF STEWART AVE. a DISTRESSED AREAS OF OWENS AVE.

FIG. 6. Trilinear Diagram of Native Soil Chemistry from Stewart Avenue and Owens Street

There is no grouping of soils from undamaged versus damaged areas, and therefore, adverse lime/soil reactions were not noticeably controlled by variations in the initial soil chemistry.

As would be expected, lime treatment resulted in major changes to soil chemistry. These changes are clearly shown by Fig. 7, a trilinear diagram of the lime-treated soils. The cations show a significant increase in relative percentages of calcium. However, it is less than would be expected from the addition of lime, suggesting that some calcium has been leached away or that it is tied up in insoluble minerals.

The relative increase in sulfate shown by Fig. 7 indicates that either additional SO^- has migrated into the soil or that total carbonate and chloride both have decreased. In fact, additional carbonate had to be introduced into the soil to form secondary calcite. Existing carbonate was already tied up in primary calcite or thaumasite. Consequently, the shift toward the S04~ apex must represent an influx of sulfate. The increased

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CATIONS PERCENT OF TOTAL Mlt.LIEQUIVAl.iNTS PER LITER

o DISTRESSED AREAS OF STEWART AVE. e HON DISTRESSED AREA8 OF STEWART AVE. D DISTRESSED AREAS OF OWENS AVE. B NON DISTRESSED AREAS OF OWENS AVE.

FIG. 7. Trilinear Diagram from Lime-Treated Subbase for Stewart Avenue and Owens Street; Although Only Calcium Was Added during Stabilization, Diagram also Shows Increased Sulfate Relative to Native Soils

sulfate concentrations after lime treatment are also apparent from stiff diagrams. A typical stiff diagram is presented as Fig. 8. The diagrams show the millequivents of each ion per liter of water extracted from the soil at a 50:1 ratio of water to soil. Again, no relationship between soil chemistry and distress is apparent.

Clay Content Examination of hydrometer data from untreated native soils along

Stewart Avenue shows a strong correlation between the percentage of clay and distress. Fig. 9 presents a graph of clay-sized particles versus distance along the alignment. On Stewart Avenue, there is a marked decrease in clay-sized particles east of Station 88+50 where distress is minimal. Where sufficient soluble silica or alumina could not be generated, the calcium reacted instead to form calcite, gypsum, and probably pozzolanic compounds.

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MILLIEOUWALENTS/UTER MILLIEQUIVALiNTS/LITER

LUKE TREATED SOIL UNTREATED SOIL

FIG. 8. Typical Stiff Diagrams for Native and Lime-Treated Soils; Increased Sulfate in Lime-Treated Soils Is Apparent Here

Mass-Volume Relationships Both the trilinear and stiff diagrams suggest that ions, beyond those

present in a given volume of native soil, were added to the system. In order to check this hypothesis, a rough estimate of the mass-volume relation­ships was developed.

In areas of severe damage, the thickness of the lime-treated subbase doubled, indicating a 100% increase in volume. Using a typical in-place dry density of lime-treated subbase (measured during construction) of 96.2 lb/cu ft, a 100% volume change would lower the dry density to 45.8 lb/cu ft, if no mass was added. The measured dry density of damaged subbase at the same location was 59.3 lb/cu ft. This suggests that 13.5 lb of new minerals were added per cubic foot of damaged subbase. Since 2 cu ft of damaged subbase grew from 1 cu ft of lime-treated soil, a total of 27 lb of minerals were added to each cubic foot of undamaged subbase.

Making the simplified assumption that all 4.5 lb of quicklime in a cubic foot of subbase reacted to form ettringite, the following analysis can be performed:

1. 4.5 lbs CaO equals 34.6 moles, yielding 34.6 moles of Ca2+ (from stoichiometry of CaO).

2. 34.6 moles Ca2+ yields 5.8 moles of ettringite (from stoichiometry of ettringite).

3. (5.8 moles ettringite) x (1,254 g/mole) = 7,273 gm (16 lb) ettringite.

The previous calculation indicates that 27 lb of new minerals were added per cubic foot of undamaged subbase, leaving a deficiency of 11 lbs. Similar calculations using the soluble sulfate present in the native soil produced very similar results. Calculations were also performed with thaumasite (1,244 g/mole) rather than ettringite, and using volume change

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PERCENT CLAY PLUS COLLOID SIZE PARTICLES

10 20 30 4.0 SO

4 o

SL ©

«S I -(0 ui

< S UJ

(0

—MOJAVE RD.

40- -PECOS DR.

a GRANULAR T R E N C H

=raBfBBLVb,

FIG. 9. Graph Showing Distribution of Clay- + Colloid-Sized Particles versus Station along Stewart Avenue; Distinctive Drop in Fine Particles East of Station 88 + 50 Correlates Well with Area of Minimal Distress

and molar volumes of ettringite and thaumasite rather than mass and density. In all cases, the results indicate that salts present in native soils are only sufficient to account for 1/2-2/3 of the mass and/or volume increase. Finally, X-ray patterns strongly suggest that the percentage of calcite and gypsum present in the native soils is not always decreased in the damaged subbase. It appears, therefore, that a significant percentage of the reactants migrated to nucleation sites, as suggested by the trilinear and stiff diagrams.

Reaction Model A simplified geochemical mechanism for lime-induced heave can be

summarized by the following series of stepped equilibrium reactions:

CaO + H20 = Ca(OH)2 (hydration of quicklime) (1)

Ca(OH)2 = Ca2+

+ 2(OH)^ (ionization of calcium hydroxide; pH rises to 12.3) (2)

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Al2Si4Q]0(GH)2 • nH20+2(OH)" + 10H2O = 2Al(OH)4~ + 4H4Si04

+ «H20 (dissolution of clay mineral, at pH> 10.5) (3)

2H4Si04 = 2H3Si04" + 2H+ = 2H2Si02.~

+ 2H+ (dissociation of silicic acid) (4)

5Ca2+ + eHjSiO^ + 4QIT = Ca5(Si6018H2) • 4H20 + 6H20 (5)

MxS04 • nH20=XMY+ + S O ^ + «H20

(dissolution of sulfate minerals; x = 1, y = 2 or x = 2, y = 1) (6)

6Ca2+ + 2A1(0H)4- + 40H~ + 3(S04)2" + 26H20

= Ca6[Al(OH)6]2 • (S04)3 • 26H20 (formation of ettringite) (7)

C02 + H20 = H2C03 (formation of carbonic acid) (8)

CaC03 + H2 C0 3 = Ca2 + + 2H +

+ 2C02 ~ (dissolution of calcite in carbonic acid) (9)

Ca6[Al(OH)6]2 • (S04)3 • 26H20 + 2H2Si04!~ + 2C0 3 "

+ 0 2 = Ca6[Si(OH)6]2 • (S04)2 • (C03)2 • 24H20 + 2Al(OH),f

+ S02~ + 4 0 H " +2H 2 0

(isostructural substitution as ettringite changes to thaumasite) (10)

Ca2+ + S 0 2 - +2H 2 0

= CaS04 • 2H20 (formation of secondary gypsum) (11)

Ca2+ + COf" = CaC03 (formation of secondary calcite) (12)

Eqs. 1-4 are normal pozzolanic reactions resulting in siliceous cemen­tation of lime-treated soils. Eq. 3 is shown for montmorillonite, one of the clay minerals identified in the Stewart Avenue soils. In fact, any clay mineral could be used in the model since the sole function of the clay is to provide a source of silica and alumina. In the presence of excessive sulfate, the rate of the forward reaction in equilibrium pair Eq. 5 approaches zero. Sulfate from any evaporite (Eq. 6) reacts with alumina generated by dissolution of any clay mineral to form ettringite (Eq. 7).

Once ettringite has nucleated, it continues to grow as the nearly pure end member until the temperature of the system drops below approximately 15°C. Below 15°C, ettringite is transformed through a series of intermedi­ate compositions to thaumasite. The change occurs by isostructural substitution of silica for aluminum and carbonate for sulfate. Secondary gypsum (Eq. 11) is likely to form at this time from calcium derived by dissolution of calcite (Eq. 9) and sulfate derived from dissolution of

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ettringite. Eqs. 7 and 10 stop as pH drops below 10.3, thereby cutting off the supply of alumina and silica (Eq. 3). Eq. 12 occurs at that time as carbonate ions are no longer being consumed by thaumasite.

The temperature control of the ettringite to thaumasite transition must be, at least, partially a function of the retrograde solubility of calcite. However, the strong pH dependence of both carbonate and silica solubility adds an additional variable to the problem. At high pH, silica is quite soluble. Calcite solubility, however, decreases rapidly at higher pH. Thaumasite growth, therefore, must be partially controlled by a balance between two nearly mutually exclusive reactions.

One unanswered question is whether or not thaumasite is even necess­ary to induce heave. In the laboratory, ettringite always forms first at any temperature when both alumina and silica are present. The transition to thaumasite may well have occurred after the heave. In Stewart/Owens, the transition was complete; no identifiable ettringite was found. In Kansas (Mitchell 1986), ettringite was blamed. This may represent a case where the damage occurred and was studied prior to the transition. Alternatively, it may be another indication of the difficulty in distinguishing ettringite from thaumasite by X-ray diffraction.

The percentage of clay-sized particles in the native soils is a major controlling factor. Virtually all severe heave occurred west of Station 88+50 where the percent of clay-sized particles averaged 28. East of Lamb Boulevard, damage was minimal and the clay-sized particles averaged 10%. Where insufficient clay minerals were present, lime-induced heave did not occur. With few exceptions, it appears that areas where the percentage of clay-sized particles was less than 10, thaumasite (or ettring­ite) did not grow sufficiently to induce heave. Exceptions may indicate areas where silica and alumina migrated to nucleation sites in the pore water or areas of soil variation between the depth of the subbase and the underlying native soils tested for grain-size distribution.

The availability and flow of pore water is the single most important factor controlling lime-induced heave. Both ettringite and thaumasite are highly hydrous minerals. Without an abundance of water, they cannot form. In addition, ions must migrate through the soil in pore water solutions to generate the observed heave magnitudes. To develop signifi­cant distress in only two years, ion migration could not have been dependent upon diffusion through the clay soil. This observation accounts for virtually all major distress having occurred near an obvious water source. Typically, heave paralleled a utility trench backfilled with highly permeable material. Localized areas of damage could often be traced to specific houses, which channeled roof and yard drainage to the street.

Heave was also observed along many construction joints, particularly in lower areas where stormwater ponded. Storm drains in the Las Vegas area are undersized with streets designed to carry excess drainage. During one small thunderstorm (November 1982), water was observed running well over sidewalks on Stewart Avenue. Ponded water in some areas exceeded a depth of 4 ft. It is suggested, therefore, that stormwater entered the subsurface along construction joints and migrated through the free-draining, aggregate base section into utility trenches. After ponding in low-lying areas of a trench, capillarity and diffusion supplied salt-rich solutions to ettringite and/or thaumasite nuclei.

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PRACTICAL APPLICATIONS

With the present state of knowledge, conditions under which lime-induced heave might occur are difficult to predict. This can be attributed to the complex physiochemical mechanism involved, the lack of experimen­tal data on soil-lime-sulfate-water reactions, and the difficulty in predicting the flow paths of near-surface water.

The potential for lime-induced heave can be predicted for the two extremes. Clearly, if the geochemical environment is lacking in soluble sulfates, no lime-induced distress will occur, regardless of the other variables. At the other end of scale, where soluble sulfates approach 1% (10,000 ppm), clay minerals account for at least 10% of the soil, and the environment is frequently saturated, lime-induced heave will occur. The difficulty, then, is determining heave potential between these extremes and establishing a safe set of guidelines than can be used by the geotechnical engineer.

It would be easiest to exclude all soils with measurable sulfate, and given the state of the art, this might be the appropriate conservative approach. The risks involved are high. With Stewart Avenue, e.g., lime treatment might have saved $300,000 in initial construction costs over some of the other options. Repair of lime-induced distress, along only 2/3 of the roadway, however, cost $2,700,000. In addition, liability to the engineer for lime-induced heave has increased with awareness of the problem. Unfortunately, excluding all sulfate soils from lime stabilization is overly conservative and may incur excessive construction costs.

Lime-induced heave can, to some extent, be predicted in the laboratory, but not by standard test procedures. Perhaps the simplest method involves extended expansion testing of the R-value (ASTM D 2844) specimens commonly used in lime-mix designs. Expansive clays in this test normally reach maximum volume change within 24 hr. Lime-induced swelling continues at a constant rate until all of the available sulfate has reacted. This can be a period of months and perhaps years. Consequently, if expansion continues beyond five to ten days and soil is known to contain sulfate, adverse chemical reactions should be suspected.

A limited amount of swelling produced by low sulfate levels can readily be tolerated, provided the R-value does not decrease significantly. At Stewart Avenue, however, lime-induced heave occurred in areas where native soils may have had an initial sulfate content as low as 700 ppm. This, however, was adjacent to a major water source. In addition, sulfate levels as high as 20,500 ppm were recorded in native soils from undamaged areas where subbase had to be removed with ajackhammer. The majority of these areas were east of Lamb Boulevard where clay content was generally low. Sulfate levels as high as 43,500 ppm were recorded in undamaged subbase. In these cases, sulfate was tied up in gypsum rather than thaumasite. The particular value of 43,500 ppm was found in the same trench as, and only a few feet away from, an area of severe heave. The heave followed a linear trend adjacent to a water main.

In summary, the results of this study indicate that lime treatment of sulfate-bearing clay soils is risky, even at relatively low sulfate concen­trations. It does appear that approximately 10% by weight, or more, clay-sized particles are needed for adverse reactions. While both sulfate

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and clay minerals are required, their threshold concentrations are compli­cated by ion mobility in soil pore water. Excessive water may mobilize and concentrate necessary ions in areas where the reactions might otherwise not occur. Since diffusion through saturated fine-grained soils is extremely slow, lime-induced heave occurs preferentially in areas where water flows more freely. This includes, but is certainly not limited to, utility trench backfill and construction joints.

If utility trenches with impermeable synthetic liners are sloped to drain and construction joints are adequately sealed, it should be possible to successfully use lime stabilization in soils with high concentrations of both sulfate and clay minerals. Excellent surface drainage would also be mandatory to prevent ponding of water.

Since cement-sulfate-clay reactions are similar to those of lime-sulfate clay, similar risks are present for soil cement. Heave in soil cements has also been documented where sulfate is present (Sherwood 1958, 1962; Lukas 1975).

Further research is in progress to better evaluate the mass-flux relation­ships involved in this extraordinary magnitude of chemical heave. Although the evidence suggests that salts present in the native soil were insufficient for a 100% volume increase, it is not clear how additional reactants migrate through low permeability soils to produce this heave in only two years. In addition, the thermodynamic parameters of ettringite and thaumasite are being evaluated for use in geochemical models to help predict the problem.

ACKNOWLEDGMENTS

The writer wishes to thank SEA, Inc., for their continued support during this study, and particularly, Larry Johnson and Jeanne Lauritzen. Roger Jacobson of the University of Nevada, Desert Research Institute, has provided numerous helpful discussions on aqueous geochemistry and is gratefully acknowledged.

APPENDIX I. REFERENCES

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Cordon, W. A. (1962). "Resistance of soil-cement exposed to sulfates." Highway Res. Board Bull. 309, Washington, D.C.

Davidson, L. K., Demirel, T., and Handy, R. L. (1965). "Soil pulveration and lime migrations in soil-lime stabilization." Highway Res. Rec, 92, 103-126.

Dinger, J. S. (1977). "Relation between surficial geology and near-surface groundwater quality, Las Vegas Valley, Nevada." thesis presented to the University of Nevada, at Reno, Nev., in partial fulfillment of the requirements for the degree of Doctor of Philosophy.

Eades, J. L., and Grim, R. E. (1966). "A quick test to determine lime requirement for lime stabilization." Highway Res. Rec, 139, 62-72.

Gouda, G. R., Roy, D. M., and Sarkar, A. (1975). "Thaumasite in deteriorated soil cements." Cement Concr. Res., 5, 519-522.

"Highway research report-lime soil stabilization study, phase II, laboratory investigation of California soils for lime reactivity." (1974). Transp. Lab. Res. Report CA-DOT-TL-2812-2-7340, California Department of Transportation, Sacramento, Calif.

Hollis, B. G., and Fawcett, N. D. (1966). "Laboratory investigation of the use

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Kollman, H. (1978). "Minerlogische untersuchungen uber ausbluhungs und treiberscheinungen and baustoffen durch sulfate." Giessener Geologische Schriften, 18, Giessen, Germany, 159 pp.

Kollman, H., Strubel, G., and Trost, F. (1977). "Mineralsynthetische unterSuchungen zu treibursachen durch Ca-Al-sulfat und Ca-Si-carbonat-sulfat hydrat." Tonindustrie Zeitung, 101(3), Germany, 63-70.

Kollman, H., and Strubel, G. (1981). "Ettringit-Thaumasit-Mischkristalle von Brenk (Eifel)." Chemical Erde, 40, Giessen, Germany, 110-120.

Ladd, C. C , Moh, Z. C., and Lambe, T. W. (1960). "Recent soil-lime research at the Massachusetts Institute of Technology." Highway Res. Board Bull. 262, 64-84.

Lambe, T. W., Michaels, A. S., and Moh, Z. C. (1960). "Improvement of soil cement with alkali metal compounds." Highway Res. Board Bull. 241, 67-103.

Lukas, W. (1975). "Betonzerstorung durch S03-angriff unter bildung von thaumasit und woodfordit." Cement Conor. Res., 5, 503-517.

Mehra, S. R., Chadda, L. R., and Kapur, R. N. (1955). "Role of detrimental salts in soil stabilization with and without cement I. The effect of sodium sulfate." Indian Conor. J., 29, 336-337.

Mifflin, M. D., and Wheat, M. M. (1979). "Pluvial lakes and estimated pluvial climates of Nevada." Nevada Bureau Mines Geol. Bull. 94, 7-8.

Mitchell, J. K. (1986). "Practical problems from surprising soil behavior." J. Geotech. Engrg. Div., ASCE, 112(3), 259-289.

Sherwood, P. T. (1958). "Effect of sulfates on cement-stabilized clay." Highway Res. Board Bull. 198, 45-54.

Sherwood, P. T. (1962). "Effect of sulfates on cement and lime treated soils." Highway Res. Board Bull. 353, 98-107.

Thompson, M. R. (1966). "Lime reactivity of Illinois soils." / . Soil Mech. Found. Div., ASCE, 92(5), 67-92.

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