Stabilized Fly Ash Base

Transcription

1 8 TRANSPORTATION RESEARCH RECORD 144 Performane Evaluation of a Cement Stabilied Fly Ash Base. H. GRAY, E. TONS, AND T. R. THIRUVENGADAM The performane of a ompated, aggregate-free, ement-stabilied fly ash base beneath a highay shoulder is desribed. A 2.7-m (9-ft) ide, m (1,5-ft) long fly ash test setion as plaed on both sides of State Highay M-54 near Grand Blan, Mihigan, in May The test setion base as onstruted using a high arbon, Class F fly ash that as stabilied ith 12 perent by eight portland ement. A number of tests ere used to monitor and evaluate the performane of the base. These tests inluded (a) Clegg impat readings on the ompated surfae of the fly ash, (b) moisture-density and unonfined ompression tests on ore samples, () elevation and vertial defletion measurements on the pavement, (d) edge break surveys, (e) a rak pattern analysis, and (n leahate analyses. The results of monitoring and evaluation tests onduted to date sho that in general the fly ash test setion has held up reasonably ell here design and ompation speifiations have been met. No idespread major problems (e.g., rumbling, disintegration, exessive heave or settlement of the pavement) have ourred during the 5-year period sine onstrution in these areas. Problems ith heave and raking of the pavement on top of the fly ash have so far been restrited to a fe loal areas. Heaving and raking in these areas our mainly during the inter and are assoiated ith frost effets. The presene of a joint that as purposely ut in the surfae earing ourse beteen the road shoulder and traveled ay on one side of the highay greatly exaerbated aruate raking that developed next to the joint. The Mihigan Department of Transportation, hih is responsible for km (9,6 mi) of federal and state trunkline highays, expets to omplete 225 km (14 mi) of road shoulder replaement annually. This shoulder replaement program ould use 3, tons of fly ash annually, or 1/8 of the total annual prodution in southern Mihigan's loer peninsula. Use of fly ash in a roaday base ourse requires areful design of a fly ash-ement mix suitable for Mihigan's demanding traffi loads and inter limate. Previous researh (J) has shon that ra fly ash exhibits unsatisfatory freee-tha harateristis, thus requiring stabiliation ith ement. Results of tests on a high-arbon fly ash (2) shoed that a ement ontent of 12 perent by dry eight of solids ould be suffiient to meet strength-durability riteria and minimie frost heaving. Design thikness alulations indiated that the required thikness of a ompated and trimmed base ourse as 2.54 m (1 in.). This design as based on ahieving an in-plae density of 98 perent of the maximum dry density based on the Modified Protor test. Protools (2) ere also developed for mixing, handling, and plaement of the fly ash mixture in the field. The laboratory ork indiated the importane of thorough mixing of ement and fly ash to ahieve uniform properties and satisfatory performane. Laboratory findings shoed that uniform ement D. H. Gray and E. Tons, Department of Civil Engineering, University of Mihigan, Ann Arbor, Mih T. R. Thiruvengadam, Consumers Poer Co., Jakson, Mih dispersion ould be ahieved by pre-blending a dry 4:6 fly ash and ement mixture. This mixture as then proportioned ith fly ash and ater to produe the desired base ourse produt ontaining 12 perent ement by eight of dry solids. A full-sale field trial onfirmed the need for this pre-blended onentrate to ahieve uniform dispersion. Fly ash used for this projet as a high-arbon, Class F, dry hopper ash from Consumers Poer Company's D.E. Kam Plan loated at Essexville, Mihigan, 8.45 km (5 mi) north of the demonstration site. The site seleted for the demonstration as a ne onstrution projet: a four-lane state highay onneting 1-75 diretly south of the ity of Flint. The trial road shoulder base ourse as plaed on both sides of m- (1,5-ft-) long setion of the four-lane highay. The ement-stabilied fly ash base ourse, mixed on-site using pre-blended 4:6 fly ash-ement mix and onditioned fly ash, as plaed and ompated in to lifts. Control of ompation as ritial. The first passes on eah lift ere made ith a 1- ton steel heel roller ithout vibration, hereas subsequent passes used vibration. A rubber tire roller as then used to lose all raks, thereby produing a smooth base surfae. The final dimension of the base ourse as 2.54 m (1 in.) thik by 2.74 m (9 ft) ide. The ompated finished surfae as proteted from drying and rain by appliation of a sprayed asphalt emulsion. An asphalt onrete leveling ourse as plaed over the fly ash shoulder 7 days after onstrution and the surfae or earing ourse as ompleted 3 days later. The finished M-54 highay as opened to traffi in June Details about the field onstrution of the fly ash base ourse and preliminary findings are presented elsehere (2-4). A 5-year post-onstrution monitoring and testing program as implemented to evaluate the performane of the base ourse under operating onditions. These tests inluded 1. Moisture-density and unonfined ompression tests on ore samples from the fly ash base, 2. Elevation and vertial defletion measurements on the pavement, 3. Edge break surveys, 4. Crak pattern analysis, and 6. Analysis of samples from both groundater monitoring ells and a leahate olletion system installed beneath the fly ash test setion. The results and findings of this monitoring program are summaried in the folloing setions. TEST SECTION DESCRIPTION The Mihigan Department of Transportation (MDOT) partiipated atively in the site seletion and onstrution planning phases of

2 Gray et al. 9 FLY ASH TEST SECTION REID ROAD SJA"DONNOS :t----t'"""'"""ll----!!---ll----!!--~l"o""'l ~-- CONTROL SECTION L GRAND BLANC ROAD M-54 DORT HWY o-- (ORA WING NOT TO SCALE) FIGURE 1 Shemati diagram shoing plan and layout of road shoulder test. the projet. A ne highay onstrution projet on Mihigan Route M-54 emerged as the most promising loation from a number of andidate sites onsidered. A m- (1,5-ft-) long setion beteen Grand Blan Road and Reid Road (see Figure 1) as seleted for a trial fly ash shoulder. The ombined length of the fly ash shoulder on both sides totaled 915 m (3, ft). An additional 915-m (3,-ft) length of shoulder adjaent to the fly ash test setion as established as a ontrol setion. At present the traffi is of medium intensity, but is expeted to gro ith time. To aommodate the traffi, the asphalt pavement as made 14.6 m (48 ft) ide ith to 3.66-m (12-ft) lanes in eah diretion. The total idth of the roaday, inluding the 2.7 m (9-ft) shoulders on eah side, as 2.1 m (66 ft). The required thikness of the fly ash base as alulated using the Amerian Assoiation of State Highay and Transportation Offiials (AASHTO) design equation. The soil support value as assumed to be 3, hih is equivalent to a California Bearing Ratio of about 3, and the minimum required sand subbase thikness as taken as 3.4 m (12 in.). The final thikness of the ompated ement stabilied fly ash base ourse for the M-54 shoulder as alulated to be about 25.4 m (1 in.). This design thikness as reasonable, based on previous experiene. The adjaent ontrol setion had a standard Mihigan Class A design ith a 12.7-m (5-in.) asphalt onrete base, 1.1-m (4-in.) aggregate base and a 6.3-m- to 7.6-m- (2Sto 3-in.-) thik asphalt surfae supported on a sand subbase. A stratigraphi profile of both the fly ash and ontrol setion is shon in Figure 2. PERFORMANCE EVALUATION TESTS Field Moisture-Density Tests The ompated, ement-stabilied fly ash base ourse as onstruted during the period May 14 to 19, MDOT drilled 1.1-m- (4-in.-) diam ore samples from the fly ash test setion on August 1987, Deember 1987, and April These dates orrespond approximately to 9-, 18-, and 27-day test samples. The ores ere sealed in plasti bags and transferred to the laboratory for. determination of their moisture ontent, density, and unonfined ompressive strength. I I FLY ASH SECTION CONTROL SECTION FIGURE 2 Stratigraphi profile of fly ash test setion and ontrol setion.

3 1 TRANSPORTATION RESEARCH RECORD 144 Samples ere obtained from 9 to 1 oring loations on either side of the highay. The loations ere seleted randomly, ith the exeption of to loations that ere ored eah time in the same viinity of stations ith knon high and lo Clegg impat readings, respetively; namely, Stations 26+ (est) and 15 + (east), respetively. Test speimens ere reovered from both the top and bottom portions of the fly ash field ores. Moisture ontents for the 9-day ores samples ranged from 26 to 44 perent (dry eight basis) ith an average value of 33.5 perent. With one or to possible exeptions, all of the ores samples exhibited moisture ontents ell in exess of the Modified Protor optimum moisture determined in the laboratory investigation to lie beteen 24 and 26 perent of dry eight for ements ontents ranging from to 15 perent dry eight. Moisture ontents measured on the 18-day (6-month) ores ere even higher, ranging from 29 to 53 perent ith an average value of 36.5 perent. Dry densities for the 9- and 18-day samples averaged 11. and 11.5 kn/u.m (69 and 72 pt) respetively. These values lie belo the target density of 12.4 kn/u.m (78 pt), and orrespond to a relative ompation of slightly under 9 perent based on the modified AASHTO test. These dry densities ere also loer than the dry densities measured on the ompated base using a nulear gauge [average = 12.8 kn/u.m (8.2 pt), range :::: 11.8 to 14.3 kn/u.m (74.3 to 89.7 pt)]. Cement ontents in the field ores varied from 2.8 to 19.1 perent dry eight. The target ement ontent as 12 perent. This variation is onsistent ith ement ontents measured on pugmill samples during onstrution, hih varied from 5.9 to 18.5 perent by eight ith an average of 11 perent. Unonfined ompression tests ere run on 5.1-m (2-in.) ube samples that ere trimmed from the ores by dry saing. The average unonfined ompressive strength at 9 and 18 days as 3369 and 2825 kpa (489 and 41 psi) respetively. These averages exeed the minimum required 7-day strength of 2756 kpa (4 psi). The average unonfined ompressive strength at 27 days as unreliable beause the sample population as too small and inluded samples that ere either shattered or too soft, and hene not usable. Although the average strength met or exeeded the minimum value required, there as onsiderable variation among samples. Strengths ranged from less than 689 kpa to about 689 kpa (1 to 1, psi). Only about half the samples tested met or exeeded the minimum target strength of 2756 kpa ( 4 psi). No additional strength gain as observed after 9 days of elapsed time. Correlations ere examined among unonfined ompressive strength and other soil properties suh as density, moisture ontent, ement ontent, and Clegg impat readings. The relationship beteen unonfined ompressive strength versus dry density at 18 days is plotted in Figure 3. Fairly good orrelations ere obtained, ith the strength inreasing ith dry density in an exponential fashion as shon. The oeffiient of orrelation for 9- and 18-day strength versus dry density as.61 and.95, respetively. Clegg Impat Tests Impat tests ere run on the ompated surfae of the fly ash base ourse using a Clegg impat tester. This devie reords the deeleration of a standard eight dropped from a fixed distane onto (ii 1 Q. :I:... 8 '"' a:... 6 a.: :::E 4 = <( Q 2 y = o o (.449x) R = o.95 o, DRY DENSITY - PCF FIGURE 3 Unonfined ompressive strength versus dry density: 18-day samples (1 psi = 6.89 kpa). the base ourse surfae. The Clegg "impat" reading obtained in this ay an be orrelated (5) ith the modulus and strength of a ompated base. This test as run every 15.2 m (5 ft) along the surfae and at different elapsed times up to 4 days folloing ompation. The test is nondestrutive, fast, and provides a good reord of the spatial variation and development of strength or stiffness ith time. Typial results of the Clegg impat test along the length of the fly ash test setion on the est side of the highay are shon in Figure 4. Strength gains tended to be higher on the south end of the projet. In addition, east side readings ere generally higher than the orresponding est side readings on the opposite side of the highay. A pronouned dip in Clegg impat values as reorded in the viinity of Stations 26+ to 27+ on both sides of the highay. These lo values indiated a eaker base at this loation and helped to identify an area that required lose srutiny as the study progressed. There are several possible reasons for the lo values at Stations 26+ to 27+. Lo ement ontents are a possible explanation; hoever, the oinidene of lo values at exatly the same loation but on opposite sides of the highay suggest other explanations as ell. The highay traverses a topographi lo at this loation. A small amount of standing ater as observed in the road shoulder trenh before plaement of the fly ash base. This moisture may have adversely affeted ompation of the fly ash and the ement set, thus explaining a muh loer rate of strength inrease relative to other loations. Edge Break Survey Another indiator of stability and durability of the road shoulder is the extent of "edge breaking" along the outside edge of the pavement. The extent of edge breaking as assessed along the fly ash and ontrol setions, respetively. Edge breaking as lassified as either major or minor. "Major" refers to rumbling, disintegration, or exessive settlement at the edge of the pavement, hereas "minor" refers to a slight raking or defletion at the edge. 9

4 Gray et al. 11 The amount of edge breaking in these to ategories as paed off along the entire length of the ontrol and fly ash setions in June of The results of the survey shoed that edge breaking as not as extensive along the fly ash test setion ompared ith the ontrol setion. This finding as true for both ategories of severity. It is interesting to note the pronouned ontrast in major edge breaking along the fly ash test setion beteen the east side (15 paes) versus the est side (19 paes). This differene in resistane to serious disintegration and rumbling at the edge is also onsistent ith the findings of the Clegg impat tests (see Figure 4) on the surfae and unonfined ompression tests on ore samples. In general, impat readings ere muh higher on the east than on the est side. The same also holds true for unonfined ompression test results. Vertial Displaement Measurements During the inter of 1988 to 1989, a visual inspetion of the fly ash road shoulder in Marh appeared to indiate that some vertial heave had ourred at ertain loations. The heave as manifest most visibly in the form of a loalied, slightly raised ron or mound ith an assoiated raking pattern on the est side road shoulder in the viinity of Station 25+. A slight amount of apparent differential heave also took plae over some distane along the joint that had been ut in the pavement beteen the traveled ay and the shoulder. This vertial heave subsided, hoever, to a very small residual value by May In order to study and doument this suspeted vertial heave more thoroughly, to series of vertial displaement measurements ere initiated in the fall of The first onsisted of measuring the vertial separation distane or offset every 7.6 m (25 ft) along the joint. The seond onsisted of running a line of levels every 3.5 m (1 ft) don the middle of the fly ash test setion road shoulder on either side of the highay. Both these measurements ere repeated periodially during sueeding years to see if frost heaving as ourring, and if so, its magnitude and extent. Results of these measurements sho that a onsiderable amount of vertial movement (or heave) has taken plae along some por- e 7 g JOINT OFFSET 2/119 LL LL J : 3 j:: a: (INITIAL READING) STATION FIGURE 5 Vertial offsets measured in 199 at joint in pavement beteen shoulder and traveled ay. tions of the shoulder. Vertial defletions ranged as high as 6.8 m (2.7 in.) along the joint in some loations after 5 years. The temporal and spatial distribution of vertial heave or offset along the joint are shon in Figures 5 and 6. Compared in Figure 5 are the vertial offsets measured in 199, some 3 years after onstrution, and ompared in Figure 6 are offsets measured in 1992, approximately 5 years later. Vertial offsets ere measured during the inter and in the late spring or early summer. The residual vertial heave along the joint measured in late spring or early summer as approximately half that reorded in February during the height of the inter hen the ie lenses in the base ourse ere fully developed. Vertial heaving or deformation inreased progressively ith time, as an be seen by omparing vertial defletions in 199 (Figure 5) versus 1992 (Figure 6). The vertial offset measurements at the joint are a good surrogate for heave-indued elevation hanges. This equivaleny as demonstrated by a strong orrespondene beteen vertial offset at the joint versus elevation hange along the enterline of the shoulder. East and est side elevation hanges along the enter WESTSIDE: m INITIAL RD 8 ::: DAY1..J 7 : DAY DAY4 : 5 ~ ~ 4 Cl Cl 3..J 2 1 e 7 g LL LL 4..J : 3 j:: a: 2 1 JOINT OFFSET 2/14/ ! STATION NUMBER STATION FIGURE 4 Clegg impat readings at different elapsed times and loations along the est-side shoulder. FIGURE 6 Vertial offsets measured in 1992 at joint in pavement beteen shoulder and traveled ay.

5 12 TRANSPORTATION RESEARCH RECORD 144 of the shoulder measured on February 1, 199, are ompared in Figure 7. The to urves are virtual mirror images, ith the exeption that vertial heave tends to be greater (approximately 2 times) beteen stations 15+ to 24+ on the est side ompared ith the east side. This finding is partiularly signifiant beause in spite of the generally higher heave on the est side, there is little or no sign of raking or distress here the shoulder abuts the roaday. In other ords, it appears that utting a jointthe proedure adopted on the east side-has ontributed largely to the subsequent vertial offset measured along the joint and assoiated raking and deformation problems (disussed in the next setion). The presene of the joint has alloed runoff from the roaday to flo don into the subbase. The problem is exaerbated hen the shoulder heaves (relative to the roaday) and presents a vertial fae that diverts runoff vertially donard diretly into the base. This infiltrating ater beomes a soure of ater for ie lense formation in the underlying base during the inter. Vertial displaement or heave is generally loer from Stations 15+ to 24+ ompared ith the area from Stations 24+ to 3+. The stabilied base in the former area tended to have higher ement ontents and ompated ;lensities. Analysis of field ores revealed relatively high ement ontents in this area (e.g., 19.1 perent at Station and 15.4 perent at Station 21 +, respetively). From Stations 25+ to 28+, the measured frost heave as virtually the same, but muh higher for both shoulders ompared ith the area beteen Stations 15 + to 24+. This larger frost heave orrelated ith lo ement ontents in the base (e.g., 2.8 perent at east side Station 26+7). This is also the same area in hih relatively lo Clegg impat readings ere reorded immediately after onstrution, as shon previously in Figure 4. Crak Pattern Survey Periodi inspetion visits ere made to the site folloing onstrution of the road shoulder base in May One of the main purposes of these visits as to look for any rak development in the asphalt pavement ap. The first rak as observed on April 13, 1988, in the viinity of Station 25+1 on the est side shoul- :E :E 4 3 a: LL 2 LL j:: 1 <( -' STATION NUMBER FIGURE 7 Comparison beteen elevation hanges along enterline of east and est shoulders, respetively. der. The raking onsisted of to narro approximately parallel raks ith a ombined length of 1.82 m (6 ft) at a distane of about.91 m (3 ft) from the traveled ay pavement. No other strutural raks ere observed in either shoulder at that time. Four major types of raking patterns have subsequently developed ill the pavement overlying the :1'Y ash base ourse during the 5-year monitoring period. These ategories inlude the folloing: Type I: Aruate-shaped raks along the joint beteen the shoulder and traveled ay resulting from traffi passing over a raised and eakened portion of the shoulder; Type II: General, random raking assoiated ith areas of under ompation or insuffiient ement ontent in the fly ash; Type III: Craks originating in and propagating aay from ore holes in the pavement; and Type IV: Craks propagating aay from joint omers. Crak groth tended to progress ith time. The harater and development of aruate (Type I) raking along the joint at Station on the east side is shon in Figure 8. The development of random (Type II) raking at Station 25+1 on the est side is shon shematially in Figure 9. The rak groth versus time at Station (est side) is illustrated in Figure 1. The temporal pattern of rak development at Station 25+1 (est side) shon in Figure 1 indiates that rak groth is most rapid during the inter months and is assoiated ith frost heaving in the base. The density and strength of ores from this area are generally muh loer than those at other loations. The ement ontent of a ore from this loation as also quite lo (2.8 perent by eight). Visual evidene during the inter indiated that loalied heaving as ourring here as ell. Sno plo striations in the pavement surfae ere learly visible here as a result of a slightly raised "mound" in the pavement surfae. Influene of Pavement Joint The traveled ay earing or surfae ourse as ontinued and extended aross the shoulder on the est side. A different proedure as folloed, hoever, on the east side. Here a.6-m (114-in.-) ide and 3.8-m- (1 i/2-in.-) deep groove as ut and sealed right above the onstrution joint here the traveled ay pavement base and the fly ash base (shoulder base) meet. This proedure (grooving) is often speified hen to different, adjaent base materials are used. This ay the shoulder an move somehat independently as a result of material and other influenes. Unfortunately, the presene of this joint also ontributed to loalied (Type I) raking problems observed in the east-side shoulder. Pronouned aruate (Type I) raks developed adjaent to the joint on the east side at Stations 25+3, 26+9, 27+2, and The raks at these loations are resent-shaped features oriented parallel to the joint. These raks oinide ith areas of maximum joint offset (see Figures 7 and 8) and generally loer strengths (see Figure 4). The raks most likely developed in response to traffi loads during the spring tha hen the shoulder as raised in a sharp vertial disontinuity at the joint. This problem as not observed in the est-side shoulder in spite of generally higher elevation hanges along the est side (see Figure 9). This finding indiates that it ould have been advisable not to ut

6 Gray et al. 13 a joint in the pavement. The presene of the joint not only tended to divert run-off ater donard into the fly ash base, it also ontributed to the raking problems observed along the east-side shoulder. In general, any kind of penetration through the pavement (e.g., grooves, joints, or holes) adversely affeted the pavement. Craks originated in and propagated aay from ore holes and joint orners as ell. The deleterious effet of the ore holes as partiularly pronouned; these holes ere often the lous of major transverse raks in the fly ash pavement. ENVIRONMENTAL MONITORING Both laboratory leahing tests and field monitoring of ater quality in observation ells and leahate olletion stations ere implemented as part of the demonstration projet. The purpose of + ROADSIDE WEST SIDE CRACK #3 STATION /22/9 SHOULDER l 1 o 1 19' 5 1/4" ROADWAY 25 FIGURE 9 Shemati diagram of raking pattern measured May 22, 199, in viinity of Station on est-side shoulder. 25 (a) R A K 2 WINTER #1 WINTER #2 L E N G T H (b) FIGURE 8 Aruate raking along joint at Station 25+3 (east side) shoing progression of raking ith time: (a) July 19, 1991, and (b) May 5, I N H E s 5 \STRuTEo MAY l....,. JMMJ S NJ M MJ SN JMMJ S NJ r , , FIGURE 1 Crak groth ith time at est-side Station (1 in = 25.4 mm).

7 14 TRANSPORTATION RESEARCH RECORD 144 the laboratory leahing studies as to asertain the potential for ontaminant generation by emented fly ash under extreme leahing onditions. The field monitoring system as installed in order to determine the impat, if any, of the emented fly ash base ourse on ater quality. Laboratory Leahing Studies In order to simulate the "orst ase" ondition, to relatively extreme leahing protools ere folloed. In the first, to samples of emented fly ash ere subjeted to the leahing proedures established by the Environmental Protetion Ageny (EPA) under the Resoure Conservation and Reovery At (EPA Method 131). The analytial results obtained on the resulting leahate are summaried in Table 1. The reommended limits established under the Safe Drinking Water At (SDWA) are inluded in this table for referene purposes. In general, the onentration of metal ions and in the leahates from the ement-fly ash samples are belo the reommended limits established by the SDWA. Only hromium and selenium slightly exeeded the SDWA standards. Even these exhibit less than an order-of-magnitude onentration above the standards. Field Monitoring The laboratory studies indiated the potential for ertain metal ions being released from pulveried, emented fly ash. Thus, partiular emphasis as plaed in the field monitoring study on metals analysis. Groundater monitoring ells ere installed adjaent to the roaday in both the fly ash and ontrol setions. Leahate samples ere taken from sub drain pipes, plaed diretly beneath the fly ash base, hih disharged into a man hole sampling box. The leahate samples ere olleted by positioning a arefully pre-leaned stainless steel buket at the pipe opening and athing the leahate as it floed from the pipe. Results from the groundater monitoring ells are unremarkable. Insuffiient time has elapsed for any leahate to reah the TABLE 1 RCRA Leahing Test Results on Cement-Stabilied Fly Ash COMPOUND LEACHATE CONC - MG/LITER SAMPLE"A" SAMPLE "B" SOWA LIMITS Arseni Barium Cadmium <.2 <.2.1 Copper <.2 <.2 1. Chromium Lead <.2 <.2.5 Merury <.2 <.2.2 Selenium Silver <.1 <.1.5 Zin <.2 <.2 5. Cyanide <.5 <.5.2 ~ 1. Fly ash stabilied ith 1% by t. Portland ement 2. Molding /: "A" = 2 %; "B" = 28 % (dry t. basis), ,.15 ~.1 s.5 Ci. FIGURE 11 arseni. LEACHATE DATA (NORTH COLLECTION STATION) /88 4/89 6/ /9 1/911/91 COLLECTION DATE Sub drain leahate test results for dissolved ells and no signifiant differenes have been observed to date beteen the bakground and test setion ells. Leahate samples from the sub drains ere analyed for a number of toxi ions inluding selenium, arseni, hromium, and admium. Typial results are shon in Figure 11, here dissolved onentrations are plotted versus olletion date for arseni. Results ere plotted only for the north olletion station, hih reeives leahate from the fly ash test setion. Leahate onentrations for dissolved admium and hromium have remained relatively onstant over time. On the other hand, selenium and arseni onentrations have tended to flutuate over time. The latter trend is partiularly notieable in the ase of dissolved arseni, as shon in Figure 11. Conentrations peaked in June 1989 and again in November It is important to note, hoever, that the maximum dissolved arseni onentration in the leahate is still ell belo the Safe Drinking Water Standards (SWDS), as shon in the folloing table: Parameter Arseni (As) Cadmium (Cd) Chromium (Cr) Selenium (Se) Sub Drain Leahate (mg/liter) a.ms SDWS (mg/liter) Only a limited number of usable samples have been obtained from the south olletion station, hih servies the ontrol setion. A omparison of leahate onentrations (Arseni, Cadmium, Chromium, and Selenium) beteen the fly ash test setion and ontrol setions on three diff~rent dates (7/9, 1/9, and 11/91), reveal little differene in onentrations beteen the to setions. Arseni and selenium leahate onentrations tended to be slightly higher in the fly ash test setion and this differene appeared to inrease ith the passage of time. These undiluted onentrations are nevertheless still ithin or near SDWS. CONCLUSIONS Post-onstrution monitoring of the fly ash-ement base has shon that if the fly ash is mixed orretly ith the speified amount of ement and ompated to the speified density, it ill

8 Gray et al. 15 perform ell. Problems ith surfae heave and pavement raking have so far been restrited to a fe loal areas. These loalied problems appear to be the result of lo density and strength in ombination ith a lo ement ontent in the fly ash base and a thin asphalt ap. A groove or joint should not be ut in an asphalt pavement beteen the shoulder and traveled ay. The joint ultimately interepts and diverts runoff and meltater into the underlying fly ash base ith serious onsequenes. The presene of a joint as the main ause of severe heave and raking in setions of pavement on the east side. Monitoring of groundater ell samples and sub drain leahate has not revealed any adverse impaet of the fly ash-ement base on ater quality to date. ACKNOWLEDGMENTS The projet as sponsored by the Eletri Poer Researh Institute and Consumers Poer Company. The Mihigan Department of Transportation funded the inremental ost of fly ash aquisition and plaement. The ontrators for the researh and development ork and follo-up environmental monitoiing ere the University of Mihigan, Environmental Control Corporation of Mihigan, and Soil and Materials Engineers. The authors gratefully aknoledge the assistane of individuals at Consumers Poer Company, Mihigan Department of Natural Resoures, GAi Consultants, Mihigan Ash Sales, Dundee Cement Company, and. the paving onstrution ontrator, Thompson-MCully Company. REFERENCES l. Gray, D. H., and Y. K. Lin, Engineering Properties of Compated Fly Ash. Journal of Soil Mehanis and Foundations Division (ASCE), Vol. 98, SM4, 1972, pp Berry, W. H., D. H. Gray, U. W. Stoll, and E. Tons. Use of Coal Ash in Highay Constrution: Mihigan Demonstration Projet. Interim Report. Eletri Poer Researh Institute, EPRI GS-6155, Palo Alto, Calif., Gray, D. H., E. Tons, and W. H. Berry. Design and Constrution of a Cement-Stabilied Coal Ash Base Course in a Highay Pavement. In Pro., International Symposium on Utiliation of Coal Mining Wastes, Glasgo, Sotland, Sept. 2-5, Gray, D. H., E. Tons, and W. H. Berry. Post-Constrution Monitoring and Performane Evaluation of a Cement-Stabilied Fly Ash Base. In Pro., 9th International Ash Use Symposium, Vol. 2: Stabiliation and Aquati Uses, EPRI GS~7162, Jan. 1991, pp. 45: Clegg; B. An Impat Testing Devie for In Situ Base Course Evaluation, In Pro., 8th Australian Road Researh Board Conferene, Perth, Australia, Vol. 8, No. 8, 1976, pp. 1-6.