LIME STABILIZATION OF ORGANIC SOILS

Transcription

1 LIME STABILIZATION OF ORGANIC SOILS Ara Arman and George A. Mnfakh, Loisiana State University The potentials of improving the engineering characteristics of organic soils fond in Loisiana ere stdied. A comprehensive literatre srvey on soil-lime reactions and the properties of organic soils as made. Laboratory stdies of approximately 1, 1 samples of inorganic and organic soils shoed that the engineering characteristics, sch as plasticity and strength, are improved by the addition of lime. At lo organic content, lime effects a redction in the plasticity index, ith the minimm occrring at abot 4 percent lime content. At higher organic content, the plasticity index contines to decrease ith increasing lime content. The dry density and optimm moistre content of soil-lime mixtres vary ith the amont of lime added to the soil, depending on the amont of fine clays. Changes in ph of soil-lime mixtres shoed that increased levels of lime treatment ere able to netralie the acidity of organic matter in the soil and initiate the soil-lime reaction. Previos stdies at the Loisiana State University have shon that the organic content of the majority of soils in Loisiana is approximately 2 percent. Based on this stdy it is conclded that the addition of lime improves the engineering characteristics of the soils stdied. REACTIONS beteen lime and soils have been stdied by several researchers (1, 2). Diamond and Kinter (3) detected 2 reaction stages: (a) a rapid process dring h ich the plastic properties-improve, bt little permanent strength develops; and (b) a later slo process of strength development ith the formation of cementitios prodcts. Other orkers (4, 5, 6) have shon that the addition of lime to a plastic soil reslts in a redction of the-plasticity index (Pl). Both the plastic limit (PL) and the liqid limit (LL) of the soil are affected. Usally, an immediate increase in the plastic limit is observed after the addition of lime. This increase varies directly ith the lime content p to some limiting amont, referred to as the "lime fixation point" (7 ). Hoever, lime has different effects on the liqid limit; some athors report decreases in this property after addition of lime (5, 6 ), and others report sbstantial increases (8 ). Other orkers ( 4, 9) have fond-both increases and decreases that depend on the type of the soil. Anday {lo) fond that strength development in soil-lime systems is comparatively slo, characteristically reqiring several eeks to many months at normal temperatres. Thompson, ho stdied the elastic properties of soil-lime mixtres, reported that lime treatment improved the shear strength (11). Soils can be divided into 2 major grops: organic and inorganic. Organic soil is defined as "any soil containing a sfficient amont of organic matter to inflence its engineering properties" (12). The organic matter is derived from the decay of plants. Clare and Pollard (13) indicate that organic materials retard the hardening of soilcement mixtres by retaining the calcim ions liberated dring the hydrolysis of the calcim silicates and alminates in the cement. On the basis of that mechanism, e shold be able to relate the "activity" of organic material to its capacity to adsorb calcim ions from soltions of high ph. Ths, soil-lime mixtres shold also be affected by the presence of organic matter. Thompson has shon that organic matter adversely affects the poolanic reaction (14). That inflence can logically be attribted to the retardation of the strength-prodcing Sponsored by Committee on Lime and Lime-Fly Ash Stabiliation. 37

2 38 poolanic reaction (15). Organic matter has ahigh base-exchange capacity (16). When lime is added to organic soil, some of the Ca++ ions are sed to satisfy the exchange capacity of the organic matter, ths redcing the nmber of calcim ions available for the poolanic reaction. Maclean and Sherood (17), in their stdy of the effect of organic materials on soil-cement reactions, indicated that some hydroxyqinine componds ere the active organic components that interfered ith the reactions by forming complexes ith the metal ions (sch as calcim). Similarly, the formation of sch complexes in soil-lime mixtres old redce the qantity of available calcim and affect the longterm poolanic reaction. A stdy of cementitios reactions beteen soil and lime and the effect of organic matter on these reactions as condcted by Ferrell (2) sing the specimens formed from soils investigated in this stdy. His reslts sho that organic soil-lime mixtres prodce cementitios prodcts similar to those reported for inorganic soil-lime reactions and that the organic matter does not appear to block the reaction that reslts in increase in the soil strength. TESTING METHODS AND SPECIMENS The testing schedle for this stdy as prepared for investigating the effects of soillime reactions on the engineering characteristics of the mixtre and the effect of organic materials on these reactions. The short-term reaction as stdied by its effect on the Atterberg limits and the long-term poolanic reaction as stdied by the change in the compressive strength. Three types of cylindrical specimens er e fabricated: mineral, nonorganic soils; organic soils obtained from natral deposits (ithot alteration of organic content); and natral organic and mineral soils (organic content predetermined throgh the varying of mineral content). Clays and silty clays (Table 1) ere sed becase clay minerals are the major components largely controlling the physical properties of stabilied soil (18). The organic soils ere obtained from different areas in Loisiana. Loose samples ere oven-dried and then crshed in a disk crsher to pass a No. 1 sieve (ASTM Designation D ). Part of the same sample as plveried to pass the No. 4 sieve for determining the Atterberg limits and the ph. The organic content as fond from dry combstion, 45 C for 6 hors (12). The plasticity of organic soil-lime mixtres as determined by placing samples ith different lime and moistre contents in plastic bags (not airtight) and cring them in a 1 percent hmidity room at 75 F for periods of 48 hors, 7 days, and 28 days. Liqid and plastic limit tests ere then performed on the cred samples. The effect of lime on the density and optimm moistre content as stdied by se of the standard compaction test (AASHO T-99). After the mixtres of organic soils, lime, and ater had been cred in the 1 percent hmidity room for varying lengths of time, the standard compaction test as performed. Changes in strength ere determined from nconfined compression tests of mixtres ithot lime and ith different lime contents. To types of specimens ere sed: one ith sfficient ater to approximate the liqid limit and one ith ater content eqal to the optimm moistre content (AASHO T-99). These 2 moistre contents ere chosen for obtaining the strength of the soil both in the eakest and in field-compacted conditions (12). Also, approximating the liqid limit alloed the mixing of very homogeneos samples. Immediately after mixing, the soil-lime-ater mixtre as placed in alminm cylindrical molds (2) ith a diameter of 2.5 in. (This diameter allos the soil samples to fit in the nconfined compression and triaxial eqipment ithot being trimmed.) Then samples ere alloed to consolidate in the mold nder a 4-lb srcharge approximating the average natral overbrden pressre of 8.34 psi (12). After the 48-hor cring and "'"'"C'll"\l;M,,...f.;n,... V'lon...,..;nA 4-hn. ""1,...,.. nn....,...,...,f...,,,a,...,:1 4-',..n.h,... n'h,...-\...,......,.....,..1.,..,..,...;1.;..,......,1,..,,,...;,.....,...,,......,...,......,....t'...,......,..,.,......, -....t'......,..., ''....._ ,... '-&&..., "'.1..1.c..t..a..i a., l:"....,...'-''-''" p &.v bags, and kept in the moistre room for the rest of the cring period. Separate samples ere cred for testing at 48 hors, 7 days, and 28 days. Varios mixtres of soil and lime ere also tested triaxially. Undrained qick tests ere performed on samples ith optimm moistre content. After cring periods of 7 and 28 days, the samples ere tested nder confining pressres ofo, 5, 1, 15, and 2psi.

3 To determine the amont of lime needed to stabilie a soil reqires a determination of the amont that ill satisfy the reactions beteen calcim and the organic and inorganic componds and the calcim absorption of the hydroxyqinines in organic soils as reported by Maclean and Sherood (17). (Soil stabiliation, in general, is defined as the changing of any physical or chemical property or both of a soil to improve its engineering performance and ths render it sitable for a particlar se.) Hilt and Davidson (? ) sed the plastic limit to determine the "lime fixation point." This method is satisfactory bt time-consming. The ph test devised by Eades and Grim (19) as sed in this stdy to determine the lime content for soil stabiliation becase it is qicker. Eades and Grim explain that "since lime is very alkaline (ph-12.4 at 25 C), the silicates, especially the clays ill be attacked, freeing silica and almina ith hich calcim reacts to form calcim silicates as long as a high alkaline condition is maintained" (19). Becase a high alkaline condition is reqired for the formation of calcim silicates,- the least amont of lime that ill prodce a ph of 12.4 is the amont reqired to stabilie the soil. The ph readings ere made on the soil-lime mixtres after cring periods of 1 hor and 7 days. All tests in the stdy ere performed by sing a minimm of 2 identical specimens. Three identical specimens ere sed for the nconfined compression, Atterberg limits, and ph determinations. Whenever the test reslts of a set of specimens varied more than ±3 percent, the tests ere repeated. 39 ) Atterberg Limits TEST RESULTS The effect of lime on the Atterberg limits is illstrated by plots of the liqid and plastic limits of organic soil-lime mixtres and the Ca(OHh content for different cring periods. The change in Atterberg limits of a typical soil (M-16, silty clay, ith 22. percent organic content) is shon in Figre 1. The net effect as a decrease in the plasticity index (Table 2). [Within a period of 48 hors, the PI decreased from 22 to 7 becase of the addition of 12 percent of Ca(OHh to soil M-16, as shon in Fig. 1.] This change in the plasticity can be attribted to the base-exchange reaction of soil andlime (the divalent calcim ions Ca++ replace the eaker nivalent ions, sch as Na+ and H+, on the clay particle srface) and the croding of the calcim ions on the srface of the clay particles, reslting in flocclation. Figre 2 shos the plasticity index and the lime content. The plasticity decreases ntil a critical point (4 percent lime) is reached. The increase in plasticity after this point can be explained by the chemical satisfaction of the reaction beteen lime and soil. Excess lime acts as a lbricant to increase the plasticity. Density and Optimm Moistre Content The changes in the maximm density and optimm moistre content of a typical organic soil (M-1) are shon in Figre 3. For this soil, the density decreased and the optimm moistre content increased as the lime content as increased. For example, the addition of 4 percent of lime redced the density of soil M-1 from 82.6 to 75. lb/ ft3 for a cring period of 7 days. Hoever, for soil M-9 (Fig. 4), an increase in the density as observed after the addition of lime. The optimm moistre content decreased. These effects are attribted primarily to flocclation and the change in gradation de to poolanic reaction that reslts in cementing fine clay particles. Figres 3 and 4 also sho that as the cring time is extended the density frther changes, ths indicating that poolanic reaction is taking place and contining. Compressive Strength Lime increases the strength of mineral and inorganic soils becase of the poolanic reaction. This increase also occrred in all the organic and inorganic soils tested in this research program. A typical increase in the nconfined compressive strength is shon in Figre 5. The changes in the nconfined compressive strength of 3 organic soil-lime mixtres, M-1, M-9, and M-16, are given in Table 3. Generally, the strength

4 Table 1. Basic soils for soil-lime mixtres. Soil Organic Liqid Plastic Desig- Content Limit Limit Plasticity nation Soil Type (percent) (percent) (percent) Index ph T-5 Silty clay T-8 Silty clay L-4 Silty loam M- Heavy clay M-1 Natral organic M-9 Natral organic M-1 Heavy clay and organic material ND' ND 7.8 M-11 Heavy clay and organic material ND ND 6.93 M-12 Heavy clay and organic material ND ND 6.77 M-16 Silty clay and organic material M-18 Silty clay and organic material 1. ND ND ND 6.88 M-2 Silty clay and organic material 2. ND ND ND 5.45 anot determined. Table 2. Atterberg limits of soil-lime mixtres M-1, M-9, and M Hor 7 Day 28 Day Ca( OH), Liqid Plastic Plas- Liqid Plastic Plas- Liqid Plastic Plas- Content Limit Limit ticity Limit Limit ticity Limit Limit ti city Soll (percent) (percent) (percent) Index (percent) (percent) Index (percent) (percent) Index M M M Figre 1. Atterberg limits of soil-lime mixtre M-16. Figre 2. Plasticity index of soil-lime mixtre M-1 at 3 cring times. 7 LL ;::- IO... ::: : ::>... "' lo ::E "' : " : l "' c >= SOIL M oc 48 Hor Cre <O _, Ca(OH} 2 CONTENT (%} 6,2 Ca(OH} 2 CONTENT (%1

5 Figre 3. Density and optimm moistre content of soil-lime mixtre M-1 at 2 cring times. Figre 4. Density and optimm moistre content of soil-lime mixtre M-9 at 3 cring times. --MAXIMUM DRY DENSITY - - OPTIMUM MOISTURE CONTENT / /.o--- / -- _.o L..l-' ' 28 Ca(OH) 2 CONTENT (3) lt ::> :;; l? ::J... 3 t:= - a :: '" '",. '" 72 SOIL M9 14 2'1. oc Figre 5. Strength of soil-lime mixtre M-16 at varying lime contents and 3 cring times. E, I....., "' => 4 Organic Content SOIL M-16 22'1. (. SDmpln pi.pcind al thlif Oplimm Mai1h r (OAhnl Ca(OH) 2 CONTENT(%) Figre 6. Strength of soil-lime mixtres M-1, M-11. and M-12 at varying lime contents and 1 cring time..,!!, I..,... "' > v; 3 :3... "' l: 2 1 ::> 7-DA.Y CURE SOIL M lo (66 '1. OC.) SOIL M ll (9.83 O.C) SOIL M ll (1263 OC) s..,,p l11 phport d ol lht ir Liqid Limi1 O '---' Co(OH) 2 CONTENT (3 ) Table 3. Unconfined compressive strength of soil-lime mixtres M-, M-1, M-9, M-16, T-8, M-18, and M-2. Ca( OH), Strength (psi) Content Soil (percent) 48 Hor 7 Day 28 Day M-" M-1" M-9' M-16' T-8' M-18' M-2' Samplos prepared ith moistre content approximating the liqid limit. bsamphts prepared ith optimm molnre content. ) Note1 % Co(OHh 1pecimen1 e loo eak ond ivld fl: O! be 1.. ted. Ca(OH) 2 CONTENT (%)

6 42 Figre 7. Strength of soil-lime mixtres T-8, M-18, and M-2 at 12 percent lime content and 2 cring times. Eo :::x: no... "' ti;!8 111 "' 14 M "- 11 1?; BO 8 tio 7ay(r1 Sampl.. miotd ilh 12'!. Co(OH) and prepared al th,ir Oplimfll Moislre (onl1nl 1 => 4 I 1 2 ORGANIC CONTENT(%) Figre 8. Failre envelopes of soil-lime mixtres M- and M-2 at varying lime contents and 1 cring time. 1 Co(OHl 2 4 _.-: o SOIL TB 3 oc 28 DAY CUU -L----- O 2 W W M W U (psi),. Figre 9. ph of soil-lime mixtre M-16 for varying lime contents and 1 cring time SOIL M-16 22,3 ( 7oy (11 Samples pr1por1d ol their Oplimm Moistr1 Co1111nt 1 o ',o---- (psi) s 12 Ca(OH) 2 CONTENT (%) " Table 4. ph of soil-lime mixtres M-, M-1, M-11, M-12, M-9, M-1, and M-16. ph With Percentages ol Ca(OHh Cring Soil Time M- 1 hor day M-1 1 hor day M-11 1 hor day M-12 1 hor day RO 11.n , M-9 1 hor day :!.!..!! e! '.! !?.d 1?. 4 n.t-.a. " n 11 nn,., 1(\.L UUU. J. 7 day M-16 1 hor day

7 increases ith lime content. The maximm strength of soil M-1 occrred at an optimm lime content of 4 percent. Hoever, the strength of the other 2 soils as still increasing at 12 percent, hich indicates that the optimm lime content as not reached. It mst be emphasied that samples shoing small increases in strength after the addition of lime ere prepared at their liqid limits. At the time of testing they ere flly satrated, they ere assmed to represent the treatment of natral deposits, and they ere sed to determine hether any benefits cold reslt from the addition of lime. The strength changes de to the addition of lime in artificial mixtres of organic and mineral soils (M-1, M-11, and M-12) are shon in Figre 6. Three specimens ere prepared from mixtres of a mineral soil (M-) and a natral organic soil (M-9). These crves indicate that, althogh the strength increases ith the addition of lime, the increase is affected by the presence of organic materials. This effect is also shon in Figre 7 for T-8, M-18, and M-2. When lime is added to the organic soil, some calcim ions are sed to satisfy the base-exchange capacity of the organic matter. Feer ions are available for the poolanic reaction. Hoever, the reslts of ph readings for the soil-lime mixtres shoed that a lime content of less than 2 percent as reqired to netralie (ph = 7) the acidity of the tested organic soils. Ths, increased amonts of lime (above 2 percent) appear to satisfy the poolanic reaction and prodce an increase in the strength (T-8, M-18, and M-2, Table 3). Shearing Strength Shearing strength as also stdied for different soil-lime mixtres. The typical shift pard of the failre envelopes after the addition of lime is shon in Figre 8. The apparent cohesion of soil-lime mixtres increases ith lime content becase of the cementing action of the lime. For most organic soils, the evalation of the apparent rp de to increasing lime content as difficlt. Hoever, for the mineral soil, the apparent angle of internal friction increased directly ith lime content. This is most probably de to changes in particle sie and shape. ph and Optimm Lime Content The ph of soil-lime mixtres (Table 4) increased directly ith lime content becase of the alkalinity of lime. When the ph of a mixtre approaches 12.4 (ph of lime at 25 C), the soil-lime reaction is chemically satisfied as discssed earlier. For most of the soils, a ph of 12.4 as not reached for the lime contents stdied. Hoever, for lime contents of more than 12 to 14 percent, little increase in ph occrred for each content increment. Apparently, even at sch high lime contents, some reaction beteen the soil and the lime as occrring. Figre 9 shos that, in a typical soil (M-16) cred for 7 days, the ph does not flctate hen the lime content is increased above 12 percent. Ths, 12 percent of lime as sfficient for stabiliation. For the same lime content and cring period, the plasticity index of this soil dropped from 22.4 to 3.1 percent; the strength increased from 77 to 18 psi. Figre 9 also shos that a lime content of less than 2 percent as needed to satisfy the base-exchange capacity of the organic matter. It as assmed that the base-exchange capacity of the organic matter as satisfied hen the ph of the soil as raised to 7 by the addition of lime. Lime added beyond this point ill not be sed to satisfy the acidity of the organic matter, and it ill be available for the poolanic reaction if the soil is reactive. The ph rose from 5.4 to 7. CONCLUSIONS The folloing conclsions and recommendations are made based on the reslts of this stdy. 1. Increases in strength ith the addition of lime, changes in physical properties of organic soils, and increases in strength ith time indicate that organic matter in soils does not block the poolanic reaction. 43

8 44 2. Changes in the ph of a soil containing 2 percent of organic matter indicate that the addition of less than 2 percent of lime is sfficient to safisfy the base-exchange capacity of organic matter in the soil (based on the assmption that a ph of 7 indicates satisfaction of the exchange capacity). 3. The plastic properties of mineral and organic soils are improved by the addition of lime. An immediate increase occrs in the plastic limit of the soil after the addition of lime. Both increases and decreases ere observed in the liqid limit. The net effect of lime on the Atterberg limits of organic soils is a redction in the plasticity index. 4. The moistre-density characteristics of mineral and organic soils are affected by the addition of lime. Both increases and decreases in the maximm density and optimm moistre content ere observed for varios soils. This fact indicates that a moistre-density test shold be performed for every soil-lime mixtre to determine the optimm moistre content to be sed in the field. 5. The addition of lime increases the strength of all inorganic and organic soils tested, as measred by nconfined compression tests. The shear strength of mineral and organic soils is improved becase of an increase in cohesion; small increases occr in the slope of the failre envelope. The increases shon in this stdy are significant in terms of original ntreated strength of organic soil. 6. Organic soils tested in this stdy are sitable for stabiliation ith lime, provided the characteristics of the end prodct of these mixtres yield adeqate strength and other engineering properties for a particlar se. REFERENCES 1. Herrin, M., and Mitchell, H. Lime-Soil Mixtres. HRB Bll. 34, 1961, pp Eades, J. L., and Grim, R. E. Reaction of Hydrated Lime With Pre Clay Minerals in Soil Stabiliation. HRB Bll. 262, 196, pp Diamond, S., and Kinter, E. B. Mechanisms of Soil-Lime Stabiliation. Highay Research Record 92, 1965, pp Lnd,. L., and Ramsey, W. J. Experimental Lime Stabiliation in Nebraska. HRB Bll. 231, 1959, pp Wang, J. W. H., Mateos, M., and Davidson, D. T. Comparative Effects of Hydralic, Calcitic, and Dolomitic Limes and Cement in Soil Stabiliation. Highay Research Record 29, 1963, pp Jan, M. A., and Walker, R. D. Effect of Lime, Moistre, and Compaction on a Clay Soil. Highay Research Record 29, 1963, pp Hilt, G. H., and Davidson, D. T. Lime Fixation in Clayey Soils. HRB Bll. 262, 196, pp Zolkov, E. Inflence of Chlorides and Hydroxides of Calcim and Sodim on Consistency Limits of a Fat Clay. HRB Bll. 39, 1962, pp Taylor, W. H., Jr., and Arman, A. Lime Stabiliation Using Pre-Conditioned Soils. HRB Bll. 262, 196, pp Anday, M. C. Cring Lime-Stabilied Soils. Highay Research Record 29, 1963, pp Thompson, M. R. Shear Strength and Elastic Properties of Lime-Soil Mixtres. Highay Research Record 139, 1966, pp Arman, A. A Definition of Organic Soils; An Engineering Identification. Loisiana State Univ., Baton Roge, Eng. Res. Bll. 11, Oct Clare, K. E., and Pollard, A. E. Magaine of Concrete Research. No. SP 57, Thmp1:n, M. R. Inflence of Soil Properties on Lime-Soil Reactions. Pblic Works, Vol. 96, No. 8, Ag Thompson, M. R. Lime Reactivity of Illinois Soil. Jor. Soil Mech. and Fond. Div., Proc. ASCE, Vol. 92, No. SM5, Grim, R. E. Clay Mineralogy. McGra-Hill, Ne York, Maclean, D. J., and Sherood, P. T. Stdy of the Occrrence and Effects of Organic Matter in Relation to the Stabiliation of Soils With Cement. Proc., 5th Internat. Conf. on Soil Mech. and Fond. Eng., Paris, Vol. 2, 1961.

9 18. Grim, R. E. The Clay Minerals in Soils and Their Significance. Proc., Conf. on Soil Mech. and Its Application, Prde Univ., Lafayette, Ind., 194, pp Eades, J. L., and Grim, R. E. A Qick Test to Determine Lime Reqirements for Lime Stabiliation. Highay Research Record 139, 1966, pp Arman, A., and Mnfakh, G. A. Stabiliation of Organic Soils With Lime. Loisiana State Univ., Baton Roge, Eng. Res. Bll. 13, )... )