Detemining soil plasticity chaacteistics fom physico-chemical popeties G. C. Fanouakis Associate Pofesso, Depatment of Civil Engineeing Technology, Univesity of Johannesbug, South Afica ABSTRACT Numeous pedological soil classification systems have been developed woldwide. These include an intenationally accepted system and vaious national systems, some of which have been incopoated into databases which include maps. Such infomation is used pimaily fo agicultual puposes. Vaious physical and chemical soil popeties ae used fo classifying soils accoding to these pedological systems. This pape poposes an appoach, based on an extensive eseach poject, which may be used to statistically detemine the plasticity chaacteistics of soils fom the physical and chemical popeties that ae used by pedological classification systems, such as the South Afican System. These plasticity chaacteistics may be used to establish the engineeing soil classification goups which may, in tun, be used as a means of apidly detemining the geneal suitability of aeas fo poposed developments, paticulaly duing the peliminay stages of tanspotation oute locations and township developments, with a esultant saving of time and money. RÉSUMÉ Un cetain nombe de systèmes de classification pedologique de sols ont été développés depat le monde entie. Ceuxci incluent celui intenationalement accepté ainsi que dives autes nationaux dont cetains ont été incopoés dans les bases de données incluant des cates. Selon ces systèmes pedologiques, difféentes popiétés physiques et chimiques sont utilisées pou classifie les sols. Cet aticle popose une appoche qui peut statistiquement ête utilisée de façon significative pou détemine les caactéistiques de plasticité de sols à pati des popiétés physiques et chimiques utilisées pa les systèmes de classification pedologique, comme le Système Sud-Aficain. Ces caactéistiques de plasticité peuvent ête utilisées pou établi les goupes de classification de sol de constuction mécanique qui peuvent à leu tou ête utilisés comme un moyen de détemination apide de la confomité généale de égions pou les développement poposés, avec une économie ésultante en temps comme en agent. 1 INTRODUCTION 1.1 Pedology Pedology is a banch of soil science which deals with the study of soils as natual phenomena including thei fomation, mophological, physical, chemical, minealogical and biological constitution. 1.2 Pedological Classification Systems A numbe of pedological soil classification systems have been established woldwide and used as a basis fo mapping soils. Such systems include an intenationally accepted system and many national systems. The Intenational accepted system, which is endosed by the Intenational Union of Soil Sciences, is the Wold Refeence Base fo Soil Classification (WRB, 1998). This system eplaced the Food and Agicultue Oganisation (FAO) system, which was oiginally developed as a legend to the Soil Map of the Wold in 1974. National systems include the Canadian System of Soil Classification (CSSC) (SCWG, 1998), the United States Depatment of Agicultue System (USDA, 2010), and the South Afican Binomial Classification System (Mac Vica et al., 1977). These national systems ae also utilised by pivate oganisations. 1.3 Pedological Databases Vaious intenational, egional and national databases of pedological data have been established. Such databases include the SOTER (Soil and Teain) digital databases fo many, mainly developing, counties of the wold (Batjes, 2002). This collection, which places emphasis on soils and adopts the WRB (1998) classification classes, compises moe than 20000 aticles, county epots, books and maps. The map collection compises in excess of 6000 small scale (1:250 000 o smalle) maps. The physical and chemical popeties included in the database ae vey detailed. This infomation, which is accessible though the intenet, has also been egionalized. Fo example the SOTERSAF database (Batjes, 2004) petains to eight Southen Afican (SADC) counties. An example of a national database is the land type suvey database which was established fo the entie county by the South Afican govenment s Institute fo Soil, Climate and Wate (fomely the Soil and Iigation Reseach Institute). The soil data included in the land type suvey include the pedological popeties and pedological classification in accodance with the South Afican Binomial system (afte MacVica et al., 1977).
The data acquied in the couse of the land type suvey ae published in the fom of maps (at a scale of 1:250 000), known as land type maps, and accompanying documents which ae known as land type memois. The land type maps show land types detemined on the basis of teain fom, soil patten and climate. These maps and thei accompanying data can be puchased at an insignificant cost. Pedological data ae used extensively and successfully pimaily fo the optimization of agicultual land fo cop poduction so much so that pedology is often incoectly egaded as pat of agicultual science. 1.4 Objectives of this Pape This pape is based on an extensive eseach poject that was initiated on the pemise that the physical and chemical popeties of a soil, which ae manifested in the pedological class of a soil, have a beaing on the engineeing popeties of a soil. In this pape, elationships ae established fo detemining the engineeing plasticity chaacteistics (liquid limit, plasticity index and linea shinkage) of soils deiving fom a ange of pedological classes of the South Afican Binomial System (afte MacVica et al., 1977) fom thei physical and chemical popeties which ae detemined fo pedological classification puposes. These physico-chemical popeties include the clay content, the pecentage of the diffeent exchangeable base cations and the cation exchange capacities (CEC). 2 THE SOUTH AFRICAN BINOMIAL CLASSIFICATION SYSTEM 2.1 Stuctue The Binomial Classification System uses mophology (the science of fom and stuctue) and composition of soils as citeia fo diffeentiation in soil classification. This system, which goups soils accoding to the similaity of the popeties used in the identification, employs two categoies, namely an uppe o geneal level consisting of SOIL FORMS, and a lowe, moe specific one, compising SOIL SERIES. 2.2 Soil Foms The uppe level consists of 41 soil foms, each defined by a unique vetical sequence of diagnostic hoizons o mateials, not moe than fou in numbe, occuing in the uppemost 1,2 m of the soil pofile. 2.3 Soil Seies Each soil fom is sub-divisible into a numbe of soil seies, vaying fom two to 36. A total of 504 soil seies constitute the lowe categoy of the classification system. Soil seies have in common the popeties of the elevant fom (that is, the pescibed hoizon sequence) but ae diffeentiated within the fom accoding to a vaiety of citeia. The citeia used fo seies diffeentiation within each soil fom include the clay content, sand gade (whee the clay content is less than 15 pecent), ph, base status (which is a measue of the degee to which the soil has been leached), pesence o absence of fee calcium cabonate and colou. 2.4 Revisions A evised vesion of the South Afican Binomial Classification System, efeed to as the Taxonomic System fo South Afica, was published by the Soil Classification Woking Goup (SCWG) in 1991. This system is a moe specific extension of the South Afican Binomial Classification System. Many pesons have found the newe system to be unnecessaily complicated and hence have not adopted it (www.itc.nl). Nevetheless, in this pape consideation is only given to the South Afican Binomial Classification System which has been solely adopted fo soil classification in the land type suvey (which was incopoated into the SOTER database) and which is still being used in South Afica. 3 RESEARCH PROCEDURE 3.1 Reseach Aea The aea studied is intesected by Longitude 25 45' South Latitude 25 45'. The aea, which is located appoximately 150 kilometes noth of the City of Rustenbug in South Afica, is appoximately 4200 squae kilometes in extent. The aea was selected fo its divesity of soil types. 3.2 The Soils Investigated in this Poject A numbe of soil types occu in the aea, diffeing fom one anothe as a esult of diffeent paent mateials and/o the diffeent soil foming pocesses to which they had been subjected. The eseach was confined to the dominant soil seies which adequately cove the ange of soil types occuing in the aea. The soil foms and seies of each soil type selected fo the eseach ae shown in Table 1. 3.3 Fieldwok The soils at 63 andomly selected sites within the study aea wee classified accoding to the South Afican Binomial System (afte Mac Vica et al., 1977). A total of 111 distubed soil samples, taken fom the A and B (whee pesent) diagnostic hoizons of these soil pofiles, wee tested to detemine the physical and chemical popeties which ae used as citeia fo pedological classification. In view of the fact that the physical and chemical popeties of C hoizons ae not eflected in the pedological classifications of pofiles which contain C hoizons, C hoizons wee not sampled.
Table 1. Soil foms selected fo eseach puposes. Soil Fom Diagnostic Hoizons Soil Type Geneal Coelation A -Hoizon B- Hoizon C-Hoizon WRB 1 USDA 2 Canadian Hutton Othic Red Apedal Shotlands Othic Red Stuctued Valsivie Othic Pedocutanic Unconsolidated Mateial Swatland Othic Pedocutanic Sapolite Feallitic and Fesiallitic (not chaacteised by the dominance of smectitic clay mineals) Fesiallitic (chaacteised by the dominance of smectitic clay mineals) Red Seies - Fesiallitic (chaacteised by the dominance of smectitic clay mineals) Non-ed Seies Planosolic Fealsols and Aenosols Luvisols Oxisols, Inceptisols, Aidosols, Entisols and Ultisols Alfisols Alfisols and Aidisols Bunisolic Luvisolic Acadia Vetic Magallitic Vetisols Vetisols Vetisolic 1 Coelations accoding to Fey (2010) 2 Coelations accoding to MacVica et al., (1977) Details of the esults, methods of analysis adopted, efeences to the elevant methods as well as bief discussions on the significance of each popety detemined ae given in the wok of Fanouakis (1999). A total of 99 samples wee analysed fo the detemination of thei liquid limit, plasticity index and linea shinkage, in accodance with TMH1 (1986). The numbe of soil pofiles and numbe of samples tested fo the detemination of thei plasticity popeties fo the soils of each fom ae shown in Table 2. Table 2. Numbe of samples tested fo thei engineeing popeties Soil fom Numbe of pofiles Plasticity chaacteistics Hutton 28 48 Shotlands 3 4 Valsivie 11 23 Swatland 4 7 Acadia 17 17 4 ANALYSIS 4.1 Investigation of possible elationships The effect of quantitative chemical popeties such as the diffeent exchangeable base cations (Ca, Mg, Na, K), S- value, base satuation, exchangeable sodium pecentage (ESP), exchangeable magnesium pecentage, exchangeable potassium pecentage, and exchangeable calcium pecentage on the plasticity chaacteistics of the soils constituting each soil fom wee evaluated with efeence to liquid limit, plasticity index and linea shinkage. The combined effect of each of the abovementioned chemical chaacteistics in conjunction with physical chaacteistics such as the clay content, silt content, and pecentage fine than 0,075 mm on the plasticity chaacteistics of the espective samples wee also evaluated. In the evaluation of the above, the fact that the physico-chemical popeties wee detemined using the faction fine than 2,0 mm and the plasticity chaacteistics using the faction fine than 0,425 mm was accounted fo. A list of the possible elationships between the plasticity chaacteistics and the vaious chemical and physico-chemical popeties of the soils of each fom investigated is given in the wok of Fanouakis (1999). 4.2 Statistical Methods None of the data was tansfomed and no multiple egession pocedues wee adopted. The egession equations and coelation coefficients not significant at the five pecent level wee ejected. The least squaes method was used fo cuve fitting puposes. Significance tests wee caied out fo each elationship established using tables of Student's t- distibution to detemine the pobability that the coelation coefficient could have aisen by chance in a sample of the size dealt with. Fishe's Z-tansfomation technique was used to combine independent coelation coefficients, which wee significant at the five pecent pobability level, to obtain an estimate of the aveage (pooled) coelation coefficient.
This pocedue took into account each coelation coefficient and the coesponding numbe of sets of values on which it was based. 5 RESULTS AND DISCUSSION 5.1 The Effect of Exchangeable Magnesium In the case of the soils of each fom studied, the magnitude of the liquid limit (LL), plasticity index (PI) and linea shinkage (LS) each coelated significantly, linealy and positively with the amount of magnesium pesent in the clay-size potion of the faction of the samples fine than 0,425 mm. This quantity which, in the succeeding discussion, will be efeed to as "magnesium in the claysize faction", is defined by Equation 1 and expessed in milli-equivalents pe 100 g of soil fine than 0,425 mm. Exch. Mg (me%) * P0,002mm/P0,425mm [1] The quantity efeed to in Equation 1 was defined specifically fo the pupose of this discussion and should not be confused with eithe of the quantities temed "exchangeable magnesium" o "exchangeable magnesium pecentage (EMgP)". The magnesium in the clay-size faction is calculated by multiplying the value of the exchangeable magnesium (expessed in milli-equivalents pe 100 g of soil fine than 2 mm, me%) by the atio of the pecentage by mass of the faction of the total sample fine than 0,002 mm (P0,002mm) to the pecentage by mass of the faction of the total sample fine than 0,425mm (P0,425mm). The definition of magnesium in the clay-size faction by Equation 1 is based on two pemises, fist, that the exchangeable magnesium occus almost exclusively in the faction fine than 0,425 mm and second that the exchangeable magnesium is distibuted unifomly within this faction. These pemises wee egaded as applicable to all the soils investigated in this poject egadless of the degee of base satuation. To test the validity of these pemises, sets of elationships between exchangeable magnesium and plasticity chaacteistics wee established based on each of the following two assumptions: Fistly, that the magnesium cations ae unifomly distibuted within the soil faction fine than 2 mm and, secondly, that the magnesium cations ae unifomly distibuted within the minus 0,075 mm faction. The elationships established wee not all significant and it can, theefoe, be concluded that the uppe size limit of the soil faction in which the exchangeable magnesium pedominates must be of the ode of 0,425 mm. Howeve, it must be emphasized that this is not because the engineeing plasticity analyses ae pefomed on the faction fine than 0,425 mm of the soil. The exchangeable cations in a soil ae adsobed onto the negatively chaged clay paticles (Bady, 1974). In view of this fact, the elationships established between the plasticity chaacteistics and the magnesium in the clay-size faction indicate that the soils eseached must contain clay paticles which ae lage than 0,002 mm in size. Futhemoe, these elationships indicate that only the exchangeable magnesium which is adsobed onto the clay paticles which ae fine than 0,002 mm in size has an effect on the plasticity. It is not unusual fo soils to contain clay mineal paticles which ae lage than 0,002 mm in size. A study of the minealogy of five weatheing pofiles developed fom Achaean Ganite, conducted by Buhmann (1990), evealed that paticles of clay mineals such as biotite occued in the silt and sand size factions. The elationships established between the plasticity chaacteistics and the magnesium in the clay-size faction wee e-established afte gouping the soils of each soil fom accoding to two diffeent abitaily selected anges of cation exchange capacity pe 100 g clay. This gouping was not caied out in the case of the soils of the Shotlands fom of which a sufficient numbe of soil samples to waant gouping was not available. The cation exchange capacity ange limits wee selected such that, fo the soils of each fom, an appoximately equal numbe of samples fell within each of the two anges. The elationships obtained afte gouping into two anges of cation exchange capacity pe 100 g of clay esulted in coelation coefficients of a highe ode and coesponding levels of significance of deceased pobability than befoe the gouping. In geneal, the geate the cation exchange capacity pe 100 g clay, the geate the plasticity displayed by the soil fo a given quantity of exchangeable magnesium cations in the clay faction. This is especially tue of soils of the Acadia fom which had a mean activity of 1,2 and in which montmoillonite clay paticles, which have a high affinity fo wate, ae known to occu. As shown in the wok of Bave (1930), the affinity of soil fo wate and its capacity fo swelling incease as the cation exchange capacity of the clay inceases. The coelations detemined fo the soils of each soil fom ae discussed in the succeeding paagaphs. 5.2 Discussion of Results 5.2.1 Soils of the Hutton Fom The mean activity of the soils of the Hutton Fom was 0,47. Of the 48 soil samples of the Hutton fom analysed, a total of 30 samples displayed plasticity. Examination of the gading and plasticity chaacteistics of these soils evealed that on the aveage soils with a clay content of less than 12 pecent did not display plasticity. This absence of plasticity is thought to be as a esult of a low content of 1:1 clays in the soils. Sixteen of the samples displaying plasticity had a cation exchange capacity within the ange 13 to 30 milliequivalents pe 100g of clay and 14 had a cation exchange capacity within the ange of 33 to 62 milliequivalents pe 100 g clay. The egession equations and statistics petaining to these elationships ae given in Table 3.
Table 3. Statistics petaining to the coelation of plasticity chaacteistics with magnesium in the clay-size faction fo soils of the Hutton fom Table 4. Statistics petaining to the coelation of plasticity chaacteistics with magnesium in the clay-size faction fo soils of the Acadia fom P* CEC/ 100g Clay (me) Regession Equation P P* CEC/ 100g Clay (me) Regession Equation P LL 13-30 y = 20,338 + 0,077x 0,914 1E-06 33-62 y = 19,711 + 9,159x 0,951 1E-06 LL 53-61 y = 35,766 + 2,752x 0,767 1,587E-02 63-90 y = 54,827 + 1,447x 0,948 3,379E-04 PI 13-30 y = 10,527 + 4,309x 0,662 5,212E-03 33-62 y = 6,917 + 5,351x 0,869 5,472E-05 PI 53-61 y = 15,865 + 1,732x 0,740 2,263E-02 63-90 y = 29,547 + 0,721x 0,794 1,862E-02 LS 13-30 y = 2,862 + 3,599x 0,877 8,047E-06 33-62 y = 2,436 + 3,573x 0,878 3,642E-05 P* = Plasticity chaacteistic coelated with Exch. Mg (me %) * P0,002 mm/p0,425 mm LS 53-61 y = 9,996 + 0,672x 0,791 1,113E-02 63-90 y = 14,622 + 0,254x 0,839 9,214E-03 P* = Plasticity chaacteistic coelated with Exch. Mg (me %) * P0,002 mm/p0,425 mm A elative compaison of the points epesenting the A hoizons with those epesenting the B hoizons (on the scatte diagams) indicated that most B hoizons occued in the lowe cation exchange capacity ange. No hoizon specific tends wee obseved fo any of the elationships. The elationships established wee all highly significant, being at the 0,52 pecent level of pobability. The coelation was positive - an incease in the magnesium in the clay-size faction was associated with an incease in each of the thee plasticity chaacteistics. A statistical analysis of the coelation coefficients of the six elationships efeed to above and the six pais of values fom which each elationship was detemined, yielded a pooled coelation coefficient of 0,874. The mean of the pobability that any of the six elationships could have aisen by chance was 0,090 pecent. 5.2.2 Soils of the Acadia Fom Of the 17 soil samples analysed, all of which displayed plasticity, nine samples had a cation exchange capacity within the ange 53 to 61 milli-equivalents pe 100 g of clay and eight had a cation exchange capacity within the ange of 63 to 90 milli-equivalents pe 100 g clay. The egession equations and statistics petaining to these elationships ae given in Table 4. A pooled coelation coefficient of 0,826 was obtained when taking into account the coelation coefficients of the six elationships and the six pais of values. The mean of the pobability that any of these elationships could have aisen by chance was 1,3 pecent. 5.2.3 Soils of the Shotlands, Valsivie and Swatland Foms The elationships between liquid limit, plasticity index and linea shinkage espectively and the magnesium in the clay-size faction fo the two anges of cation exchange capacity pe 100 g of clay, of the soils of these foms wee analysed accoding to the six goups shown in Table 5, justified by the similaity of these soils. The soils compising each goup wee sub-divided into one of two abitaily selected anges of cation exchange capacities pe 100 g clay (except in the case of soils of the Shotlands fom). Hence a total of 36 elationships wee established. The pooled coelation coefficients as well as the minimum, maximum and mean levels of significance fo the six goups ae shown in Table 5. Fom the pooled coelation coefficients and mean pobabilities shown in Table 5, it is evident that the diffeences between the pooled coelation coefficients and mean pobabilities of the six soil goups ae so small as to be negligible. In the case of soils of the Shotlands, Valsivie and Swatland foms, which had a mean activity of 0,52, qualitative physical chaacteistics such as unifom colou, pedominant colou and the pesence of continuous clay skins on ped faces (cutans) did not bea any elationship to the soil's plasticity chaacteistics. Theefoe, the engineeing plasticity chaacteistics of soils of the Shotlands, Valsivie and Swatland foms, can be detemined fom the magnesium in the clay-size faction of each soil sample, togethe with the cation exchange capacity pe 100 g clay of that sample by using the elationships established when the test esults fo the soils of all thee of the above foms wee gouped. This is tue only fo soils with physico-chemical popeties within the anges of the soils investigated, including a low exchangeable sodium pecentage (ESP). The egession equations and statistics petaining to these elationships ae given in Table 6.
Table 5. Pooled coelation coefficients and coesponding levels of significance Seies Pooled Levels of Significance (%) Min. Max Mean Shotlands 0,965 3.3 5,3 4,5 Valsivie and Swatland Red Valsivie and ed Swatland Non-ed Valsivie and non-ed Swatland Shotlands, ed Valsivie and ed Swatland Shotlands, Valsivie and Swatland (Luvisolic) 3 0,889 <1E-04 0,006 0,002 0,828 0,003 0,025 0,978 0,979 0,003 2,4 1,277 0,826 0,003 0,383 0,135 0,885 <1E-04 0,001 0,001 3 Geneal equivalent accoding to the Canadian Classification System (SCWG, 1998). Table 6. Statistics petaining to the coelation of plasticity chaacteistics with Magnesium in the clay-size faction fo soils of the Shotlands, Valsivie and Swatland foms P* CEC/ 100g Clay (me) Regession Equation LL 25-49 y = 27,639 + 3,961x 0,918 <1E-06 52-94 y = 26,362 + 4,605x 0,913 <1E-06 PI 25-49 y = 10,830 + 2,340x 0,862 8,624E-06 52-94 y = 10,930 + 2,172x 0,862 8,624E-06 P Of the 34 samples included in these elationships, 17 had a cation exchange capacity within the ange of 25 to 49 milli-equivalents pe 100g of clay and the othe half had a cation exchange capacity within the ange of 52 to 94 milli-equivalents pe 100 g clay. A elative compaison of the points epesenting the A hoizons with those epesenting the B hoizons (on the scatte diagams) indicated that most B hoizons occued in the lowe cation exchange capacity ange. No hoizon specific tends wee obseved fo any of the elationships. 6 CONCLUSIONS This eseach showed that the liquid limit, plasticity index and linea shinkage of soils can be statistically significantly estimated fom the physico-chemical data of soils, fo a ange of pedological goups. All the elationships established fo estimating the plasticity chaacteistics of the soils of the Hutton fom (Bumisolic Ode accoding to the Canadian Soil Classification System - CSCS) and Shotlands, Valsivie and Swatland foms (Luvisolic Ode accoding to the CSCS) soils wee highly significant (P < 1 %). The elationships petaining to the Acadia fom (Vetic Ode accoding to the CSCS) wee all significant (P < 5 %). With the aid of quantitative pedological data, the plasticity chaacteistics of soils, which ae in tun used to classify a soil fo vaious intended engineeing usages, can be estimated. Hence this infomation may assist in the identification of and avoidance of lage aeas of unfavouable soils in pojects such as township development and tanspotation oute alignment, esulting in a saving of time and money. Convesely, aeas o outes coveed by favouable soils can be located, hence poviding the famewok fo moe detailed testing equied fo the final engineeing design. The findings of this eseach ae valid fo soils of the pedological classifications investigated and thei paticula physical, chemical and minealogical popeties only, since the eseach poject was intended to seve as a pilot study. The indications ae that additional simila studies coveing soils of the pedological classifications eseached but with diffeent chemical popeties as well soils with othe pedological classifications with vaying chemical popeties could undoubtedly lead to the extension of this wok and impovement of its univesality. Finally, it is hoped that this eseach has succeeded in emphasizing the inteelationships between pedogenesis and the engineeing behaviou of soils and in suggesting an appoach fo the intepetation of pedological data fo engineeing puposes. LS 25-49 y = 5,827 + 1,335x 0,875 4,222E-06 52-94 y = 5,351 + 1,298x 0,864 7,789E-06 P* = Plasticity chaacteistic coelated with Exch. Mg (me %) * P0,002 mm/p0,425 mm
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