Jair A.Baldovino,Eclesielter B.Moreira,Wagner Teixeira,Ronaldo L.S.Izzo,Juliana L.Rose

Department of Civil Construction,Federal University of Technology-Paraná,Campus Curitiba,Curitiba,Brazil

1.Introduction

In the field of civil engineering,various methods are adopted in order to improve the geotechnical properties of soils to meet the requirements for mechanical stability and maintenance cost reduction.Clayey soil stabilization by different additives may be considered as one of these methods,because the substitution of inappropriate soils by adequate ones has become increasingly expensive and ecologically unsafe.Besides,stabilization by cement is not preferable due to its growing cost and environmental concerns related to its production(al-Swaidani et al.,2016).Luting materials such as lime have been successfully employed in geotechnical engineering for soil stabilization and mechanical improvement,especially for clayey and silty soils.

Lime types commonly used for fine-grained soil stabilization include high-calcium hydrated lime,calcite lime,monohydrated dolomitic lime and dolomitic quicklime.When the lime is added to clayey soils,there are two pozzolanic reactions:cation exchange and flocculation-agglomeration.Soil improvement techniques may also be used in the foundation of small buildings and in soils that present low support capacity or low volumetric stability as such soils may cause severe construction events(Ingles and Metcalf,1972).

Researchers have been trying to understand the influence of different amounts of lime on the mechanical behaviors of these types of soils.Table 1 summarizes the effect of lime stabilization on the behavior of some soils reported in the literature.Usually,the amount of lime used to improve soils is between 3%and 9%,and the curing time used ranges from 7 d to 360 d.The most used tests to study the mechanical behavior of soils improved with lime include splitting tensile strength,unconfined compressive strength,California bearing ratio,and direct shear and triaxial tests.

The soils of the Guabirotuba geological formation are located in Curitiba City(Brazil)and its metropolitan region,and are predominantly fine-grained.Most of these soils cannot be directly employed for sub-base and base pavement layers,support of shallow foundations and protection of hillsides,because their physico-mechanical properties do not meet the specificities of current regulations.

The study of soil improvement in regions such as Curitiba under investigation in this paper is fundamental,given that its utilization in pavement layers would entail lower costs,both financially and environmentally.Since the unused soil is placed in landfills,which generates costs on excavation and transportation to dumps,whether regular or irregular,thus diminishing the service life of dumps.This also creates a need for exploitation of(non-renewable)natural materials in mineral deposits of good quality to serve as structural materials for pavement layers.

Table 1Summary of the effect of lime stabilization on the behavior of some soils reported in the literature.

This research intends to determine the influence of different lime contents on the unconfined compressive strength(qu)and splitting tensile strength(qt)of a soil from the Guabirotuba formation at different curing times.

2.Materials and methods

2.1.Materials preparation

2.1.1.Soil

The soil used for the present study is mainly observed in the third horizon of the Guabirotuba formation,which covers the area of Curitiba City and its metropolitan region.Specifically,the soil specimens were also sampled in a neighboring city,Fazenda Rio Grande,in the state of Paraná (PR),Brazil(25°41′3.9′′S and 49°18′32.5′′W).The soil is composed of 35.5%lime(＜0.002 mm),39.5%silt(0.002-0.075 mm)and 25%fine sand(0.074-0.42 mm)(Fig.1).The natural moisture content of the soil found in the field is approximately 40%.

Table 2 presents the physical properties of the soil.According to the unified soil classification system(USCS),this soil is classified as thick sandy clay.

2.1.2.Lime

This study employed hydrated dolomitic lime(CH-III),mainly composed by calcium and magnesium hydroxides(Ca(OH)2and Mg(OH)2)produced in the city of Almirante Tamandaré(PR,Brazil).The accumulated percentage of lime grains retained in the#200 sieve was 9%,which was in accordance the standard with NBR 7175(2003),specifying that 15%of this type of material must be retained by the#200 sieve.The density of the lime is 2.39 g/cm3.

2.1.3.Soil-lime

Lime contents of 3%,5%,7%and 9%were used in relation to the soil’s dry weight.

2.1.4.Water

Distilled water was used for both specimen molding and soil characterization tests.

Fig.1.Grain size distribution curve of soil studied.

2.1.5.Physical indices

ASTM D4318-10e1(2010)was used for the determination of liquid limit(LL),plastic limit(PL),and plasticity index(IP)of soil.ASTM D854-14(2014)was used to obtain the specific weight of soil(Gs).

2.1.6.pH tests

Beforemixing the soil with lime,an investigation was conducted to define the minimum lime content as a function of pH value.The methodology,called ICL(initial consumption of lime)method,proposed by Rogers et al.(1997)was used,which creates avariation curve of pH value versus lime content.

The ideal pH value is obtained when it reaches a maximum constant value.Fig.2 presents the variation of pH value in relation to the lime content used.

It can be observed that after the pH value reaches 12.5 at 3%lime content,the pH value remains constant,regardless of the increase in lime content.Therefore,the initial lime content used in this research was considered as 3%.Taking into account the research on soil-lime behavior and the pH stabilization value resulting from lime addition,the lime contents of 5%,7%and 9%were selected in this study.

2.1.7.Standard Proctor test

Soil compaction tests were undertaken with the application of standard effort according to ASTM D698-12e2(2012).

Fig.3 presents clay compaction curves subjected to standard effort.For the clay under study,compaction with standard effort resulted in a maximum dry unit weight of 13.8 kN/m3and an optimum moisture content of 31%.These values were used for the remaining specimens.

Table 3 shows that there are variations in the maximum dry unit weight and the optimum moisture content for different lime contents(3%,5%,7%and 9%).As the lime contents were low,the variations observed were very small.

2.1.8.Molding of tested specimens

For unconfined compression and splitting tension tests,specimens of 100 mm in height and 50 mm in diameter were molded.The clay was first dried in a hothouse with a temperature of(100 ± 5)°C,and then mixed with different lime contents(0%,3%,5%,7%and 9%).Subsequently,static compaction was applied to the mixture for the molding of specimens at the optimum moisture,in two layers in a stainless steel mold(50 mm in internal diameter,100 mm in height,and 5 mm in thickness).The specimens were wrapped by transparent plastic to prevent loss of moisture.Then they were placed into the moist chamber to be cured for 15 d,30 d,60 d and 90 d at an average temperature of 25°C.For the unconfined compression test,the specimens had to follow a set of conditions:the specimen dimensions with tolerances of±0.5 mm in diameter and±1 mm in height,the apparent dry unit weight(γd)with a tolerance of±1%of target value,and the moisture content(w)with a tolerance of±0.5%of target value.

One hundred and twenty specimens were tested with standard effort,60 for unconfined compression tests and 60 for splitting tension tests.For each of the four curing times and four lime contents,as least three specimens were tested,and the results obtained could not differ by 10%.For both the unconfined compression and the splitting tension tests,a load frame(WILLE GEOTECHNIK UL60)was used with the maximum capacity of 5 kN,as well as calibrated rings with the axial load of 4.5 kN,axial loading capacity of 5 kN,and loading speed of 1 mm/min.

2.2.Testing methods

2.2.1.Unconfined compression tests

The procedures for unconfined compression tests were in accordance with the standard ASTM D2166/D2166M-16(2016).

Unconfined compressive strength is the material’s ultimate loading bearing capacity,or the pressure at which a specific cylinder strain of 20%is reached when the axial stress-strain curve does not present a maximum peak.The unconfined compressive strength(qu)is calculated according to the following expression when the axial stress-strain curve reaches a maximum peak:

wherePRis the failure load at the peak of the axial stress-strain curve,andATis the corrected cross-sectional area of specimen.

2.2.2.Splitting tensile tests

The procedures for splitting tension tests were in accordance with the Brazilian standard NBR 7222(2011).

The splitting tensile strength(qt)is calculated according to the following expression:

whereDandHare respectively the diameter and height of specimen.

2.2.3.qt/quratio

One important reference for the mechanical behavior of limestabilized soils is the ratio of porosity to the volumetric lime content(η/Lv),whereLvis defined as the ratio of lime volume to the total volume of specimen,and the porosityηis calculated by the following equation(Consoli et al.,2012):

whereVsis the total volume of specimen;γdis the dry unit weight of specimen;Ssis the soil content;andGsandGlare the specific weights of soil and lime grains,respectively.

Table 2Physical properties of the soil.

Fig.2.Results of pH test.

Fig.3.Clay compaction curves with different lime contents.

Table 3Dry unit weight and optimum moisture content values for soil-lime mixtures in compaction with standard effort.

3.Results

3.1.Unconfined compressive strength

The unconfined compressive strength(qu)obtained as a function of lime contentLat curing times of 15 d,30 d,60 d and 90 d are presented in Fig.4.

Fig.4.Unconfined compressive strength(qu)versus lime content(L)curves.

As shown in Fig.4,an increase in the ultimate compressive strength can be observed for the soil-lime mixtures with higher lime content.Small addition of lime can significantly increase the unconfined compressive strength,indicating that the lime has an major influence on the strengthqu.Meanwhile,quincreases with the curing time.

The unconfined compressive strength presents an exponential relation with the lime content.Comparing the lime contents of 3%and 9%,as shown in Fig.4,it can be found that for a 90-d curing time,thequvalues increase by 115%on average,and for 60-d and 30-d curing times,quincreases by 110%,and for a 15-d curing time,quincreases by 80%.

The curing time also has an influence on the increase inquas a function of lime content,as shown in Fig.5.The unconfined compressive strengthqudisplays a linear relationship with the logarithm of curing time.Fitting curves are obtained withR2=0.87 at worst andR2=0.98 at best.The most significant increase inquwas obtained with the addition of 9%lime,followed in turn by 7%,5%and 3%.

3.2.Splitting tensile strength

The splitting tensile strength(qt)obtained as a function of lime content for curing times of 15 d,30 d,60 d and 90 d is presented in Fig.6.

Similar to the unconfined compressive strengthqu,it was observed that forqt,lime addition has a significant impact on the finalqtvalues of soil-lime mixtures,and the fitting curves also present exponential trends.The curing time also influences the splitting tensile strength,as shown in Fig.7.

By comparing 3%and 9%lime contents,it can be seen that for the 90-d curing time,the splitting tensile strengthqtincreases by 260%;and for 60-d,30-d,and 15-d curing times,qtincreases by 230%,140%and 130%,respectively.

Based on the previous data,it can be found that bothquandqtincrease as the lime content and the curing time increase.

3.3.qt/quratio

Figs.8 and 9 present respectively the unconfined compressive strengthquand the splitting tensile strengthqtin relation toη/Lvfor different lime contents and curing times.

A decreasing exponential behavior can be noted in Figs.8 and 9 due to an increase inLvand,consequently,an increase in theη/Lvratio.

Fig.5.Unconfined compression strength(qu)versus logarithm curing time curves.

Fig.6.Splitting tensile strength(qt)versus lime content(L)curves of specimens.

According to these results,qu/qtcan be calculated for different curing times,as listed in Table 4.

Thus,the ratios ofquto η/Lvandqtto η/Lvcan be taken as the parameters to evaluate principally theqt/quratio for various lime contents used.As indicated above,theqt/quratio increases with the curing time.Theqt/quratio may also be visualized by the functiony=f(x)orqt=f(qu).As a result,the ratio may be obtained depending on the type of soil and the curing time of luting material.The ratio varies from 0.1 to 0.15 according to Thompson(1965).Consoli et al.(2010,2012)found values of 0.15 and 0.16 for sandcement mixture and lime-treated silt cured for 28 d,respectively.

Fig.10 presents the variation inqtas a function ofqufor the specimens in the present study.

Fig.7.Splitting tensile strength(qt)versus logarithm curing time curves.

Fig.8.Relations between quandη/Lvfor different lime contents and curing times.

Fig.9.Relations between qtandη/Lvfor different lime contents and curing times.

Table 4qu/qtvalues for different curing times.

Fig.10.quversus qtratio.

Table 5Effect of lime stabilization on the behavior of some soils reported in the literature.

In the present study,theqt/quratio was calculated for different curing times.Exponential curves forquin Fig.8 present different correlation coefficients varying fromR2=0.8(at worst)toR2=0.97(at best);likewise,forqt(Fig.9)curves,the correlation coefficients vary fromR2=0.78 toR2=0.89.Taking these correlation coefficients into consideration,theqt/quratio for a 15-d curing time may be calculated.

A linear tendency is observed for all lime contents and curing times used withR2=0.93,which follows the equationqt=0.2698qu-59.31.The equation can be used as mix design relationship.For the sandy clay studied,any values of splitting tensile and compressive strengths can be obtained if the lime content is increased or the porosity is reduced.In addition,any value of compressive strength can be calculated if the splitting tensile strength is known,which can be time-effective and reduce the cost in materials for other tests.

For example,in the construction of pavement layers,where it is intended to achieve a defined resistance in a required time,the equations in Figs.5-10 can be used to determine the porosities(compaction energy)and the minimum lime content,depending on the curing time to achieve the planned strength,taking into account the lowest cost.

4.Discussion

Table 5 shows the effect of different lime contents and curing times on the unconfined compressive strength(qu),splitting tensile strength(qt)andqt/quvalues of several soils reported in the literature for comparison with the results in this paper.As shown in Table 5,the researchers used lime contents between 1%and 12%of specimens molded with dry unit weights of 14-19.5 kN/m3and curing times of up to 360 d.

The maximum compressive and tensile strengths were obtained at higher lime content,longer curing time and higher unit weight.According to Consoli et al.(2012,2014),the increase of soil-lime mixture strength is directly related with the decrease of pores and the increase of lime volume.Thompson(1965)obtained the compressive strength of 11,156 kPa and tensile strength of 1427 kPaat 7%lime content.These are due to the high dry weight in compaction test,around 18 kN/m3.

Other authors such as Consoli et al.(2012)and Calik and Sadoglu(2014)obtained higher values of strengths compared with results in this paper.This is explained by the dry weight of compaction used by the researchers mentioned above,being higher than that in this paper.Another explanation may be the amount of clay particles,due to the reaction between lime and clay.A final argument would be the differences of lime applied,differing from the calcium and magnesium contents according to each of local manufacturers.

Ghobadi et al.(2014),by testing a clayey soil of low plasticity,obtained the unconfined compressive strength of only 6%of the

result in this research.The difference may be illustrated by the reaction between clay and soil.In Ghobadi et al.(2014),the soil has only 12%of clay,whereas 35.5%of clay is reported in the present study.

5.Concluding remarks

The goal of the present study was to measure the influence of hydrated lime addition on unconfined compressive and splitting tensile strengths of a sedimentary soil of the Guabirotuba geological formation(Curitiba,Brazil).Bothquandqtvalues increase as the curing time increases.For 90-d curing time,the maximumquandqtvalues were obtained.At 90-d curing time,the unconfined compressive strengthquincreases by 400%for 9%lime content,260%for 7%lime,240%for 5%lime,and 170%for 3%lime.In the same condition,the splitting tensile strengthqtwas also observed to increase by 600%for 9%lime,440%for 7%lime,240%for 5%lime and 170%for 3%lime.The results show increasing strengths of the soil-lime mixtures like some soils reported in the literature listed in Table 1.

The increasing rate ofqtis higher than that ofqufor 7%and 9%lime contents,but it is the same for 3%and 5%lime contents,which shows that the lime content and curing time have greater influences onqtthan onqu.

Using theη/Lvratio to calculatequ/qtvalue was considered to be satisfactory.It was verified that this ratio(qu/qt)ranged from 0.17 to 0.2(for 15-d and 90-d curing times,respectively).Finally,the use of lime for stabilization of soils with low mechanical resistance was observed to be an efficient technique in geotechnical engineering.

Conflicts of interest

The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

The authors are thankful to Universidade Tecnológica Federal do Paraná and to the financial support given by CAPES-Brasil,Funda??o Araucária do Paraná and CNPq.

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