Hydrated Lime Effects on Geotechnical Properties of Clayey Soil

Cohesive soils present difficulties in construction projects because it usually contains expansive clay minerals. However, the engineering properties of cohesive soils can be stabilized by using various techniques. The research aims to elaborate on the influences of using hydrated lime on the consistency, compaction, and shear strength properties of clayey soil samples from Sulaimnai city, northern Iraq. The proportions of added hydrated lime are 0%, 2.5%, 5%, 7.5% and 10% to the natural soil sample. The results yielded considerable effects of hydrated lime on the engineering properties of the treated soil sample and enhancement its strength. The soil's liquid limit, plasticity index, and optimum moisture content were decreased with the increase of hydrated lime percent. The soil's other geotechnical properties such as plastic limit, maximum dry density, and unconfined compressive strength were increased with the hydrated lime content increase. The oedometer test results produced a notable decrease in the compressibility characteristics of the lime-treated soil sample. Hence, hydrated lime is successfully contributed and can be considered as an effective material to improve the strength, compressibility, and consistency properties of the cohesive soils in Sulaimani city.


INTRODUCTION
In order to obtain an eligible soil foundation for construction projects, its geotechnical characteristics might require some improvement. Therefore, various common methods for soil stabilization exist globally nowadays, which can be used for improving weak foundation soil. Many techniques may perform soil stabilization; replacement of a part of the weak soil by better materials is one of these techniques. However, stabilization materials might be expensive and/or environmentally insecure. For instance, cement is one of the not desirable stabilization material (Al-Swaidani et al., 2016; Al-Hadidi and AL-Maamori, 2019). Then, lime is effectively utilized for soil improvement purposes to stabilize soil's weak geotechnical engineering characteristics. Mechanical properties of soils such as clayey and silty soils were successfully improved by using lime. Notably, lime is used in many civil engineering projects such as road layers, earth embankments, soil foundations, and piles (Al-Rawas and Goosen, 2006). The carried-out studies in the literature on the soil's geotechnical engineering properties improvement yielded significant properties modification. For soils compaction properties, the addition of lime caused the optimum moisture content to be decreased and the maximum dry density to be increased (Bell, 1996 For soils consistency parameters, in one hand, lime decreases the plasticity index, reduces shrinkage cracking, and increases the workability and shrinkage limit (Guney et al., 2007;Bell, 1996 Chen and Lin, 2009). Moreover, the capability of the stabilized soil by lime in terms of strength is significantly affected. This is resulted due to the deduction in the voids percent because of the added lime percent increase (Consoli et al., 2012(Consoli et al., , 2014. For soil compressibility properties, in some studies such as (Rao and Shivanada, 2005), the yield stress increase by the increase in the lime content. Similarly, both of (Sut-Ünver et al., 2018, and Vukicevic et al., 2019) found that 5% lime content is the crucial percent, which improved both of compressibility and rebound indexes. Lime used in some works with other materials such as the study of (Hayder, 2016). The addition of the composite of cement and quick lime can improve the compressibility by reducing both of compression rebound ratios. This study presents the effect of hydrated lime on the Atterberg limits, compaction characteristics, compressibility parameters, and unconfined compressive strength of Sulaimnai city, Northern Iraq

Soil
The used soil in this study was obtained from a site, namely Barika soil, Fig.1, situated in the Sulaimani Governorate, northern Iraq (Latitude = 35.393755 and Attitude = 45.595097). The natural soil was collected at a depth of 0.5 to 1 m from the natural ground level and is usually consolidated clay. The undisturbed and disturbed soil samples were excavated and then extracted, placed in plastic bags, and transported to the geotechnical laboratory for testing. Extreme  -422, 2007). The soil's particle size distribution shown in Fig.2 and the other properties are summarized in Table 1.

Hydrated Lime
The used hydrated lime in the current study is locally-available lime typically used for construction purposes. It is obtained from the Karbala Lime Factory located in the southeast of Baghdad, Iraq. The chemical and physical properties of the used lime are presented in Table 2.

METHODS OF TESTING
The collected soil sample was divided into five equal parts with the same natural properties. Then, each part was mixed with 0%, 2.5%, 5%, 7.5%, and 10% of hydrated lime, which was replaced from the total dry mass of the natural soil sample, and then the required water content was added. The mixtures were stored in waterproof containers for 24 hours to allow for homogeneity and mature. By using the maximum dry density and optimum moisture content (Table 1), the testing samples were prepared for all the proposed geotechnical characteristics of this study. These prepared samples were performed for all of the above selected hydrated limes percentages. The performed laboratory tests were particle size distribution, specific gravity, Atterberg limits, standard Proctor compaction, and unconfined compression tests. All these laboratory tests were conducted on the natural and stabilized soil samples, respectively.

Atterberg Limit Tests
Liquid limit (LL), plastic limit (PL), and plasticity index (PI) were obtained following the method given in the ASTM D4318 (2000). Variations in the plasticity index of the natural soil before and after the addition of hydrated lime were then studied. The natural soil sample was air-dried and sieved through No. 40. The required percent of hydrated lime was added after that, de-ionized water was added to the soil-lime mixture, and the paste was left in an airtight container for 24 hours to be matured. The consistency limits test then performed on the prepared paste at room temperature.

Compaction Tests
The method given in the ASTM D698 (2000) was applied to determine the maximum dry density (MDD) and the optimum moisture content (OMC) of the soil samples. Two compaction series were performed by using de-ionized water. The first one was for the determination of the natural soil compaction parameters (OMC and MDD). The second one is for the compaction properties determination of the stabilized sample by hydrated lime.

Unconfined Compression Tests
Unconfined compressive strength tests were conducted according to ASTM D2166 (2000). Field dry density (1.56 g/cm 3 ) and natural moisture content (18.68 %) were chosen to prepare the Journal of Engineering Volume 26 November 2020 Number 11 155 unconfined compression test samples. All the prepared samples were kept in plastic bags to prevent any moisture change due to evaporation.

One-Dimensional Consolidation Test
ASTM D 2435-2011 was chosen to be considered for the conduction of compaction tests by using an oedometer device. The sample size is 50 mm in diameter and 20 mm in height. The testing style is consisting of cumulative time-dependent loading. A seating load (5.0 kPa) is applied to obtain a suitable contact between the porous stone on the soil sample and the loading cap, also, by taking into consideration that the pressure is equal to the vertical overburden pressure corresponding to the depth of sample 0.5 to 1 m from the natural ground level. The vertical static loading consists of 25, 50, 100, 200, 400, and 800 kPa with a load increase ratio of / =1.0 is applied, and each loading left for 24 hours. Besides, vertical static unloading steps are also performed.

Results of consistency tests
The Atterberg limits test results regarding Barika CL soil and hydrated lime mixtures at various percentages are presented in Table 3 and Fig. 3. The reduction in liquid limit and plasticity index is a consequence of exchanges between the free calcium of the hydrated lime and the absorbed cations of the clay mineral. This leads to a decrease in the size of the diffused water layer surrounding the clay particles. The decrease in the size of the diffused water layer enables closer contact among the clay particles resulting in flocculation of these particles, which yielded in significant reductions in LL and PI.

Effect of hydrated lime on unconfined compressive strength (UCS)
UCS tests were performed on the natural and stabilized Barika CL soil samples with hydrated lime. The stabilized samples prepared at various percentages of hydrated lime are shown in Fig. 4. The UCS of hydrated lime-treated samples develops rapidly with the hydrated lime percentage increase until the optimum hydrated lime content is reached. The soil samples in this study exhibit a rapid initial increase in the UCS with the addition of hydrated lime. The added optimum percentage of the hydrated lime to the natural soil sample further increased its UCS values from 174.23 kN/m 2 to 960.85 kN/m 2 . In comparison, similar results have been observed by various researchers who studied soils UCS properties such as (Elhassan, 2006). The addition of hydrated lime might generate bonds among soil particles, which strongly bind the particles together and resist strongly any externally applied forces. This is notably can be noticed in the obtained increase in the UCS value due to 10% hydrated lime.

Results of compaction tests of the natural and stabilized Barika CL soil with hydrated lime
The results for the compaction tests performed on the untreated and treated Barika CL soil samples with hydrated lime were measured via using the standard proctor compaction method. The addition of hydrated lime at various percentages to the soil samples increases their maximum dry density and reduces their optimum moisture content for the same compaction effort, as shown in Fig. 5. A similar trend of behavior has also been observed for the case of hydrated lime treated clay in the study of (Ingles and Metcalf, 1972). Added hydrated lime absorbs some percent of the added water for compaction purposes. This was resulted in a significant decrease in the required amount of water to achieve the MDD. Similar to the previous improved properties, hydrated lime might generate bonds among soil particles, which strongly bind the particles together and resist strongly any externally applied forces. This is notably can be noticed in the obtained increase in the MDD value due to 10% hydrated lime.

Results of consolidation tests of the natural and stabilized Barika CL soil with hydrated lime
From applied pressure-void ratio (e-log p) graphs, all the added hydrated lime percentages, in addition to the natural soil's graph, are given in Fig. 6, and all test results are shown in Table 4. With the addition of hydrated lime up to 10% the value of Cc and mv decreases with the increase of lime content by 89.61% and 79.41%, respectively. The expansion index (Cr) decreases with the increment of lime percent by 69.54%, but the rapid increase occurs at 7.5% after that, the value of the expansion index starts to decrease up to 10%. Generally, the compressibility properties (Cc, Cr, and Cv) decrease distinctively as the lime content increases due to suction and cementation bonding. These results are in agreement with the findings of (Shareef 2016, Sut-Ünver et al., 2018, and Mavroulidou et al., 2013). Similar to the previous improved properties, hydrated lime might generate bonds among soil particles, which strongly bind the particles together and resist strongly any externally applied forces. This is notably can be noticed in the obtained reductions in the values of Cc, Cr and Cv due to 10% hydrated lime.  Figure 6. Applied pressure-void ratio relationships for the natural and stabilized Barika soil samples with various percentages of hydrated lime.

Consistency Properties Correlations
A good correlation was obtained between liquid limit as a function of hydrated lime content in the form of a linear equation with the coefficient of determination R 2 equal to 0.8021, as shown in The best relationship between plasticity index (PI) and used hydrated lime content, as compared to the plastic limit (PL) and used hydrated lime content, can be seen in Fig. 8 in the form of polynomial equations with the coefficient of determination R 2 equal to 0.8181 and 0.7204 respectively and the corresponding non-linear equations as follows:

Unconfined compressive strength (qu) values correlations
The best trend line between qu versus used hydrated lime content plot has given a high coefficient of determination R 2 equal to 0.9818 in the form of a non-linear equation as shown in Fig. 9

Compaction Parameters Correlations
The relationship between maximum dry density (MDD) and the used hydrated lime content, in addition to the optimum moisture content (OMC) relationship with the used hydrated lime content, can be seen in Figs. 10 and 11. The best trend line for the MDD versus HL plot is the linear correlation with a high coefficient of determination R 2 equal to 0.9792, as shown in Fig.10

Consolidation Characteristics Correlations
Compression index property (Cc) correlated with hydrated lime content yielded an excellent relationship, which can be noticed from the obtained R 2 = 0.9622 (Fig. 12). At the same time, the correlation between hydrated lime percent with expansion index (cr) and coefficient of consolidation (Cv) individually showed lower R 2 , which are 0.7383 and 0.7753, respectively (Figs.  13 and 14). The obtained corresponding linear equations are as follows:

RESEARCH OUTCOMES COMPARISON
The results of the conducted geotechnical laboratory tests on the selected CL soil samples yielded insignificant outcomes. The obtained improvements have notably worked with this type of cohesive soils. From Table 5, the perfect percent for stabilization purposes is 10%, which is clearly within the obtained range of lime perfect percent for soil stabilization. Although the achieved percent in this study is higher than most of the listed recent lime stabilization researches, it may be due to various reasons. These reasons might be various, such as particle size distribution, clay minerals percent, soil parent materials, impurities, soil depth, and the used lime composition. More on that, the examined geotechnical properties in this study are more than some of the listed researches, which some of them just checked few properties. Hydrated lime may work perfectly for some of the soil geotechnical properties as those properties may require materials such as lime to decrease water absorption and increase the particle's densification, which can be noticed in the improved consistency and shear strength characteristics. Hence, lime stabilization works notably to stabilize the geotechnical properties of soils in general. Besides, and from the outcomes of this research, lime significantly works to stabilize cohesive soils (CL soil), which is replaced by a part of the problematic composition of that soil and substituted by capable materials to stick to soil particles. Locally-available hydrated lime check to stabilize local CL soil, it strengthened this soil and resulted in favorable changes happened in this soil's natural properties. The mentioned properties improved successfully (10%)

CONCLUSIONS
Based on the test results, the following conclusions can be drawn:  Cl soil consistency properties varied in their response to stabilization by hydrated lime. For the 10% of hydrated lime content, liquid limit and plasticity index reduced significantly by 28 % and 73 %, respectively. While the plastic limit slightly increased by 6%.  The stabilized soil maximum dry density and optimum moisture content were notably improved. The stabilization yielded a 13% increase in maximum dry density and a 37% decrease in the optimum moisture content due to 10% of hydrated lime content.  A significant increase of the CL soil's unconfined compressive strength value, was found for the 10% of hydrated lime content to be 81% increase.  The obtained coefficient of determination, R 2 , for the established relationships between the measured CL soil geotechnical characteristics and the selected hydrated lime content, indicate that these expressions are suitable for the determination of the compaction and strength characteristics for CL soil stabilized with hydrated lime.  The addition of hydrated lime yielded in a valuable decrease in the CL soil's compressibility characteristics values (Cc, Cr, and Cv), which yielded in 89%, 69%, and 79% reductions, respectively.  Overall the experimental outcomes presented the significant role of hydrated lime on the CL soil geotechnical properties. 10% hydrated lime is the best percent to be used for stabilization of the selected CL soil.