Effect of Combined Stabilization by Lime and Cement on Hydraulic Properties of Clayey Soil Selected From Mosul Area

This paper is aimed to study some of the hydraulic properties (permeability and water retention) of a stabilized clayey soil selected from Mosul area. The optimum lime stabilization percentage was found to be 4 %. Cement amounts of 6 % and 18 % are needed to achieve a minimum unconfined compressive strength of 1400 and 5600 kPa respectively “suggested limits for stabilized soil suitable as a base course”. For the same indicated range of strength, a combination of lime-cement ratios of 1-3 % and 2-9 % respectively were found to be sufficient to achieve this limit. Permeability of natural soil was found to increase with treatment of lime, cement and lime-cement. A maximum increasing in permeability about 17 fold was found using 4 % lime. While the permeability was 1.2 folds as a minimum increase when treating the soil with 18 % cement. The soil-water characteristics curve shows that (S-Shape) curves were maintained for natural and stabilized soil. دٌوغلاو جسىٌلات ةكشولا دٍثثرلا شٍثأذ حعاسد حٍعذٌهلا صئاصخلا ضعت ىلع حٍكٍلوسذٍهلاو لصىولا حٌٌذه يه جسارخه حتشرل باطخ ظٌسدإ لٍهع .د يسىثدلا فلخه ذوحه رارعأ ذعاغه طسذه لصىولا حعهاخ / حعذٌهلا حٍلك / حًٍذولا حعذٌهلا نغق حصلاخلا حٌرافٌلا( حٍكٍلوسذٍهلا صئاصخلا ضعت شٍثأذ حعاسد ىلإ ثحثلا فذهٌ )ءاولات ظافرحلإاو يةه جساةرخه حةرثثه حةٌٍٍي حةتشرل لصىولا حٌٌذه . جسىٌلل ىلثولا دٍثثرلا حثغً دادٌإ نذ ًهو 4 % ًةرلا دٌوةغلل ىةلثولا دةٍثثرلا حثةغً داةدٌإ نةذ اةوك ،جسىً ىًدلأا ذحلا ققحذ 14 نع/نغك 2 ىلعلأاو 65 نع/نغك 2 ًةهو ، شةطلل طاةعلأا حقثطل سىصحولا شٍغ ياغضًلاا حهواقول 6 % و 18 % جسىٌلا حٌضول ىلثولا دٍثثرلا حثغً دادٌإ نذ كلزكو ،ًلاىرلا ىلع داةدٌإ ًة حةعثرولا حةقٌشطلا ظفٌت دٌوغلا يٍرثةغٌلا ىأ ذةخو ذةقو ،دٌوةغلل ىلثولا حثغٌلا 3-1 % و 9-2 % حةلهاعولا حةتشرلا خشةهظأ .دةٍثثرلل ىةلثولا ةةغٌلا ًةه جسىٌلاو دٌوغلا ،جسىٌلات فٌلا ً ًجداٌص دٌوغلا .ح اةضولا داىةولا حثةغً يةع شظٌلا ضغت ،حٍعٍثطلا حتشرلا حٌرافً يع حٌرا حثغٌت حلهاعولا جراوٌلل دًاك حٌرافٌلا ً ىلعلأا جداٌضلا ىأ ذخو % 4 دوذحت جسىً 17 لةقلأا جداةٌضلا دةًاك يٍح ً .جشه حثةغٌلا ذةٌع حةٌرافٌلا ًة 18% ًلاىةح دٌوةع 1.2 حةتشرلا حٍةصاخ خاةٌٍحٌه دةظ اح .جشةه اةولا ىةلع حةلهاعولا حةتشرلل ء لكش يأ ماعلا لكشلا (S-Shape) حتشرلا حٍصاخ ًٌحٌول ةٍذشرلا طوً ىاك .حعوسذولا خلااحلا عٍودلو حةٍحاً يةه ءاةولا .خا اضولا حثغً جداٌص عه دادضٌ ءاولات ظافرحلاا حٍلتاق Received: 27 – 6 2011 Accepted: 16 – 1 2012 Al-Rafidain Engineering Vol.20 No. 5 December 2012 140 Introduction The soil at any particular locality could be unsuited, wholly or partially, to the requirements of the construction engineer. A basic decision must therefore be made, whether to [6]: 1. Accept the site material as it is, and design to standards sufficient to meet the restrictions imposed by its existing quality. 2. Remove the site material, and replace with a superior material. 3. Alter the properties of the existing soil so as to create a new site material capable of better meeting the requirements of the task in hand, and this is known as soil stabilization. Soil stabilization is the mechanical, physical, or chemical treatment of a soil designed to increase or maintain stability of the soil or otherwise to improve the engineering properties of the soil, enabling the material to serve as a better construction material [4]. Therefore there are different ways to stabilize soil [4]: 1. Mechanical Stabilization (compaction). 2. Chemical stabilization involves addition of chemicals to the soil, particularly lime, cement, asphalt. Sometimes using two types of stabilizers or more and this known as combined stabilization. 3. Physical stabilization refers to addition of particulate material to the soil, for example clay and sand. The most methods of soil stabilization are: lime, cement, and asphalt. In this paper combined stabilization by lime and cement of soil selected from Mosul area in Iraq was studied. Lime will primarily react with medium, moderately fine, and fine-grained soils to produce decreased elasticity, increased workability, reduced swell, and increased strength [1,12]. The strength of lime-treated soil develops with increasing addition of lime until an optimum lime content is reached beyond which the strength continues to increase at a reduced rate or begins to decline. [3]. A wide range of soil types may be stabilized using Portland cement. It is generally more effective and economical to use with granular soils due to the ease of pulverization and mixing and the smaller quantities of cement required [9]. Fine-grained soils of low to medium plasticity can also be stabilized with cement, but not as effectively as coarse-grained soils. The strength of soil treated with cement increasing with increase in cement content, where a linear relationship can be used to approximate the relationship between strength and cement content [7]. Sometimes combined stabilization is used for the following reasons [7]: 1. Improving of the engineering properties of a certain soil to achieve best using. 2. Economic or environmental reasons. 3. One of the stabilizers in the combination compensates for the lack of effectiveness of the other in treating a particular aspect or characteristic of a given soil. While cement cannot be used alone for highly plastic soils, lime can be used first to initiate action exchange and flocculation-agglomeration reaction and to produce immediate changes by reducing the plasticity and improving the workability of these soils. Addition of cement then ensures rapid strength development of the mixture. This is especially advantageous when rapid strength gain is required under cooler weather conditions [7]. Soil-water characteristics curve one of the important hydraulic properties; it is the relation between water content and soil suction. It is central to the engineering behavior of an unsaturated soil, and can be used as a means of deriving and linking soil behaviors such as permeability, shear strength and volume change [8]. The Main aim of this paper is studying the effect of combined stabilization on some hydraulic and engineering properties; the permeability and the soil water characteristic curve are studied as important properties which describe the changes which happened in the structure of soil selected from Mosul area when Khattab: Effect of Combined Stabilization by Lime and Cement on Hydraulic -----141 treated with lime-cement. The most important studied variable in this paper are: admixtures percentages, effect of curing times which reached to 90 days, and water content of specimens. Materials and Methods


Introduction
The soil at any particular locality could be unsuited, wholly or partially, to the requirements of the construction engineer. A basic decision must therefore be made, whether to [6]: 1. Accept the site material as it is, and design to standards sufficient to meet the restrictions imposed by its existing quality. 2. Remove the site material, and replace with a superior material. 3. Alter the properties of the existing soil so as to create a new site material capable of better meeting the requirements of the task in hand, and this is known as soil stabilization. Soil stabilization is the mechanical, physical, or chemical treatment of a soil designed to increase or maintain stability of the soil or otherwise to improve the engineering properties of the soil, enabling the material to serve as a better construction material [4]. Therefore there are different ways to stabilize soil [4]: 1. Mechanical Stabilization (compaction). 2. Chemical stabilization involves addition of chemicals to the soil, particularly lime, cement, asphalt. Sometimes using two types of stabilizers or more and this known as combined stabilization. 3. Physical stabilization refers to addition of particulate material to the soil, for example clay and sand. The most methods of soil stabilization are: lime, cement, and asphalt. In this paper combined stabilization by lime and cement of soil selected from Mosul area in Iraq was studied. Lime will primarily react with medium, moderately fine, and fine-grained soils to produce decreased elasticity, increased workability, reduced swell, and increased strength [1,12]. The strength of lime-treated soil develops with increasing addition of lime until an optimum lime content is reached beyond which the strength continues to increase at a reduced rate or begins to decline. [3]. A wide range of soil types may be stabilized using Portland cement. It is generally more effective and economical to use with granular soils due to the ease of pulverization and mixing and the smaller quantities of cement required [9]. Fine-grained soils of low to medium plasticity can also be stabilized with cement, but not as effectively as coarse-grained soils. The strength of soil treated with cement increasing with increase in cement content, where a linear relationship can be used to approximate the relationship between strength and cement content [7]. Sometimes combined stabilization is used for the following reasons [7]: 1. Improving of the engineering properties of a certain soil to achieve best using. 2. Economic or environmental reasons. 3. One of the stabilizers in the combination compensates for the lack of effectiveness of the other in treating a particular aspect or characteristic of a given soil. While cement cannot be used alone for highly plastic soils, lime can be used first to initiate action exchange and flocculation-agglomeration reaction and to produce immediate changes by reducing the plasticity and improving the workability of these soils. Addition of cement then ensures rapid strength development of the mixture. This is especially advantageous when rapid strength gain is required under cooler weather conditions [7]. Soil-water characteristics curve one of the important hydraulic properties; it is the relation between water content and soil suction. It is central to the engineering behavior of an unsaturated soil, and can be used as a means of deriving and linking soil behaviors such as permeability, shear strength and volume change [8]. The Main aim of this paper is studying the effect of combined stabilization on some hydraulic and engineering properties; the permeability and the soil water characteristic curve are studied as important properties which describe the changes which happened in the structure of soil selected from Mosul area when treated with lime-cement. The most important studied variable in this paper are: admixtures percentages, effect of curing times which reached to 90 days, and water content of specimens.

Natural Soil
The clayey soil was selected from Al Hadbaa quarter (Mosul city). The basic soil properties are summarized in Table 1. This clayey soil was classified as CH. The device of permeability test was made locally; the dimensions of compaction mold are 9.7 cm in diameter and 3.85 cm in height. It contains two outlets at the bottom of device to supply water during the test, and single outlet at the top to permit the water to exit. This device was made such that to help obtaining higher available degree of saturation by expulsion air bubbles from top. Soil-water characteristics curves were established within suction range of (0-1,000,000) kPa. Saturated solution method was used for the application of suction range (2700-325000) [10]. The other part of the curve was completed using the osmotic solution method, an osmotic membrane submerged in different solutions of Poly Ethylene Glycol (P.E.G) to give solutions of (50,450) kPa. Small chunks (5-10) g of statically compacted soil samples were brought to saturation in distilled water. The soil chunks were then placed in desiccators of different chemical solutions, each one of them will cause certain suction. Equilibrium time was found to be 45 days for saturated solution method, and 21 days for the osmotic solution method. Soil Treated with Cement Portland cement was used in this paper; the method proposed by (Ingles & Metcalf 1972) [6] was used in this paper to select the optimum cement content to stabilize soil. This method involve finding the unconfined compressive strength of cement stabilized soil samples treated with different percentages at a temperature of 25 o C and 7 days curing time. It is clear from Table 2, that a limit of unconfined compressive strength of (14 to 56) kg/cm 2 and of cement treated soil is suitable to be utilized as a road base.
Purpose U.C.S [1] C.B.R [2] Swell Loss in wet/dry test [3] kgf/cm 2 (Ibf/In 2 ) per cent per cent Road sub-base, formation backfill for trenches etc. Road sub-base, base for light traffic [4] Base for heavy traffic [4] Building blocks Embankment protection Floodways (too strong for general use under thin surfacing) 3.5-10.  142 [1] U.C.S cured seven days at constant moisture content. The loss of strength on soaking should not be more than 20 per cent. [2] C.B.R. -Soaked four days. [3] Durability test appropriate only where moisture penetration is likely to occur. Higher cement contains may be required to meet this criterion. [4] Lower strengths may be adequate for well-drained areas in the tropics.
Results showed that, 6 % and 18 % cement were the percentages that attain the lower and upper limit of unconfined compressive strength of road base respectively as shown in Figure ( The liquidity and plasticity limits of cement treated soil are shown in Table 3, the mellowing period was 10 minutes. It is clear that the addition of 6% and 18 % cement turn the soil to non-plastic.

Soil Treated with Lime
Hydrated lime Ca(OH) 2 was used (71.3% Ca(OH) 2 & 6.1% CaO). Stabilization percent (optimum lime percent) was found to be 4% lime based on dry weight using Edges & Grim protocol [5] and Thompson mixture design [12]. To compare the soil samples treated with lime with that treated with cement, the same curing conditions of (temperature 25 o C and curing time 7 days) were applied. Figure (2) shows that 4 % lime should be used to obtain the uppermost of unconfined compressive strength (1135 kPa). This value does not satisfy the lower limit of the unconfined compressive strength of soil used as a road base, Table 2. However, this percentage was choose in this paper to study the effect of lime on the permeability of stabilized soil and on the soil-water characteristics curve. The liquid and plastic limits of lime treated soil was shown in Table 4, the mellowing period was one hour. It is clear that a reduction of liquid limit values and decreasing the plasticity index with addition of lime percentage, and the soil turn to non-plastic at 3 % lime.

Soil Treated with Lime-Cement
Lime-cement mixture of (1-3, 2-9) % cured at a temperature of 25 o C for 7 days were selected on the basis of achieving the lower and upper limit of unconfined compressive strength of soil treated with cement used as a road base, Table 2 [6].  (Table 2) could approximately be achieved using 1 % and 2 % lime respectively.   (6) and (7) show that (2-9) % lime-cement are the required amount to obtain the upper limit of unconfined compressive strength stated in table 2. It is worth mentioning here that, soaking of unconfined compressive strength of soil samples treated with lime-cement for 7 days show a strength loss not exceeding 20 %.
The liquid and plastic limits of soil treated with lime-cement percentages added to it are shown in Table 5, the mellowing period was 10 minutes. It is clear that the soil turn to nonplastic at both percentages.

Results and Discussion
Permeability of Natural Soil The samples were prepared at different moisture content and different dry unit weight as shown in Table 6. Figure 8 shows a reduction in the permeability with increasing both of dry unit weight and compaction moisture content. From the test, it is found that the coefficient of permeability (k) of natural soil compacted at maximum dry unit weight (MDD) and optimum moisture content (OMC) is equal to 7.9 * 10 -8 cm/sec. Hence, the soil is classified as impermeable [11] depending on soil type and coefficient of permeability.   Figure 9 presents the variation in the coefficient of permeability according to curing time initial placement conditions (water content and dry unit weight). Results show an increase in permeability of soil treated with lime at all curing conditions compared with permeability of natural soil. This could be explained by the change in soil structure of lime treated soil; the cation exchange with (Ca++) ions produced from lime addition is expected to causes a reduction in the ionic layer in treated soil, thus creating a flocculated structure. Also, this Figure shows that this effect in the dry side of compaction curve was larger than that at the wet side. In the other side, Table 7 indicates an increase between 1.2 to 17 folds in the wet side (2 days curing time) and in the dry side at (28 days curing time) respectively compared with the permeability of natural soil.   146 Figure 10 again shows increasing in permeability due to cement addition at all curing periods compared with the permeability of natural soil. This could be attributed to the same reason that previously cited at lime addition. The samples that treated with 6 % cement showed a higher permeability than that treated with 18 % cement, where the maximum increase in permeability are about (8 to 12) folds compared with the permeability of natural soil (in the dry side and at curing time 28 days) respectively. This may be due to the formation of a hardened materials in the samples treated with 18 % cement and the presence of an additional un reacted amounts of cement.   On the other hand, the samples treated with 1-3 % lime-cement showed a higher permeability than that treated with 2-9 % lime-cement, where the more increase in permeability about (10 to 13) folds compared with the permeability of natural soil (in the dry side and at curing time 28 days) respectively as shown in Figure 11. This is due to the same reason that previously cited in the soil treated with cement. Comparing between permeability of the soil samples treated with cement that achieving the lower and upper limit of unconfined compressive strength with permeability of the soil samples treated with limecement; The samples that treated with 1-3 % lime-cement showed small increase in permeability compared with the samples that treated with 6 % cement, where the more increase in permeability is about (12 to 13) folds compared with the permeability of natural soil (in the dry side and at curing time 28 days) respectively. The treated samples with 2-9 % lime-cement showed small increase in permeability compared with the samples that treated with 18 % cement, where the more increase in permeability is about (8 to 10) folds compared with the permeability of natural soil (in the dry side and at curing time 28 days) respectively. It is worth noting that, all the cement treated soil samples showed permeability values less than that treated with lime. Figure 11. Relationship between coefficient of permeability and moisture content of treated soil samples with (1-3, 2-9) % lime-cement at different curing times.

Effect of Moisture Content on Permeability of Treated Soil
The results shown in Figure 12 indicate an increase in the permeability of treated soil compared with that of natural soil under the same compaction conditions. This increasing covers all the moisture percentages. The results showed also, that the permeability of treated soil with certain percentage of additives compacted in the wet side were less than that compacted in optimum moisture content and in the dry side. This could be attributed to the flocculation-agglomeration beside the role of moisture in the formation of soil structure.
Here, the particles arrangement changes from the parallel structure to the flocculated structure which consequently increase the voids ratio and permeability. A similar results was mentioned by (Khattab, S. A. A. 2002) [10]. On the other hand, the maximum increase in the permeability was noted in the lime treated soils with 4 % compared with that of natural soil and cement and lime-cement treated soils in all the studied cases. As example, the lime addition to samples prepared at the dry side leads to 17 folds increase in permeability (curing time 28 days), while, an increase of 9 and about 5 folds were noted for the samples compacted at the optimum moisture content and wet side respectively at 7 days curing time.
More variation of the coefficient of permeability of the treated soil in the dry side was between 3.42 * 10 -5 cm/sec (soil treated with 4 % lime) to 1.52 * 10 -5 cm/sec (soil treated with 18 % cement) at curing time 56 days, where at the optimum moisture content was between 7.21 * 10 -7 cm/sec (soil treated with 4 % lime) to 3.6 * 10 -7 cm/sec (soil treated with 18 % cement) at curing time 7 days. Finally more variation in the wet side was between 4.23 * 10 -7 cm/sec (soil treated with 4 % lime) to 2.11 * 10 -7 cm/sec (soil treated with 18 % cement) at curing time 7 days.

Effect of Curing Time on Permeability of Treated Soil
Figures (13, 14 and 15) present the effect of curing times on the values of coefficient of permeability. It is clear that the coefficient of permeability increases at the earlier curing times (between 7-28) days followed by a reduction in different rates with increasing of curing times for all the studied cases. This could be explained by the flocculation process in the first days and after that to the formation of the pozzolanic reaction products that cause closing some of water paths. Identical results are noted (Khattab, 2002) [10], while (EL-Rawi, & Amir, 1981) [6] noticed a continuous reduction in permeability with increasing of curing times. 150 The more increasing in permeability of the treated soil samples was at curing time 28 days in the dry side, where the more increasing in permeability of the treated soil samples was at curing time 7 days at the optimum moisture content and in the wet side. This may be attributed to the effect of moisture percentage on the chemical reaction operations for lime-cement and cement hydration.

Effect of Additives on SWCC for Treated Soil
It is shown from Figure (16) that the water holding capacity increases with the addition of additives. The SWCC`s for treated soil samples with 4 % lime were the closest to SWCC for natural soil and located in the lower part of the figure. While the SWCC for treated soil samples followed the sequence of increasing the water holding capacity as; 1-3 % lime-cement, 4 % lime, 6 % cement, 2-9 % lime-cement and finally the SWCC for treated soil samples with 18 % cement in the upper part of the figure. This could be explained by the fact that the higher percentages of additives lead to a more hardened products compared with the small percentages. These products will be expected to construct a more open structure, and the excessive amount of additives act as fine material which fill some pores, and hence, increasing water holding capacity for soil. It could be also noted that there was an interaction between SWCCs at high suction ranges, Figure (16). This behavior was due to the insufficient amount of water for the reactions to proceed because of the high suction applied Figure 16. SWCC of treated soil samples that prepared at maximum dry unit weight and optimum moisture content.  [2]. Finally, Table (8) indicates that the air-entry value and residual suction were increased with additives percentages, and the boundary effect zone and transition zone were also expanded. This could be attributed to the addition of lime and/or cement to soil and its above stated effects on the soil structure  (17 to 21) show the effect of curing times on the SWCCs. It was noticed that SWCCs move upward as curing time proceeds, where the suction value was increased at any value of moisture content at curing time proceeding from (7-90) days. This is expected to be due to the curing time that causes the formation of a more hardened products and more open soil structure, leading to a structure with high water holding capacity. On the other hand, it was noted that, in general, there is a divergence of SWCCs till 28 days in boundary effect zone and transition zone, followed by convergence till reaching 90 days curing time in boundary effect zone and transition zone. This could be attributed to the decrease in the rate of hardened materials formation in the time between 28 and 90 days compared with that from 0 to 28 days. Finally, a convergence of SWCCs was observed at residual zone of treated soil specimens.

CONCLUSIONS
The following conclusions could be obtained from this work considering the limits and soil type of this study 1. The combined stabilization using more than one stabilizer was found to be more economic over than using single stabilizer alone for the selected clayey soil from Mosul city. Soil stabilized with (2-9) % lime-cement was found to attain nearly an identical strength of that treated with 18% cement. 2. Permeability of natural soil was found to increase with treatment of lime, cement and lime-cement. 3. More permeability is noticed for the samples compacted in dry side of the compaction curve for both natural and treated soil, while less permeability is observed for the samples compacted in wet side of the compaction curve of the treated soil. 4. All the studied conditions on soil-water characteristic curves had (S) shape curve for natural and treated soils. 5. Water holding capacity for the treated soil was increased due to the increase in additive percentages. 6. Air-entry value and residual suction were increased with additive percentages, while the boundary effect and transition zones were also expanded.