Tensile Strength of Natural and Lime Stabilized Mosul Clay Al-Layla

The Purpose of this study is to investigate mainly the tensile stress properties of natural and stabilized clayey soil selected from Mosul area .The tensile strengths of compacted specimens of natural soil and lime stabilized soil are obtained using the flexural test (third-point loading). The tensile and the compressive stress-strain curves of both soils are evaluated. The compressive strength on a portion of the beam is determined for both soils. The results reveal that both the tensile and compressive strengths increase with the addition of lime and with the increasing the curing time .Furthermore ,the results show that the tensile strength is more sensitive to lime stabilization than the compressive strength .The stress-strain curves of the stabilized specimens are rather irregular .All specimens, natural and stabilized show sudden type of failure .The method of analysis used for determining the tensile stressstrain curves are the direct method of analysis.

Tensile stresses can develop in highway pavement layers, at some distance from the wheel load. When the tensile stresses reach or exceed the tensile strength of the pavement layers, cracking takes place which results in excess deflections and more stresses reaching the sub-grade layer. Therefore, the higher the tensile strength of the pavement layers, the more sound roads.
The natural soils have low tensile strength compared with the compressive strength. To improve the tensile strength properties, the soil may be stabilized.
It is well known that the lime stabilization of a clayey soil improves its strength and other engineering properties [1,2,3]. The tensile strength is one of the main properties which the pavement designer consider and always try to select a material with edquate tensile strength . The tensile characteristics can be measured by direct method, indirect method (Brazilian) and by bending method (flextural). Because of the slab action of the pavement layers, the result of flexural tension test are more appropriate than other methods to evaluate the tensile properties of the pavement materials [4].
Leonards and Narain [3] carried out flextural tension tests to investigate the tensile properties of compacted clays. They reported that the tensile strain decreased with the increase of compactive effort at comparable moisture contents. For a fixed compaction energy, the tensile strain increased with increasing moisture content up to the optimum.
Addanki et al [5] performed indirect tension test on a compacted well graded granular soil to study the effect of water content and compactive effort on the tensile strength. They concluded that the tensile strength decreased with an increase in water content, while the tensile strength increased with increasing the compactive effort for water content below the optimum and decreased slightly for water content above the optimum.
Lushnikov et al [6] conducted a series of direct tension and unconfined compression tests on compacted loams and clay soils. They showed that the moisture content have a significant effect on the values of modulus of elasticity in tension and compression.
Ajaz and Parry [7] reported that the values of the initial tangent modulus from the flextural test of clay is greater in tension than in compression and their values are influenced by the moulding moisture content. Thus the use of identical values for tension and compression in analysis of soil structure is not justified.
Jaro [8] measured the tensile strength of sub-base material stabilized with cement. He found that the tensile strength of stabilized material with 4% cement increased as the fine material increased up to 14% and then decreased. For 8 and 12 percent of cement, the maximum strength occurred at 6% of fine material then decreased.
In this study, the tensile strength characteristics of natural and lime stabilized clayey soil selected from Mosul city were determined. The clay was stabilized with 2, 4 and 6 percent lime and cured for 7 and 30 days at room temperature of (25 ± 2 o C ).
The tensile properties were obtained using the simple beam with thirdpoint loading. The results of the tests were analyzed using the direct method of analysis.
The soil used in this study is a clay obtained from Al-hadba , a District in Mosul city. The compaction characteristic of the soil was established using modified compactive effort. The soil was stabilized using 2, 4 and 6 percent lime, and the compaction curves of soil lime mixture were obtained.
The flextural test was conducted on untreated and stabilized soil using prismatic beam (50.8 * 50.8 * 305 mm). The specimens were prepared by compacting the soil at the optimum moisture content in four layers using special square base hammer weighing (17.1)N and falling from (28)cm. to obtain the modified compactive effort. The compacted specimens were wrapped in serval plastic bags to secure constant water content during the curing periods.
The specimens were cured at room temperature of (25 ± 2 o C ) for 7 and 30 days. At the end of curing periods the specimens were weighted and it was found that the weight was almost constant with variation not more than 2 gm. out of about 1600 gm.
The specimen was mounted in the compression machine as shown in Fig.(1) and a load was applied at rate of 0.127 mm/min (0.005in/min). The deflections at the center of the beam (top and bottom) with applied load were recorded every (1min.) and the flextural strength properties were evaluated.
In addition to the evaluation of the flextural strength, the compression strength was determined using a portion of the beam which was free from cracks or any seen defect. This test was performed according to (ASTM D 1634-631997 ) [11].
The direct method of analysis is used to calculate the tensile stress at the bottom and the compressive stress at the top of the beam from the applied bending moment. In this method it is assumed that the plane section remain plane after bending, that is the elongation and contraction of longitudinal axes are proportional to their distances from the neutral axis. The value of deformation modulus in tension may differ from that in compression (hence the neutral axis is not necessarily at the mid-height of the beam) and no creep occurs during bending.
Duckworth [12] derived the following Equations for tensile stress (σ t ) and compressive stress (σ c ) The Strain of the beam is found from the following Equations:- The compaction characteristics of untreated and the treated soil with different percentage of lime is shown in Fig.(2). The maximum dry unit weight decreases with the addition of lime and the optimum water content increases. This behavior of the lime treated soils have been reported by several investigators and is due to agglomeration of soil particles and the affinity of lime to water [2,13,14]. The results of the tensile and compressive strength of the compacted specimens, at the optimum water content, are presented in Table (3) and in Figures (3 and 4) The tensile strength of the untreated soil is 180 kPa. When 2% and 4% of lime was used and the specimens cured for 7 days, the tensile strength increased to 218 and 430 kpa respectively, then decreased to 328 kpa at 6% lime.
On the other hand, when the soil cured for 30 days, the tensile strength increased to 233, 504 and 560 kPa when 2, 4, and 6 % lime was used respectively.
The increase in the tensile strength with the increase of the percentages of lime is due to the reaction between lime and clay which is a function of the amount of lime, type of the clay minerals and curing conditions.
The reduction in tensile strength when the soil is treated with 6% lime and cured for 7 days is due to the extra lime which acts as a fill material with the short curing period .This extra lime is the result of uncompleted reaction in 7 days between lime and clay minerals .Similar behavior was found in compressive strength with the same percentage of lime [ Table  (3) and fig.(4)].
The effect of lime on the tensile strength ( ) and compressive strength ( ) can be visualized by considering the ratio ( / ) as shown in table (3). For untreated soil the ratio is 15%. When the lime is added and the specimens cured for 7 days the ratio become 14.5, 23.4 and 23.4% for 2, 4 and 6% of lime respectively. This means that the relative increment of increase in compressive strength is more than that in tensile strength for 2% lime, while for 4 and 6 % lime the relative increment in tensile strength is more than that in compressive strength. The same behavior in the ratio ( / ) was found for the specimens cured for 30 days. The tensile and compressive stress that occurred at bottom and top of the beam respectively during the flexural tension tests are calculated using equation 1 and 2. These values are used to draw the stress-strain curve for both the untreated and lime stabilized specimens as shown in Figs. (5 and 6). In general the curves of stabilized specimens are irregular in their shape, such relations were observed by Ajaz and Parry [7] for compacted clay and Jaro [8] for sub base material stabilized by cement. It is believed that such irregularities in the stress-strain curves are due to progressive type of failure which took place during the test or due to non uniform distribution of lime in the specimens .All the specimens showed sudden type of failure.