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Abd El-Aziz, Abo-Hashema, and El-Shourbagy

The Effect of Lime-Silica Fume Stabilizer on Engineering Properties of Clayey Subgrade

By

Dr. Magdy A. Abd El-Aziz / Dr. Mostafa A. Abo-Hashema
Assistant Professor, Dept. of Civil Engineering / Assistant Professor, Dept. of Civil Engineering
Cairo University – Fayoum Branch / Cairo University – Fayoum Branch
Ph. +2010-527-3502, Fax. +208-433-4031
Dr. Mahmoud El-Shourbagy
Associate Professor, Dept. of Civil Engineering
Mansoura University

ABSTRACT

The engineering properties of clayey subgrade soils may need to be improved to make them suitable for construction using some sort of stabilization methods. Stabilization of pavement subgrade soils has traditionally relied on treatment with lime, cement, or waste materials such as Silica Fume (SF). Most transportation agencies, however, are hesitant to specify these nontraditional stabilizers without reliable data to support vendor claims of product effectiveness. The main objective of this study is to investigate the effect of the engineering properties of clayey soils when blended with lime and SF. A series of laboratory experiments have been implemented for varieties of samples: 1%, 3%, 5%, 7%, 9% and 11% for lime and 5%, 10% and 15% for SF. The results from the Lime-Silica Fume (LSF) blend confirmed that the blend would diminish swell behavior of clayey soils. Their Plasticity Index and swell potential would decrease from 40.25% to 0.98% and from 19.0% to 0.016%, respectively, when subjected to a LSF blend of 11-15%. Their California Bearing Ratio value would increase from 3.0% to 17.0% at a LSF blend of 5-15%. Their internal friction angle concerning shear strength parameters would enhance from 5.80 to 24.75. Soil cohesion increased as well from 55.52 kN/m2 to 157.54 kN/m2. At LSF 5-10%, consolidation settlement was lowered from 0.025 to 0.007. All of these results can be summarized to say that by blending Lime and Silica Fume together, the engineering properties of clayey soils can be enhanced.

Keywords

Engineering Properties, Soil Stabilization, Subgrade, Clayey Soils, Silica Fume, Lime

INTRODUCTION

A difficult problem in civil engineering work exists when the subgrade is found to be clay. Clay soils having high clay content have a tendency to swell when their moisture content is allowed to increase [1]. This moisture may come from rains, floods, leaking sewer lines, or from the reduction of surface evaporation when an area is covered by a building or pavement. Frequently, these clayey soils cause the cracking and breaking up of pavements, railways, highway embankments, roadways, foundations and channel or reservoir linings [2].

Most of the agricultural roads in the Egyptian road network were originally constructed as a canal or drain embankments. These embankments are molded with high percent of clay and they generally have high swell potential and are not favorable when used for construction where construction damages are possible [3]. When civil engineers are faced with possible construction damage, a need for improving the engineering properties of the soil is justified using some sort of stabilization methods. Stabilization of pavement subgrade soils has traditionally relied on treatment with lime, cement, and special additives such as Pozzolanic materials. Pozzolanic materials, such as Fly Ash, Silica Fume, and Rice Husk Ash, which are regarded as wastes may be used for soil improvement as indicated in recent research [4]. The Silica Fume is found to be 40% cheaper than that of Portland cement.

Most transportation agencies, however, are hesitant to specify such these nontraditional stabilizers without reliable data to support vendor claims of product effectiveness. The main objective of this study is to investigate the effect of blending clayey soils with lime and SF together on their engineering properties. A series of laboratory experiments have been implemented on varieties of samples by blending both Lime and Silica Fume together. These samples were 1%, 3%, 5%, 7%, 9% and 11% for lime and 5%, 10% and 15% for SF.

Soil Stabilization

Definition

Soil Stabilization in its broadest sense implies the improvement of both durability and strength of soil. Soil may be improved by adding the deficient particle sizes to give a more satisfactory grading. A process known as mechanical stabilization or in other instances cement, lime, bitumen, or special additives are used to bind or waterproof the particles of soil and so increase its strength and durability. Cement treatment is most applied to road stabilization especially when the moisture content of the subgrade is very high. Waste materials or nontraditional stabilizers, such as Silica Fume and Fly Ash, are also sometimes applied for stabilization. When the soil has been treated with any of the mixtures mentioned above, thus is called stabilized soil [5]. This study focuses on using Lime and Silica Fume together as a stabilizer material for clayey soils.

Lime Stabilizer

Calcium hydroxide (slaked lime) is most widely used for stabilization. Calcium oxide (quick lime) may be more effective in some cases; however, the quick lime will corrosively attack equipment and may cause severe skin burns to personnel. Ingles (1972) recommended the criteria of lime mixture as show in Table 1 [6].

Silica Fume Stabilizer

Silica Fume (SF), also known as micro-silica, is a byproduct of the reduction of high-purity quartz with coal in electric furnaces in the production of silicon and ferrosilicon alloys. SF is also collected as a byproduct in the production of other silicon alloys such as ferrochromium, ferromanganese, ferromagnesium, and calcium silicon [7]. Before the mid-1970s, SF was discharged into the atmosphere. After environmental concerns necessitated the collection and landfilling of Silica Fume, it became economically justified to use Silica Fume in various applications.

SF consists of very fine vitreous particles with a surface area on the order of 20,000 m2/kg (215,280 ft2/lb) when measured by nitrogen absorption techniques, with particles approximately 100 times smaller than the average cement particle. Because of its extreme fineness and high silica content, Silica Fume is a highly effective pozzolanic material [7,8]. Silica Fume is used in concrete to improve its properties. It has been found that SF improves compressive strength, bond strength, and abrasion resistance; reduces permeability; and therefore helps in protecting reinforcing steel from corrosion.

SF is available in two conditions: dry and wet. Dry silica can be provided as produced or densified with or without dry admixtures and can be stored in silos and hoppers. Silica Fume slurry with low or high dosages of chemical admixtures are available. Slurried products are stored in tanks with capacities ranging from a few thousand to 400,000 gallons (1,510 m3) [9,10].

SF is a pozzolanic material that could be potentially used in Egypt, considering it is sufficiently produced and is widespread. When SF was allowed to burn under controlled temperature, higher pozzolanic properties were observed. Silica is a main mineral of SF. When reacted with lime, it will form a bonded gel [Ca (SiO3)]. The composition of SF minerals is shown in Table 2 [11].

Reaction Mechanism of Pozzolanic Materials

Lime reacts with any other fine pozzolanic component (such as Silica Fume) to form calcium-silicate cement with soil particles. This reaction is also water insoluble. The cementing agents are exactly the same for ordinary Portland cement. The difference is that the calcium silicate gel is formed from the hydration of anhydrous calcium silicate (cement), whereas with the lime, the gel is formed only by the removal of silica from the clay minerals of the soil. The pozzolanic process may be written as:

Ca (OH)2 + SiO2 C-S-H (1)

Ca (OH)2 + Al2O3 C-S-H (2)

(Note: Calcium Silicate Hydrate, C-S-H, is cemented material).

The silicate gel proceeds immediately to coat and bind clay lumps in the soil and to block off the soil voids in the manner shown by Figure 1. In time, this gel gradually crystallizes into well-defined calcium silicate hydrates such as tobermorite and hillebrandite. The micro-crystals can also mechanically interlock. The reaction ceases on drying, and very dry soils will not react with lime or cement. The mechanism of the reaction can be represented as:

NAS4H + CH ® NH + CAS4H ® NS + degradation product* (3)

NH + C2SH** 8 (2CH)

Where: S = SiO2, H = H2O, A = Al2O3, C = CaO, N = Na2O.

* As silica is progressively removed, calcium aluminates and alumina are formed residually

** Or CSH

Materials Tested

Stabilizer Product: Lime and Silica Fume

The stabilizer materials used in this study were Lime and Silica Fume (LSF). Lime material was taken from a quarry located in Beni-Suif, Egypt and Silica Fume products were brought from Ferrosilicon Alloys Company (Edfo-Komombo), Aswan, Egypt. Hydrated lime in powder form was used for the lime material. It is noteworthy that SF has a high degree of water absorption. When SF reacted with lime, it will form a bonded gel [Ca (SiO3)]. The composition of SF minerals is presented before in Table 2.

Test Soils

The clayey soils involved in this research were dug from an area in Fayoum, Egypt. The soil sample was disturbed. The physical characterization of clay sample is presented in Table 3. The soil is classified as Silty Clay (Gs = 2.625 with 90.25% fines) with expansive behavior. The chemical element of tested materials is presented in Table 4.

Specimen Preparation

Test samples were mixed from pulverized, air-dry soil and de-ionized water. Treated specimens were prepared following the nine-step protocol outlined here. Untreated control specimens were prepared in the same manner, but without the addition of the stabilizer product (LSF).

  1. Using the modified Proctor compaction test [12], the Optimum Moisture Content (OMC) for compaction was determined for the untreated soil. These values are listed in (Table 3).
  2. Based on the product literature, the recommended percentages of the stabilizer product were as follow: 1%, 3%, 5%, 7%, 9% and 11% for lime and 5%, 10% and 15% for SF.
  3. The test soil was pre-moistened to a natural moisture content = 70.65 (Table 3). The soil was mixed dry of optimum at this point to allow for the water that would be added with the stabilizer in Step 6.
  4. The pre-moistened soil was allowed to mellow for at least 16 hours in a sealed container.
  5. The mass of stabilizer needed to achieve the recommended OMC in the treated sample was measured out.
  6. The stabilizer was thoroughly mixed with the soil sample, which was then allowed to stand for 1 hour in a covered container. If there were no evaporation losses, the soil water content would now be equal to the OMC.
  7. The treated soil was compacted using a modified Proctor effort [12], extruded from the compaction mold, and sealed in a plastic bag.
  8. The compacted soil was cured in a sealed plastic bag at room temperature for 7 days.
  9. The cured sample was trimmed to an appropriate size for testing. If the specimen water content was more than 3% above or below the OMC, new specimens were prepared using adjusted initial water content. Almost all test specimens were within ±2% of the target OMC.

A seven-day curing period was selected as a reasonable delay to allow reactions between the stabilizer and the soil prior to conducting the evaluation tests. Laboratory assessments of soil stabilizers often include a 28-day cure following treatment; the additional three weeks may, depending on the stabilizer, yield additional changes in the soil properties. However, it is expected that significant changes due to an effective soil treatment should be measurable at seven days.

It is noteworthy that all specimens, whether untreated or treated, were compacted at the same optimum water content. In this study, however, the same water content was used to prepare all specimens of a given soil, so that the effect of the stabilizer on the measured soil properties could be distinguished from the effects of varying the water content. For the same reason, the samples were maintained at constant water content during the curing period.

Laboratory Experiments

A series of laboratory experiments have been conducted for varieties of samples. These experiments are to measure the engineering properties of the soil as follow:

·  Physical Properties

o  Specific Gravities

o  Consistency Limits

o  Swelling

·  Grain Size Distribution

·  Compaction Characterization

·  California Bearing Ratio

·  Triaxial Test

·  Consolidation Test

Analysis of the Results

Physical Properties

Based on the laboratory experiments, the soil is classified as Silty Clay (Gs = 2.625 with 90.25% fines) with expansive behavior. Soil with PI > 35% is classified to have very high swell potential [1], where LL and PI of the sample is respectively 72.45% and 40.25%. The effect of blended LSF on the physical properties of soil can be shown as in Figure 2, Figure 3, and Figure 4.

Specific Gravities

Specific Gravities (Gs) for the untreated and treated soils were determined and the results were plotted in Figure 2. As shown in Figure 2, by adding LSF the specific gravity of the soil decreases. This indicates that the soil is lighter than that of its natural conditions.

Consistency Limits

The Atterberg limits for the untreated and treated soils were determined following ASTM D 4318 [13]. The treated soil samples were allowed to cure for seven days before testing but were not compacted. The results are plotted in Figure 3 (a), (b), and (c). Figure 3 shows the influence LSF has on consistency limits. Figure 3(c) shows that the PI of the soil decreases when lime content is increased.