Silica Fume Influence on Behavior of Expansive Soil

. In this investigation, silica fume (SF) was used as an additive material to study its influence on the characteristics of expansive soil. The soil used in this research was prepared in a lab by mingling bentonite (50 percent by soil dry weight) with natural soil. According to (USCS), the prepared soil was classified as (CH) with a liquid limit of 67.5 percent and a plastic limit of 29.3%. Expansive soil was mixed with different percentages of (SF), (8, 9, 10, 11, 12, and 13%), and experiments were conducted for treated and untreated soil to examine the impact of (SF) on Atterberg's limits, compressibility, swelling, and shear strength parameters. The outcome of this test shows that the use of silica fume raised Atterberg's limits (L.L, P.L, and P.I) and lowered compressibility and swelling percentage, as well as shear strength parameters (c,φ), were altered by increasing the angle of internal friction (φ), and decreased the cohesion (c). From these results, it is concluded that (SF) has a positive effect as an additive material to enhance the geotechnical properties of expansive soil.


INTRODUCTION
The expansive soil is distinguished by its capacity to expand when wet.The presence of clay minerals like montmorillonite is mostly to blame for the expansion of soil [1].Desert and semi-desert sections of the globe are affected by soil swelling.For these countries, evaporation levels are higher than the average annual precipitation.Adding water can stimulate the soil with the potential for swell.Semi-arid regions are characterized by short periods of precipitation accompanied by long periods of clouds, resulting in periodic swelling and deflation [2].The soil heave resulting from soil swelling potential is a multi-factorial phenomenon involving a mixture of a type of matter, clay minerals' shape and quantity, microfabric, initial water content, and dry density [3].Swollen soil issues can include the cracking and collapsing of sidewalks and building foundations, roads, and canals.As well as that, it harms irrigation systems' water pipelines, tank liners, and sewage lines [4].
When a soil material is improved and stabilized, its bearing capacity is boosted, and its strength and durability are enhanced [5].Some techniques of treatment may be necessary to make expansive soils appropriate for building purposes.Stabilizing these soils has been the subject of several studies.One of these methods is using pozzolanic materials [6], such as lime, cement, waste materials (fly ash, rice husk ash, etc.), and metakaolin [7][8][9][10][11].Additionally, a new technology known as the precipitation of calcite induced through microorganisms (MICP), which employs ureolytic bacteria to stabilize soil, has been developed [12].Many researchers studied the effectiveness of silica fume as a stabilizer to modify expansive soils.This additive material (SF) may be used alone [13 -19] or mixed with other materials to enhance the properties of expansive soil, such as (scrap tire rubber fiber, rice husk ash, cement, lime, etc.) [20][21][22][23][24][25][26].This study aims to investigate silica fume's influence on the behavior of expansive soil.

MATERIALS 2.1 Soil Used
The natural soil was blended with 50% bentonite in order to create the expansive soil in the lab.The natural soil was brought from a region east of Baghdad.The physical and chemical parameters of the soil were specified via a series of experiments on both natural and prepared soil.Table 1 contains the test results.(USCS) defines prepared soil as (CH).The grain size distribution and outcomes of compaction are shown in Figures 1 and 2

Silica Fume (SF)
Silicon and ferrosilicon alloys may produce (SF) as a byproduct of the process.Every year, the globe produces almost 100,000 tons of (SF).Due to the high surface area and silica content, silica fume is Pozzolanic and finer than fly ash.(SF) were emitted into the atmosphere until the mid-1970s [23].

PREPARATION OF MIXTURES
For 24 hours, the natural soil was dehydrated at 105 degrees Celsius to remove moisture.The natural soil was first blended with 50% bentonite to create expansive soil.Then, the required percentage of silica fume (8, 9, 10, 11, 12, and 13) % was added as a percentage (total dry weight of the prepared expansive soil) to the prepared soil and mixed manually.Also, the appropriate quantity of water (for optimum water content) at room temperature in a slow stream was mixed by hand until the water dispersed throughout the mixture.

RESULTS AND DISCUSSIONS 4.1 Silica Fume's Effect on Atterberg's Limits
The outcome of the Atterberg limits tests for treated and untreated prepared expansive soil are displayed.inFigure 3, from which it can be noticed that L.L, P.L., and P.I increased with increasing silica fume percentage (8,9,10,11,12, and 13) %, depending on the soil condition.This is attributed to the soil type, the silicate clay mineral's relative concentration, and its associated exchangeable cat ions in the samples [27,28].In addition, Atterberg's limit was raised due to the excess of (SF) without aluminous materials [24].

Silica Fume's Effect on Compressibility
The consolidation test was done for treated and untreated prepared expansive soil to explain the impact of (SF) on compressibility characterizes.Figure 4 shows the relationship between silica fume content (8, 9, 10, 11, 12, and 13) % and compressibility characterized (swelling (expansive) index (Cr) and compression index (Cc)).It can be seen that (Cc) and (Cr) decrease as the amount of silica fume added rises.An explanation for this action may be found in clay minerals' relationship with silica fume particles and the addition of low plastic content.Calcium silicate gels are produced by the reaction of activated silica with calcium and hydroxide, the clay mineral content of the composite samples has decreased as a consequence of this chemical reaction [16,[29][30][31].

Silica Fume's Effect on Shear Strength Parameters
The direct shear test was carried out on untreated and treated prepared expansive soil in order to study the impact of (SF) on shear strength parameters (c and φ).The outcome of these tests is presented in Table 2 and Figure 5. From this, it is noticed that the cohesion (c) decreased as the percentage of silica fume increased.While (φ) was increased as the percentage of (SF) increased.This behavior results from a chemical reaction between soils and (SF) and internal friction of silica fume particles.

4.4Silica Fume's Effect on Swelling
A swelling test was done for treated and untreated prepared expansive soil using consolidation cells to determine the impact of (SF) addition on soil swelling.The influence of (SF) addition on swelling percentage is shown in Figure 6, from which it can be noticed that the swelling percentage decreases by increasing the percentage of (SF) This is due to the interaction between (SF) particles and the soil minerals.

Figure 1 :
Figure 1: Grain size distribution of prepared soil.

Figure 2 :
Figure 2: Relationship between dry unite weight and moisture content of prepared soil.

Table 1 :
. Physical and chemical properties of soil used.

Table 2 :
Shear strength parameters for untreated and treated soil.