XRD and SEM Studies of Chemically Treated Expansive Soil Subgrades

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XRD AND SEM STUDIES OF CHEMICALLY TREATED EXPANSIVE SOIL SUBGRADES

Manchikanti Srinivas Research Scholar, Department of Civil Engineering, JNTU, Kakinada–533 003, India. E-mail: manchisri@yahoo.com G.V.R. Prasada Raju Professor, Department of Civil Engineering, JNTU, Kakinada–533 003, India. E-mail: gvrp_raju@yahoo.com

ABSTRACT: This paper reports the outcome of a field investigation into the effects of potassium chloride, calcium chloride and ferric chloride on some geotechnical properties to ascertain their suitability for use as a modifier or stabilizer in the treatment of expansive soil. Test tracks were constructed whose subgrades were treated with potassium chloride, calcium chloride and ferric chloride. Chemical tests were carried out (on the samples collected at a depth of 0.5 m from potassium chloride, calcium chloride and ferric chloride-treated test tracks) after the test tracks were exposed to one season of drying and wetting. Samples were also collected from the untreated test track, to observe the relative performance between treated and untreated test tracks, by carrying out X-ray diffraction and Scanning Electron Microscope tests. The results of chemical and fabric studies, have indicated that ferric chloride is the best alternative for treating the expansive soil when compared with potassium chloride and calcium chloride. 1. INTRODUCTION

Many researchers Al-Hamoud (1995), Srivastava et al. (1997), Rao (2000), Murty & Raju (2001), Tiwari et al. (2001) and Lopez-Lara et al. (2007) have documented the influence of additives on the properties of expansive soil through the study of XRD and SEM data.

The site selected for the test tracks was situated very close to the Civil Engineering Department of JNTU-Kakinada. Test tracks 3 m long and 1.5 m wide were constructed.

The X-ray diffraction data and the details of photomicrographs of SEM of the untreated sample, potassium chloride-treated sample, calcium chloride-treated sample and ferric chloride-treated sample, along with their interpretation are below.

2. RESULTS OF X-RAY DIFFRACTION

The X-ray diffraction data for the various treated and untreated samples are shown in Figures 1–4 and the observations made from them are presented below.

The layered structure of the untreated soil has expanded along the 'c'-axis, because of the exchangeable cation and due to interlayer solvation. The charge density for an abundant mineral, in the soil, is low. Both the cation-exchange capacity and total surface area are high.

Therefore, this clay maximises all the characteristics associated with colloidal materials, such as particle size, adsorption, plasticity, cohesion, shrinkage and swelling. This mineral is stable in neutral, slightly leached soils, where silica and basic cation activities are high. This all, confirms the presence of montmorillonite, in the untreated soil. The ‘d’ values confirm this further, as shown in Figure 1.

Fig. 1: X-ray Diffractogram of Untreated Soil

IGC 2009, Guntur, INDIA

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Fig. 2: X-ray Diffractogram of Potassium

Chloride-Treated Soil

Fig. 3: X-ray Diffractogram of Calcium

Chloride-Treated Soil

Fig. 4: X-ray Diffractogram of Ferric Chloride-Treated Soil

The intensities have increased for potassium chloride-treated soil when compared with that of the untreated soil. The intensities have increased for calcium chloride-treated soil when compared with that of the potassium chloride-treated soil, and much more, when compared with that of the untreated sample, which is all evident from the X-ray data.

3. RESULTS OF SOIL-FABRIC TESTS

The photomicrographs obtained for various treated and untreated samples are shown in Figures 5, 6, 7 and 8, and the observations made from them are presented below.

The photomicrograph of the untreated specimen was compared with those of the treated specimens. The aggregations seen in the treated specimens are absent in the untreated specimen.

Fig. 5: Scanning Electron Micrograph of the Untreated Soil

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Fig. 6: Scanning Electron Micrograph of the Potassium

Chloride-Treated Soil

Fig. 7: Scanning Electron Micrograph of the Calcium

Chloride-Treated Soil

Fig. 8: Scanning Electron Micrograph of the Ferric

Chloride-Treated Soil

The treated specimens clearly indicated the crystalline format with faces and edges visible, in the photographs, whereas the untreated specimen remained as it is, without any modi- fications.

This is more so, in the FeCl3-treated specimen compared to other treated specimens. In the untreated specimen’s photo- micrograph, large void spaces are present, whereas the void space distribution in the treated specimens seems to be more uniform with continuous intra-assemblage pore spaces.

The porosity of the soil sample reduced with the addition of KCl, CaCl2 and FeCl3, and porosity was the least for the FeCl3 specimen.

On addition of the chemicals, the water in the voids of the expansive soil was removed, and the K+, Ca2+ and Fe3+ ions got separated from the chlorides and they got lodged in the micropores of the soil (due to the adsorption phenomenon), thereby reducing the porosity.

When the soil was again saturated, the treated soil did not swell, as these ions have formed a wall like structure in the inside wall of the pore spaces of the soil, which would prevent swelling, and when the soil is again dried, the shrinkage is under control, as the ions will not allow shrinkage. Ferric chloride reduces the ability of the soil to take in water, much more, than when compared with the soil samples treated with KCl and CaCl2.

The fabrics seen in the case of KCl-treated and CaCl2-treated specimens are less oriented, when compared with FeCl3-treated specimen.

The clay layers are stacked one above the other. The void space distribution in the untreated specimen also indicates that under the application of pressure and treatment, the aggregated particles have a chance to move closer to each other and readjust themselves to new positions.

The fabrics seen in case of KCl-treated and CaCl2-treated specimens clearly demonstrate that the structure is not properly arranged, in between the cation-to-cation and cation-to-anion, whereas FeCl3-treated sample clearly indicates that the texture orientation is arranged in order, after the treatment.

The microfabric of the FeCl3-treated specimen also explains, the void ratio of the FeCl3-treated specimen is the least amongst the KCl-treated, CaCl2-treated and untreated specimens. The photomicrograph of the FeCl3-treated soil shows that the edges of particles have become straight which indicates stability and the soil became strong and it’s swell-shrink behaviour has been controlled.

The chemical formula of montmorillonite consists of alumina, silica and molecules of water. Aluminium is in 4, 6, 8 chemical coordination and silicon is in 6 chemical coordination. After FeCl3 treatment, Aluminium moves into 6 coordination. This means oxygen maintains the same distance from Al and Si, and which means the bond is more strong. After the treatment, the mineral becomes stable, as water molecules are removed. The cation-anion distances are neutral. The ionic structures become stable.

4. CONCLUSIONS

(a) It is evident from the results of the X-ray diffraction test, that there is a presence of montmorillonite.

(b) After the chemical treatment of the soil, it is observed that the soil’s swell-shrink behaviour is controlled.

(c) The chemical treatment of the soil has reduced the soil’s porosity.

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(d) The results of fabric studies, have indicated that ferric chloride is the best alternative for treating the expansive soil when compared with potassium chloride and calcium chloride.

REFERENCES

Al-Hamoud, A.S., Basma, A.A., Husein Malkawi, A.I. and Al, Bashabsheh (1995). “Cyclic Swelling Bahaviour of Clays”, Jr. of Geotechnical Engineering, 562–565.

Lopez-Lara, T., Zepeda-Garrido, J.A. and Castario, V.M. (2007). “A Comparitive Study of the Effectiveness of Different Additives on the Expansion Behaviour of Clays”, Electronic Journal of Geotechnical Engineering, http://www.ejge.com/1999/Ppr9904/Ppr9904.htm, visited on July 07, 2007.

Ramana Murty, V. and Prasada Raju, G.V.R. (2001). “XRD, SEM and AAS Studies on Stabilised Expansive Clay”, Indian Geotechnical Conference-2001: 48–51.

Srivastava, R.K., Joshi, D.K., Srivastava, K., Singh, J., Tiwari, R.P. and Shukla, N.K. (1997). “SEM Analysis and Geotechnical Characterisation of Industrial Waste-Expansive Soil Interaction Behaviour”, Indian Geotechnical Con- ference-97, 409–410.

Subba Rao, K.S. (2000). “Swell-Shrink Behaviour of Expansive Soils-Geotechnical Challenges”, Indian Geotechnical Journal, 30(1): 1–68

Tiwari, R.P. and Srivastava, R.K. (2001). “Expansive Soil-Industrial Waste Interaction Behaviour”, Indian Geo- technical Conference–2001: 207–210.