Field Simulation Technique to Enhance the Mechanical Strength and Elemental Composition of Soft Clay Soil Using Thermal Treatment

Abstract

This paper aims to improve the strength of soft clay soil using thermal treatment through a laboratory study that simulates the field application. The laboratory work consisted of preparing a soft clay (Cu = 14 kPa) inside a metal box (92.5 × 92.5 × 92.5) cm3. Boreholes of diameter 43 mm, with different lengths, spacing, and arrangements, were made inside the soil to work as a guide for heating pipes which connected to a controlled heating system. A novel heating system, using gas as a heat source, was developed and manufactured. After the end of the treatment periods, a load was applied until failure on a (20 × 20) cm2 square footing. Various parameter spacings (3, 4, and 5 times the outer diameter of the borehole), depths (1, 1.5, 2, and 2.5 times the width of the model footing), arrangements (square, circular, and triangular), and heating periods (2, 4, 6, 8, and 10 h) were investigated. The results showed the strength and behavior of the soil when subjected to the heated boreholes at different spacings, depths, and heating times, which were determined to be three times the outer diameter of the borehole, two times the width of the square footing, and eight hours, respectively, while the effect of the arrangement of the heated borehole casings was small. Also, a cone penetration probe (CPT) conducted on the heated soil showed that the unconsolidated shear strength (Cu) increased from 14 to 360 kPa and then decreased to 140 kPa (as an average with depth). In contrast, the average angle of internal friction (Ø) increased from 0 to 52 degrees and decreased to 16 degrees (as an average with depth) from the center of the heating model to the furthest point affected by heating. The EDS formula showed that components such as silicon, aluminum, and iron decreased at 300 °C and increased at 400 °C in the treated soils. The calcium content increased at 200 °C and then decreased sharply at 400 °C. The carbon percentage increased at 300 °C and decreased at 400 °C. The elemental proportions showed little change or remained stable at temperatures between 400 °C and 600 °C.