Experimental study on physical properties of soft soil after high temperature exposure

• A novel heating device was developed to study the physical property of soft soil.
• The heat transfer mechanisms of soft soil in high temperatures were studied.
• The volume decreases as the soft soil is exposed to high temperatures over 100 °C.
• Dry density and saturation change as the soft soil is exposed to high temperatures.
• The micromechanisms of soft soil after high temperatures over 100 °C were studied.

The temperature effect induced by tunnel fire is important in geotechnical engineering. Due to the difficulties of extinguishing a fire in a tunnel, the fire duration time could be more than 120 min and after the heating the temperature of the soil around the tunnel could exceed 100 °C. However, the physical and mechanical properties of soils exposed to temperatures in the range of 100 °C to 200 °C have been rarely explored. A custom high-temperature apparatus was developed for measuring soft clay specimens with variations of mass, moisture content, dimensions and temperature distributions exposed to high temperatures of 105 °C, 120 °C, 150 °C and 200 °C for 150 min and 240 min, respectively. These measurements are performed after high-temperature exposure, and the water in both liquid and vapor phases are allowed to escape the specimens when the pore pressure exceeds 100 kPa during the heating process. The results show that the volume change, saturation and dry density of the specimens vary nonlinearly with the ambient temperature and are affected by the exposure time. The results also show that the variation of temperature within the specimen may be divided into four stages characterized by a rapid temperature rise stage followed by a plateau at 100 °C with another rapid temperature rise and a final plateau corresponding to the ambient temperature. Finally, novel changes in particle agglomeration and cluster shapes of the exposed samples were observed using a Scanning Electron Micro-scope.