A Study Of Temperature Effect On The Structural Strength Of A Clayey Soil Using The Micropenetrometer

Nowadays, environmental issues such as global warming, urban heat island effect and the geological disposal of nuclear waste have become more and more significant. Correspondingly, temperature effects on the strength of the clayey soils due to such problems gained much attention. Lots of researches at home and abroad have shown that temperature effect of clayey soils is very complicated, In order to further understand the temperature effect on the strength of the clayey soil, this paper presented an experimental study using a modified micropenetrometer (SMP-1). Soils samples in this study are taken from the piedmont slope in Nanjing pukou district, classified as eluvial expansice soil. Laboratory penetration tests using two penetration rates were conducted on disturbed soil specimens whose degree of saturation (Sr) and dry density (pd) were controlled (with the temperature range of20℃-50℃). Research achievements are listed as follows:(1) The existing micropenetrometer (SMP-1) has been improved. According to the requirements of the test, a water bath was installed on the instrument to ensure a constant temperature during the whole proess.It is shown that the temperature effect on the structural strength of soil can be clearly revealed by the penetration tests using themicropenetrometer (SMP-1).(2) In general, for specimens studied in this paper, the increasing temperature reduced the structural strength of the soil and evident thermal softening behaviour was observed. Conversely, the strength of the soil increased with decreasing temperature, indicating that the soil was hardened by decreasing temperature.(3) Saturation of the soil can affect the temperature effect on the structural strength of the clayey soil. During the heating process, for specimens with the same dry density, the thermal softening behaviour is reduced with increasing Sr and vice versa. At higher Sr (i.e.80%,90%) the decreasing rates were close to zero. Therefore, the phenomenon of thermal softening will be more significant in less saturated soil.(4) During the heating process, for samples with the same saturation, the thermal softening behaviour of structural strength is more significant in denser soils and less significant in softer soils.(5)The end resistance was higher with the penetration rate at10mm/min, but the thermomechanical behaviour of soil was not affected by the penetration rate.(6) The mechanism of the thermal softening behavior is analyzed. In general, increasing temperature results in thermal expansion of soil aggregates as well as pore water and lead to a reduction of the strength of these components. Moreover, the soil expansion induced by heating also contribute to the soil particles softening as well as the softened contacts between aggregates. All of which result in a decrease of the end resistance.(7) The thermomechanical behaviour of soil is irreversible. It can be explained as follows:Increasing temperature results in thermal expansion of the absorbed water of hydrophilic mineral particles such as montmorillonite in soils. When the vertical stress gets lower, this thermal expansion leads to a desrease of the interaction force between the particles. Particles move from a unstable position to a stable position, resulting in changes in the structure of the soil. Thus thermal softening phenomenon is irreversible.