Abstract:

Organic matter is a common component in soil and has a significant effect on the thermal properties of soil. To clarify the influence and mechanism of organic matter on the thermal conductivity of clay, bentonite was used as the matrix to prepare soil samples with different organic matter contents by adding humus. Thermal conductivity coefficient λ was measured by thermal probe, and the variation of λ with dry density ρ_d, water content w (degree of saturation S_r) and organic matter content w_u were examined. By analysing the adsorption characteristics of bentonite to organic matter, the influence mechanism of organic matter on soil thermal conductivity was discussed. The results show that the thermal conductivity coefficient λ  increases with the increase of dry density ρ_d water content w and degree of saturation S_r when w_u is the same. When the dry density ρ_d of saturated soil is the same, l decreases with the increasing of w_u, but is not a monotone linear decline. It can be divided into three stages: sharp descent stage, transition stage, and gentle descent stage. The approximate range of w_u corresponding to the transition stage is 7.5% to 15.0%. The saturated adsorption capacity w_u,1 of bentonite to compact organic matter is about 8%, and that of w_u,2 to compact + stable organic matter is about 14.0%. In other words, the two thresholds (w_u,1, w_u,2) of the variation of organic matter occurrence form are consistent with the w_u corresponding to the transition section of the λ-w_u relation curve. Mechanism analysis shows that in addition to the content of organic matter, the change of organic matter occurrence form is also an important factor affecting the thermal conductivity of the soil. The applicability of several theoretical prediction models of thermal conductivity for clay soil containing organic matter was analysed.

Key words: thermal conductivity, cohesive soil, organic matter content, organic matter form, thermal conductivity prediction model