Thermal properties of soils are important in buried structures contact problems. Although laboratory is distinctly advantageous in measuring the thermal conductivity of soil under ideal condition, given the ability to simulate relatively large-scale in place of soil bed, the field thermal conductivity of soil is not yet commonly used in many types of research. The use of only a laboratory experiment to estimate thermal conductivity may be the key reason for overestimation or underestimation it. In this paper, an intensive site investigation including field thermal conductivity tests for six different subsoil strata were performed using a thermal probe method (TLS-100) to systematically understanding the effects of field dry density, water content and soil type. Results were obtained from the alluvial plain lands in the middle part of Iraq, in an attempt to find a correlation between different soil characteristics and the thermal conductivity. It is shown that clayey soil generally had lower thermal conductivity than sandy soil. Thermal conductivity can potentially be affected by the proposed soil low or high plasticity. It is evident that in general, the measured field thermal conductivity value for the lean (low plasticity) silty clay increases with an increase in depth due to the increase of the degree of saturation; however, decreases with an increase in depth for the fat (high plasticity) silty clay. The field water content of the soil in the study obtained here increases so does the thermal conductivity of the soil for most the sites. Further investigations are required, to understand the effects of other environmental conditions with the seasons. This is especially helpful to the future of geotechnical engineering when designing geothermal systems. © 2021 Elsevier Ltd. All rights reserved.