The geothermal field variation around the shallow section of cross-river highway shield tunnels will influence the engineering geological condition of the river beach where the tunnel has gone through.A case study on Nanjing Yangtze River tunnel is proposed in this paper with developing the numerical models using finite difference method.The measured ground surface temperature and inner segment temperature are employed as external alternating heat source and internal alternating heat source

respectively.The variation process of the typical shallow-buried section of the tunnel is then simulated and analyzed.The results show that the initial temperature-constant geothermal field is transferred to daily temperature-variation one owing to the heat transferring effects.The heat transferrs from the tunnels to the soil mass around in high air temperature period and vice versa in low air temperature period.An obvious varying temperature zone is discovered.Its highest temperature increase reaches 7.14℃

and the highest peak-to-valley value of temperature fluctuation approaches 10℃.This zone is located within the distance of 5mto tunnel exterior and ranges between the two tunnels.The distribution characteristics of geothermal field will be preliminarily stable after 20 years of tunnel operation.In addition

an annual fluctuation model for geothermal field in relative high temperature zone is developed

which could be employed to calculate the temperature around the tunnels with different distances in different times.The model can be used to estimate the initial temperature value around the shallow section of the tunnel under the extreme heat transfer conditions like fires.