Variability of the internal friction angle with temperature is studied in a boundary value problem numerically simulating the impact of a cylindrical heat source on the stress and strain in soil mass in which it is emplaced. Even if the thermal increase of the internal friction in that temperature range is quite modest (less than 20% in terms of the critical state parameter, M), it affects quite significantly the effective stress path near the heat source. The effective stress path approaches the yield locus and the critical state at a significantly higher principal stress difference values for the variable internal friction than for the M = cons. case. The “mean effective stress distance from the critical state” is substantially reduced during heating, in the closest vicinity of the heat source. In the case of small perturbations of any parameter this proximity may lead to a potentially unstable or statically inadmissible behaviour. The solutions obtained allow one to identify zones of influence around the heat source of several variables of interest. The presented results may be of relevance to the design of prototype in situ installations and their monitoring, and eventually of actual facilities of the nuclear waste disposal.