The very high radiation fluences expected at the High Luminosity LHC (HL-LHC) impose new challenges in terms of design of radiation resistant silicon detectors. The choice to use p-type substrates to improve the charge collection efficiency involves an optimization of the strip isolation to interrupt the inversion layer between the n $^+ $ implants, limiting the breakdown voltage. To this purpose, TCAD modelling and simulation schemes, already developed and validated at typical LHC fluences have to be adapted to take into account ...