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Capital mine development is often faced with limited geotechnical databases and designers are faced with more or less accurate estimates of missing parameters. GSI classification if often used with numerical modelling and its rounding unit is ±5 as suggested by its creators. In situ stresses are usually estimated in such manner that vertical component is equal to the weight of the rocks above, while horizontal components may vary in wide range, starting with ratio to vertical component of 0.3 and even be several times higher than vertical component. Influence of estimate error of GSI and horizontal stress is analyzed for the Cukaru Peki location near Bor in Serbia. Zone in the rock mass valued with GSI of 40 at depth 160m is analyzed for the change of GSI value of ±5 and horizontal stress ratio between 0.5-1.5. Change of the unsupported length of decline and shotcrete layer thickness is tracked for different values of input parameters. Finally, best case and worst case scenarios are analyzed with results showing that shotcrete layer thickness could vary in range between 4-33cm, and unsupported lengths between 0.6-2m.