Site investigation in combination with field and laboratory testing, plays a vital role in characterizing the soil profile for geotechnical design in order to reduce uncertainty. In spite of this, site investigations are often limited in scope due to high costs. In this paper, conditional random fields are used to examine the influence of soil strength mean, standard deviation and spatial correlation length on the risk of slope design failure for different levels of site investigation scope. An undrained slope example is used to illustrate how the proposed approach can be used to assess the risk reduction that can be obtained as the scope of a site investigation is increased. By combining the cost of site investigation with the cost of slope failure, the results indicate that there exists an optimal site investigation scope, beyond which the cost of additional boreholes does not justify the cost savings due to reduced slope failure risk.