The Northern Marmara Motorway is a project that aims to alleviate the heavy traffic congestion in the provinces of İstanbul, Tekirdağ, Kocaeli and Adapazarı. Within the 5th section bounded by “the Motorway Port Connection Road” and “İzmit Intersection”, mass movements of slide (Y1, Y2) and flow (Y3) types were observed along the route. This study investigated the causes of mass movements in the Korucu Formation, which consists of sandstone and shale alternation. It also evaluated the support systems to prevent these movements. The analysis considered project criteria, both static and dynamic conditions, types of mass movements and triggering factors. The study identified a combination of factors, including the water table and surface waters, which lead to progressive weathering and mass movement. Stability analyses were conducted for specific right-cut slope sections. These analyses included assessments of soil structure, soil–rock mechanics, engineering geology and geotechnics, as well as examination of field and laboratory test results. These analyses aimed to comprehensively investigate and understand the factors influencing the occurrence of mass movements, particularly for km: 170 + 300–170 + 400, km: 170 + 640 and km: 175 + 297–175 + 463. At Y1, pile retaining walls are proposed using Slide2 software to reduce the slope angle from 22° to 17°. At Y2, a translational landslide occurred with recommendations for the adjustment of the slope angle and protective measures considering the disturbance factors (D = 0.3 and D = 0.5). Y3 was a flow-type movement that required protection of the slope with riprap due to the different geological conditions and disturbance factors. This study underlines the need for a comprehensive geological analysis and structural measures to ensure safety in these areas.