The underpass project at the intersection of Tahliyah Street and Medina Road has been implemented in order to reduce heavy traffic that occurs at one of the most critical intersections in Jeddah. The soil improvement works comprise of design and implementation of a bottom plug by means of tangent jet grouting in order to form an impervious blanket beneath the excavation pit of the underpass. The purpose of the bottom plug is to minimize underground water seep into the underpass excavation and protect existing structures and foundations of the existing Medina Road Bridge. The soil improvement project was designed by Kasktas Arabia Ltd. Within the scope of the project; 8,065 nos of jet grout columns with a total drilling length of 138,500 m were completed under an excavation area of 12,000 m2. The works were completed as of December 2020. The results of the Lugeon (Lu) tests carried out after the improvement works verifies that grout improvement by jet grouting works is satisfactory.