Abstract:
With a population of nearly 2 million, Warsaw, the capital of Poland, has become one of the most congested cities in Europe. To support growing transportation needs of the city, the government has invested in the extension Metro Line II in Warsaw approximately 240m € through the European Investment Bank. Within this context, Line II “ C21” metro station was surrounded by a diaphragm wall having a perimeter length of 1, 998m as part of the construction works. To mitigate the water ingress and risk against the settlement of the adjacent buildings, the soil was plugged with 50, 000 m3 big diameter jet grout columns. The diameter of the jet grout columns was selected as 3.5m and placed with a triangular grid of 2.6m by the foundation specialist. However, the soil formation mainly consists of silty fine sand where the fine content ratio is < 20% and SPT-N values ranges between 20 to 25. As per the design requirement, the jet grout columns had to be executed with a height of 2m and 34-36m below the ground surface close to the shoring toe level. To form these columns by achieving to the required high energy level, a special jet grout monitor with a double fluid system, mounted on a single pass rig, was utilized by the foundation specialist. The diameters of the columns were regularly checked by both Cyljet® and painted steel techniques. Moreover, the columns and their intersection zones were also cored in terms of the QA/QC testing schedule. In short, this paper summarizes the execution and testing details of the big diameter jet grouting plug system, which was successfully applied and tested by the foundation specialist for a permeability level of 5E-8 m/s, beneath the extension Line II C21 metro station. Keywords: Warsaw metro project, big diameter jet grout, improvement, soil bottom plug