A ring with a conical hole is placed over a tube to be extracted. Wedges are inserted between the ring and the tube. As the ring is lifted with hydraulic cylinders, the wedges grip and extract the tube. This kind of device is used, for example, in civil engineering for extracting stop tubes during the construction of diaphragm walls.
Diaphragm walls are usually constructed in the ground for foundation forms or for bordering working pits. First, a narrow trench (usually 60 to 100 cm wide and 10 to 30 m deep) is excavated. Then concrete is poured into the trench to form the diaphragm wall. A stop tube is lowered into the trench to limit the length of the trench for each pour. This makes a panel of diaphragm wall 2-4 m long. The stop tube is removed after the concrete has set. An extracting device, working with hydraulic cylinders, is used to separate the tube from the solid concrete, then the tube is removed from the trench with a crane.