The types of extrusion:
the study of extrusion phenomena was perfected by analysing maps of displacement at given points measured topographically on a large number of faces in different types of ground and under a very wide variety of stress strain conditions.
• It was established from these studies that the advance core extrudes from the face according to tree basic types of deformation depending on the type of material involved and the stress it is subject to.
• cylindrical extrusion: the face presents translational movement parallel to the alignment of the tunnel with intensity increasing from top to bottom;
•spherical dome-like extrusion: maximum extrusion at the Centre of the tunnel or immediately below it;
•combined cylindrical domelike extrusion: the wall of the face presents movement which is a combination of the two previous types of movement described. It is the type of extrusion which is encountered most frequently in practice.
•if, however the stress state is high, then on the contrary, it will be necessary to act above all on the core reinforcing it with longitudinal intervention and calibrating radial measures after the passage of the face appropriately. If a medium is stressed in the failure range, it is of primary importance to stiffen the advance core with preconfinement of the cavity, which may be supplemented with appropriate action to confine the cavity after the passage of the face. In these cases, experience (and that described in the previous paragraphs is particularly significant) advises
• working ahead of the face on the form and volume of the advance core by creating a protective crown of improved ground around it. This method was used effectively on the construction of the Vasto tunnel to advance through particularly difficult ground.
Author:
Pietro Lunardi
Italy
EDIT -Rakesh Debnath
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