These are the realization means or external manifestations of NATM. If we want to understand NATM, we must understand it in essence. NATM is not a construction method, but a change of design concept.
Simply put, before the appearance of NATM, people thought that all supporting structures were passively subjected to surrounding rock pressure. NATM completely changed this concept. Surrounding rock is both an active load and a load-bearing structure. The guiding ideology of the New Olympic Development Association is to make full use of the bearing capacity of the surrounding rock and protect it as much as possible, so smooth blasting and weak blasting are adopted to disturb the surrounding rock as little as possible. Bolt-shotcrete support can seal the surrounding rock in time, minimize the looseness of surrounding rock and control the deformation of surrounding rock. Monitoring measurement can directly monitor the convergence deformation of surrounding rock, because the finite element analysis of tunnels, especially deep-buried tunnels, is basically a lie. When you enter the tunnel and look at the front of the tunnel, you will despair of the model.
Before the appearance of NATM, the primary support or temporary support of the tunnel was mostly to ensure the safety in the construction process, and the lining (integral lining) was a pure supporting structure. At present, NATM tunnels mostly adopt composite lining of primary support and secondary lining, both of which are designed according to permanent structures. Theoretically, the primary support bears all the loads, and the secondary lining belongs to the safety reserve. In the actual design, considering the current domestic understanding of NATM concept, construction level and construction status, the surrounding rock pressure of some secondary linings was tested, ranging from 30% to 50%.
Of course, these are all changes in people's ideas. The surrounding rock where the tunnel project is located exists objectively and remains unchanged from beginning to end, and will not change because of our different understanding of the surrounding rock. Therefore, I personally think that even before the appearance of NATM, the construction of many tunnels reflected NATM's ideas. Today's NATM- NATM tunnel construction method didn't come into being until Austrian scholar Professor Labouche Weitz formally put forward this idea for the first time and applied for a patent. In recent years, with the improvement of bolting and shotcreting technology, measuring technology and blasting technology, NATM has also made great progress and become an indispensable method for tunnel engineering design and construction.
You got it? The essential difference is the understanding of tunnel surrounding rock. Before NATM, surrounding rock was regarded as a simple load, while NATM thought that surrounding rock was both a load and a load-bearing structure.