to mean different things. New terms have arisen, and alternative names for certain aspects of NATM have been adopted as its use has spread. That is partly caused by an increased use of the tunneling method in the United States, particularly in soft ground shallow tunnels. Other designations are seen for this modern tunneling style; Sequential Excavation Method (SEM) or Sprayed Concrete Lining (SCL) are often used in shallower tunnels. In Japan, the terms Centre Dividing Wall NATM or Cross Diaphragm Method (both abbreviated to CDM) and Upper Half Vertical Subdivision method (UHVS) are used. The Austrian Society of Engineers and Architects defines NATM as "a method where the surrounding rock or soil formations of a tunnel are integrated into an overall ring-like support structure. Thus the supporting formations will themselves be part of this supporting structure." Some engineers use NATM whenever they propose shotcrete for initial ground support of an open-face tunnel. The term NATM can be misleading in relation to soft-ground tunnels. As noted by Emit Brown, NATM can refer to both a design philosophy and a construction method. Key features Key features of the NATM design philosophy are: The strength of the ground around a tunnel is deliberately mobilized to the maximum extent possible. Mobilization of ground strength is achieved by allowing controlled deformation of the ground. Initial primary support is installed having load-deformation characteristics appropriate to the ground conditions, and installation is timed with respect to ground deformations. Instrumentation is installed to monitor deformations in the initial support system, as well as to form the basis of varying the initial support design and the sequence of excavation. When NATM is seen as a construction method, the key features are: The tunnel is sequentially excavated and supported, and the excavation sequences can be varied to efficiently address the specific rock conditions being encountered. The initial ground support is provided by shotcrete in combination with fibre or welded-wire fabric reinforcement, steel arches (usually lattice girders), and sometimes ground reinforcement (e.g. soil nails, spiling). The permanent support is typically a cast-in-place concrete lining placed over a waterproofing membrane. There is a quick closure of the invert, that is, the bottom part of the tunnel, to create a structural ring that takes advantage of the rock or soil arc naturally created on the top part of the tunnel section. Safety The 1994 Heathrow Airport tunnel collapse led to questions about