About the best tunnel construction methods and the minimum thickness of rock overburden for a "self-supporting" tunnel

Which are the best tunnel construction methods?

Tunnel construction methods include drill and blast, shield, cut and cover, tunnel boring machine (TBM), New Austrian Tunneling Method (NATM), also known as Sequential Excavation Method (SEM), road header, sunken tube, and jack. The more important question is how to figure out which method is best for a particular project. The selection of tunnel construction methods is influenced by a variety of factors ranging from geological to economic to sociological. Because ground conditions such as whether the tunnel passes through rock, soil, or mixed ground can drive the construction method decision, because some methods are more efficient in certain ground conditions, geological factors frequently affect economic considerations. Shallow tunnels are frequently built using cut and cover techniques, particularly in urban areas. Tunnels that cross bodies of water can be bored or built with sunken tubes, depending on the depth of the water, ground conditions, and tunnel length. In urban areas, jacked tunnels have been used to build tunnels beneath active transportation facilities. An engineer with experience in various tunnel construction methods will be best equipped to assist an owner/operator in selecting the best tunnel construction method (s) for their project. 

A good reference for more information on the tunnel construction methods is highlighted in Bickel and Kuesel's "Tunnel Engineering Handbook." (https://www.springer.com/gp/book/9781461380535).

The minimum thickness of rock overburden for a "self-supporting" tunnel

The minimum thickness of rock overburden for a "self-supporting" tunnel is determined by several factors, including (1) ground strength, (2) applied loads, (3) opening geometrics, and (4) time. The strength variables include the strength of the intact rock (the solid rock portion) as well as the rock mass (bulk strength of the rock mass, including discontinuities - generally much less than the intact strength). Bedding/foliation, weathering characteristics, and groundwater conditions all contribute to strength.

Overburden loads, in situ tectonic stresses (often in the form of high horizontal stresses), hydrostatic pressures, variable loads due to nearby, active excavations, seismic loads, and possibly surface surcharge loads are among the applied loads (for shallow tunneling applications). The geometry of the opening (size, shape, and span dimensions) and its alignment relative to loading and geologic setting (orientation and dip) all have a significant impact on self-supporting performance. Finally, when evaluating stand-times for specified unsupported spans, time must be taken into account.

Self-supporting tunnel designs necessitate intimate knowledge of the rock mass to determine maximum unsupported span capacity and the stand-time of a given span dimension for the tunnel's intended use and environment. As a result, the amount of rock cover required varies greatly from one set to the next. A shallow tunnel was driven through massive granite (with few discontinuities), for example, may be able to stand unsupported for decades (centuries?) with spans extending up to 10-20m or more.

Other factors may influence the minimum overburden requirements in addition to simply being able to drive the intended tunnel dimension as an unsupported tunnel. For example, to protect surface structures, rock overburdens of sufficient thickness to tolerate caving without causing surface subsidence (piping) may be required if the tunnel collapses. In other cases, rock inter burdens may be required to separate tunneling from overlying aquifers to reduce both water in the tunnel and the risk of capturing or contaminating the aquifer.