Tunnels and their stability mechanisms
can be defined as underground construction for the connection between two
destinations to build up an effective means of transportation. Tunnels are made
for various purposes such as Railway transportation, for highways, for water
supply and for communication cables etc. Tunnel construction has several
benefits e.g. less disturbances to surrounding environment and reduced communication
distances which results in saving of energy and resources.
What’s a Tunnel Made of?
is underground passage constructed through the ground which may be consists of
rock or different type of soils. Construction process involves excavation,
ground temporary support system and muck removal, erection of permanent support
such as lining and provision of other facilities which are required for smooth
operations. So one can say that tunnel is made of rock or soil which is
supported by permanent supports such as lining to provide adequate stiffness
and support to structure against damage and collapse. Various methods based on geological,
environmental and economic conditions etc. are available for tunnel
construction. New Austrian Tunneling method (NATM), Shield Tunnel method and
Cut & Cover method are the famous method of tunnel construction.
Tunnel Stability in the Ground
is a thin walled closed excavated cross section and according to peck’s theory,
tunnel get stability in a ground through deformation of its shape and/or support
from surrounding ground makes it stable (Fig.1). After excavation a loosened zone is created and
stress redistributes outside the loosened zone due the formation of ground arch
(Fig.2), only few part of ground which is present in loosened zone, imparts
load on the support or lining because of this arch action. Trapdoor experiment (Laboratory experiment) reveals this stress redistribution phenomenon. Figure 3 shows that ground will
deform upon loading in order to attain stability. This is important to realize
that if tunnel supporting pressure will be less, than tunnel will deform more
so in case of urban area tunneling, strong lining is required. Fenner-Pacher curve
can also be seen to understand the ground stability which explains relationship
between supporting pressure and displacement of excavated surface (Fig. 4).