Civil Engineering Reference
In-Depth Information
Tunnelling in soft ground
25.1 Tunnel construction
Tunnels are built for transport of people, cars, trains and water, for storage and for
mining. They may be deep or shallow, in rock or in soil, in urban or rural environments.
They may be built by boring or by cut and cover methods or by sinking them into the
bed of a river. In Fig. 25.1(a) the tunnel is advanced by mining the ground from
inside the tunnel. In Fig. 25.1(b) the tunnel is constructed as a pair of retaining walls
with a roof; the design is really design of retaining walls discussed in Chapter 24.
In Fig. 25.1(c) tunnel sections are floated into place, sunk into a trench, connected
together and covered by fill.
Some tunnels are deep and so they are mostly in rock and construction does not
affect nearby buildings or other tunnels unless they leak and alter the groundwater
conditions. Deep tunnels are for mining or road and rail connections through moun-
tains. The construction problems are mostly excavation of strong rock and support of
fractured rock in the roof of the tunnel. Other tunnels are relatively shallow: the ratio
of their depth to diameter is less than about 10. These include tunnels for underground
rail and road connections and water supplies in cities. Shallow tunnels are often built in
soils and close to existing buildings and underground structures. The term soft ground
is used in tunnelling to describe soils and weak rocks which require support to prevent
collapse and damaging ground movements during construction and throughout their
lives.
In this chapter I will discuss engineering design of shallow tunnels in soft ground.
The criteria for design of a tunnel in soft ground are essentially the same as those
for foundations described in Chapter 22 and retaining walls described in Chapter 24.
Firstly it is necessary to investigate the ultimate limit state: there must be an adequate
factor of safety against collapse. Secondly it is necessary to investigate the serviceability
limit state: the ground movements caused by construction of the tunnel must not
damage nearby infrastructure.
25.2 Construction of bored tunnels in soft ground
Tunnels in soft ground have stiff and strong permanent linings to prevent collapse and
excessive movement. They are usually bored and often their cross-section is circular, as
