CHAPTER 11 DESIGN AND SUPPORT OF TUNNELS ...

CHAPTER 11 DESIGN AND SUPPORT OF TUNNELS: OPERATIONAL CRITERIA AND
PRINCIPAL SUPPORT TYPES
Introduction
When a tunnel is excavated in all but the most competent of ground
conditions it is an inevitable consequence that some form of support will
be required if the tunnel is to retain adequate stability and/or maintain
sufficient dimensions to facilitate its use in the intended manner.
The form and function of the support will vary according to a wide range of
factors apart from just geotechnical considerations and it would seem that
for every different tunnel there is a different lining solution [11.1].
Tunnel linings: main types. Tunnel linings are grouped into three main
forms some or all of which may be used in the construction of a tunnel:
Temporary ground support
Primary lining
Secondary lining [11.2]
Temporary ground support. In rock tunnels where the ground has insufficient
stand-up time to allow the construction of the primary lining some distance
behind the face, then some form of temporary ground support applied at the-
tunnel face is required eg rock bolts, shotcrete and steel sets.
Such support is not required in soft ground in conjunction with a shield
driven tunnel as temporary ground support is provided by the body of the
shield itself.
Primary lining. A primary lining is the main structural component of the
tunnel support system which is required to sustain the loads and
deformations that the ground may induce during the tunnel's intended
working life. A further function performed by the primary lining is the
control of water egress (exit) and ingress (entrance).
Secondary lining. Various tunnels require smooth bore profiles for their
intended use, eg sewer and water tunnels or aesthetic finishes for public
usage, eg highway and pedestrian tunnels. Erosion and corrosion protection
for the primary lining and further waterproofing may also be required, all
of which are provided by secondary linings.
The terms temporary, primary and secondary linings are not always defined
on a consistent basis in various texts [11.3].
Requirements for tunnels
Prior to the discussion of support types and design principles it is
important to differentiate between the two very different areas of
application of tunnelling, namely civil and mining engineering together
with the relative support requirements in each of these categories.
Civil engineering tunnels
Tunnels in the civil engineering category are generally driven at
relatively shallow depths by comparison to those situations involving
mining tunnels.
Civil engineering tunnels require higher tolerances in terms of the final
tunnel profile and its ensuing stability.
Many civil engineering tunnels relate to public usage and high density
traffic with projected working lives in excess of 100 years.
As a result, long term stability is desired coupled with an appropriately
high factor of safety (>2.0).
Other factors which are critical to the design and use of civil engineering
tunnels include line, grade, special surface finishes and prevention of
water inflow or outflow in respect of the tunnel structure.
Mining engineering tunnels
Mining engineering tunnels' function is to provide access to the workings
for men, materials and ventilation in addition to egress for men and the
mined minerals/rock products.
In general, the traffic using mining tunnels is of relatively low density
whilst the projected working lifetime is relatively short when compared to
civil engineering tunnel applications with perhaps the exception of mine
shafts.
It is only the major tunnels in mining situations which need to be
maintained open for the full life of the mine, whereas the majority of
mining tunnels which are associated with mineral exploitation may only need
to remain functional for a period of time of generally less than 10 years.
As a result, the acceptable levels of deterioration and deformations for
mine tunnels are higher than civil engineering tunnels and also any repair
work is far easier to facilitate.
The use of secondary linings is also precluded (omitted) as aesthetics is
not a consideration for mine tunnels.
In general, the function of a mine tunnel support is simply to keep the
tunnel adequately stable and/or open to a sufficient size thus allowing the
mining operation to take place relatively unhindered in an acceptable
condition of safety, typical factors of safety being 1.2-1.5.
It is clear then that the roles of civil and mining engineering tunnels are
very much different in many aspects and this leads to different
philosophies in terms of formulating acceptable support designs.
Although, the methods used in arriving at the final designs may be similar
in both fields, the interpretation of the results will be quite different.
Therefore, both approaches are worthy of separate explanation.

Civil Engineering Tunnels: Design Considerations
Civil engineering tunnels are constructed in the full spectrum of possible
ground conditions ranging from soft ground to massive unjointed rock,
coupled with various states of in situ stress and hydrogeology (Table
11.2).
However, the dimensions and details of a tunnel lining for civil
engineering purposes are rarely governed only by geotechnical
considerations and the loads and deformations that will be imposed on the
lining [11.4].
The user needs are the only real basis for constructing a tunnel and that
the designer should first consult the user or client commissioning the
tunnel construction [11.3].
This is due to the fact that the user may require certain aspects to be a
part of the tunnel design, eg conformation to an existing tunnelling
system, intended service life and the reduction of capital outlays by
accepting higher levels of maintenance.
Water. Given that the requirements of the user are established, the first
consideration in designing a tunnel lining is that of water. If the lining
has to resist hydrostatic pressures either externally or internaily then
this will in general govern the lining design and control the success of
the tunnelling operation [11.4].
A tunnel specification will often state that it should be completely dry
for its intended operation, but such a condition may not be economically
feasible. Therefore, consideration should be given to the tunnel's intended
use and its sensitivity to water leakage either in or out of the tunnel.
O'Rourke [11.3] summarises a number of considerations which should be taken
into account regarding water infiltration into various types of tunnel,
these being presented in Table 11.3.

Table 11.2 Ground classification in relation to tunnel design [11.2]

|Classification |Uniaxial |Ground types |
| |compressiv| |
| |e | |
| |strength | |
| |(MPa) | |
|Soft ground | |(a) Recent alluvium and glacial drift |
| | |depositsincluding water-bearing sands, |
| | |gravels, silts, and clays, and boulder clay. |
| | |(b) Eocene, Cretaceous and Jurassic stiff |
| | |fissured clays |
|Very weak to |up to 50 |Low strength rocks including shales, |
|moderately | |Cretaceous Chalk, Triassic (Keuper) Marl and |
|strong rock | |Jurassic rock formations. |
|Strong rock |50-100 |Many Triassic and Permian rock formations, |
| | |sandstones and medium strength Carboniferous |
| | |Coal Measures. |
|Very strong and|above 100 |The hard Carboniferous and older rocks, the |
|extremely | |limestones and harder rocks. |
|strong rock | | |



Table 11.3 Summary of infiltration considerations for several types of
tunnel [11.3]


|Tunnel type |Considerations regarding tunnel infiltration |
|Rapid transit |Need to evaluate corrosion. Deterioration possible for |
| |rails, track fasteners, wooden ties, train control and |
| |signalling equipment, metallic fixtures, electrical |
| |installations, and reinforced concrete. Initial costs of|
| |leakage control must be evaluated relative to long term |
| |maintenance. |
|Subway stations,|Avoid wet patches. Intercept (stop) drips on walking |
|pedestrian |surfaces. Promote clean surfaces. Strict limits imposed |
|walkways |on discoloration and dulling of surface finishes. |
|Highway |Need to minimise traffic hazards. Promote clear |
| |visibility and clean surfaces. Avoid freezing. Some |
| |water introduced by vehicles and periodic spraying to |
| |wash internal surfaces. Drainage system capacity usually|
| |governed by fire-fighting water criteria. |
|Water conveyance|Infiltration rates for waste-water tunnels often set |
| |byfederal and state regulations. During periods of high |
| |flow, tunnels may be affected by internal pressure. |
| |Exfiltration should be evaluated under conditions of |
| |internal pressures.