Solution
Robert answered on
Dec 26 2021
SOLUTION
A)Expected geology and geotechnical conditions along the
Tunnel route:
Tunnels are underground passages or routes used for different purposes. They are made by
excavation of rocks below the surface or through the hills or mountains. Like dams reservoirs
tunnels also are very important project for civil engineering aspect. So the site selection or best
possible site is made after carefully considering the relative advantages or disadvantages. As
the safety success and economy of tunneling depend heavily on the various geological
conditions prevailing at site. As usual the important geological factors which infers that litho
logical, structural and ground water conditions. The relevance of these aspects discussed
elow:
A. The type of the rock and their strength and deformation behavior
The first basis of classification of rock is the geological classification according to which rocks
are divide into three classes i.e.
a) Igneous rock
) Sedimentary rock
c) Metamorphic rock
a) Igneous rock
The crystalline nature of the igneous rocks signifies high compressive strength with
potential difficulties in rock excavations process, but can also indicate the mark
competence with the advantages of decreased support needs to achieve an
acceptable degree of stability.
Localized and relatively thin intrusives are usually fine-grained and often possess
high strength and significant resistance to weathering by comparison to the coarser
grained igneous types with similar mineral constituents.
Igneous rocks consisting of volcanic tuff and pumice, can be particularly weak and
porous and whilst usually exhibiting low strength values with ease of excavation,
they can be subjected to rapid weathering with accompanying loss of competency
and can also give rise to the significant ground water problems.
) Sedimentary rocks
The effects of stress and advanced weathering, and weakening by the action of
water can give rise to the problems especially where such rock type contains
appreciable clay minerals.
The banded characteristics are sometimes responsible for marked variation in
strength, deformation and permeability in different directions.
c) Metamorphic rocks
Rock types such as quartzite, ma
le, dolomite ma
le, and hornfels generally
exhibits random distribution of minerals and display minor foliation and are
elatively more competent.
Rock containing micaceous minerals have well defined planes of weakness and
can easily split along these planes of weakness and show very rather properties
in terms of both strength and deformation properties.
B)Groundwater aspects
The presence of groundwater is recognized as a major hazard in addition to
causing operational difficulties in respect of tunnel construction works.
Predicting with accuracy the likely water inflow qualities is however, difficult,
and detailed monitoring and regular reviews of conditions together with
adoption of special measures such as de- watering or injection programmes
need consideration.
Encountering large quantities of water in weak ground conditions can lead to
apid formation of cavities around the tunnel excavation and can be produce
the potential for the significant quantities of wet and loose ground to flow the
tunnel.
Some tunnel projects have experienced problems from the relatively warm(
greater than 30-35
0
c) groundwater which can impair the environmental
conditions within the tunnel.
So ground occu
ence should be assessed during the site investigation stage.
There are some example of problem due to groundwater excavation during
tunnel design.
The Thames tunnel :- flooding occu
ed five times and the tunnel
was reclaimed each time by dumping clay and gravel in the rive
ed
and pumping out the water and digging out the dedris . what happened
was that water softened the silt and clay came in ever-increasing
volume and ultimately led to the flooding of tunnels. It was
accompanied by the methane causing minor explosion causing illness
and death of some tunnelers.
C) Geological discontinuities and associated strength and deformation behavior
There are many discontinuities but among them followings are the major discontinuities which
affects the tunnel design are:
a)Folds
) Faults
Although folds occur in all types of rock but it is more conspicuous in layered
ock due to deformation of rock mass.
This of course depends on the degree of fracturing which is almost associated
with folding fractured and folded rock, therefore present more serious problem
in tunnels; particularly during excavation.
To overcome from this problem, it needs stronger supports and besides,
fractures rock pieces immediately su
ounding excavation tend to dislodge
creating hazards.
The degree of fracturing is more in stronger rocks although this depends on the
depth of excavation and/or in situ stress condition.
Folds are sometimes the natural traps of natural gases, which might be harmful
to the persons working in tunnels.
Trough of fold accumulates water during excavations causing pumping
problems.
Figure below Presents example of the influence of folded rock masses on the
location of tunnel whilst excavation is in progress.
) Faults
Are associated displacements along the plane of the rapture caused by tectonic stresses
The following points are based on the Wahlstrom who has discussed faults in relation to
tunneling.
1 Repeated intermittent movements occur at several sites, particularly where tectonic
and igneous activities are still present.
2. Faults are frequently prefe
ed paths for groundwater movement but may act as
hydrological ba
iers of 7 below. Consequently internal erosion can occur and is
particularly pronounced with certain rock types, eg. Limestone, whilst significant
wall- rock alteration is likely with other rock types, e.g. igneous, feldspathic,
sandstones etc.
3. Frictional effects of movement along the fault plane can induce wall- rock alternate in
addition to chemical reaction from water circulation.
4. The width of the fault zone is related to the geological and tectonic history and rock
types. Fault zones can be tens of meters in width even where relatively minor
displacement has occu
ed between strata and is possibly indicative of several
eversals in movement over long periods of time.
5. Fault filling and gouge characteristics differ quite markedly and often reflect the
degree of influence of groundwater movement .
6. Brescia filling is characterized by its fragmented nature derived from relatively
competent rocks,. Such filling can exhibit voids but often contains fine materials.
The
eccias and in-filling materials can originate from upper or lower horizons
depending upon the motive force causing movement of the fault filling.
7. Rock in a crushed and comminuted state caused by the grinding action of relative
movement along the fault plane is commonly refe
ed to as gouge. Water assists in the
eakdown of some rocks and the fault gouge can often contain clay minerals which can give
ise to plastic deformation in to underground excavations by virtue of time – dependent
ehavioral properties and swelling pressure effects. Consequently humid conditions and
groundwater contact with fault gouge can initiate progressive collapse.
D) Rock temperature
1. Increase with depth.
2. Temperature increases about 10c for every 60-80metres in geologically stable
areas and 10c for 10-15 meters in volcanically active areas.
3. The simplon tunnel experienced very high temperature i.e. 560c at a depth of
2134 meters.
Alpine tunnels
a sudden increase in temperature from 270c to 450c and even 630c occu
ed due to
the sudden release of methane gases.
� Effective ventilations is perhaps the only means which can alleviate the problem.
E) Topographical conditions
Is responsible for modifying stress conditions at the surface. One principal
stress is zero at surface and other is parallel to it which is directly related to the
slope of the ground.
One obvious changes that tunnel construction may
ing about is the change
in the situ stress conditions as well as ground water conditions.
B) Classify the expected conditions according the RMR
and Q
As pointed out Barton and Bieniawski in T&T Fe
uary, 2008, rock engineering classification
systems play a steadily more important role in rock engineering and design. The main
classification systems for rock support estimates, the Q and the RMR (Rock Mass Rating)
systems, use some of the most important ground features or parameters as input. Each of
these parameters is classified and each class given values or ratings to express the properties of
the ground with respect to tunnel stability.
The RMR system
Significant revisions to the RMR system have been made in 1974, 1975, 1976, and 1989; of
these the 1976 and the 1989 versions of the classification system are mostly used. The RMR
value is found from RMR = A1 + A2 + A3 + A4 + A5 + B
where A1 = rating for the uniaxial compressive strength of the rock material; A2 = rating for the
RQD; A3 = rating for the spacings of joints; A4 = rating for the condition of joints; A5 = rating for
the ground water conditions; and B = rating for the orientation of joints.
From the value of RMR in the actual excavation, the rock support can be estimated from an
excavation and support table (for tunnels of 10m span). Bieniawski (1989) strongly emphasises
that a great deal of judgement is used in the application of rock mass classification in support
design.
The Q system
The Q system for estimating rock support in tunnels is based on a large database of tunnel
projects. The value of Q is defined by six parameters combined in the following equation:
Q = RQD/Jn × J
Ja × Jw/SRF
where RQD = the actual values of RQD;
Jn = rating for the number of joint sets; Jr = rating for the joint roughness; Ja = rating for the
joint alteration, Jw = rating for the joint or ground water, and SRF = rating for the rockmass
stress situation.
Together with the ratio between the span or wall height of the opening and the stability
equirements to the use of the tunnel or cavern (excavation support ratio called ESR the Q
value defines the rock support in a support chart.
Here,
This structural pattern results in typical minimum block sizes in the range of 150 to 750 mm
The groundwater table is below the proposed station alignment.
In situ stresses are assumed to be hydrostatic (σv = σH = σh) and γrock =
23kN/m3
so after calculation we get that result that
INPUT PARAMETERS
RMR
Q
ROCK
UNIAXIAL COMPRESSIVE STRENGTH A1=19 -
DEGREE OF
JOINTING
RQD A2=12 RQD=90
AVERAGE OF YOUR SPACING -
BLOCK SIZE - -
JOINTING
PATTERN
NUMBER OF JOINT SETS - Jn=10
ORIENTATION OF MAIN JOINT B=-8 -
The Q-system was developed from over 1000 tunnel projects, most of which are in Scandinavia
and all of which were excavated by drill and blast methods. When excavation is by TBM there is
considerably less distu
ance to the rock than there is with drill and blast. Based upon study of
a much smaller data base (Barton, 1991) it is recommended that the Q for TBM excavation be
increased by a factor of 2 for Qs between 4 and 30.
C)THE PREFER METHOD FOR TUNNELING:
Tunnel boring machine (TBM)
Tunnelling construction involves three main processes, namely excavation, dirt removal
and tunnel support (Ruwanpura, 2001). The construction of a tunnel (using TBM) begins
with the excavation and liner support of the vertical shaft. In the...