Transcript Preliminary Studies and Design Considerations

Preliminary Studies and Design
Considerations
Geological surveys
• Any tunnel project will require investigations and studies
on a number of different aspects related to construction
and operation.
• The most important phase of preliminary work in
tunnelling is careful exploration of geological conditions.
• The geological and hydrological environment decisively
affects both the loads acting on the tunnel and the
choice of the preferable tunnelling method to be
employed. In the most general and simplified sense, the
major problem during tunnel construction is the ground
(i.e. rock or soil) behaving differently than anticipated.
• Mechanized methods have greater rates of progress but
require more specific data. Mechanized construction
requires a large capital investment by a contractor, and
delays become costly.
• A geologist with local experience must be consulted
when considering the first draft plans for the tunnel or
other underground structure.
• The help of the geologist will be invaluable in the
selection of the first choices. The information gained
from large-scale geological maps is of a general
character only and no detailed picture of geological
conditions can be obtained unless detailed soil and rock
explorations are made. The basic identification of "hard"
or "soft" ground is important, but equally important is the
determination of the transition zones between "hard" to
"soft", as well as the potential for both extremes to exist
in the same place, i.e. mixed face.
• The general path of the tunnel is governed by existing traffic or
transportation interests, while the exact location is controlled by the
geological conditions prevailing in the area.
• An important consideration in selecting the location is the location of
the tunnel portals. These acting as retaining walls, are especially
sensitive to adverse stratification which may result in a tendency to
sliding. On the other hand they are to be built in the most weathered,
weakest surface crust.
• The more carefully and accurately the geological conditions of the
proposed location and its environment are explored, the more
confidently the plans of the tunnel can be prepared and tunnelling
methods selected, i.e. essentially, the more rapidly and economically
can the tunnel be constructed.
The purposes of geological
exploration :
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The determination of the origin and actual
condition of rocks;
The collection of hydrological data and
information on underground gases and soil
temperatures;
The determination of physical, mechanical and
strength properties of rocks along the
proposed line of the tunnel;
Determination of geological features, which
may affect the magnitude of rock pressures to
be anticipated along the proposed locations.
Explorations should be extended
to:
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Investigation of the top cover
Determination of the position and quality of
subsurface rock
Surface drainage conditions
Position, type and volume of water and
gases contained by the sub-surface rocks
Determination of the physical properties
and resistance to driving of the rocks
encountered.
The sequence of geological explorations referring to
tunnel constructions may be divided into three groups:
(a) Investigations
of a general character prior to planning,
which should include the bibliographical and statistical
survey of morphology, petrography, stratigraphy and
hydrology of the environment. This should be completed
by a thorough field reconnaissance and by surface
explorations. The field reconnaissance on foot where
possible will amplify and crystallize previous data
obtained from preceding bibliographical study. From
aerial photographs not only much of the above data may
be spotted, but the trained observer by identifying the
vegetative plant types can often draw conclusions
concerning the gross chemical characteristics and thus
the origin (igneous or sedimentary) of the underlying
bedrock, not to mention the clearer tracing of fault
outcrops, folds, etc.
(b) Detailed geotechnical (subsurface)
investigations parallel to planning but prior to
construction, by which an improved information
should be obtained on the physical strength and
chemical properties of rocks to be penetrated, as
well as on their condition (weathering,
fissuration, relative density, consistency).
Information on the location and dip of layers,
folds, faults, bedding planes, and joints, as well
as on the location, quantity and chemical
composition of under-ground waters associated
therewith is of paramount significance. The
determination of gas occurrence and rise in rock
temperature in both location and extent is
similarly important.
(c) Geological investigations should be
continued during construction, not only in the
interests of checking design data but also for
ascertaining whether the driving method
adopted is correct or needs to be modified.
For this reason, a pilot heading should be
driven in advance of the working face to
explore actual rock conditions and to take
rock samples on which strength tests and
chemical analyses can be performed, and
occasionally for the in-situ measurement of
rock stresses.
• All results of preliminary geological surveys should be united in the
geological profile. The main items to be indicated in the geological
profile are the location and depth of boreholes, exploration shafts,
drifts etc., together with all information on the rock obtained otherwise.
• Beside the bore log in the tunnel axis and the location of the tunnel,
the geological profile should display all rock types, their condition
(fissured, weathered, etc.), detailed information on stratification,
folding and fault zones and, where possible, even strength properties.
Hydrological conditions (groundwater table, intercalated aquifers,
artesian water level, etc.) must also be shown, together with water
gouges, springs and water-bearing layers. A very important
supplementary feature of the geological profile is the curve of the
estimated internal temperatures.
• Geological profiles of underwater and urban tunnels
would be incomplete without the indication of the bottom
of ground-surfaces, of the extent of level fluctuations, of
the riverbed material, its physical properties, especially
its impermeability. In addition to these the weight,
foundation conditions of major buildings on the surface,
elevations of possible access roadways, the location of
public utilities, elevations of various groundwater stages
together with the pertinent heads should also be entered.
• The object of the survey preceding actual tunnel
construction is, essentially, to furnish preliminary
information on all circumstances affecting the site,
location, construction and dimensions of the tunnel, in
particular the quality and position of the layer to be
penetrated, on rock and water pressures and on water,
gas and temperature conditions within the mountain.
Rock temperatures in mountain interiors
• Temperatures on the surface of the Earth's crust are subject to wide
variations and are governed primarily by external conditions, such
as season, geographical location, climate, etc.
• Temperature fluctuations may exceed 50 °C. These surface
fluctuations, however, become less and less perceptible in the
temperature of rock with increasing depth below the surface and are
no longer effective below a depth of 20-25 metres. Below this
crust affected by external influences there is a consistent
increase in rock temperature with depth.
• The rate of increase is not uniform and is governed by several
factors. It is measured by the geothermal step defined as the
vertical distance over which there is a temperature increase of
1 °C. The inverse of this is the geothermal gradient, expressing
the temperature increase for every 1 m depth.
• The geothermal step depends on several factors, the principal one
being the material of the mountain itself, i.e. the thermal conductivity
of the rock. The higher the conductivity, the higher is the value
of the geothermal step.
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The value of the geothermal step is lower in loose, frozen and dry rocks and
may be reduced by chemical processes that may take place in the rock. The
step is reduced and consequently rock temperature is increased by gases
trapped in the rock.
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Temperatures are further frequently increased by mineral oil, coal and
especially by ore deposits, i.e. they reduce the value of the geothermal step.
Temperatures increase similarly as a result of fissuration caused by rock
pressures, or of the increase in porosity. The influence of porosity can,
naturally, be traced back to the presence and movement of air in the voids.
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An influence still greater than that of air on thermal conductivity is the
infiltration of meteoric water, which, apart from the approximately 25 times
larger thermal conductivity of water, results in the expulsion of air from the
voids and the wetting of rock surfaces.
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The value of the geothermal step is considerably affected by the topography
of the terrain. Under otherwise identical conditions the geothermal step is
higher under hills than under valleys. Accordingly, the lines connecting
points of the same temperature (geoisotherms) will be more widely spaced
under bills than under valleys.
• During the construction of the Great Appenine tunnel under a cover
depth of some hundred metres, temperature suddenly increased in
clay-shale from 27 °C to 45 °C and exceptionally to 63 °C as a
consequence of gas inrush (4000 g/lit CH4 content).
• Finally, the value of the geothermal step is affected to a
considerable extent by the stratification and dip of the rock layers as
well. Heat in rocks is conducted better in a direction parallel to their
stratification bedding, or shelving than perpendicular to it. For this
reason the geothermal step is higher in steeply inclined, or vertically
stratified rock layers than in almost horizontally bedded ones.
• Dense stratification, i.e. a close succession of thin layers, tends to
minimize the value of the geothermal step owing to the insulating
effect of layer interfaces.
• Maximum temperatures in the tunnel depend, finally, on its length,
as will be demonstrated by the following theoretical considerations
and by the tabulated values.
Stini has given the following values for the long European
tunnels:
Tunnel
Depth (m)
Geothermal step (m/°C)
• Simplon
2100
65.3
• St. Gotthard
1725
85.3
• Mont Cenis
1565
104
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The temperature likely to be encountered in the interior
of the mountain is governed, according to Andreae, by
the following factors:
The position of the geoisotherms under the mountain
ranges (geothermal step);
The soil temperature on the surface over the tunnel;
The thermal conductivity of the rock and hydrological
conditions;
The elevation of the tunnel;
The annual mean temperature of the ground surface (to) can be determined from the annual mean
temperature of the air (lt) as:
to = lt + k = lto –(h1/X) + k
Where;
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lto = the annual mean air temperature at a known location
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hl = the height difference between the point under consideration and the one with the known mean
temperature lto
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X = the height difference causing a l °C drop in air temperature (150-220 m).
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k = a correction factor expressing the difference between the air temperature and terrain
temperature, given by Bendel in the following form
lt
h1
h
lto
A
Addit
to
Elevation (m)
0
500
1000
1500
2000
2500
k
0,8
1,0
1,3
1,7
2,3
3,0
C
T
Exit
For the temperature (T) within the tunnel to be built at depth h we may write
T = lt + k + ((h- c)/ G )
Where;
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G= the geothermal step
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lt = the annual mean air temperature
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c = the thickness of the cover affected by the external temperature
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h = the total overburden over the tunnel
Geological profile of the St. Gotthard
tunnel
Geological strata
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Tunnel construction is simplified, accelerated and
less costly by the uniformity and soundness of rock.
The greater the variation and fracturization of layers,
the more involved, expensive and time consuming
the tunnelling methods will be. Mountain formations,
devoid of stratification are much more favourable for
tunnelling than mountains composed of several
layers, or shales, or granular masses of varying
degrees of solidification.
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The adverse effects of stratification and shaling are
the more pronounced, the more distinct and the
thinner the individual layers are. The direction
(strike) and dip of the layers are of paramount
importance.
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The location of the layers in space can be described
in terms of strike and dip. Strike may be defined as
the direction of the horizontal extension of the layer,
i.e. the direction of the horizontal straight line that
can be drawn on the layer. The dip is the inclination
of the layers and is perpendicular to the strike.
• In strata that are simply tilted both dip and strike are relatively
constant over wide distances, but in folded beds variations from
regional dip and strikes are numerous.
• Where the tunnel axis is perpendicular to the strike of a steeply
dipping rock stratum the excavation of the tunnel is likely to
succeed under favourable rock pressure conditions. However,
where the tunnel axis is parallel to the strike higher rock
pressures may be expected to occur.
• In general, steeply dipping strata facilitate the penetration of
atmospheric effects into the interiour of the mountain, producing
a loose crust of increasing thickness. Otherwise, steeply
dipping, or even vertical layers may be advantageous as far as
strength is concerned.
Figure 2.5: Tunnel location in relation to various stratifications
• When driving the tunnel perpendicular to the stratification (i.e. to the
strikes) each individual stratum must act as a girder with a span
equal to the width of the cross section, and with a considerable
depth (figure 2.5 a). The only disadvantage of such stratification is
the generally poor efficiency of blasting operations.
• When, on the other hand, the tunnel axis is parallel to the strikes
and bedding planes of the vertical strata (figure 2.5 b), bridge action
is limited to the extent until the shear strength (due to friction and
cohesion between adjacent layers) is fully mobilized, while the
inherent bending strength of the layer is not utilized unless an
appropriate span is developed in the longitudinal axis of the tunnel.
Folded strata
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The folding of strata creates pressure on the core and tension in the crown of the
fold. Anticline and syncline folds are of special significance in tunnel driving.
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Both terms denote a wave-like fold, but whereas a syncline is the
trough of the wave, the crest is called the anticline. If circumstances
necessitate that tunnels follow the strike, they should always be
located in the anticline, since on passing through the crest of the fold
they will then be subject to lower pressures. In the syncline, however,
they would be exposed to overpressure from both sides and in
addition the accumulation of water there would increase the danger of
inrushes, in the anticline the water would tend rather to seep away
from the tunnel.
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For tunnels running perpendicular to the strike uniform pressure conditions will also be slightly
disturbed, although over a rather considerable length, both in synclines, and in anticlines. In
anticlines the entrance sections of the tunnel will be subjected to higher pressures and the
central portions to lower ones, whereas in tunnels in longitudinal synclines the pressure
conditions will be reversed
Not only the dip and strike but also the sequence of layers plays an important role in tunnelling.
Uniform stratification will usually afford easy conditions both for driving and for constructing the
final tunnel section, whereas serious difficulties are likely to be encountered where strata are
highly variable. Instead of a continuous type of lining a system composed of adjoining rings
should be adopted in this case.
Tunnels are not insensitive to earthquake damage therefore particular care should be devoted to
seismic activity during geological investigation. Tunnels driven in hard rock that intersect no
active faults present fewest difficulties in terms of seismic activity. The flexibility ratio of the tunnel
will be high and the tunnel will move with the ground, although stress concentrations can prove a
problem. The greatest problems associated with seismic shaking tend to occur when the tunnel
is constructed in soft ground (liquefaction), as is often the case with immersed tube designs,
unless sufficient degree of flexibility is built into the structure. The best solution to the problem of
placing a tunnel through an active fault, is not to. Active faults should be avoided for
transportation tunnels. For conveyance tunnels, the philosophy must be to evaluate and accept
the displacement likely to take place and facilitate repairs into the design. One way of doing this
is to 'over bore' the tunnel so that, even if the maximum earthquake induced displacement occurs
the tunnel is still of sufficient diameter to allow it to fulfil its function.
In conclusion, the purpose of geological investigation is essentially to provide in advance
information on pressures likely to act on the tunnel, on conditions to be expected during driving,
e.g. water pressures and temperatures.
Hydrological survey
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Water is a governing factor in tunnel loads as well as in construction
possibilities and conditions.
The effect of water on tunnels reveals itself in three respects:
a) Static and dynamic pressure head: loading action.
b) Physical: dissolving and chemical: modifying action.
c) Decomposing and attacking action: harmful against certain linings.
Generally seeping and moving water exerts more harmful action, than
standing or banked up backwater. Which quantities and what kind of water
will enter the tunnel during construction depends primarily on the character
and distribution of water-conveying passages. The length and depth below
the terrain surface of the cavities, precipitation and local geological
conditions are also important.
• The passages may extend along surfaces, as e.g. filtrations appearing in
fissures and joints, where one dimension of the conveying cross-section is
negligibly small in comparison with the other. They may again be tube-like,
ranging in size from cavities of several metres in diameter down to tiny
seepage ways called "threadlike" water passages.
• For a sound judgement of hydrogeological conditions, the
cognition of waterstoring rocks in the geological formations is
indispensable. A permeable layer, when e.g. lying in anticline
formation will bear no or very little water, on the other hand,
will be able to store very considerable quantities of water
when lying in a syncline formation. Also the different degree of
rock weathering, varying with the respective areas is
influencing water-bearing qualities, just as the tectonic past is
of importance, because more water must be expected in the
disturbed or dislocation zones.
• More water will percolate as a rule in longitudinally disturbed
zones, than in transversely disturbed ones and in wide
disturbed and detrital zones the rate of flow is bigger on the
sides, than in the middle. In addition the crumbled rock
particles become gradually saturated and softened thus being
turned into a more or less muddy condition. This may lead
eventually to inrushes of water and mud.
• A governing principle of tunnel alignment: waterlogged areas
and spots should be possibly avoided by any underground
cavity.
• Groundwater and the water of intercalated aquifers, where the
voids of the rock are saturated with a coherent mass of water
extending over the entire thickness of the layer, or at least over a
considerable part of it is the most dangerous in tunnelling.
• If possible, the tunnel should not be located under the
groundwater table. However, where construction in such a layer
is unavoidable special tunnelling methods and techniques must
be resorted to (shield driving, dewatering by compressed air). If
the tunnel can be located above the groundwater table then only
drainage of periodically percolating meteoric water should be
provided.
• In tunnels under the groundwater table rain-like dripping from the
roof and entrance of water through fissures of sidewalls can be
expected. The volume of water entering the tunnel in such cases
depends exclusively on its height, relative to the groundwater
table, and decreases with this hydraulic head. In the figure below
location 3 is the least favourable.