Preliminary Studies and Design Considerations

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Preliminary Studies and Design Considerations
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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.

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• 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.

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• 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.

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The purposes of geological exploration

• 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.

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Explorations should be extended to

• 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.

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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.

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• (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.

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• (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.

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• 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.

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• 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.

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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.
• 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.
• 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.
• 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.

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• 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.

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• 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
• 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

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• 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
• lto the annual mean air temperature at a known
  location
• hl the height difference between the point
  under consideration and the one with the known
  mean temperature lto
• X the height difference causing a l C drop in
  air temperature (150-220 m).
• k a correction factor expressing the difference
  between the air temperature and terrain
  temperature, given by Bendel in the following
  form

lt
to
h1
h
lto
C
A
T
Addit
Exit

• For the temperature (T) within the tunnel to be
  built at depth h we may write
• T lt k ((h- c)/ G )
• Where
• G the geothermal step
• lt the annual mean air temperature
• c the thickness of the cover affected by the
  external temperature
• h the total overburden over the tunnel

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Geological profile of the St. Gotthard tunnel
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Geological strata

• 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.
• 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.
• 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.

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• 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.

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• 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.

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Folded strata

• 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.
• 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.

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Hydrological survey

• 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.

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• 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.

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• 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.