preliminary studies and design considerations. geological surveys any tunnel project will require...

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Preliminary Studies and Design Considerations

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Page 1: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

Preliminary Studies and Design Considerations

Page 2: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

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.

Page 3: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

• 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.

Page 4: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

• 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.

Page 5: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

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.

Page 6: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

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.

Page 7: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

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.

Page 8: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

(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.

Page 9: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

(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.

Page 10: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

• 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.

Page 11: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

• 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.

Page 12: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

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.

Page 13: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

• 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.

Page 14: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different
Page 15: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

• 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.

Page 16: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

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;

Page 17: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

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

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

A

T

h

C

Addit Exit

lt

tolto

h1

Elevation (m) 0 500 1000 1500 2000 2500

k 0,8 1,0 1,3 1,7 2,3 3,0

Page 18: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

Geological profile of the St. Gotthard tunnel

Page 19: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

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.

Page 20: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

• 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.

Page 21: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

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.

Page 22: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

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.

Page 23: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

• 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.

Page 24: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

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.

Page 25: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

• 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.

Page 26: Preliminary Studies and Design Considerations. Geological surveys Any tunnel project will require investigations and studies on a number of different

• 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.