The underground space provides provisions for sewage systems, rapid mass transportation, groundwater and geothermal energy (Bobylev, 2016) and protection from adverse weather conditions and natural hazards. The space is useful for the urban-built environment in various ways as it provides space for land usage and construction of essential and useful structures such as car parks, recreational facilities, museums and aquariums, protective structures, pipeline systems, water lines, etc. In hills and mountains, underground tunnels serve as a passageway and give protection against landslides.
However, underground constructions have certain limitations which are prevalent during and after construction (Chow et al., 2002). During construction, problems of noise and vibrations arise due to the excavation. Another notable limitation is the increase in surface congestion due to roads being blocked, traffic diversions and traffic jams. During post-construction, fear of entrapment and unfamiliarity of the people adjusting to the new underground structures are two issues. The operational phase of an underground metro system causes vibrations of such high impact that it tends to shake the urban-built structures and sometimes leads to crack formations in them.
The feasibility of construction of an underground structure depends on the location and geology of the ground where it is to be constructed (Gong et al., 2016; Li et al., 2017). These effects involve the potential noise and vibration waves that are generated, affecting people and properties of the urban-built environment. Vibration from underground metros and railways is generated mostly at the rail-wheel interface and propagates through the ground into the structural foundation of the urban-built environment (Yang et al., 2013; Vladimir et al., 2017). These disturb the nearby houses, sensitive electronic equipments, measuring installation set-ups etc. (Celebi et al., 2013). The vibration occurs due to difference of frequency levels between the surface and the underground spaced. Therefore, consideration of ground vibrations and ground movements is necessary during the design and construction of underground structures.
Structures represent an important field for understanding the vibrational impact. These come under excitation when their wavelength matches with that of ground vibrations (Dawn et al., 1979). These can be reduced to their minimum values, provided appropriate control measures are taken (Berta, 1994). Ground geology is a major factor governing the impact of vibration. Soft soils tend to produce larger deformation than hard soils. Phenomena like Soil Layering enhance the magnitudes of vibrations due to wave reflection from stiff soils (Watts et al., 2000). The induced vibrations caused by underground metros and railways have high energy and low frequency but would not affect buildings if their frequency falls within the audible limits of 20 Hz (Hildebrand, 2004). Damage to structures due to differential settlement of foundations is basically caused by repeated dynamic excitations having low amplitudes (Karg et al., 2010). Therefore, ground vibrations require serious attention and need to be measured for appropriate and economic design of the underground structure so that it can function well and cause no disturbance to the urban-built environment.
The Underground Space
- Provision for activities or infrastructures that are either difficult to construct above ground or are undesirable there (Godard, 1995)
- Protection to anything that is placed underground be it humans or materials
- Enhancement of serviceability, amenity and safety of urban areas
- Rapid mass transit system in urbanized areas: Urban metro systems involve underground tunnels which serve as a rapid and flexible mode of transportation in the busy cities. Ex: Kolkata Metro Rail Corporation
- Storage: Natural resources like oil, Liquefied Petroleum Gas, Liquefied Natural Gas as well as regular essentials like food grains and packed food are all kept underground for storage and conservation.
- Sewage and waste water disposal: Pipelines are installed underground in the urban-built environment to serve as medium for sewage transport from pipelines of houses.
The various methods of excavation for underground construction are as follows:
- Drilling and Blasting: This method is preferable for very hard ground, grounds with varying properties, underground structures of less length, caverns, stations etc. This method involves drilling blast holes followed by the operations of charging, ignition, ventilation, scaling and mucking.
- Hydraulic and Pneumatic Hammers: This method is suitable for weak rocks and its approach is like Drilling and Blasting without using explosives. Vibration and fumes are also avoided in this method. Dust particles are treated by water spraying.
- Gripper TBM: This is used for excavating competent and hard rocks. Rock is fragmented by chipping between cutters.
- Roadheader: This excavating machine is highly effective in hard grounds
- Tunnel Boring Machine (TBM): This is useful for rock and firm soil without need of a face pressure as it has a mechanical face support.
- Earth Pressure Balance Tunnel Boring Machine (EPB-TBM): Useful for soft soils where the TBM progresses forward by balancing earth pressure while excavating soil at the same time. The TBM is equipped with a full-faced cutterhead for ground cutting.
- Slurry/Hydro-shield TBM: The TBM is fitted with a full face cutterhead which provides face support by pressurizing boring fluid inside the cutterhead chamber.
- Double shield TBM: This machine uses the features of gripper and shield in a TBM and serves as a quick method of excavation.
Urban-built environment includes roads, bridges, buildings, dams, sanitary facilities etc. Underground constructions have not only helped in controlling surface congestion but have benefitted the natural and urban-built environment in multiple ways.
Benefits of underground construction (Godard, 1995; Rönkä et al, 1998; Durmisevic, 1999; Chow et al., 2002; Petroutsatou et al., 2010; Sterling et al., 2012).
- Location: They provide space for constructions that are unwanted above ground such as car parks. An underground structure such as subways builds proximity to the existing urban-built facilities. It is also helpful when it is impossible to construct structures on unbuildable sites.
- Rapid Transportation System: It facilitates fast commuting.
- Water pipelines and Sewage system: The entry of freshwater and exit of wastewater from houses are served by the underground pipeline system. The sewage system also runs on underground pipeline network. Egg-shaped underground sewer tunnels are constructed for the very same purpose.
- Storage: The limited number of access points of underground structures makes it more convenient to store materials and prevent entry of intruders. To some extent, ammunitions can also be stored underground. It is highly useful for food storage as the ground has an insulating nature and does not hamper the quality of food.
- Protection: Underground structures provide excellent protection against fire because the ground is incombustible and provides thermal insulation to the structures beneath it. Similarly, during earthquakes, the underground structures are constrained to move along with the ground motions. The urban-built environment is also protected by underground structures from severe weather conditions like storms, cyclones, tsunamis, floods etc. Man-made disasters like explosions, radioactive fallouts or even times of war can also be countered.
- Thermal isolation: A uniform thermal temperature is maintained up to a depth of 500m for most places. The moderate temperature and the slow response of large thermal mass of earth are significantly helpful towards energy storage and conservation. Conduction losses from buildings are lessened in cold climatic conditions. The requirement of energy for tempering air infiltration is lessened. Thus, energy conservation can be better achieved by the urban-built environment if some of the urban-built systems such as offices or homes are shifted underground as per convenience.
- Preservation: An underground structure has lesser visual impact than an equivalent surface structure. It is important in siting facilities when industrial locations are proposed to be shifted to the urban-built environment. This also helps in preservation of the natural ecosystem of the urban-built environment as trees will not be required to be cut and lesser damage will be inflicted on the ecological cycle. The run-off waters from rain are reduced and chances of soil erosion are also reduced as a result.
- Economic Savings: An underground infrastructure does not require much maintenance as its property is unaltered by external disturbances due to the isolating and insulating nature of ground. This leads to economic savings.
- Recreation: Underground space serves for the urban-built environment to establish recreational facilities such as museums and aquariums. This also is helpful in terms of preservation, surface space saving and storage.
- Fear of entrapment, a major psychological problem would affect people when shifted to underground, due to unfamiliarity with the new location and its systems, for example, ventilation systems of underground structures.
- Dwelling in underground structures result in loss of connection with the natural world. There is no facility for sunlight or for observing weather changes. Lack of openness in the underground structure leads to poor ventilation.
- Development of positive pressure during fire can reduce the fire resisting capabilities of construction materials.
- Lack of oxygen levels in case of fire.
- Induced vibrations due to underground transport result in noise and disturbance in the urban- built environment. Ground movements are caused due to induced vibrations which eventually lead to surface settlement. All these three phenomena lead to surface cracks on walls and floors of buildings.
- In case of explosions, deep underground facilities have limited points of connection to the surface. A person has to climb upstairs to escape and there is also difficulty in venting out poisonous fumes caused by fire.
- Heavier gases may seep underground and be present in heavier concentration than at surface. Grounds containing poisonous gases and chemicals cause health problems. Thus, ventilation is an important criterion.
- Traffic management is a major issue during underground construction of a project that can take 10-15 years for completion. During the construction period, roads get blocked and traffic has to be diverted. This results in enhanced traffic volume which eventually leads to surface congestion. Sometimes traffic diversion leads to longer travel distances which lead to wastage of time and fuel (Bhutani et al., 2016).
One of the most significant aspects affecting the urban-built environment are ground vibrations, vibration induced noise and ground movements. The underground construction has led to interference in the regular orientation of ground and its behavior with the urban-built environment. It leads to disruption of balance of ground stress and results in stress redistribution, which in-turn lead to ground vibrations and ground movements. Researchers have observed that movement of the ground located above underground structures are more in cohesionless than cohesive grounds (Fattah et al., 2011). These phenomena ultimately result in displacement, deformation, settlement and collapse of the surface buildings, bridges, roads etc. (Yuan et al, 2011). The problem of ground movement is usually of minor significance but where surface structures are present, can lead to significant damage.
Ground vibrations are transmitted through the ground on to the urban-built environment like buildings, bridges, roads etc. (Berta, 1994; Yang et al., 2018). If not designed properly, these vibrations would tend to form cracks on surface structures and cause the walls and floors to shake (Connolly et al., 2016). The vibrations produced during railway or metro operation in an urbanized area are generated as a result of dynamic forces, track unevenness and the quasi-static track displacement influenced by successive axle loads (Jones et al, 1996). The main sources of vibrations resulting from underground constructions are the operations of pile driving, dynamic compaction, blasting and heavy machineries.
The different types of vibration include the P-waves, the S-waves and the R-waves. Vibration waves travel initially as compression waves (P-wave) and shear waves (S-wave) which propagate radially outward and are transmitted through the ground into the surface structures. When these waves reach the surface, they are generated as Rayleigh waves (R-wave) which experience viscous damping (Verhas, 1979). The diffraction and reflection of bulk waves tend to amplify these vibration waves. Heavy freight trains produce ground vibrations with predominant frequency of 4-30 Hz which may lead to damage of property if the amplitude of these vibrations is high enough (Jones et al, 1996).
Both ground vibration and its re-radiated noise are undesirable for humans and result in distress. In the urban-built environment, it is natural that the ground vibrations and their noise levels are high. Ground-borne noise, which generally occurs when these ground induced vibrations tend to vibrate the surface structures, generally has an appreciable frequency between 50 and 160 Hz (Hood et al., 1996). The vibration generated within the premises of the urban-built environment due to an underground moving train has frequency signals in the range of 22.5 Hz to 90 Hz (Vladimir et al., 2017). These negative impacts from the railway lines may be due to various reasons like construction of a new line, upgrading an exist ing rail line or changing rail switches. An environmental impact assessment is hence necessary before measuring vibrations of buildings and underground tunnels.
The properties of the structures of urban-built environment change due to the effect of vibrations as additional stresses are induced in them. Their features depend on the geology, materials constituting the building, and heterogeneity and uncertainty of soil deposits at site. These residual strains arise from the foundation movements associated with moisture changes in soil, thermal movements associated with temperature changes in material, humidity changes and substandard building construction. This leads to expansion, shrinkage and swelling of structure, and also results in a decrease in its lifespan. Ground vibrations with PPV value more than 15 mm/s damage the structures of urban-built environment.
Measurement of Ground Vibrations
The measurement of ground vibrations is essential so as to ensure a proper design and construction approach for the underground structure. The energy propagated in the soil due to rail traffic dominates at low frequency as it touches its peak when the frequency of waves ranges between 10-20 Hz. The effect of damping is also low at low frequencies and hence not much of ground attenuation is required. In order to mitigate the ground vibration levels caused due to underground structures, it is important to measure them. The following criteria are needed to be measured for proper assessment ground vibration:
- Vibratory force produced due to a running train on an underground structure
- Attenuation of ground vibrations between the underground structure and ground surface
- The interaction between the ground vibration and building structures
It is essential to have a parameter in order to measure ground vibrations. One such parameter is the Peak Particle Velocity (PPV) having measurement units of mm/s. PPV refers to the movement of particles present within the ground and not the ground surface. Yarmohammadi et al.(2018) studied the average PPV values of some jet-grouted overlapping columns after their construction. The variation in the average PPV values with the number of columns for each length of column is shown in Fig.4.
There were columns of two different lengths of 2 m and 3 m who’s average PPV values were studied and then compared with the standard limits recommended by Han (2015). These recommended data of Han is given in Table 1 and Table 2.
|Table.1. PPV values and their impact (Han, 2015)|
|PPV (mm/s)||Level of Perception|
|Table.2. Recommended values of maximum PPV as per structure (Han, 2015)|
|Structure||Recommended maximum PPV (mm/s)|
|Industrial buildings||3 - 5|
|Residential buildings||5 - 15|
|Sensitive structures||20 - 40|
Mohanan et al. (1989) studied the noise levels of functional metros of the Calcutta Metro Rail. A variation of the noise levels at different locations of the underground tunnels of Calcutta metro system has been shown in Fig.5.
Note: All loudness values are measured in decibel units
L10 and L50 - noise level exceeding 10% and 50% time of measurement duration respectively
L90 – A-weighted sound level equalled or exceeded 90% of the time of measurement duration
Leq – Equivalent noise level
Ground vibrations and its induced noise should be maintained under limits as per standard code provisions. Their magnitudes vary in different locations. Table 3 gives the general guidelines on the limits of noise exposure that the World Health Organization (WHO) had made. The data in Table 4 gives the limiting vibrations for cosmetic damage and relate to transient vibrations which do not give rise to resonant responses in structures and low-rise buildings. The sources of vibration measured for these limiting values were underground constructions, piling, ground compaction, road and rail traffic etc.
|Table 3. WHO recommended Exposure Noise Levels|
|Environment||Recommended maximum Leq Level (dB(A))|
|Table 4. Transient vibration guide values for cosmetic damage at building base (BS 5228)|
|Type of Building||Peak component velocity in frequency range of predominant pulse|
|4 to 15 Hz||15 Hz and above|
|Industrial and Heavy commercial buildings||50 mm/s at 4 Hz and above||50 mm/s at 4 Hz and above|
|Residential/ light commercial buildings||15 mm/s at 4 Hz increasing 20 mm/s at 15 Hz||20 mm/s at 15 Hz increasing to 50 mm/s at 40 Hz and above|
Table 5 addresses the effect of construction-induced vibrations on both surface and underground structures for both short- and long-term duration. Vibration measurements were taken from the foundation level of a building for short term vibration and at the top of roof for long term vibrations.
|Table 5. Limiting values of vibration velocity for evaluation of building damage for short-term and long-term impact (DIN 4150, Part 3)|
|Peak Velocity (mm/s)||Location of measurement|
|Office and Industries||1||20||-||Foundation|
|1||-||10||Top Floor, Horizontal|
|Domestic houses and similar construction||1||5||-||Foundation|
|1||-||5||Top floor, horizontal|
|Other buildings sensitive to vibrations||1||3||-||Foundation|
|1||-||2, 5||Top floor, horizontal|
Tables 6 & 7 give the Swiss standard SN 640 312 (1979) which considers the effect of transient and continuous vibrations on buildings. Table 6 categorizes the different types of buildings whereas Table 7 gives the maximum vibration levels for continuous vibrations as per SN 640 312.
|Table 6. Category of Buildings as per SN 640 312 classification|
|Building Category||Building Type|
|I||Reinforced concrete structures for industrial purposes, bridges, towers|
|Subsurface structures such as caverns, tunnels with or without concrete lining|
|II||Buildings with concrete foundations and concrete floors, buildings made of stone and concrete masonry/ blocks.|
|Subsurface structures, water mains, tubes and caverns in soft rock|
|III||Buildings with concrete foundations & basement, timber floors, masonry|
|IV||Especially vibration-sensitive structures and buildings requiring protection|
|Table 7. Maximum vibration levels for continuous vibrations (SN 640 312)|
|Building Category||Frequency Range (Hz)||Recommended Vibration Velocity (mm/s)|
Underground constructions are accompanied by ground movements which are obtained at the surface in a trough form. Ground movements are a clear proof of the effect of deformation caused by underground construction. Such effects occur in several phases like the excavation phase, construction phase and post construction phase. The post construction phase is time-dependent and arises from effects like creep, consolidation, pore water pressure distribution, and recompaction (Rankin, 1988). This may also lead to a sudden catastrophic failure of the structures of the urban-built environment.
The different causes of ground movements (Fattah et al., 2011; Lu et al, 2009) are as follows:
- Elastic and plastic deformations of soil
- Running of soil
- Excessive excavation of ground
- Ploughing, yawing or negotiating curves
- Deflection of supports and linings of the underground structures
- Vibrational effect of excavating machines
- Soil Consolidation of disturbed soil
- An operational underground metro system
Ground movements in the form of surface settlement cause several impacts. During underground construction, there is an increase in effective stress in drawdown affected areas due to pore pressure reduction caused by drawdown. Underground construction causing ground movements result in heaving and thus, the foundation level of the structure is disturbed. As a result, the buildings or other urban-built structures undergo settlement. There is also a fall in the water table.
The following tables show the variation of ground movements for two different soils i.e, stiff clays and soft clays in tunnels of different sizes located at different depths. Stiff clays cannot be moulded and have high strength ranging between 75-150 kN/m2. So, the vibration waves travel quicker due to rigidity of the medium. Soft clays are relatively weak having strength between 20-40 kN/m2 and can be moulded very easily by slight application of finger pressure.
In Tables 8 and 9, the ground movement is found to vary mostly in the range between 0.1 to 10 mm except for one or two readings where an excessive deformation of 25 mm was observed for stiff clays. For soft clays, the deformation value was found abnormally excessive (20-25 mm) in one of the readings when compared with the other two readings. This suggests that underground constructions in soft soils have a greater risk for the urban-built environment. Better methods of excavation like usage of Earth Pressure Balance Tunnel Boring Machine (EPB-TBM) should be implemented for such low strength soils. Deeper underground locations are better for underground activities to maintain minimum risk for the urban-built environment.
|Table 8. Measured Ground Movements for Stiff Clays (Rankin, 1988)|
|Tunnel Diameter (m)||Tunnel Depth (m)||Ground Movement (mm)|
|Table 9. Measured Ground Movements for Soft Clays (Rankin, 1988)|
|Tunnel Diameter (m)||Tunnel Depth (m)||Ground Movement (mm)|
Challenges of the Urban-Built Environment
The urban-built environment has faced many negative impacts due to underground constructions mostly in the form of ground vibrations, noise and ground movements. This issue thus requires serious attention. The underground induced vibration waves need to be mitigated to minimum levels so that their generation does not affect. The building vibrations can be sensed by humans when their frequency ranges between 1 Hz and 80 Hz. The ground-borne vibrations start shaking the walls and floors of a building when their frequency ranges between 16 Hz and 250 Hz. This in turn causes re-radiated and structure induced noise (Schevenels et al., 2017). Ground induced vibrations and noise needs to be mitigated at their source, their transmission path and the neighboring structures.
Vibrations can be mitigated at the source by grinding the rails, using soft rail pads, under-sleeper pads and floating slab tracks (Nelson, 1996; Loy, 2004; Hilderbrand, 2004; Lombaert et al., 2006). Other measures include modification of features of the dynamic vehicle, reduction of the wheel-rail unevenness, and usage of resilient elements such as ballast mats. Such mats result in a low resonance frequency which effectively reduces the free-field vibrations (Lombaert et al., 2006). The mats significantly reduce the impact by high frequency vibrations. It spreads the vibration waves over a large area to reduce the impact at a particular point. However, the degree of reduction is dependent on the angle between the track and the line that connects the source and receiver. Along the transmission path of vibrations, the best way of mitigation is to impede the propagation of ground vibrations.
This can be achieved by constructing open trenches (Karlstrom et al., 2007), construct soft and stiff buried wall barriers (Woods, 1968; Massarsch, 2005; Kattis et al, 1999; Coulier et al., 2015), subgrade stiffening (Andersen et al., 2005), install wave impeding blocks (Takemiya et al., 1994; Peplow et al, 1999) and wave reflectors (Krylov, 2005; Dijckmans et al., 2015). Stiff wall barriers can be formed by using concrete slab (Ahmad et al., 1992) or sheet pile (Dijckmans et al., 2016) in soil or by stiffening soil by jet grouting (Coulier et al., 2015). The installation of these walls reflects the incident waves causing a reduction in wave transmission. They cause reduction in wave propagation when the trace wavelength is less than bending wavelength of barrier wall. Their performance to mitigate the vibration further depends on the ground properties and size of wall used (Schevenels et al., 2017). The receiving end of ground vibrations is the urban-built environment itself. Mitigation measures involve improvement of foundation design and base isolation. However, these measures are structure dependent and vary for different structures.
The vibration waves can be mitigated by using barriers that modify the propagation medium. While providing materials for mitigating the vibrations it should be maintained that they are neither soft nor have high stiffness values as such features cannot generate significant impact (Auserch et al., 2017). The application of a resilient mat below the track slab gives rise to a new resonant frequency of the system and results in attenuation of energy from track to the ground (Colao et al., 2017). The degree of mitigation depends on the type of ground medium. Likewise, the floating slab on a stiff soil shows better mitigation of vibration waves than a stiff soil. Table 10 presents the different noise mitigation measures and their effectiveness.
|Table.10. Mitigation measures with their contribution to Noise reduction (Tumavice et al., 2010)|
|Mitigation Measure||Noise Reduction|
|Retrofitting with K-blocks||8-10|
|Retrofitting with LL – brake blocks||8-10|
|Accoustic rail grinding||1-3|
|Noise insulated windows||10-30|
The transmission speed of underground vibrations can be reduced by incorporating a high resilient element in its path such as rubber spring can be used under the rails, sleepers or the supporting plates. Foam rubber mat can also be used under the ballast as a resistant material to counter underground vibrations (Costa et al., 2010). These measures turn out to be the most effective when the materials are made as soft as their safety limits permit and also when the mass between the resilient material and the rail is made very large, keeping cost under consideration. In this way, the resonant frequency of the mass-spring system can be maintained very low (10-20 Hz). This will provide a good isolation medium at frequencies of 40 Hz or higher in order to control the ground induced vibrations and noises.
However, a low ratio of dynamic stiffness to the static stiffness should be maintained till a frequency of 50 Hz as it leads to a small static deflection and good vibration isolation. The pronounced resonances of any plate between rail and elastic element must be avoided as it tends to deteriorate the isolation of a plate-spring system. This can be done by using small plate elements that are connected only by damping materials. Other methods to control the underground rail induced vibration include construction of thicker walls of the underground structure or create ditches or resilient mounts under the buildings and bridges (Heckl et al.,1996).
Underground structures are important and beneficial for urban-built environment in several ways starting from rapid mass transportation to food storage, oil and LPG storage, protection from natural disasters and adverse climate, environmental preservation and energy conservation. In-spite of several advantages, it has some disadvantages like psychological disturbance and isolation for people. Problems of traffic maintenance during underground construction are a major drawback. Problems like ground vibrations, vibration induced noise and ground movement arise during underground constructions, which excite the buildings, leading to distresses. It is, therefore, important to design and establish underground structures appropriately. In addition, use of EPB-TBMs for underground excavation in soft soils, underpinning, sheet piling, usage of resilient materials between rails etc. are needed.
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