Stiff to hard clay under high cover may move in combination of raveling at execution surface and squeezing at depth behind surface. When exposed at steeper slopes, they run like granulated sugar or dune sand until the slope flattens to the angle of repose.
Clean, dry, granular materials. Apparent cohesion in moist sand, or weak cementation in any granular soil, may allow the material to stand for brief periods of raveling before it breaks down and runs. Such behavior is cohesive running. Flowing A mixture of solid and water flows into the tunnel like a viscous fluid. The material may enter the tunnel from the invert as well as from the face, crown, and walls, and may flow for great distances, completely filling the tunnel in some cases.
Below the water table in silt, sand, or gravel without enough clay content to give significant cohesion and plasticity. May also occur in highly sensitive clay when such material is disturbed. Figure 9. Typical air-rights structure tunnel. This difference is due to the increased dynamic compressive and tensile strengths and the increased displacement capacity at ultimate stress.
For reinforced concrete, dynamic strength magnifica- tion factors as high as 4 in compression and as high as 6 in tension for strain rates in the range of to per second have been reported by Grote et al.
For steel members, the U. Army recommends that dynamic yield strength 10 percent greater than the static yield strength be used [Ref. When the blasting induced peak overpressure is greater than the dynamic strength of the lining materials, the lining is con- sidered overstressed.
Therefore, estimation of the blasting induced peak overpressure provides a critical input in tunnel lining vulnerability assessment. Breach failure potential may be determined by comparing breach threshold thickness and effective thickness of the tun- nel lining. The liner may be considered breachable when the effective thickness of the liner is less than the breach thresh- old thickness.
The effective thickness of the lining includes the final lining thickness and the thickness of the portion of the initial support system that can be considered a permanent application, such as shotcrete. Breach threshold thickness of normal reinforced concrete with a strength of 4, psi 2,, kilograms per square meter for a spherical deto- nation is shown in Figure Breach threshold thickness is expressed as a function of explosive charge weight and set- back distance i.
Note that Figure 11 is not applicable for contact charges. This information allows a rough assessment of the tunnel lining vulnerability to an explosion inside the tunnel. Joint Failure Joints between immersed tube segments or between the end tube and the connecting structures e.
Path to progressive failure. Figure Breach threshold thickness for reinforced concrete [Ref. The tremie joints in one particular underwater tunnel are steel formed in soil trenches and rock encased in rock trenches. For these tremie concrete joints, the steel reinforcement and the steel plate were welded and continued through the joints after internal dewatering. Thus, in this case, they are as strong as the main body of the tunnel. The tremie concrete is anticipated to provide additional resistance to loading resulting from blast waves.
Many tunnels in the United States have used temporary gaskets that may form a seal, but the load is carried on solid stop bars.
The two most recently built tunnels in the United States have used Gina-type joints that have soft noses and bodies capable of carrying the compressive load. Particularly in seismic areas, the flexible joints are designed to carry expected shear and tension loads and may sometimes be referred to as seismic joints.
In such cases, a joint cannot open or have offset dis- placements under seismic loading conditions, which could lead to life-threatening ingress of water. This type of joint presents potential weakness for ingress of water and flood- ing under blast wave conditions resulting from detonation of an explosive.
The resistance of the joints is therefore the same as the tunnel lining. Cross Passageway Failure The general lining response of cross passageway tunnels subject to blast loading is approximately the same as described above. Special attention should be given to the fol- lowing considerations: 1 high stress concentration may occur at the junctions with main tunnels and 2 given the same amount of explosive charge, the resulting blast peak pressure in a cross passageway tunnel may be greater than that in the main tunnel due to its smaller cross-sectional geome- try.
Therefore, cross passages are more vulnerable to damage. In general, however, from an operational standpoint, cross passageway tunnels are not considered to be more critical than the main running tunnels because 1 there is generally more than one cross passageway tunnel i. Portal Failure From a stability standpoint, the tunnel portal area is gen- erally one of the critical locations due to the inherent slope stability problem.
Landslide, rock fall, or even collapse at and near tunnel portals may be triggered by certain extreme events, such as earthquakes and blast waves, thereby blocking the passageway and potentially affecting structures or facili- ties at the top of the slope. Tunnel portals are therefore con- sidered to be particularly vulnerable during such extreme events. However, at the portal, the blast is less confined and the energy will dissipate. To stabilize the portal area, soil anchors or rock reinforcement systems are often used.
Other remedial measures, such as flattening the earth slopes or using various ground improvement treatments, may also be effec- tive. Nevertheless, the damage potential of a portal failure is generally considered to be less than that of a tunnel lining fail- ure because the repair for a portal failure can be done in the open space. In addition, flooding is normally not an issue when a portal is damaged or collapses, so the repair time and associated costs are relatively low compared with the other parts of the tunnel.
Ground Soil and Rock Failure Blasting may also cause the geological media surrounding the tunnel to yield or fail, particularly when the tunnel liner is breached or in unlined tunnels such as those constructed in sound rock. The post-yield behavior of the surrounding geological media depends on the types of the materials encountered and their characteristics under high-energy transient loads. When sand and gravel are saturated with water, semi-flowing to flowing conditions may occur.
Flooding of the tunnel could also happen if the surrounding material is very porous such as gravel or rock fill under a high groundwater level. This is particularly true for immersed tube tunnels. The material falling into the tunnel should be confined to the area where the liner is breached.
It may yield without losing its coherence and thus provides self-support capability for a short duration. Fresh fractures could be generated, thereby resulting in some loosened rock pieces falling into the tunnel. Chunks of rock loosened by the explosion could fall into the tunnel. Water Inflow and Flooding Transportation tunnels are intensively concentrated and interconnected in urban areas.
Therefore, failure of an under- water tunnel ranging from collapse or complete inundation with water due to local breaching of the liner may lead to flooding in the underground transportation system. Flooding may also introduce large quantities of sand, silt, gravel or shear zone debris.
Significant lengths of tunnel can become filled with debris or mud in short periods of time, causing tunnel structures to become buried. In addition, loosening of the soil under foundations can undermine structures above or adjacent to the tunnel. Progressive Failure Failure of the tunnel liner and surrounding ground may cause instability of adjacent underground utilities and dam- age to surface structures by piping and differential settle- ments. Flooding of the entire transportation system may also be considered a progressive failure.
Fires in tunnels may lead to a high risk of explosive spalling of the concrete liner, particularly for concrete with high mois- ture content, such as shotcrete, or for high-performance or high-strength concrete with low permeability. Explosive spalling occurs in the temperature range where chemically bound water is released from the concrete.
Explosive spalling of high-performance or high-strength concrete is directly related to internal pressures generated during the attempted release of chemically bound water. Lawson et al.
Using results from a combination of a heat transfer analysis and a nonlinear structural analysis conducted for a range of service loads, concrete mixes, and fire types, Caner et al. The effects of temperature-induced material degradation and ground tunnel liner interaction were considered in these analyses. Caner et al. They should be removed or replaced if they are found to be deficient. The depth of fire-damaged concrete may be determined by using heat transfer analyses and should be verified by condition assessment.
Voids and spalls should be patched with patching materials of similar characteris- tics as the concrete mix design used for the original tunnel to maintain its structural integrity.
The new reinforcement should be properly spliced to the existing reinforcement. An evaluation of the need for major repair should be determined on a case-by-case basis. Furthermore, with the more permeable concrete, the chance of explosive spalling may be minimal in the event of another fire.
For practicality, spray-on insulation materials may be used to patch the damaged area. Various tunnel ventila- tion systems and configurations were operated to evaluate their respective smoke and temperature management capa- bilities. The fire sizes ranged from For fires below For the The areas that resulted in exposed reinforcing steel were repaired with reinforced shot- crete. Insulated boards with high- temperature resistance were installed to protect the tunnel surfaces.
A total longitudinal distance of 75 meters was pro- tected. The boards were installed along the first 25 meters downstream of the fire site. Ceramic curtains were installed beyond the boards; 9 meters upstream and 41 meters down- stream were covered.
Significant spalling of the tunnel material occurred both upstream and downstream of the passive fire protection system. Earthquake Effects on Tunnels Underground structures are generally less vulnerable to earthquakes than surface structures, such as buildings and bridges, because the surrounding ground confines under- ground structures.
As long as the surrounding ground is sta- ble and experiences only small ground deformations, the tunnel tends to move along with the surrounding ground and maintains its structural integrity. The area experiencing this shaking may cover hun- dreds of square miles near the fault rupture. As the ground is deformed by the traveling waves, any tunnel structure in the ground will also be deformed.
Each of these instabilities can be potentially catastrophic to tunnel structures, although the damage is usually localized. It is often possi- ble to design a tunnel structure to account for ground instability problems, although the cost may be high. Vulnerability Screening for Geotechnical Hazards and Threats. The discussions above show that it is important to perform a tunnel vulnerability screening study for ground failure potential i. The objective of the vulnerability screening process is to identify which sec- tions of the tunnel structures may have risk of poor perform- ance during earthquakes.
For sections identified to have low earthquake risk, no further evaluations are required. Other- wise, further assessments may be needed. Both methods compare the soil liquefaction resist- ance through SPT or CPT data with the earthquake induced dynamic stresses. Detailed information about liq- uefaction and the recommended procedures for evaluating liquefaction procedures are documented in the report from the workshop sponsored by the National Center for Earthquake Engineering Research NCEER [Ref.
Evalu- ations should focus on the following areas: 1 at tunnel portals in soil as well as in rock , 2 in shallow parts of the tunnel alignment adjacent to soil slopes, and 3 in areas where existing slopes have displayed signs of movement under static conditions. The commonly used pseudo-static method of analysis can be used for evaluating the seismic stability in areas of concern. If a pseudo-static seismic sta- bility analysis indicates an insufficient safety margin against the landslide movements, then a more refined deformation- based method of analysis should be used to estimate the The impact of the potential slope movements on the affected structures should then be assessed.
In general, it may not be economically or technically feasible to build a tunnel to resist potential faulting dis- placements, particularly if the magnitude of the fault dis- placement is large e. However, avoidance of faults may not always be possible, especially for tunnel systems that are spread over large areas.
In highly seismic areas such as California, it may be inevitable for the tunnel to cross a fault. The design approach to this situation is to accept the displacement, localize the damage, and provide means to facilitate repairs.
Tunnel Response to Ground Shaking. The response of a tunnel to seismic shaking motions may be described in terms of three principal types of deformations: 1 axial deformation, 2 curvature deformation, and 3 ovaling for circular tunnels such as bored tunnels or racking for rectangular tunnels such as cut-and-cover tunnels. Axial deformations are induced by components of seismic waves that propagate along the tunnel axis i.
When the component waves pro- duce particle motions parallel to the longitudinal axis of the tunnel, they cause alternating axial compression and tension strains, as illustrated in Figure 12A.
Curvature deformations result from component waves that produce particle motions in the direction perpendicular to the tunnel axis. The cur- vature deformation results in bending and shear demands on the tunnel structure, as shown in Figure 12B. The oval- ing or racking deformation i. Vertically propagating shear waves are generally considered the most critical type of waves for this mode of deformation, as shown in Figure 13 [Ref. Dowding and Rozen reported 71 cases of tunnel response to 67 Figure Longitudinal deformation of tunnels.
Transverse ovaling and racking of tunnels. The permanent ground supports ranged from no lining to timber, masonry brick, and concrete linings.
Owen and Scholl documented additional case histories making a total of , including cut-and-cover tunnels and culverts in soils [Ref. Damage initially inflicted by earth movements, such as faulting and landslides, may be greatly increased by continued reversal of stresses on already dam- aged sections. Using the data presented above as well as additional data from the Kobe, Japan, earthquake with a moment mag- nitude of 6.
The damage state is presented as a function of ground shaking levels represented by peak ground acceleration and tunnel lining types. The data apply only to damage due to shaking. Data for cut-and-cover and immersed tunnels are not included in the figure.
If an immersed tube tunnel is breached, the result could be rapid flood- ing in the tunnel and potential flooding of significant portions of the underground transit system if they are connected. Even a large blast inside a tunnel in good rock will likely induce only limited local damage and could be easily repaired within a reasonably short period.
Tunnels constructed in soil tend to be more vulnerable than those in rock. Tun- nel structure elements in very soft soil will induce larger bending and shear demands under blast loading condi- tions. Underwater tunnels surrounded by very porous material such as immersed tunnels backfilled with gravel or rock fill are particularly vulnerable to the inflow of large volumes of water mixed with surround- ing materials.
Deeper cover provides better tunnel protection from both interior and exterior explosions. For a tun- nel on land, better tunnel performance can be expected when it is surrounded by a dry geological medium i. As mentioned previously, Figure 11 presents a rough estimate of the required tunnel liner thickness for reinforced con- crete as a function of the explosive charge weight and the charge standoff distance. Table 12 presents relative severity ratings of tunnels based on some of the critical factors discussed above.
The infor- mation in this table is based on recent tunnel security proj- ect experience and expert opinion. This chart has been prepared in a qualitative manner, and therefore should be used as such. Empirical correlation of seismic ground shaking induced damage to bored tunnels [Ref. The depth charge is dropped and det- onated above an immersed tube tunnel. In addition to the size of the hazard or threat, other critical factors considered in the damage potential rating included type of tunnel construction, ground condition, ground support sys- tem, and soil or rock overburden thickness.
Tables 14, 15, and 16 present structural vulnerabilities to the most likely hazard or threat scenarios for road tunnels, transit tunnels, and rail tunnels, respectively. These tables basically combine the information given in Table 3 hazard and threat scenarios with the information given in Table 13 damage potential ratings for transportation tunnels. The hazards and threats pre- sented on the left side of the tables include very large, large, medium, and small IEDs and large fires.
All of the hazards and threats were developed further to identify hazard and threat scenarios that include hazard and threat, path to target, tactical delivery device, and ultimate target. The right side of the tables contain each of the major tunnel types: immersed tube, cut- and-cover, bored or mined in soft to firm ground, bored or mined in strong rock, and air-rights structure tunnels. Each row represents a unique hazard or threat scenario.
If that sce- nario poses danger to a certain type of tunnel, then that inter- secting cell describes the physical vulnerability PV , the operational vulnerability OV and the damage potential DP.
The damage potential is presented in terms of the rating abbre- viations given in Table 13 from A to F. Of these systems, many are not visible but are nonetheless Table Relative severity ratings in transportation tunnels. Transported by foot. Transported by car. Transported by truck. Damage potential ratings for transportation tunnels. Structural vulnerabilities to most likely hazard or threat scenarios for road tunnels. Structural vulnerabilities to most likely hazard or threat scenarios for transit tunnels.
Structural vulnerabilities to most likely hazard or threat scenarios for rail tunnels. Life safety includes all of the systems, equipment, and facil- ities required to provide protection during an emergency to the tunnel and its inhabitants. Electrical includes both normal and emergency power for ancillaries, systems, and train traction. Command and control includes traffic, train, and system control, along with signals.
Communications includes all communications systems required to make the tunnel functional and safe. To create the above five primary categories of systems, the research team started with an initial list of safety systems serv- ing road, transit, and rail tunnels. Table 17 shows this initial list of safety systems, along with the tunnel functions associ- ated with each system. After careful review of the data in this table, the research team made several decisions.
One decision was to combine the categories of passenger rail tunnels and freight rail tunnels in this report because the vulnerabilities and damage potentials are similar. The other decisions involve the elimination of some elements such as emission control, emission monitoring, and normal lighting because they do not affect the vulnerability of particular tunnels.
In the end, the research team decided on the above five primary categories of systems. These revised primary categories are depicted in Table Table 19 pro- vides a subjective evaluation of the different impacts and mit- igation requirements. System paralysis can occur if a coordinated attack is aimed at specifically related systems. Such threats may cause synergistic effects and may require systemwide checks to be conducted before tunnel operations are resumed.
Tables 20, 21, and 22 subjectively highlight the impact of system element disruption on each of the transportation tun- nel function types. These subjective impact ratings are based on single-point attacks. In the case of multiple-point or coor- dinated attacks, the disruption to the tunnel systems would obviously become more severe.
This assessment was combined with the system element impact list to develop the draft guidelines. The results of the combined assessment and list are presented in Table 23 as a list of potentially critical locations where each of the tunnel systems is vulnerable.
Tables 25, 26, and 27 present system vulnerabilities to the most likely hazard or threat scenarios for road tunnels, tran- sit tunnels, and rail tunnels, respectively. These tables com- bine the information given in Table 3 hazard and threat scenarios with the information given in Table 24 vulnera- bilities of critical locations. All of the hazards and threats were developed further to identify scenarios that include hazard or threat, path to target, tactical delivery device, and ultimate target.
Each of the hazard or threat scenarios was considered for each of the five primary system categories presented in Sec- tion 4. Each row presents a unique set of vulnerabilities both physical and operational and a set of damage poten- tials. This should provide the owner or operator with a clear guide to the types of hazard and threat scenarios possible for tunnels.
Each of the criti- cal systems has been assessed, and a set of vulnerabilities and damage potentials have been identified for each reasonable hazard or threat. To determine the countermeasures available to the tunnel owner or operator, the research team applied comparative analysis to the hazard and threat scenarios to discern com- mon themes.
From this analysis, it was determined that the This category includes all fixed fire suppression systems such as sprinklers, mist, and deluge systems.
Fixed fire suppression systems are only in ancillary facilities. There are three road tunnels in the United States with sprinkler systems in the roadway. There are some U. Fixed fire suppression systems are only in stations and ancillary facilities. Traction power is in all transit and rail tunnels with electrified train vehicles. Initial categories of safety systems. Revised categories of safety systems. The research team then analyzed the damage potential of a disturbance emanating from each of the four major categories of sources.
Damage is the loss of use of the tunnel. Minor dam- age may result from a disabled car blocking one lane, and major damage may result from a fire that closes the tunnel to traffic. The scope of the functional loss is significant, and the damage potential reflects the potential percentage loss of the tunnel use. The percentage loss of the tunnel use is important, more so than the hazard or threat that triggered the incident.
Given this importance, the research team began to match the greatest damage potential, or potential loss of use of the tun- nel, to the hazards and threats.
The research team finally sum- marized the hazards and threats that have the greatest damage potential, or the potential for total loss of tunnel use. Large fires and explosive devices had a similar damage potential as that of all other hazards and threats examined.
Fire, as a primary or secondary hazard i. An explosion can cause similar dis- ruption to the tunnel. Each of these main hazards and threats exhibited damage potential to both the structure and systems of the tunnel.
Therefore, the hazard and threat platforms were fully described as a series of scenarios, including the type and size of hazard or threat, the tactical delivery device, and the tar- geted tunnel element.
A lengthy list of scenarios was com- pressed to reflect the common hazard and threat platforms. The vulnerabilities of various tunnel types to these hazard and threat scenarios, as well as the relative damage potential, appear in Tables 14, 15, and 16 for road, transit, and rail tun- nels, respectively. The vulnerabilities of various tunnel safety system types to the same set of hazard and threat sce- narios, along with relative damage potentials, appear in Tables 25, 26, and 27 for road, transit, and rail tunnels, respectively.
These tables present the groundwork for the presentation of countermeasures, which is discussed in the next chapter. Degree of impact on safety and operations. Disruptive impacts in road tunnels. Disruptive impacts in transit tunnels. Disruptive impacts in rail tunnels. Vulnerabilities of potentially critical locations.
Vulnerabilities of critical locations. Assumes perpetrator gets inside 3. Assumes transverse system or longitudinal with fans housed in central location 4.
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