With the increase in population of cities and progress of technology, peoples demand toward a comfortable life has also increased. Traffic jam in big cities and shortage of parking lots have made necessary the need for use of underground spaces more than ever.
The use of underground metro trains as a fast and safe vehicle could be appropriate alternative for passengers. However, passage of trains with appropriate speed in between stations produces vibrations that may sometimes be disturbing for people living in the region. These vibrations, which are one of the most serious concerns related to region close to transportation systems, could cause buildings to shake and make rumbling sounds to be heard inside buildings. In addition to human annoyance, these vibrations could affect old buildings and sensitive equipments. They could cause fatigue in the materials of ground and structure, create differential settlement, crack in walls, resonance and other difficulties. Therefore, in designing new railroad, evaluation of vibrations due to movement of trains is one of the important parameter in selecting the route and the type of rail system.
Intense disturbance to human being in residential area occurs when the level
of vibrations reach to 85 dB (FTA, 2006). However, according
to ISO-2631-2 standard, human response to ground-borne vibration is very complicated
and depends on many factors. Howarth and Griffin (1988)
showed by different experiments that the number of train passage and the duration
of vibrations in addition to vibrations magnitude, play role on human disturbance.
Precise evaluation of subway induced vibrations requires complete information
about details of railing system, site geology and other information related
to the sources producing and transmitting vibrations from metro line to buildings.
Many of these information may not be available during initial stage of design,
therefore, application of numerical modeling, using reasonable and simplifying
assumptions would be very useful and informative to prediction of vibrations
due to passage of metro trains. Fortunately considerable research on this subject
have been conducted throughout the world in recent years e.g., Gupta
et al. (2008), Forrest and Hunt (2006a,
b), Degrande et al. (2006a,
b), Clouteau et al. (2005), Hemsworth
(2000) and Sheng et al. (1999) that makes
this task obtainable for cases that are in the preliminary stages of the project.
In this research, using numerical modeling in Plaxis v8, induced vibrations
due to passage of trains in Ahwaz city subway currently under construction is
evaluated and the regions with high vibration potential, are identified. The
route of line 1 of Ahwaz metro with the approximate length of 23 km connects
NE region of the city to SW region by passing through the downtown area and
crossing Karoon River. Along this route, 8 stations were selected, each one
representative of part of the route. Dynamic analysis on each station was performed
and vibrations created on the ground surface due to passage of train were predicted.
In addition, the effects of different parameters on these vibrations were evaluated.
The findings of this research may be used by authorities for future planning
and designing appropriate railing system and etc.
BASICS OF METRO TRAINS VIBRATIONS
Exact assessment of ground vibrations created during passage of underground trains requires complete knowledge of parameters that affect magnitude of these vibrations. Therefore, it is necessary that the process of vibration creation and factors influencing these vibrations to be exactly evaluated.
Some common sources of ground-borne vibration are trains, buses on rough roads
and construction activities such as blasting, pile-driving and operating heavy
earth-moving equipment. These vibrations cause tangible movement of the building
floors, rattling of windows, shaking of items on shelves or hanging on walls
and rumbling sounds insides rooms. Vibration is perceived directly or it is
sensed indirectly as re-radiated noise. The frequency range of interest for
subway induced vibrations is 1-80 Hz and for the re-radiated noise it is 1-200
Hz (Gupta et al., 2007). Disturbance due to these vibrations occurs when
they exceed the threshold of human perception. The range of vibrations that
disturb the human are much less than the range that cause disturbance to the
regular buildings (FTA, 2006).
The human body responds to an average vibration amplitude and because the net
average of a vibration signal is zero, the root mean square (rms) amplitude
is used to describe the smoothed vibration amplitude. The root mean square of
a signal is the square root of the average of the squared amplitude of the signal.
The average is typically calculated over a one-second period. The use of unit
of decibel (dB) used for describing the vibrations is also customary.
In general, subway induced vibrations include three basic parts namely source
of vibration, route of propagating waves and receivers of vibration. These three
parts are shown in Fig. 1. The knowledge of how these parts
could affect vibrations is very effective in predicting and lessening of vibrations.
||Propagation of vibrations due to movement of metro trains
into ground and buildings (FTA, 2006)
Vibration source: The train wheels rolling on the rails create vibration energy that is transmitted through the track support system into the transit structure. In fact this part includes all the parameters related to train performance and also the train route. Factors such as train speed, train suspension system, roughness of rail surface and wheels and rail supporting system all affect vibrations. Jointed rails, rough rails and impact of wheels on rails all cause severe increase in vibrations in the source.
Degrande et al. (2006a, b)
measured the vibrations on the rail and also on the axle of train wheels
and observed that the produced impact during the passage of wheel from the joint
of the rail increases linearly with the increase of train speed. Such that 200%
increase in speed of the train created an increase in vibrations of about 4
to 5 dB.
Vibration path: After creation of vibration in the source, these vibrations
propagate into the surrounding medium. Soil and subsurface conditions are known
to have a strong influence on the level of ground-borne vibrations. Among the
most important factors are the stiffness and internal damping of the soil and
depth to the bedrock. Experience with ground-borne vibrations indicates that
vibration propagation is more efficient in stiff clay soils and that shallow
depth to the bed rock seems to concentrate the vibration energy close to the
surface and which can result in ground-borne vibration problems at large distances
from the track. Factors such as layering of the soil and depth to water table
can also have significant effects on the propagation of ground-borne vibration
Soil layering will have a substantial, but unpredictable, effect on the vibration levels since each stratum can have significantly different dynamic characteristics. Therefore in evaluation of vibrations in numerical methods inclusion of all soil layers present at the site, even those with small thickness could lead to more exact prediction.
The presence of the water table may have a significant effect on ground-borne
vibration, but a definite relationship has not been established (FTA,
2006). Unterberger (2004) using Flac 4.0 showed that
there was no distinct relationship between the changes in the ground water table
and the vibrations created due to the passage of trains.
Yi-Qun Tang et al. (2008) using continuous dynamic
monitoring by means of embedded earth pressure piezometers and pore piezometers
around the tunnel studied the response frequency and stress amplitude of the
saturated soft clay with the distance from the tunnel due to the subway vibration
loading. Also they proposed A formula for the attenuation of the dynamic water
pressure response in the soil.
Vibration receiver: The vibration of the transit structure excites the adjacent ground, creating vibration waves that propagate through the various soil and rock strata to the foundations of nearby buildings. The vibration propagates from the foundation throughout the remainder of the building structure.
The receiver building is a key component in the evaluation of ground-borne vibration since ground-borne vibration problems occur almost exclusively inside buildings. The vibration levels inside a building are dependent on the vibration energy that reaches the building foundation, the coupling of the building foundation to the soil and the propagation of the vibration through the building. The general guideline is that the heavier a building is, the lower the response will be to the incident vibration energy.
In this research, the prediction of subway induced vibrations on the ground surface in Ahwaz city, is made using the computer code Plaxis V8. The tunnel, train loading and surrounding soil are modeled in plane strain condition.
Engineering geology of the city: Soil conditions of Ahwaz subway route
are taken from geotechnical reports supplied from general contractor Keyson
Co. of Iran. By careful study of all geotechnical boring logs of the metro route,
eight soil profiles at the location of the metro stations were selected as representative
of the whole route to be used in dynamic analysis. According to Fig.
2, the selected soil profiles were named as N1 to N8. By precise assessment
of geotechnical profiles of these stations, as shown in Appendix, the selected
soil profiles could be divided in two distinct parts.
|| Plan view of the Ahvaz metro route
First one, the soil profiles
from north half of the route mainly consisted of fine grain clay and silty layers
over the bedrock formation consisting of red marl, siltstone and sandstone at
a shallow depth. Second part, the soil profiles from the south half of the route
on which the bedrock formation falls below a depth of 40 m under young alluvial
deposits due to the presence of the Ahwaz fault. The young alluvial deposit
consists of layers of fine to medium sand, clay and silt with low to medium
Determination of Rayleigh damping coefficient: It is very clear that
damping in soil and structure affects the amount and form of dynamic response
of the system very much. Although, a lot of research in this subject have been
done in the past, however little information is available about the determination
of damping parameters. Rayleigh damping coefficient is defined as:
where, coefficient α is related to the effect of mass on system damping and coefficient β relates the effect of stiffness on system damping.
For high values of β, vibrations with high frequencies are damped. The coefficient α and β could be determined from the damping ratio Di, which is related to vibrations with frequency ωi. The relationship between these parameters is as follows:
Because the strains developed in the soil due to vibrations created by passage of trains are generally low we can assign a constant value for D in the Eq. 2, then:
Now, we can determine natural frequencies of soil layer in first and second mode using empirical equations and then obtain D and from that, damping coefficient α and β are computed. As it can be seen due to very low strain level, low damping ratios are obtained. By this method values of α and β for all soil layers in different stations are calculated. The values of α and β are in the range of 0.008-2.353 and 0.0083-0.00005, respectively.
Determination of train dynamic load: vibrations due to the train passage on the rail are resultant of several different mechanisms. The most important of these mechanisms are deformation of the rail system due to passage of the wheels, roughness of the rail and the wheels and the rail joints. There are several methods for determining train dynamic loading as an input for dynamic analysis. These methods include, pseudo static load function, analytical load function and direct measurement of train dynamic load. In pseudo static load function method movement of a determined load causes an oscillating load function in one section of the route. In this method the effects of roughness of rail and wheels, the geometry of rail and wheels and impacts due to train braking close to stations are ignored. In direct measurement method, by attaching several velocity meter sensors at an appropriate place near to the train track, the vibrations induced by train movement at rail-subgrade level are directly measured and after some correction, the obtained time history is used as an input loading for dynamic ground response analysis.
In this research, the third method is used and the particle velocity time history obtained from measurement in Hasanabad station in Tehran metro is used as a dynamic load input in dynamic analysis. It should be mentioned that at the time of this research, Ahwaz metro project is at its initial stage and precise information about type of wagons, rail system and subgrade system under rail is not yet known and there may be some differences between the two projects, yet the use of this method as compared with other methods is more precise. In this method all the mechanism that cause the vibration during the passage of train such as train speed, rail joints, roughness of rail and wheels, rail and wheel geometry, impacts due to train braking and etc. are recorded by measurements in Hasanabad station. For recording vibrations seismograph SSR-1 equipped with three short period seismograph SS-1 from Kinemetrics Co. was used. The SSR-1 equipment depending on number of canals was capable of recording 0.03 to 1000 samples per second. The seismograph SS-1 record the velosity with one second period with damping ratio of 0.7.
In order to record vibrations at the same time two sensors was attached, one on the tunnel floor at a distance of 3 m from the rail and second one on the ground surface. The vibrations were recorded at time interval of 0.01 sec for period of 600 sec that include the passage of train. In order to minimise traffic noise, the measurement were performed during the weekend.
In this research the time history of velocity related to the vertical component
of vibrations was selected as dynamic loading and it was applied as vertical
vibration to tunnel floor in numerical modeling. It should be mentioned that
because the measurement was close to the metro station, train has been decelerating,
therefore the measured vibration should include the vibration due to braking
of the train.
|| Measured vibrations at rails level, during the passage of
train at Hasanabad station Tehran metro
||View of modeling in Plaxis
Therefore, the selected recording that lasts for 20.48 sec includes
all the factors responsible for vibrations during the passage of train. This
time history is shown in Fig. 3.
The vibrations measurement in Tehran metro is obtained only at one speed of
train and information about vibrations at other train speed was not at hand.
Therefore in order to determine the dynamic loading for different speeds of
train, empirical method given by US Department of Transportation that is based
on many measurements is used to calculate vibrations at different speed of train
at a distance of 3 m from the axis of the rail. Then the velocity time history
obtained in this way is scaled to the same level of vibration obtained in empirical
method. The average speed for train in Tehran metro is 60 km h-1
and the maximum speed is 80 km h-1. If we assume the same range of
speed for Ahwaz metro, the above mentioned method was used in this research
to predict the vibrations due to train speed of 20 to 100 km h-1.
|| Geometry characteristics of 2D model
Numerical modeling in plaxis: Here, a 2D model of tunnel, surrounding soil and dynamic loading of train with the use of Plaxis V8 is introduced in order to evaluate the vibrations propagation due to train passage to the ground surface. The geometry characteristics of 2D model (Fig. 4) are shown in Table 1 and the train dynamic loading is applied to the model as an uniform distributed loading at the rail location.
In this model, two phases of analysis is defined. The first phase consists
of elasto-plastic analysis of tunnel excavation and placing the tunnel lining
and the second phase includes dynamic analysis related to the passage of train
inside the tunnel.
Geotechnical properties of soil layers at different stations used in numerical modeling are shown in appendix and Mohr-coulomb criterion was selected as the soil behavior model. Concrete lining properties used in the modeling are (E = 20000 MPa) and (Poisson ratio = 0.15).
In dynamic analysis, in addition to static boundary conditions, reflection of waves at the model boundaries should be considered. In fact some special boundary conditions have to be defined to account for the effect that in reality the soil is a semi-infinite medium. Without these special boundary conditions the waves would be reflected on the model boundaries, causing perturbation. To avoid these spurious reflections, the static boundaries of the model (which do not exist in reality) are taken sufficiently far away to avoid direct influence of the boundary conditions and also absorbent boundaries are specified at the bottom and right hand side boundary.
In the model 15 nodes triangular elements were used in finite element mesh.
|| Fine element generation in Plaxis model
For determining the optimum size of elements in order to get reasonable precise
result in a minimized time, four different meshing pattern were analyzed and
the results of analysis with very fine and fine meshing were very close to each
other therefore, fine meshing pattern were chosen (Fig. 5).
Vibrations at Darvazeh station (N3): As we observe from borehole in
this location (Appendix), geotechnical profile consists generally of mudstone,
sandstone and siltstone. The rock formation in this location belongs to Aghajari
formation. Figure 6 shows the velocity loading applied to
tunnel floor and Fig. 7 to 11 show diagram
of vertical particle velocity at points A, B, C and D at the surface ground.
The position of these points is shown in Fig. 4. As we compare
vibrations at rail level and at the surface ground, for example point A, an
increase in vibrations level is observed. Also by comparing the results for
point A~D we observe a decrease in vibrations with distance from the tunnel.
Figure 11 shows RMS velocity in terms of decibel. In Fig.
11, average vibrations produced at the surface ground at different distances
from rail axis during the passage of train with speed of 80 km h-1
can be seen. According to Fig. 11, vibrations at point A
(zero distance from the rail on ground surface) is equal to 84 dB, at point
B (10 m) = 79.7 dB, at point C (20 m) = 77.5 dB and at point D (30 m) = 73.2
One of the noticeable point of the analysis at this location is that the amount
of decrease in vibrations with distance away from the tunnel is low as it is
shown in Fig. 11. The diagram is flat as compared with those
in other stations shown later in the study. The reason for this phenomenon is
the presence of the bedrock formation close to the surface of ground. Actually,
vibration energy is concentrated in the surface layers and is propagated horizontally.
|| Velocity loading applied to tunnel floor, train speed = 80
km h-1, Darvazeh station (N3)
|| Computed velocity with time at point A, train speed = 80
km h-1, Darvazeh station (N3)
||Computed velocity with time at point B, train speed = 80 km
h-1, Darvazeh station (N3)
||Computed velocity with time at point C, train speed = 80 km
h-1, Darvazeh station (N3)
Vibrations at Kargar station (N8): Geotechnical profile at this station that is shown in Appendix consists of young alluvial deposits including clayey, silty and sandy deposits. The first 2 m of soil consist of fill materials then it turns to brown medium to firm silty clay to depth of 3.8 m, to medium fine sand to depth of 5.9 m, to medium to firm brown sandy silt to depth of 8.5 m, to firm to very firm brown clay to depth of 10 m and finally to dense sand to depth of 35 m.
Figure 12 shows the result of analysis for train with speed
of 80 km h-1 at point A located above the axis of the rail on the
As we compare Fig. 12 with Fig. 6, we
observe that the vibrations at tunnel level, has been damped as they reach the
surface of ground at this station.
|| Computed velocity with time at point D, train speed = 80
km h-1, Darvazeh station (N3)
|| Ground surface vibrations at different distances from rail
axis, train speed = 80 km h-1, Darvazeh station (N3)
|| Computed velocity with time at point A, train speed = 80
km h-1, Kargar station (N8)
Figure 13 shows RMS velocity
in terms of decibel at different points away from the axis of the rail. As it is shown in Fig. 13, the level of vibrations rapidly
decreases with distance away from the axis of the rail. For example at distance
30 m from the rail (point D), vibrations have decreased by 17 dB. The most important
reason is significant geometric damping of the waves due to the presence of
bedrock formation at much deeper elevation at this location. In fact, propagation
of waves downward into the ground, without any considerable reflection, can
cause a rapid decrease of vibration level at the surface with distance away
from the axis. This prediction is completely different from N3 station explained
||Ground surface vibrations at different distances from rail
axis-train speed = 80 km h-1, Kargar station (N8)
||Ground surface vibrations at different distances from rail
axis-train speed = 80 km h-1 all stations
We can also observe that the level of vibrations in this station is
lower than those in station N3. These differences are due to different layering
and material properties at the two stations.
Vibrations of ground surface along the metro route: The results of analysis for other stations at speed of 80 km h-1 are shown in Fig. 14. By comparing vibrations of ground surface at different stations from Northeast (N1) to Southwest (N8) along the metro route we can see a general decrease in the level of vibrations. However, there are some exceptions to this general trend. For example, in station N7 the level of vibrations is slightly higher than that in station N8 despite of the similarity in soil profile. This phenomenon could be due to the presence of layer of dense sand at the depth of 18 m at the station N7 which acts like a bedrock formation and causes reflection of waves to the surface layers. In general, the level of vibrations in North part of the Ahwaz metro route is higher than that in Southern part. This observation is consistent with the depth of rock along the route. Geotechnical boring logs of the metro route, in selected stations are shown in Appendix.
Decrease in vibrations with distance from the rail axis: One of the
noticeable points in the surface ground vibration curves in Fig.
14 is the slope of these curves, which indicates the rate of decrease of
vibration with increase in distance from the axis of the rail.
||Comparison of slop of curves of ground surface vibrations
with distance-speed = 80 km h-1
The slope of
curves in Northeast of the route is very low. This means that the vibrations
due to passage of train could affect even the buildings in far distances. On
the other hand the slope of the curves in Southwest of the route is higher which
means vibrations damp very fast with the distance from the axis of the rail
and they could only affect the buildings in close distance. In Fig.
15, this point is clearly observed for stations Zeytoon (N1) and Kargar
One of the most important reasons for this difference in behavior between the ground surface vibrations curves in Northeast and Southwest stations is the depth of bedrock. The shallow depth of bedrock in Northeast part of the route causes effective propagation of vibrations to the ground surface. Major parts of waves will be reflected back to the surface as they hit the bedrock and therefore with multiple reflections of waves they propagate horizontally in surface layers.
In Fig. 16 and 17, particle velocity
vectors after passage of train from station N3 and N8 are shown. The difference
in the pattern of vectors in Fig. 16 and 17
can be seen clearly.
Effect of train speed on vibrations: The speed of train along the route
between stations varies, therefore in order to predict exact vibration at each
point along the route it is necessary to evaluate the ground vibrations at different
train speed. Figure 18 shows changes in speed of train with
time and with the traveled distance between two metro stations.
As it is observed in this Fig. 18, the train begins to travel
at the station from zero speed, after a distance of about 250 m it reaches to
maximum speed of 80 km h-1 and it travels at this constant speed
for a distance of about 600 m. Then at a distance of about 250 m from the next
station then train begins to decelerate until it reaches speed of zero when
it arrives at the station. This is repeated in other parts of the route between
In order to evaluate the effect of train speed on vibrations, train dynamic
load related to speed of 20, 40, 60, 80 and 100 km h-1 was applied
to the model at stations N1 to N8 and dynamic analysis was performed.
||Concentration of vibration energy at surface layers due to
reflection of waves during impact with rock-Darvazeh station (N3)
|| Particle velocity vectors, Kargar station (N8)
||Changes in distance and speed of train with time between two
19 shows the changes of the ground surface vibrations in terms of rms velocity
at a distance of 20 m from the axis of tunnel at different speed of train. According
to Fig. 19, when train speed becomes twice, the ground vibrations
increases by about 4 to 6 dB. This result is consistent with experiments performed
by US Department of Transportation. Figure 20 and 21 also show the changes in peak particle velocity at the ground surface with distance
from the axis of the rail at different train speed. it is observed that PPV
at the ground surface decrease with distance from the axis of the rail and this
decrease at further distances is not much affected by train speed.
Effect of depth of ground water table on vibrations: Depth of ground
water table along the route of Ahwaz metro is very close to the ground surface.
Also because of seasonal floctuation of ground water table, it is necessary
to evaluate this effect on the level of vibrations. Therefore, dynamic analysis
with train speed of 80 km h-1 was performed at Kargar station (N8)
with different depth to the ground water table. Figure 22
shows the results of this analysis. According to these results, there is no
regular relationship between the ground surface vibration and depth of ground
water table. For example, vibrations at a distance of 30 m from the axis of
the rail, when depth of ground water table is at 3.8 m is equal to 57.9 dB.
||Changes in level of ground surface vibrations with speed at
point C (20 m from the axis of rail)
||Maximum particle velocity at different distances from the
axis of the rail train speed = 80 km h-1
||Maximum particle velocity at different distances from the
axis of the rail train speed = 40 km h-1
With lowering the water table the vibrations is increased such that when water
table is at depth of 11.5 m the vibrations at a distance of 30 m from the axis
of the rail amount to 63.65 dB and with further lowering of water table, the
level of vibration decreases again. Therefore, For precise evaluation of the
ground surface vibrations, it is necessary that the depth to the water table
to be determined exactly.
Resonance during passage of train: One of the important aspects in evaluation
of subway induced vibrations that should be taken into account is determination
of natural frequency of railing system and frequency content of dynamic loading
||Changes in vibrations at different depth of water table-Kargar
When the magnitude of the two frequencies are close to each other,
the occupance of resonance is probable. In other word the rail system, rail
support and tunnel lining should be designed in such way that natural frequency
of the whole system is far enough from prominent frequency of dynamic loading
In order to determine the natural frequency of the railing system we used trial
and error method in which a harminic sinusoidal loading with different frequencies
is applied to the railing system for 2 sec. Then for evaluating natural frequency
of the system in free osilation, it was allowed to osilate without outside loading
for 2 sec. This loading function is shown in Fig. 23. After
that, the ground surface vibrations due to this loading were obtained. At Zeytoon
station (N1) harmonic loading with frequencies of 1 to 5 Hz was applied to the
model and the resulting response is shown in Fig. 24. As
it is observed in Fig. 24, the ground surface vibration due
to this loading pattern with frequency of 1 Hz is irregular and the amplitude
of vibration is very low. With increase in loading frequency, the vibrations
become more regular and the amplitude is increased. As it is clear from Fig.
24 vibrations at loading frequency of 3 Hz have highest amplitude and at
higher frequencies the amplitude of vibration is decreased and the vibration
becomes irregular again indicating the occupance of resonance at frequency of
3 Hz. Therefore from this observation we can conclude that the natural frequency
of the system in this station is 3 Hz. By repeating this procedure for other
stations, it is concluded that natural frequency of the railing system along
the metro route in Ahwaz geology is about 2-3 Hz.
In order to determine frequency content of train loading function, using fast
Fourier transformation, time domain function is converted to frequency domain
as shown in Fig. 25. As it is observed from Fig.
25, the predominate frequency of train loading is in the range of 10 to
25 Hz. Therefore, comparing the predominant frequency of train loading and natural
frequency of the system along the Ahwaz metro route, it is concluded that the
probability of resonance occupance is very low.
|| Sinusoidal loading with frequency of 3 Hz and maximum ampilitude
of 0.001 m
||Ground surface vibrations at a distance of 20m from the axis
of the rail due to harmonic loading at different frequency-Zeytoon station
||Train loading function at frequency domain
According to dynamic analysis performed, the following conclusions are reached
in regard to subway induced vibrations in Ahwaz geology.
||With comparison of the ground surface vibrations in different
stations along the Ahwaz metro route, it was observed that in general the
level of vibrations in Northeast part of the route are higher than the southwest
part. One of the most important factor responsible for this observation
is the shallow depth of the rock and also soil profile in the Northeast
part of the route
||Assuming allowable vibration of 75 dB for residential building
and 30 to 70 passage of train each day, it seems that when the speed of
train is 80 km h-1 in Darvazeh station (N3) at a distance of
25 m from the axis of the rail, Zeytoon station (N1) at a distance of 30
m and Naft station (N2) at a distance of 10 m from the axis of the rail,
the vibrations at the ground surface become more than 75 dB and appropriate
measures should be taken to decrease these vibrations
||Because of shallow depth of rock in Norheast part of the route,
vibration energy due to passage of train is concentrated in the surface
layers and these vibrations affect the buildings in far distances from the
axis of the rail
||The speed of train is an inportant factor in vibrations due
to passage of train.According to the result of this research, with twice
increase in speed of train, the ground surface vibrations increase by 4
to 6 dB which is consistent with other researches
||Maximum particle velocity at the ground surface decreases
with distance from the axis of the rail and this decrease is not affected
by the speed of train at a distance about 15 m from the axis of the rail
||The depth to water table is one of the effective factors in
level of propagated vibrations to the ground surface. However, according
to this research there is no distinct relationship between vibrations and
depth of water table
||According to this research the natural frequency of the railing
system and soil profile along the Ahwaz metro route is about 2 to 3 Hz and
the predominant frequency of train loading is determined to be 10 to 25
Hz, therefore, it is concluded that probability of the resonance occurrence
along the Ahwaz metro is very low
Appendix: Boring logs of stations and soil parameters used in dynamic
|| Geotechnical properties of soil layers at different stations
used in numerical modelling