7
GROUNDBORNE NOISE IMPACT
7.1
Potential
ground-borne noise impacts likely arising from the Project during both the
construction and operation phases have been evaluated and the results are presented
in this section.
Environmental
Legislation, Standards and Guidelines
7.2
Construction
ground-borne noise is under the control of the Noise Control Ordinance (NCO),
the Environmental Impact Assessment Ordinance (EIAO), and their subsidiary
Technical Memorandum.
7.3
Noise
arising from general construction works of the Project during normal daytime
(0700-1900 except general holidays and Sunday) is governed by the EIAO-TM. With
reference to the Technical Memorandum for the Assessment of Noise from Places
Other Than Domestic Premises, Public Places or Construction Sites (IND-TM)
under the NCO, the criteria for noise transmitted primarily through the
structural elements of the building or buildings should be 10dB(A) less than
the relevant acceptable noise level (ANL). These criteria apply to all
residential buildings, schools, clinics, hospitals, temples and churches.
7.4
In
restricted hours (i.e. between 1900 and 0700 on a normal working day or at any
time on a general holiday and Sunday), the construction noise is controlled by
the Technical Memorandum on Noise from Construction Work other than Percussive
Piling (GW-TM). Similarly, the ground-borne noise criteria shall be limited to
10dB(A) below the respective ANL. Application for Construction Noise Permit
(CNP) is required for construction activities involving the use of powered
mechanical equipment (PME) carried out in restricted hours unless a CNP has
been obtained.
7.5
The
construction ground-borne noise criteria for the representative ground-borne
NSR of the Project are tabulated in Table
7.1 below.
Table 7.1 Construction
Ground-borne Noise Criteria
Type of NSR /
Assessment Point |
Ground-borne
Noise Criteria, (Leq
30 min, dB(A)) [a] |
||
|
Daytime
(0700-1900 hrs) |
Daytime during general holidays and Sundays and all
days during Evening |
Night
|
|
|
Domestic
premises, hotels and service
apartments |
65 |
55 |
40 |
|
Schools |
60/55 [c] |
55 |
[b] |
Note:
[a]
Ground-borne noise would not be affected by any Influencing Factors. Thus, Area
Sensitivity Rating of B will be considered for identifying criteria during
restricted hours.
[b] No sensitive use/activity
during this period.
[c] A 5dB(A) reduction
to the ground-borne noise criteria is recommended for school during examination
period.
Operation Phase
7.6
With
reference to the IND-TM, the criteria for noise transmitted primarily through
the structural elements of the building or buildings should be 10dB(A) less
than the relevant acceptable ANL.
The same criteria are applied to all residential buildings, schools,
clinics, hospitals, temples and churches.
The criteria applied for ground-borne rail noise assessment are
summarised in Table
7.2.
Table 7.2 Operational
Ground-borne Noise Criteria
Type of NSR / Assessment Point |
Ground-borne
Noise Criteria, (Leq
30 min, dB(A)) |
|
|
Day
& Evening |
Night
|
|
|
Domestic
premises, hotels and service apartments
|
55 |
45 |
|
Schools |
55 |
[a] |
Note:
[a] No sensitive use/activity during this period.
Identification of Ground-borne
Noise Sensitive Receivers
7.7
In
order to evaluate the ground-borne noise impacts from the Project during construction
and operational phases, representative existing and planned/committed noise
sensitive receivers within
7.8
Under
the assumption of worst-case scenario, one representative NSR was identified
for the assessment of construction ground-borne noise impact due to the use of
PME for excavation works and rock chiselling works for diaphragm wall
construction. The identified NSR is presented in Table
7.3
and shown in Figure No.NEX2213/C/361/ENS/M52/501.
Table 7.3 Representative
Noise Sensitive Receivers for Construction Ground-borne Noise Assessment
|
NSR No. |
Description |
Uses |
Horizontal Distance to
the Work Site (m) |
Nearest Site |
|
HH7 |
The
Metropolis Residence |
Service
Apartment |
90 |
Construction
of HUH |
7.9
Locations
of representative NSRs for operational ground-borne noise are presented in Figure No. NEX2213/C/361/ENS/M52/601
and summarized in Table 7.4.
Table 7.4 Representative
Noise Sensitive Receivers for Operational Ground-borne Noise Assessment
|
NSR
No. |
Description |
Land
Use |
Area
Sensitive Rating |
|
HH2 |
|
Residential Premises |
B
[a] |
|
HH3 |
The |
Educational
Institutional |
B
[a] |
|
HH5 |
Planned PolyU Phase 8 |
Educational
Institutional |
B
[a] |
|
HH7 |
The Metropolis Residence |
Service
Apartment |
B
[a] |
Note:
[a] Ground-borne noise would not be affected by any
Influencing Factors. According the
7.10
Potential
construction ground-borne noise impacts would arise from modified rigs during
rock chiselling for diaphragm wall construction. The modified rigs would be
used for soil excavation with grab bucket. Minor rock chiselling would be
required in order to toe in the diaphragm wall into the rock. In this case,
grab bucket would be replaced by a
chisel.
7.11
When
trains operate in tunnels that are located in close proximity to occupied
structures, there is a possibility that vibrations associated with train
passbys will be transmitted through the ground and structure, and be radiated
as noise in the occupied spaces within the structure. The transmitted noise
through the structure may have impact to the NSRs.
Ground-borne Noise Prediction Methodology
7.12
The prediction methodology is recommended by the U.S.
Department of Transportation and Federal Transit Administration[1]
. This projection methodology has
been previously used for Ground-Borne Noise & Vibration Assessment for
approved Kowloon Southern Link (KSL) EIA[2]
(EIA Register No. EIA-098/2004).
7.13
The
main components of the proposed prediction model for ground-borne noise are:
l
Vibration
source level from operation of modified rig during rock chiselling works;
l
Vibration
propagation through the ground to the structure foundation;
l
Vibration
reduction due to the soil/structure interface;
l
Vibration
propagation through the building and into occupied areas; and
l
Conversion
from floor and wall vibration to noise.
7.14
The
vibration level Lv,rms at
a distance R from the source is
related to the vibration source level at a reference distance Ro. The conversion from vibration levels to
ground-borne noise levels is determined by the following factors:
Cdist: Distance
attenuation
Cdamping: Soil damping loss across the geological media
Cbuilding: Coupling loss into building foundation
Cfloor: Coupling
loss per floor
Cnoise: Conversion
factor from floor vibration levels to noise levels
Cmulti: Noise
level increase due to multiple sources
Ccum: Cumulative
effect due to neighbouring sites
7.15
The
predicted ground-borne noise level Lp
inside the noise sensitive rooms is given by the following equation.
Lp = Lv,rms + Cdist +
Cdamping + Cbuilding + Cfloor + Cnoise
+ Cmulti + Ccum
Reference Vibration Source (Lv,rms)
7.16
In
view of similar nature of rock chiselling by modified rig and rock breaking by
hydraulic breaker, the source term of the modified rig is assumed same as that
of hydraulic breaker adopted in the approved EIA study for the Kowloon Southern
Link project (EIA-098/2004) as shown in Table
7.5. The assumptions
adopted in the present assessment are provided in Appendix 7.1.
Table
7.5 Reference
Vibration Level
|
Plant |
Vibration (rms) at reference distance of
5.5m from source [a] |
|
Hydraulic
Breaker |
0.298 mm/s |
Note:
[a]
Extracted from KSL GSA 5100 Environmental Impact Assessment & Associated
Services - Environmental Impact Assessment Report, Register No.: EIA-098/2004
Soil Damping Factor (Cdamping)
7.17 Internal losses of soil would cause the vibration amplitude to decay against the propagation distance and the decay relationship is based on the equation set out in the Transportation Noise Reference Book[3].
V(R) = V(Ro) ´ e-2pf h R/2c.
7.18
The
velocity amplitude V is dependent on the frequency f in Hz, the soil loss
factor, h, the wave
speed c in m/s, the distance R from the source to the NSR. The properties of soil materials are
shown in Table 7.6.
Table 7.6 Wave
Propagation Properties of Soil
|
Soil
Type |
Longitudinal
Wave Speed c, m/s |
Loss
Factor, h |
Density,
g/cm3 |
|
Soil |
1500 |
0.5 |
1.7 |
|
Rock |
3500 |
0.01 |
2.65 |
7.19
No damping
attenuation was applied for propagation in rocks. All NSRs were assumed to have
a piling foundation on rockhead.
Coupling Loss into Building Structures (Cbuilding)
7.20
The
coupling loss into building structures represents the change in the incident ground-surface
vibration due to the presence of the piled building foundation. The empirical values with reference to
the “Transportation Noise Reference Book”, 1987 are given in Table 7.7. In addition, a coupling loss correction of -18 dB from bedrock
to pile should be adopted. However, the correction from bedrock to pile depends
on actual site condition and correction of zero dB is assumed for conservative
approach.
Table 7.7 Loss factor
for Coupling into Building Foundation
|
Frequency |
Octave Band Frequencies, Hz |
|||||
|
16 |
31.5 |
63 |
125 |
250 |
500 |
|
|
Loss factor for coupling into building foundation, dB |
-7 |
-7 |
-10 |
-13 |
-14 |
-14 |
Coupling Loss per Floor (Cfloor)
7.21
The
coupling loss per floor represents the floor-to-floor vibration transmission
attenuation. For multi-storey
buildings, a coupling loss of 2 dB reduction per floor was assumed in this
report for a conservative assessment to account for any possible amplification
due to resonance effects.
Conversion from Floor Vibration to Noise Levels (Cnoise)
7.22
Based
on FTA Manual, a -27 dB correction for conversion of vibration (re: 10-9
m/s) in room walls, floors and ceiling to noise (re: 20 micro Pa) was assumed
in this study.
Multiply Source Factor (Cmulti)
7.23
This
represents the increase in noise level due to multiple noise sources. The ground-borne noise levels from
construction plant are summed logarithmically in accordance with standard
acoustic principles to obtain the total ground-borne noise level at the area of
interest.
Cumulative Effect (Ccum)
7.24
The
cumulative contribution from other nearby concurrent sources, such as (SCL (TAW
- HUH)) and KTE at Hung Hom have been added.
7.25
The methodology
for the operational ground-borne noise impact assessment has been conducted in
accordance with the procedures outlined in FTA Guidance Manual for detailed
vibration analysis. This
methodology has been adopted for approved West Island Line EIA Study (EIA
Register No. EIA-153/2008). The
ground-borne noise levels in NSRs have been calculated as follows:
L
= FDL + TIL + TOC + TCF + LSR + BCF + BVR + CTN + SAF
where
|
L |
Train passby
noise level, in dB |
|
FDL |
force density level,
in dB re 1 lb/in1/2 |
|
TIL |
trackform
attenuation or insertion loss, relative level |
|
TOC |
turnout and
crossover factor |
|
TCF |
vibration
coupling between the tunnel and the ground for soil based tunnels, relative
level |
|
LSR |
line source transfer
mobility, in dB re 1 (uin/s)/(lb/ft^0.5) |
|
BCF |
adjustment to
account for building coupling loss, in dB |
|
BVR |
building
vibration amplification within the structure, in dB |
|
CTN |
conversion from
vibration to noise within the building, in dB |
|
SAF |
safety factor
to account for wheel/rail condition and uncertainties in ground conditions,
in dB |
Force Density Levels (FDL)
7.26
The
vibration source levels (force density levels, FDL) for the existing SP1900 EMU
were obtained from passby measurements on the up track through Pat Heung Depot
in previous rail project. The
deterioration in rail and rolling stock condition has already been taken into
account in FDL obtained by measurements under rough rail condition. In accordance with the previous KSL
approved EIA report, comparisons of FDL obtained from the SP1900 EMU to other
Hong Kong transit trains, including old East Rail EMU, as well as several other
heavy rail EMUs in operation in the United States, indicated that the SP1900
FDL was 5 dB to 10 dB higher than the maximum FDL levels for the other trains.
The level adopted is based on previous approved EIA as shown in Appendix
7.2. Speed correction is
applied to the FDL using an empirical relationship,
Trackform
Alternatives or Insertion Loss (TIL)
7.27
Trackform
attenuation has two components: the magnitude of the attenuation and the
frequency above which attenuation occurs (resonance frequency of the trackform). Generally, more compliant trackform
support and more massive elements in the trackform would result in a greater
magnitude of attenuation occurring at lower frequencies. Thus, floating slab trackform (FST)
would produce significantly more attenuation at lower frequencies than a
resilient baseplate. However,
greater compliance in the trackform support results in greater mobility of the
rail, which would require careful examination of changes in rail geometry under
loading, and consideration of associated fatigue and component life expectancy.
In addition, more massive trackform elements would take up more space in
tunnels and may cause spatial incompatibilities that are difficult to be
overcome in the design. The TIL for existing MTR trackforms in previous
approved EIA has been adopted where appropriate.
7.28
The
ground-borne noise levels at NSRs have been calculated initially with direct fixation
track without trackform insertion loss. If noise exceedances
would be predicted, low noise trackforms including low stiffness fasteners,
floating slab track, etc would be considered. The attenuations provided by
different low noise trackforms would be included in the calculation to
determine the appropriate trackforms for meeting the criteria. The type of vibration
mitigating trackform has often been grouped into three categories listed below:
·
Type
1: A medium attenuation baseplate or booted dual sleepers based on a bonded or
non-bonded compression style baseplate with a resilient elastomeric element
having static stiffness of about 25 kN/mm, to be fitted atop the concrete
sleepers or atop the invert;
·
Type
2: A high attenuation baseplate or booted dual sleepers includes
i.
a
bonded “Egg” style baseplate with a resilient elastomeric element having static
stiffness in the range of 7 kN/mm to 14 kN/mm, to be fitted atop concrete
sleepers or on the invert;
ii.
the
Pandrol Vanguard baseplate having static stiffness on the order of 3kN/mm to
5kN/mm; or
iii.
resiliently
supported sleepers whose resilient support pad is manufactured from natural
rubber and has a static stiffness in the order of 8kN/mm to 12 kN/mm - an
alternative for tangent, or near-tangent track only.
·
Type
3: A floating mini slab trackform (FST) with loaded resonance frequency of
about 16Hz
Tunnel Coupling Factor (TCF)
7.29
Generally
heavier transit structures lower the vibration levels. With reference to FTA Manual, vibrations
induced by train in Cut and Cover (CC) tunnels and Stations are 3dB and 5dB
less than that in bored tunnel in soil.
For bored tunnel in soil, the TCF depends on the soil properties. Due to
lack of comprehensive data on different soil strata, TCF has been
conservatively assumed to be 0dB. Thus, the TCF used is 0dB, -3dB and -5dB for
bored tunnel, CC tunnel and stations respectively.
Turnout and Crossover Factor (TOC)
7.30
At
points and crossings, where the wheel transitions from one rail to another, the
sudden loading/unloading of the leading and trailing rails results in increased
broad band vibration levels over that of plain line continuous rail. While it
is not possible to grind the rails through either the points or crossings,
surface deterioration would often be evident. For standard level turnouts and
crossings receiving average maintenance, the FTA Manual has recommended a
correction of 10dB. For modern inclined turnouts in good condition, where
impact loads are lessened, a correction of 5dB would be appropriate. These
corrections would be adopted in this study.
Line Source Response (LSR)
7.31
The
LSR determines the vibration levels or attenuation in the ground as a function
of distance caused by an incoherent line source of unit force point impacts,
with line source (train) orientated along the alignment. Thus, the basic quantity required for
the determination of the LSR would be the vibration response caused by a unit
point source impact, which has been defined as the Point Source Response
(PSR). Given that the PSR would be
along the alignment, the LSR would follow directly by incoherent integration of
the PSR values over the train length.
However, the determination of the LSR from force point impacts in
numerous boreholes along the alignment over the length of the alignment is
neither practical nor affordable.
Thus, idealised assumptions of transverse isotropy and layer-wise
homogeneity are invoked, which allow PSR obtained from a single borehole to be
taken as representative along the alignment near a building receiver and used
in the calculation of LSR.
7.32
Soil
mobility has already been measured in
Table 7.8 Typical PSR to
be adopted for the Representative NSR
|
Selected
NSR |
Selected
Borehole |
|||||
|
NSR
No. |
Rock(R)
/ Soil-borne(S) |
Approx.
Track Depth [m] |
Approx.
Rockhead Depth [m] |
Borehole
No. |
Borehole
Depth [m] |
Rockhead
Depth [m] |
|
HH2 – |
S |
11 |
30-40 |
WIL D095 |
10.4 |
23 |
|
HH3 - Cheung On Tak Lecture Theatre |
S |
13 |
40-50 |
WIL D095 |
10.4 |
23 |
|
HH5 - Planned Poly U Phase 8 |
S |
15 |
35-40 |
WIL D095 |
10.4 |
23 |
|
HH7 - The Metropolis Residence |
S |
11 |
21-31 |
WIL D095 |
10.4 |
23 |
7.33
PSR is
numerically interpolated between setbacks to create contour surface in
frequency and distances. The Line
Source Response (LSR) is then determined by numerical incoherent integration of
the PSR along the length of the train centred on the receiver for each
individual 1/3 octave bands.
where s =
perpendicular setback
d
= depth to top of rail
l
= train length
Building Coupling Factor (BCF)
7.34
In general,
larger and heavier structures have greater vibration attenuation than smaller
and lighter structures. The recommended BCF established within FTA Manual would
be followed. Receivers in this
study would be divided into 5 types according to its structures and would have
different BCF attenuation as below:
·
Type 0
– Large Masonry with spread footings
·
Type 1
– 2-4 storeys medium sized structures
·
Type 2
– 1-2 storeys complexes
·
Type 3
– Single family detached residences
·
Type 4 – Large
7.35
The
BCF for different types of structure is shown in Appendix 7.4 which
indicates that larger and heavier structures have greater vibration attenuation
than smaller and lighter structures. In fact, the extent of the attenuation is
governed by the difference in mechanical impedance between the soil and the
foundation, with impedance being determined by differences in mass and
stiffness within the soil and foundation.
For structures founded on rock, there would be no impedance contrast
between the soil and the foundation and therefore the BCF has been considered
to be zero.
Building Vibration Response (BVR)
7.36
The
BVR has been determined by two factors as described below:
·
Resonance
amplification due to floor, wall and ceiling spans: With reference to the FTA Manual, a 6 dB correction would be considered to
account for structural resonances of typical reinforced concrete buildings. The spectral correction has been provided
in Appendix 7.5.
·
Floor-to-floor
attenuation: A floor-to-floor attenuation of 2 dB reduction per floor would be
assumed. Where there is a
multi-floor occupancy, only the structure borne noise impact on the lowest
occupied floor is considered.
Conversion to Noise (CTN)
7.37
A +2
dB correction for conversion of vibration (re: 10-6 in/s) in room
walls, floors and ceiling to noise (re: 20 micro Pa) would be assumed in this
study.
Safety Factor (SAF)
7.38
To
tackle the problem of differences in overall predicted and measured A-weighted
noise levels, a safety factor would be applied in the model. As a conservative
approach, a 10 dB safety factor would be adopted to account for uncertainty and
variation in ground characteristics.
7.39
Construction
ground-borne noise level associated with operation of the modified rigs,
hydraulic breakers and piling rigs were predicted and summarized in Table
7.9. For the worst case scenario, it was
assumed in the calculation that the PMEs would be operated simultaneously. Detailed calculation and assumptions are
provided in Appendix
7.1.
Table
7.9 Predicted Construction
Ground-borne Noise Impact
|
NSR
No. |
Description |
Predicted
Ground-borne Noise Levels Leq(30mins), dB(A) |
Noise Criteria for daytime (0700-1900), Leq(30mins), dB(A) |
Criteria Achieved? |
|
HH7 |
The Metropolis Residence |
48 – 50 |
65 |
Yes |
7.40
As
shown in Table 7.9,
construction ground-borne noise levels at The Metropolis Residence (HH7) would
comply with the day time (0700-1900) noise criteria of 65 dB(A). Adverse ground-borne construction noise impact
due to the use of PME would not be envisaged.
7.41
In
case of any construction activities to be conducted during restricted hours
(i.e. between 1900 and 0700 on a normal working day or at any time on a general
holiday and Sunday), it is the Contractor’s responsibility to ensure compliance
with the Noise Control Ordinance (NCO) and the relevant technical memoranda.
The Contractor will be required to submit CNP application to the Noise Control
Authority and abide by any conditions stated in the CNP, should one be issued.
7.42
The
ground-borne noise levels at NSRs have been calculated based on direct fixation track. The
prediction results are summarized in Table 7.10.
Sample calculation and assumptions
have been provided in Appendix
7.6. The predicted noise contribution from the Project
at the worst-affected NSR is <20dB(A), which is insignificant and down to
the ambient level.
Table 7.10 Predicted Operational
Ground-borne Noise Impact (Unmitigated)
|
NSR
No. |
Description |
Predicted
Ground-borne Noise Levels, Leq(30mins)
dB(A) |
Noise Criteria, Leq(30mins) dB(A) |
Criteria Achieved? |
|
HH2 |
|
<20 |
45 (with night-time operation) |
Yes |
|
HH3 |
Cheung On Tak Lecture
Theatre |
<20 |
55 (with day/evening operation only) |
Yes |
|
HH5 |
Planned Poly U Phase 8 |
<20 |
55 (with day/evening operation only) |
Yes |
|
HH7 |
The Metropolis Residence
|
<20 |
45 (with night-time operation) |
Yes |
7.43
The potential cumulative ground-borne noise impacts
with SCL (TAW - HUH) and KTE have also been assessed. Ground-borne noise impacts
assessment results for SCL (TAW - HUH) and KTE have been extracted from both
EIA reports for cumulative assessment. The cumulative results are shown in Table 7.11.
Table
7.11 Cumulative
Operational Ground-borne Noise Impact
|
NSR
No. |
Description |
Predicted Ground-borne Noise Levels
arising from SCL (MKK – HUH), Leq(30mins) dB(A) |
Predicted Ground-borne Noise Levels
arising from SCL (TAW – HUH), Leq(30mins) dB(A) |
Predicted Ground-borne Noise Levels
arising from KTE, Leq(30mins) dB(A) |
Cumulative Ground-borne Noise Levels, dB(A) |
Criteria, Leq(30mins) dB(A ) |
Criteria Achieved? |
|
HH2 |
|
<20 |
36 |
<20 |
36 |
45 (with night-time operation) |
Yes |
7.44
Cumulative noise levels due to the Project, SCL
(TAW - HUH) and KTE at NSR would be 9 dB(A) below the criteria. Adverse impact
from the Project is therefore not anticipated.
7.45
During
abnormal and emergency operations of SCL (MKK - HUH), the train service would
be interrupted or stopped. Therefore, the train frequency at that period would
be lower than the scheduled timetable. Hence, the ground-borne noise impact
would not be worse than the above prediction, which has been conducted based on
the worst case scenario.
Recommended Mitigation Measure
7.47
The predicted operational ground-borne noise at all
identified NSR would comply with the noise criteria. Mitigation measures are not required.
7.48
The prediction was based on a conservative
approach. However, provisions
have been allowed in the design of the tunnel for installation of any necessary
contingency mitigation measures.
Based on the size of the tunnel proposed under the Project, these contingency measures may be as follows:
·
Medium attenuation
baseplates (Type 1) –– additional attenuation
of about 5 to 10dB(A);
·
High
attenuation baseplate or booted dual sleepers (Type 2) additional attenuation of
about 10 to 15dB(A);
or
·
Floating
mini slab trackform (Type 3) –– additional attenuation
of about 20 to 30dB(A).
Environmental
Monitoring and Audit Requirements
7.49
The predicted construction ground-borne noise would comply with the noise criteria.
Therefore, environmental monitoring is considered not necessary during
construction phase.
7.50
Prior
to the operation phase of the Project, a commissioning test should be conducted
to ensure compliance of the operational ground-borne rail noise
levels with the noise criteria.
Details of the test requirements are provided in a stand-alone EM&A
Manual.
7.51
During
construction phase, projections of ground-borne noise at identified
NSR/assessment point have been performed, based on a methodology recommended by
the US Department of Transportation. It was found that the predicted
construction ground-borne noise at representative NSR (Metropolis Residence)
would comply with the noise criteria stipulated in the EIAO.
7.52
During
operation phase, projections of ground-borne noise
at identified NSR have been performed, according to the methodology recommended by the US
Department of Transportation. Based on direct fixation track, the predicted
operational ground-borne noise at all the identified NSR would comply with the
criteria. The predicted noise contribution from the Project at the
worst-affected NSR is 19dB(A), which is insignificant and down to the ambient
level. Thus, adverse impact from the Project is not anticipated. Cumulative
noise levels from the Project, SCL (TAW - HUH) and KTE at the worst-affected
NSRs would be 9dB(A) below the noise criteria. Provisions have been allowed in the
design of the tunnel for installation of any necessary contingency mitigation
measures. Based on the size of the tunnel proposed under the
Project, these contingency measures could be medium attenuation baseplates
(Type 1), high attenuation baseplate or booted dual sleepers (Type 2) or
floating mini slab trackform (Type 3).