15.1
The purpose of this
assessment is to identify the potential impacts during both construction and operation
of the proposed Project tunnels on the restored Ngau Tam Mei Landfill
(hereinafter also referred to as the “restored NTML” or “Landfill”); and
conversely to determine the potential impacts of the Landfill on the proposed
Project tunnels.
15.2
Of particular concern
in this regard is the identification and assessment of the potential adverse
effects that the proposed XRL tunnels may have on the integrity of various
components constructed in fulfilment of the North-west New Territories
Landfills Restoration Contract (Contract No. EP/SP/30/95) with the
Environmental Protection Department (EPD) (hereinafter referred to as the
“Restoration Contract”) to restore the Landfill to a controlled,
environmentally sustainable condition.
Furthermore, mitigation measures are proposed for the identified
impacts. Potential impacts that the
restored NTML may have on the proposed Project tunnels are also identified and
assessed, with mitigation measures proposed.
15.3
The following
sections describe the history of the restored NTML; the proposed Project tunnel
configuration and construction methods; identification and assessment of the
potential impacts of the restored NTML on the proposed Project tunnels, and the
proposed Project tunnels on the restored NTML; with mitigation measures
proposed for the identified potential impacts; followed by conclusions and
recommendations and a list of references.
15.4
The NTML is located
approximately 6km to the northeast of Yuen Long city centre. It is approximately 1.7ha in plan area and
was formed in a natural stream valley generally oriented with the central-axis
from northeast to southwest; various developments to the north and west, and
open hillside space to the south and east.
To the northwest is the
15.5
From the various
available records and reports, it appears that waste disposal activities had
occurred at the Landfill on an informal basis as early as 1963, with more
formal waste placement occurring between 1973 and 1975. From historic aerial photographs and
topographic survey information, it is reported that a total of approximately 90,000m3
of waste was disposed of at the Landfill prior to its closure.
15.6
The NTML is a
valley-fill site, generally configured as two platforms (the Upper Platform and
Lower Platform) with the Upper Slope between the Upper Platform and the Lower
Platform, and the Lower Slope between the Lower Platform and the Landfill
toe. The larger, Upper Platform is
generally between elevations +32mPD and +36mPD, and gently slopes from
northeast to southwest at nominally a 4% gradient. The smaller, Lower Platform is between
elevations +24mPD and +26mPD, with a slightly steeper slope of approximately 8%
across the width toward the southwest.
The toe of the Landfill is located at approximately elevation
+16mPD. The plan view configuration and
cross section through the Landfill are shown in Figure No. NOL/ERL/300/C/XRL/ENS/M61/201 and Figure No. NOL/ERL/300/C/XRL/ENS/M61/202, respectively.
15.7
The Landfill was
restored in 1999 under the Restoration Contract, with the restoration works
generally consisting of placement of a “high integrity” capping system over the
two platforms; minor modifications to the existing leachate management system
to provide for collection and transport to an off-site treatment facility;
installation of a passive landfill gas (LFG) ventilation system; and on-going
monitoring of several component features, including surface settlement,
groundwater and leachate levels and quality, and landfill gas migration.
Tunnel
Configuration and Construction Method
15.8
The Project is an
underground railway approximately 26km long, extending from
15.9
The portion of the
alignment passing beneath the Landfill consists of twin, parallel tunnels, each
9.35m in external, cut diameter, with 16m of separation, centre-to-centre,
between. The rail level within the
tunnels is to be approximately –23.5mPD, making the crown level approximately
-16mPD [Ref 1.]
15.10
Based on the
preliminary geological/geotechnical assessment performed to-date, it is
expected that rockhead will extend into the tunnel horizon as it passes beneath
the Landfill [Ref. 1]. Although various
methods will be employed to construct the tunnels throughout their length,
including: drill-and-blast, mining (also
referred to as “boring”) and cut-and-cover; it is anticipated that an
approximately 120m length passing beneath the Landfill will be bored in
rock. Specifically in this regard, it is
anticipated that a closed-face Tunnel Boring Machine (TBM) of the Earth
Pressure Balance Machine (EPBM) or slurry-type will be used [Ref 1].
15.11
The design life of
the completed railway system is, consistent with MTRC standard practice, 120-years.
15.12
The typical tunnel
configuration is shown in Figure No. NOL/ERL/300/C/XRL/ENS/M61/112.
15.13
Consideration is
herein given to the potential impacts that the tunnels may have on the restored
NTML; and conversely the potential impact that the NTML may have on the
tunnels. A detailed review of the
various components of the NTML restoration works has been performed, with a
particular emphasis on the construction and operation of the proposed tunnels
and the integrity of the restored NTML system components. Of the various components included in the
restoration works (refer to Section 15.7 herein); waste slope stability, ground
surface gradient, capping system integrity, leachate and landfill gas
management system integrity, and infrastructure (surface drains and road)
integrity have been identified as potentially being impacted by the
tunnels. In addition, groundwater laden
with waste constituents as it passes through the Landfill and off-site
migration of landfill gas have been identified as the potential impacts of the
restored NTML on the proposed tunnels.
The potential hazard of landfill gas migration from the restored NTML to
the tunnels during the construction and operation phases are discussed in
Section 14. A brief description of the
identified impacts follows:
·
Vibrations generated by construction of the tunnels using
TBM, and on-going operation of the tunnels will have a potential impact on the
stability of the Landfill slopes;
·
Formation of the tunnels may impact the settlement of the
Landfill surface and consequentially impact the ground surface gradients, the
integrity of the capping system components, the leachate and landfill gas
management systems, and the infrastructure (surface water drainage network and
the road);
·
Formation of the tunnels may impact the local hydrogeologic
regime, including the groundwater and/or leachate levels and the groundwater
flow; and
·
Exfiltration of leachate from the Landfill into the
groundwater will potentially impact the quantity of drainage into the tunnels
and/or the integrity of the materials used to construct the tunnels.
15.14
A detailed evaluation
of these impacts, including the assessment methodology and criteria, is
presented in the following sections.
Assessment Methodology and Criteria
Slope Stability
15.15
The stability of the
NTML slopes under the vibrations that are determined to be potentially induced
during construction and operation of the Project has been assessed by several
analytical methods, both qualitatively and quantitatively. A preliminary assessment employing the
infinite slope method provided a conservative, quantitative estimate of the
safety factor under static conditions, without consideration of vibrations from
the tunnels. However, due to the nature
of the analysis, the infinite slope method is limited to only a qualitative
indication of the stability of the slopes under the influence of the induced
vibrations. The more traditional method
of analysis by force-moment equilibrium has been used, employing a
pseudo-static coefficient to represent the expected maximum vibration
levels. These analytical approaches were
supplemented by an empirical method reportedly developed specifically for TBM
tunnelling in rock which estimates the upper bound peak particle velocity at a
receptor as a function of the distance from the source TBM machine. This value was then compared to a threshold
value below which the induced vibrations were considered to have negligible
impact on slopes and structures.
15.16
Each of these methods
has been employed on the primary slope through the Landfill, a cross-section
extending from the Upper Platform through the Upper Slope, Lower Platform and
Lower Slope, extending beyond the toe of waste and Landfill boundary (refer to Figure Nos. NOL/ERL/300/C/XRL/ENS/M61/201 and NOL/ERL/300/XRL/ENS/M61/202). As may be possible by each analysis method,
the potential of both tunnels being formed by TBM’s simultaneously passing
beneath the site has been considered.
15.17
An estimate of the
magnitude of the ground borne vibrations generated during construction of the
tunnels was provided by the Design Engineer in the form of peak particle
velocity. This magnitude has been
independently checked, revised and used in the assessment; then converted to
another form, a gravitational acceleration coefficient, for use with the
force-moment equilibrium analysis method.
15.18
It is herein assumed
that the vibrations generated during construction using the TBM will represent
the “worst-case” scenario, and it is anticipated that the effect of the vibrations
from operations will be less severe, subject to confirmation during the
detailed design stage. The vibrations estimated from construction by TBM will
therefore be employed throughout this Section.
15.19
Results from the infinite
slope analyses performed under the “dynamic” conditions representing the
vibrations induced from the formation of the tunnels are qualitative, producing
an indication that the slopes will move either in conjunction with the
surrounding vibrating ground surface, or that there will be a net displacement
between the two features, considered to be a failure of the slope. A positive indication of the slope moving
synchronously with the surrounding ground is required. The results of the force-moment equilibrium
stability analyses under pseudo-static conditions were, because of the
quantitative results produced, compared to the minimum Factor of Safety of 1.2
required under the Restoration Contract.
The upper bound peak particle velocity determined from the empirical
analysis method was compared to the reported threshold limit value; 5 mm/s;
below which the generated vibrations are considered to have negligible effect
on slopes and structures [Ref. 1].
Settlement
15.20
The impact of
Landfill settlement induced by construction of the tunnels was assessed by
first estimating the settlement of only the waste mass due to degradation as a
function of time. This was done by
adopting and extending the predictive model developed by the Restoration
Contractor [Ref. 6]. The settlement
induced by only formation of the tunnels passing beneath the Landfill was
estimated by using a commonly employed model for such applications that assumes
the shape of the settlement trough resulting from tunnel formation to be an
inverted Gaussian curve, the shape of which is dependent on the assumed ground
conditions at the tunnel level and in the soil/waste above. The estimated settlements resulting from each
of the waste degradation and tunnel formation components were then superimposed
and totalled to develop a composite settlement profile through the
Landfill. This profile was then used to
determine:
·
the potential for
gradient reversal across the Landfill ground surface, as well as the potential
for excessive stresses induced in the geomembrane component of the capping
system across the Landfill platforms; and
·
the potential for
damage to the existing leachate management and passive landfill gas management
systems, surface water drainage features and the road.
15.21
The results of the
settlement analyses were compared to various criteria for each of the Landfill
features and systems. In accordance with
the Restoration Contract, a minimum ground surface gradient of 1% must be
maintained after settlement. Similarly,
positive gradients are to be maintained within the surface water management
features. The estimated stresses/strains
potentially developed in the geomembrane capping system component were compared
to the commonly adopted conservative maximum; the yield strain; of the specific
material installed. The effect of the
estimated settlement on the component materials of the leachate and landfill
gas management systems was evaluated. In
each instance, the net effect of the additional settlement caused by only
formation of the tunnels was compared to the result of settlement only by
degradation of the landfilled waste mass.
Groundwater/Leachate Levels, Flow and Quality
15.22
The potential impacts
of the formation and operation of the Project on the local hydrogeologic regime
(groundwater and leachate levels and flow) around and within the Landfill were
assessed through application of a fundamental drawdown model employing Darcy’s
Law and based on the proposed tunnel boring methods and lining
design/materials.
15.23
The potential impact
of the Landfill on the tunnels was assessed by comparison of the leachate
and/or down-gradient groundwater quality; particularly pH, sulphate and
chloride concentrations; with known levels of resistance and/or degradation of
the materials proposed for construction.
A model was employed to predict the potential corrosion of reinforcing
steel that may be used in a concrete tunnel lining from chlorides in the
groundwater.
15.24
The criteria to be
employed to determine the potential degree of the impact of the groundwater/
leachate on the tunnels vary with the constituent concentration of contaminants
as they are present at the tunnel locations.
The commonly accepted ranges of pH and sulphate (less than 5.5 or 6; and
400mg/L, respectively) were used as a basis for comparison and determination of
the degree of potential concern for attack of concrete materials that may be
used. The results of the model used for
estimating potential corrosion of reinforcing steel exposed to chlorides in the
groundwater were used to determine the required level of protection, if any.
Effect of Vibrations on Slope Stability
15.25
Several approaches
have been considered and are presented in this assessment of the stability of
the restored NTML slopes subjected to the vibration conditions imposed by
construction of the tunnels using TBM’s and ultimately operation with the
passing of trains. The methods performed
and described in the assessment presented include:
·
a preliminary analysis by the infinite slope method;
·
an estimation of the peak particle velocity generated with
comparison to an adopted threshold limit value; and
·
an analysis by the more traditional force-moment equilibrium
method with application of a pseudo static coefficient to simulate the forces
contributed by the induced ground vibrations.
15.26
The details of the
assessment by each of these methods are provided below.
15.27
The configuration of
the restored NTML is shown in plan view on Figure
No. NOL/ERL/300/C/XRL/ENS/M53/001; with the critical cross-section employed
for assessment of slope stability shown on Figure
No. NOL/ERL/300/C/XRL/ENS/M61/202.
Infinite Slope Method
15.28
Infinite slope
stability analyses were performed to provide an initial indication of the
potential effect that ground borne vibrations generated from construction and
operations of the tunnels may have on the restored NTML waste slopes. The analyses performed were comprised of two
(2) components:
(a)
a quantitative assessment of the minimum Factor of Safety of
the waste slopes under “static” conditions based on the conservative
assumptions that the slopes are infinitely long and configured at the maximum
slope inclination shown in the “as-constructed” drawings produced by the
Restoration Contractor [refer to Appendix 15.1 for a copy of select
“as-constructed” drawings of the restored NTML]; and
(b)
a qualitative assessment of the same slopes under “dynamic”
conditions by relative comparison of the anticipated ground acceleration
experienced during construction and operation of the tunnels to a maximum allowable
ground acceleration. Note that this
assessment is qualitative in nature due to the fact that the infinite slope
analysis method is limited to determining non-quantifiable, relative ground
movements between the Landfill slope in question and the surrounding ground.
15.29
The results of the
quantitative infinite slope analysis indicate that the restored NTML slopes are
stable to a minimum Factor of Safety of 1.5 under “static” conditions, without
consideration of the vibrations induced by external sources such as the tunnel
construction or their use by trains.
This is higher than the minimum Factor of Safety of 1.2 required by the
Restoration Contract [Ref 5]. As
anticipated, because of the conservative nature of the analytical method used
in this assessment, the minimum Factor of Safety obtained is less than the
value of 1.73 determined by the Restoration Contractor during the works design.
15.30
The subsequent
qualitative analysis provides an indication that the waste mass within the
Landfill will move together, in synchronization with, the adjacent ground
surfaces. Therefore, there will be no
net differential movement or displacement between the Landfill slopes and the
surrounding ground under the considered “dynamic” conditions of TBM tunnel
construction and train operations. Under
these conditions, the Landfill waste slopes are considered stable.
15.31
A copy of the
infinite slope analyses performed for both the “static” and the “dynamic”
conditions described herein is included in Appendix 15.2 for reference.
Peak
Particle Velocity Method
15.32
An estimate of the
vibrations in the form of peak particle velocities generated during
construction was developed and presented in Working Paper No. 42 [Ref. 1]. The estimate made is based on use of the
Hiller and Bowers equation, in which the upper bound of the peak particle
velocity at the receptor; in this case the restored NTML slopes; is a function
of an exponential of the distance between the TBM and the receptor, multiplied
by a constant:
V
= 180 x-1.3
Where: V = the upper
bound Peak Particle Velocity (PPV) resultant (mm/sec) at the receptor; and
x
= the distance (m) between the TBM source and the receptor.
15.33
In performing the
same analysis in association with this assessment, the vibration source to
receptor separation distance is taken as approximately 32.0m (from -16.0mPD at
the crown of the tunnel TBM bore [Ref. 1], to +16mPD at the lowest point along
the Landfill base), with the resulting estimate of the generated peak particle
velocity being 2.0mm/sec. Additionally,
although the likelihood appears remote, a more conservative scenario would be
the condition in which two TBM’s are in operation beneath the Landfill
simultaneously. The effect is assumed to
be additive, thus doubling the peak particle velocity to 4.0mm/sec; however
still less than the 5mm/sec cited in Working Paper No. 42 [Ref. 1] and implied
in GEO Report No. 15 [Ref. 3] as the limiting value below which there is
“negligible impact on adjacent slopes and structures.” [Ref. 1]. It could therefore be concluded from this
analysis that the vibrations generated from tunnel construction and operation
will not have adverse impact on the slope stability of the restored NTML.
Force-Moment Equilibrium Method
15.34
A detailed stability
analysis of the restored NTML slopes subjected to the potential vibrations
generated from construction and operation of the tunnels was also performed
employing the more traditional force-moment equilibrium method, with a
pseudo-static coefficient applied to model the disturbing effect of the
vibrations generated as an equivalent inertia force.
15.35
An assessment of the
magnitude of potential vibrations was first performed. From recent literature [Ref. 4] it was
determined that the magnitude of the particle velocity generated by the TBM
during construction could be in the order of 0.25mm/s. However, more conservatively, and consistent
with the peak particle velocity method previously described herein, the maximum
peak particle velocity was taken as 2.0mm/s as estimated using the Hiller and
Bowers equation.
15.36
The pseudo-static
ground acceleration was then determined by considering the vibrations as a
single degree of freedom system with a magnification factor applied to the peak
particle acceleration while assuming the most conservative conditions for the
slope height (6m for the Upper Slope); vibration frequency (30Hz); and damping
factor (0.5). The resulting
pseudo-static ground acceleration coefficient was determined to be the 0.019
times gravitational acceleration; most commonly expressed as “0.019g”. This
coefficient was then applied directly as an additional force to the slope
stability analysis performed using the SLOPE/W computer software.
15.37
Two (2) sets of
pseudo-static slope stability analyses were performed to model the conditions of
a single TBM and, the “worst-case” scenario of a TBM in each of the twin
tunnels beneath the Landfill. The
resulting minimum Factors of Safety corresponding to the pseudo-static
conditions modelling a single and two TBM’s simultaneously are 1.7 and 1.6,
respectively; well above the minimum value of 1.2 required by the Restoration
Contract [Ref 5].
15.38
It should be noted
that the slope stability analyses under “static” conditions (without
application of a pseudo static coefficient to model vibrations) were performed
as part of this assessment to serve as a “calibration” and comparison to the
analyses submitted in association with the Restoration Contractor design. The results of the “static” analysis
performed herein (Factor of Safety of 1.8), compares favorably with the results
reported by the Restoration Contract (Factor of Safety of 1.73) [Ref. 6].
15.39
The results of the
pseudo-static slope stability analyses performed are included with the
cross-section on Figure No.
NOL/ERL/300/C/XRL/ENS/M61/202 with the detailed analytical results included
in Appendix 15.2.
Effect of Tunnel Formation on Landfill Settlement
15.40
Settlement of
landfill surfaces is a common occurrence, with two primary causes: in the short-term, due to the consolidation of
the waste from the weight of materials placed above; and, in the long-term, due
to the decomposition of the disposed waste.
As waste placement ceased in the Landfill in 1975, nearly 35-years ago;
and the restoration was performed in 1999, approximately 10-years ago; it can
reasonably be assumed that the short-term settlement from waste consolidation
has been realized. Therefore, without
additional, external influences, any future settlement would only be from the
decomposition of the waste disposed within.
Construction of the tunnels is an additional, external influence to be
considered.
15.41
As with any
settlement across a Landfill surface, the potential detrimental effects are:
·
Alteration of the ground surface gradients with, in a
“worst-case” scenario, a reversal of gradients resulting in ponded water,
increased infiltration and therefore increased leachate generation.
·
Increased stresses and strains in the waste and/or capping
system, particularly the geomembrane component, resulting in rupture (in the form
of a hole or tear) and increased infiltration and leachate generation.
·
Damage to the existing leachate and landfill gas management
systems, surface water drains and road.
15.42
Each of these
potential effects has been investigated and assessed. Settlement of the Landfill due to only the
decomposition of the waste is first considered; then settlement resulting from
only the formation of the tunnels is estimated; and finally the total
settlement determined, along with an assessment of the effect on the Landfill
surface gradients, the integrity of the capping system components and damage to
the leachate and landfill gas management system, surface water drains and road.
Landfill Settlement Due to Waste Decomposition
15.43
Under the Restoration
Contract (Contract No. EP/SP/30/95), the platforms and the side slopes of the
restored NTML have been restored with a minimum slope inclination of 1% (as
measured from the horizontal). A typical
plan view and the corresponding cross section of the “as-constructed”
conditions of the restored NTML are provided in Figure No. NOL/ERL/300/C/XRL/ENS/M61/201 and Figure No. NOL/ERL/300/C/XRL/ENS/M61/202, respectively.
15.44
Settlement of the
landfilled waste as a function of time was assessed by the Restoration
Contractor, through analysis, as part of the Restoration Contract [Ref.
6]. A copy of the analysis is included
as Appendix 15.3 for reference and completeness.
15.45
The typical Landfill
cross-section presented in Figure No.
NOL/ERL/300/C/XRL/ENS/M61/202 has been analyzed for the long-term,
“secondary” settlement that has occurred since 1999 and that will occur in the
future through year 2049 due to decomposition of the waste. This analysis is based on the assumption that
the predicted settlement rate exceeds the actual, observed rate; and therefore
is conservative. The details of this
analysis are included in Appendix 15.3 and the results are presented in Figure No. NOL/ERL/300/C/XRL/ENS/M61/204.
15.46
The results of this
analysis, which is based on somewhat conservative assumptions, indicate a slight
decrease in the slope inclination through the entire Landfill cross section;
this generally due to the decrease in waste thickness from the maximum toward
the middle of the Landfill, to a minimum at the toe of the Landfill slope.
15.47
The slope inclination
across the Upper Platform generally decreases slightly from the original 4% to
approximately 2% through the 50-years of analysis from 1999 through 2049. Similarly, the slope inclination across the
Lower Platform decreases from the original 8% to approximately 6% over the same
design-life. Because of the relatively
steep slope inclination prior to settlement, the slope inclination over the
side slopes generally decreases only slightly with the estimated settlement.
Settlement Due to Tunnel Formation
15.48
Settlement caused by
the tunnels could alter the restored (post-settlement) Landfill surface
gradients by potentially increasing them or reducing them; determined as a
function of the orientation of the alignment of the tunnels relative to the
Landfill configuration (location of the platforms and slopes) as shown in Figure No. NOL/ERL/300/C/XRL/ENS/M61/203.
15.49
A detailed analysis
of the potential settlement has been performed based a commonly employed
approach attributed to Peck in 1969, with further developments since that
time. The approach assumes that the
shape of the settlement trough above the tunnel and perpendicular to the
alignment is represented as an inverted Gaussian curve, the ultimate shape of
which is dependent on the geologic/ground conditions at the tunnel face and
above; and determined based on the assumptions for two parameters: the volume loss factor (VL); and
the trough width factor (k).
15.50
Conservative values
have been assumed for both the volume loss factor (VL = 1.2% [Ref. 7];
and the trough width factor (k = 0.5 in the soil and/or rock beneath the
Landfill and 1.0 in the waste and soil cover through the Landfill profile. Additionally, the analysis superimposes the
affect of both tunnels, each excavated to a diameter of 9.35m; separated by 16m
centre-to-centre [Ref. 1].
15.51
The details of the
settlement analysis performed for the tunnels beneath the restored NTML are
included in Appendix 15.3 and the results are presented in Figure No. NOL/ERL/300/C/XRL/ENS/M61/204. In general, the magnitude of settlement
anticipated as a result of tunnel construction ranges from a maximum of
approximately 30mm along the centreline between the twin parallel tunnels; to
nominally 20mm, and 5mm at a distance of 20m, 40m from the centreline, respectively. This magnitude is minimal when compared to
the settlement estimated to result from waste decomposition, which typically
ranges from approximately 250mm to 750mm.
15.52
From inspection, the
qualitative effect that the potential settlement induced by the construction of
tunnels has on the Landfill surface is summarized in Table 15.1.
15.53
The results of the
analysis of potential settlement caused by the construction of the tunnels are
superimposed on the post-settlement configuration of the Landfill resulting
from only waste degradation (Figure No.
NOL/ERL/300/C/XRL/ENS/M61/204), to determine the combined long-term affect
on the final configuration of the Landfill cross section as shown.
Table
15.1 Summary of the Impact of Only Settlement
Resulting from Tunnel Formation
|
Location |
Description of Impact on Surface Gradient |
|
Upper Platform |
Surface gradients will
increase from the centre of the Landfill toward the crest of the Upper Slope. |
|
Upper Slope |
Slope inclination will increase as the toe of slope is
closer to the centreline of the tunnels and therefore has a higher magnitude
of settlement. |
|
Lower Platform |
Surface gradients across the northeastern portion of the platform
will increase, whereas surface gradients across the northwestern portion of
the platform will decrease. |
|
Lower Slope |
Slope inclination will decrease as the crest of the slope
is closer to the centreline of the tunnels and therefore has a higher
magnitude of settlement. |
Resulting Landfill Surface Gradients
15.54
In review of the
potential final configuration of the Landfill, it is obvious that the
settlement caused by the construction of the tunnels alone is minimal by comparison
to that resulting from the degradation of waste. Additionally it appears that the surface
gradients across the platform will be reduced to a minimum of nominally 2%;
whereas the inclination of the side slopes will be slightly flatter than the original,
but remain relatively steep. The
flattening of the slopes and platforms as described is not unexpected and, if
it remains within the order of magnitude described herein, is acceptable.
Integrity of Geomembrane Component of Capping System
15.55
Differential
settlement of the Landfill surface can result in stresses and strains to be
induced in the geosynthetic components of the capping system layers;
specifically the geomembrane layer. The
magnitude of strain in the geomembrane resulting from the differential
settlement caused by the combined effect of waste degradation and the
construction of the tunnels is less than 1% as shown in Figure No. NOL/ERL/300/C/XRL/ENS/M61/204.
15.56
The geomembrane reported
to be used in construction of the capping system [Ref. 6] is a linear low
density polyethylene (LLDPE) material, also sometimes referred to as “very
flexible polyethylene (VFPE)”. This
material is typically considered to have no definable yield stress or strain,
and a break elongation in excess of 800% as measured in uni-axial tension. For application to the NTML, it can very
conservatively be assumed that the LLDPE geomembrane material used has the same
yield elongation characteristics as high density polyethylene (HDPE), nominally
12%.
15.57
With an estimated
potential settlement-induced strain of less than 1% and an allowable strain of
at least 12%, the geomembrane component of the capping system should not be
adversely affected by the additional settlement caused by tunnel construction
or by the total settlement resulting from the combination of waste
decomposition and construction of the tunnels.
Integrity of Leachate and Landfill Gas Management Systems and Infrastructure
15.58
The leachate
management system consists of a simple piping network installed at the base of
the Landfill, and a concrete chamber near the toe of the Lower Slope (refer to
Appendix 15.1). As a result, it will not
be subject to settlement from degradation of the overlying waste, but only the
settlement caused by construction of the tunnels. The resulting differential settlement is
estimated as less than 0.1% along the pipeline length (refer to Figure No. NOL/ER:/300/C/XRL/ENS/M61/204);
a magnitude which is not anticipated to result in significant change in the
flow gradient or increased compressive or tensile stresses in the pipe
material.
15.59
The existing landfill
gas management system consists of a network of vertical passive ventilation
wells across the Upper Platform; and horizontal pipes installed in relatively
shallow trenches with vertical passive ventilation risers generally aligned
around the perimeter of the Upper Platform, along the toe of the Upper Slope
and diagonally across and down the Lower Slope (refer to Appendix 15.1). The vertical ventilation wells across the
Upper Platform are likely to be subjected to significant settlement from
degradation of the waste, but are located too distant from the tunnel alignment
to be significantly influenced by its construction (refer to Figure No. NOL/ER:/300/C/XRL/ENS/M61/204). Similarly, the network of passive horizontal
trenches and vertical risers around the perimeter of the Upper Platform likely
have a limited depth of waste beneath and are relatively distant from the
tunnel alignment; therefore they will be subject to limited settlement and even
less differential settlement. The
horizontal pipes along the toe of the Upper Slope will be subjected to
substantial settlement (estimated to be more than 400mm) from waste degradation
and less than 30mm from formation of the tunnels, but limited differential
settlement. The horizontal pipe across
the Lower Slope is installed at a relatively steep gradient (approximately 13%)
and thus will be affected only to a limited extent by both waste degradation
and tunnel construction settlement. The
pipe aligned across the width of the Lower Platform will be the most critical
in terms of settlement impact; however the settlement resulting from formation
of the tunnels is anticipated to result in a gradient reduction of less than
0.1% across an graded at more than 6%, and therefore relatively
inconsequential.
15.60
Based on the
configuration of the surface water management system (refer to Appendix 15.1)
it is anticipated that the performance will be remain relatively unaffected by
the settlement resulting from the formation of the tunnels. Many of the surface water features are
oriented across the crest or toe of the Upper and Lower Slope, or directly
down-slope. Again, the most critical
location on the site is anticipated to be across the Lower Platform, where
formation of the tunnels is estimated to alter the designed gradient only
slightly; from the initial design of approximately 6%, remaining at more than
5.9% after settlement.
15.61
The concrete-paved
road is generally constructed outside of and along the waste boundary (refer to
Appendix 15.1); and therefore is not subject to settlement induced by waste
degradation. It will, however, be
subjected to settlement caused by formation of the tunnels. The resulting differential settlement along
the northeast-southwest trending portion of the road is estimated to be less
than 0.1% (refer to Figure No.
NOL/ER:/300/C/XRL/ENS/M61/204); a magnitude which is not anticipated to
result in significant additional tensile or compressive stresses, or
significant additional cracking of the concrete surfaces.
Impact of Groundwater on Tunnels
15.62
There are two
potential concerns with regard to the effect that the restored NTML may have on
the tunnels; both related to the flow of groundwater. Specifically in this regard are the concerns
of:
(a) the quantity; and
(b)
the quality of groundwater appearing at the tunnels; with
the potential consequences being:
-
the need to manage the inflow volume (during both the
construction and operation stages);
-
the collection and discharge of contaminated groundwater
(during the construction and operations stages); and
-
degradation of reinforced concrete or other
materials/elements used in construction of the tunnels (operations stage).
15.63
A brief description
of a groundwater model and monitoring data available for the Landfill is
presented in the following sections, followed by an assessment of these
concerns.
Groundwater Model
15.64
Because the restored
NTML does not have a base lining system (there is a relatively impermeable
component in the capping system, but only across the two platform areas), the
local groundwater regime can be modeled as that provided in the Hydrogeological
Impact Assessment Report [Ref 8].
15.65
During the wet season
the groundwater recharges from the permeation of rainfall through the
superficial deposits and weathered rock below the areas of predominantly
natural habitat (vegetated hill slopes) and agricultural land [Ref. 8]. During the dry season, there is a reduction
in rainfall with a corresponding reduction in permeation through the
superficial layers. As a result, the
flows are reduced, with a subsequent decline in the groundwater table. Regional and localized groundwater flow is
from the topographic high areas to the topographic low areas throughout the
entire year.
15.66
There is a localized
groundwater mound within the hill immediately to the east of the Landfill, with
flow radially outward [Ref. 8], and thus generally west to east across the
Landfill site. The unnamed hill therefore
represents an area hydrogeologically up-gradient of NTML; with flow toward the
relatively flat areas to the west that are down-gradient from the
Landfill. The areas immediately adjacent
to the Landfill, particularly to the north, can generally be hydrogeologically
considered as side-gradient.
15.67
Based on this model,
the areas up-gradient from the Landfill should be free from contamination with
waste constituents and thus provide a baseline or background for the
concentrations of the various constituent parameters. Conversely, the down-gradient areas to the
west of the Landfill are expected to potentially have traces of the same
constituent parameters as the waste mass; the concentrations of which will vary
as a function of time and distance, both vertically and horizontally, from the
Landfill. The side-gradient areas will
commonly demonstrate concentration levels between that of the up-gradient and
down-gradient locations, but typically closer to that of the down-gradient,
with the exception of a zone immediately adjacent to the landfilled waste mass.
15.68
Hydrogeologic
principles applied generally to landfilled waste, and more particularly the
restored NTML, suggest that the flow of contaminants from the disposed waste
mass is through a combination of transport mechanisms, including advection,
diffusion and dispersion. Various
models, many of them relatively complex, exist to simulate the transport of the
various constituents as a function of time and distance from the disposed
waste. However, a more fundamental
approach has been taken for the assessment described herein, with the results
providing a range which does not warrant more complex modelling.
Available Data
15.69
The Restoration
Contract requires that groundwater and leachate levels and quality be monitored
at designated locations and at specified frequencies during the contract
duration. There is, as a result, a
series of six (6) groundwater monitoring wells around the Landfill:
·
one (1) up-gradient well (Well No. GW1);
·
four (4) down-gradient (Well Nos. A458, DH403, DH404 and
DH405); and
·
one (1) side-gradient (Well No. DH407).
15.70
Information on both
the groundwater level/elevation and the quality have been provided by the
Restoration Contractor for the period from October 2006 through July 2008,
based on select constituent parameters required by the Restoration
Contract. There is also a series of two
(2) leachate monitoring wells (Well Nos. DH401 and DH402/402A), for which there
is monitoring information on the level/elevation and quality within the waste
mass for the same time period. The
locations of the groundwater monitoring wells and the leachate monitoring wells
are shown on Figure No.
NOL/ERL/300/C/XRL/ENS/M61/203.
15.71
Additional
groundwater sampling from a borehole drilled along the alignment and level of
the tunnels in the vicinity of the Landfill was performed in early 2009 in
conjunction with a supplementary investigation associated with this
Project. The results of the subsequent
groundwater quality testing are included in Appendix 15.5.
Quantity of Groundwater
15.72
The quantity of
groundwater appearing at the alignment is generally a function of several
factors, including:
·
the local geology;
·
the soil/rock strata in which the tunnel is to be
constructed;
·
the depth of the tunnel below the groundwater table; and
·
the overall hydrogeologic setting of the area.
15.73
A description of the
groundwater regime, including a conceptual groundwater model and a qualitative
impact assessment is included in the “Interim Hydrogeological Impact Assessment
Report” [Ref. 8].
15.74
Of the factors
identified, most (including the local geology, the soil/rock strata along the
Project tunnel alignments and the depth of the tunnels below groundwater) are,
by their nature, generally unaffected by the presence of the restored
NTML. These therefore do not need to be
considered further herein. The remaining
factor, the hydrogeologic setting is considered in this assessment.
15.75
As reported, the
general hydrogeologic setting along the tunnel alignment (and all of
15.76
The topographic
setting of the restored NTML is within a relatively small valley, between
approximately elevations +15mPD and +35mPD at the toe of the western slopes of
the isolated unnamed hill that rises to an elevation of approximately
+85mPD. This setting, and the
groundwater and leachate level data obtained as part of the ongoing monitoring
programme of the Restoration Contract, suggests that the groundwater has risen
relative to the original level, likely due to the presence of the Landfill; but
generally only within the area of the former valley now filled with disposed
waste. Therefore, the local and regional hydrogeology remain practically
unaffected.
15.77
Groundwater flow into
the tunnels during and after construction is related to the method of
construction and the designed tunnel lining system, respectively; and is not
directly related to the presence of the waste within the restored NTML. It is understood that groundwater inflow will
be “effectively precluded” [Ref. 1] both during construction, by use of a
closed-face Tunnel Boring Machine (TBM); and during operations, by a permanent
precast concrete tunnel lining.
15.78
With no groundwater
or leachate flow into the tunnels, the existing hydrogeologic regime will not
be altered. As a result the groundwater
level surrounding the Landfill, the leachate level within the Landfill, and the
groundwater and leachate flow will practically be unaffected.
Quality of Groundwater
15.79
The quality of the
groundwater is of concern in two (2) regards; the quality of water that could
potentially drain into and be discharged from the tunnels during construction and/or
operation phases; and the potential impacts that the quality of groundwater may
have on the materials selected for construction of the tunnels.
Impact on Collected Groundwater
15.80
It is understood that
the inflow of groundwater will be controlled during construction by use of a
closed-face Tunnel Boring Machine (TBM); either an Earth Pressure Balance
Machine or a slurry type machine. A
closed-face machine of this nature reportedly “…effectively precludes the ingress
of water into the tunnel during...construction.” [Ref. 1]. As a result it is anticipated that
groundwater inflow into the tunnel during construction will be controlled by
the selected boring machine and construction method, to the extent that none
will be collected from the area along the alignment immediately adjacent to the
restored NTML.
Impact on Tunnel Materials
15.81
It is understood that
the tunnels will be lined with a precast reinforced concrete segmental lining
having joints sealed with an elastomeric gasket and hydrophilic strip [Ref. 1]. In this regard, there are two (2)
construction materials of potential concern for assessment of the impact of the
restored NTML on the tunnels, specifically:
the reinforced concrete to be used as the tunnel lining; and the
specific materials to be used as the joint sealant. The detailed assessment of each of these
materials is presented as follows.
Precast Reinforced Concrete Material
15.82
Based on the
composition of “typical” landfill leachate [Ref. 7] and the leachate quality
monitoring data for the restored NTML (refer to Appendix 15.4), there are three (3) primary leachate
constituents of concern with regard to the use of concrete for the tunnel
lining: pH, sulphate, and chloride.
15.83
It is commonly
accepted in landfill engineering that the aggressiveness of most, if not all
municipal solid waste leachate constituents decrease with time after placement
(increasing “age” of the waste). This is
the case for each of the three parameters of concern. It should be noted that there is an initial
increase of the values of most, if not all parameters, while waste is still
being placed and shortly thereafter; say within 1-year; after which time the
values decrease through time.
15.84
It is commonly
considered that concrete can be vulnerable to acid attack at pH levels of less
than approximately 5.5 or 6. The pH of
the groundwater down-gradient of the Landfill was tested in association with
the recent supplementary investigation performed for this Project (refer to
Appendix 15.5). The pH was measured at
7.2; therefore it is not anticipated that the concrete tunnel lining will be
subject to acid attack.
15.85
Sulphate attack of
concrete can occur when sulphate in the groundwater reacts with the
constituents in the cement used to produce the concrete mixture. This is generally considered of minimal
concern when the sulphate concentration in groundwater is less than 400mg/L
[Ref. 4]. The results of the recent (May
2009) testing performed in association with this Project indicate sulphate
levels of 8mg/L (refer to Appendix 15.5). Therefore there is practically no
concern for sulphate attack of the precast tunnel lining.
Impact on Concrete Reinforcement
15.86
The average chloride concentration in the groundwater
monitoring wells down-gradient from the restored NTML was approximately 30mg/L
as measured from October 2006 through July 2008. However, the results of the more recent (May
2009) testing performed in association with this Project indicate a chloride
concentration of 9mg/L along or near the alignment and level of the tunnels in
the vicinity of the Landfill (refer to Appendix 15.5). These more recent results appear more
directly relevant and, for practical considerations, suggest a chloride
concentration which is at or only slightly elevated from the background level
as measured up-gradient from the Landfill.
15.87
As the more recently
tested chloride concentration levels are, by
Joint Sealant Material
15.88
As the tunnel has not
yet been designed in detail, the elastomeric gasket and hydrophilic sealant
materials between the concrete tunnel lining segments have not yet been
specified. It is therefore recommended
that these materials be selected based on proven chemical resistance/chemical
compatibility with liquids containing the constituents and in the concentrations
found in the Landfill down-gradient monitoring wells or a sample obtained from
within the tunnel bore such as has been tested in May 2009. Based on the typical constituents and
concentrations found down-gradient of the restored NTML, and the range of
polymeric and natural materials available for production of the sealant
materials, it is anticipated that the issue of resistance/compatibility can be
appropriately and effectively addressed through the process of design and
proper selection.
Mitigation
of Potential Impacts
15.89
The assessment performed
herein concludes that the anticipated ground vibration generated by the TBM
during construction and the trains during operation of the Project will have
minimal impact on the restored NTML slopes.
The impact is a slight reduction in the minimum Factor of Safety
obtained in analysis of the stability of the slope. The resulting Factor of Safety remains
comfortably above the minimum specified level; therefore no mitigation measures
are necessary.
15.90
Similarly, the
assessment performed for settlement of the NTML indicates that, even based on
conservative assumptions, the potential ground movements generated by formation
of the XRL tunnels will have minimal impact on the settlement of the Landfill,
particularly compared to the magnitude of settlement resulting from degradation
of the waste. Specifically in this
regard, the anticipated settlement from the construction of the Project tunnels
is conservatively estimated at a maximum of approximately 30mm as compared with
a maximum of 450mm due to waste degradation.
The generally favourable orientation of the Project tunnels with the
overlying Landfill configuration; along with the relatively minor magnitude of
estimated settlement; is not anticipated to result in a gradient reversal or
the generation of excessive stresses:
across the restored final ground surface; with the geomembrane component
of the capping system; in the leachate and landfill gas management systems, the
surface water management system or the road.
As a result, no mitigation measures of these components and systems are
necessary.
15.91
The assessment
performed for the potential impact of the restored NTML on the tunnels
indicates that the groundwater quality down-gradient of the Landfill will
likely contain only trace constituents from the waste mass that could
potentially cause deterioration of the tunnel concrete lining or the
elastomeric joint sealant between segments.
It was determined that each of the three constituents of potential
concern; pH, sulphate and chloride; the concentration/level of each is likely
to be sufficiently low that there would be no adverse impact on the tunnel
concrete lining.
15.92
Assessment of the
potential impact of the groundwater quality on the elastomeric gasket and
hydrophilic sealant are very dependent on the actual materials selected for
use. It is therefore recommended that
the gasket and sealant materials be reviewed for chemical resistance to the
various constituents and concentrations in the leachate/groundwater; and
potential suppliers of these materials be provided with the chemical
composition of the groundwater so as to propose compatible gasket/sealant
materials.
Conclusions
and Recommendations
15.93
The existing restored
NTML slopes are stable (with a minimum Factor of Safety of 1.8). The calculated minimum Factor of Safety will
be reduced slightly to approximately 1.6 under the imposed conditions of
conservatively large vibrations assumed during the construction of the tunnels
in rock. As a result, no mitigation
measures are necessary.
15.94
Long-term settlement
of municipal solid waste placed in landfills results from the
naturally-occurring process of decomposition and gas generation. The surface of the NTML is expected to settle
up to 750mm over the course of the modelled duration. Based on commonly employed modelling methods
and under conservative assumptions, it is anticipated that the construction of
the tunnels beneath the Lower Platform of the Landfill will not cause more than
30mm of additional settlement. Given the
location of the tunnels beneath the Landfill, the constructed gradients across
the Landfill surface, and the anticipated settlement pattern; it is anticipated
that the construction of the tunnels will not cause adverse impacts (such as
reversal of the ground surface gradients; rupture of the geomembrane component
of the capping system, damage to the leachate and landfill gas management
systems, additional damage to the surface water management features or damage
to the road) to the restored NTML, and therefore no mitigation measures are
anticipated to be required.
15.95
It is indicated by
others that use of Tunnel Boring Machine (TBM) methods and a permanent
segmental concrete liner with elastomeric gasket/hydrophilic sealant will
prevent inflow of groundwater into the tunnels during the construction and
operation phases. Based on this design
assumption, it is anticipated that the groundwater and leachate levels and
flows will remain virtually unaffected and therefore will not require
mitigation measures of any kind.
15.96
Furthermore, based on
the currently reported leachate quality within the waste and, more
specifically, the groundwater quality down-gradient of the Landfill, it is
unlikely that leachate will have an adverse impact on the concrete lining of
the tunnels; therefore mitigation measures will not be necessary.
1.
Arup-Atkins Joint Venture; “Working Paper No. 42 –
Tunnelling Impact on Ngau Tam Mei Landfill”; Consultancy Agreement No.
NEX/2102; Express Rail Link – Preliminary Design of Tunnels & Associated
Structures; January 2009.
2.
Arup-Atkins Joint Venture; “Deliverable D3.24A, Draft
Preliminary Design Final Report”; Consultancy Agreement No. NEX/2102; Express
Rail Link – Preliminary Design for XRL Tunnels and Associated Structures;
December 2008.
3.
Wong, H.N. and P.L.R. Pang; GEO Report No. 15 – “Assessment
of Stability of Slopes Subjected to Blasting Vibrations”; Geotechnical
Engineering Office, Civil Engineering Department, The Government of the Hong
Kong Special Administrative Region; 2000.
4.
Carnevale, M, G. Young and J. Hager; “Monitoring of
TBM-induced ground vibrations”; North American Tunneling ’00; Balkema; 2000.
5.
Binnie Black & Veatch HK Ltd.; “Design Criteria Design
Submission”; Contract No. EP/SP/30/95 – North-West New Territories Landfills
and
6.
Binnie Black & Veatch HK Ltd.; “Ngau Tam Mei Formation
Design Submission”; Contract No. EP/SP/30/95 – North-West New Territories
Landfills and
7.
Guglielmetti, Vittorio; Piergiorgio Grasso; Ashraf Mahtab;
and Shuli Xu; “Mechanized Tunnelling in Urban Areas – Design Methodology and
Construction Control.
8.
Arup-Atkins Joint Venture; “Interim Hydrogeological Impact
Assessment Report”, Consultancy Agreement No. NEX/2102; Express Rail Link –
Preliminary Design for XRL Tunnels & Associated Structures; Deliverable No.
D3.1R; August 2008.
9.
Technical Memorandum Standards for Effluents Discharged into
Drainage and Sewerage Systems, Inland and Coastal Waters.
10.
Rowe, R. Kerry; Robert M. Quigley, Richard W.I. Brachman and
John R. Booker; “Barrier Systems for Waste Disposal Facilities”; 2nd
Edition; Spon Press; 2004.\
11.
“Final Civil
Engineering Scheme Report”, Consultancy Agreement No. NEX/2102, Express Rail
Link; Document No. NEX2102-DED-AAV-CS-0029-101, Deliverable No. D3.10C as cited
in email communications.
12.
Skalny, Jan; Marchand, Jacques and Odler, Ivan; “Sulfate
Attack on Concrete”, 2002.
13.
Ismail, Mohamed and Masayasu Ohtsu; “Corrrosion rate of ordinary
and high-performance concrete subjected to chloride attack by AC impedance
spectroscopy”; Construction and Building Materials; 20 (2006), pgs 458-469.
14.
Geotechnical Engineering Office, Civil Engineering and
Development Department, The