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Tunnelling

Adit mining in high permeability, interbedded sandstone, Red Line, Dubai MetroAbstract

Adits connecting annex shafts and a bored tunnel are constructed for the Dubai Metro Red Line for the purpose of ventilation and passenger escape. A critical part of the design is to mitigate the risk of face instability due to the mining of adits to connect the bored tunnel and annex shafts, which is carried out in calcareous sandstone, whose strength and stiffness are equivalent to very densely compacted soil or extremely to very weak rock material. Overall instability due to global shear failure of the adit faces is classified as acceptably low in risk, but local piping failure due to the possible existence of weak layers of cohesionless materials could lead to overall stability problems. To limit water ingress and piping risks, dewatering via deep well pumping and horizontal drains has been proposed. Pumping trials have been carried out, with back analyses performed to create calibrated seepage models.

The Road & Transport Authority (RTA) constructed the Dubai Metro Red Line, the first railway line in the United Arab Emirates. The Red Line, together with the Green Line, civil works were built by JTMJV (Japanese Turkish Metro Joint Venture). Atkins was Contractor’s Designer responsible for all tunnel design including temporary and permanent works.

The line, which is 52.5km in length, has 26 stations, two depots, and three annex shafts. Two earth pressure balance tunnel boring machines (EPBMs) were used to construct a 5.3km long underground section.

The Dubai Metro project includes three annex shafts, each of which has two adits connected to the bored tunnel to provide ventilation (ventilation adits) and an emergency escape from the bored tunnel into the shaft (escapeway adit). The ventilation adits have a span of 8.2m and are 7.5m in height. The escapeway adit is smaller, with a span of 3.7m and a height of 5m.

The size and shape of the adits were optimised to minimise the bending moment generated on the bored tunnel

lining and to encourage the arching effect of the ground, especially for the ventilation adits, the height of which is almost the same as that of the bored tunnel. A section of the adits is shown in Figure 1, viewed from within the tunnel.

The adits for annex shafts 1 and 2 were constructed underneath existing roads, whereas the adits for annex shaft 3 were constructed underneath an existing six-storey building. It was important therefore to prevent excessive ground settlement due to adit excavation, especially for annex 3. The layout of annex 3 is shown in Figure 2.

The annex shafts were constructed using permanent diaphragm walls with the top-down excavation method. The bored tunnel was constructed by the EPBM, which passed beside the annex shafts.

The adits were constructed by the mechanical mining method with lattice girders and shotcrete for temporary support. Permanent support was provided by cast in situ reinforced concrete.

A temporary ring beam bracing system was installed within the affected section of the bored tunnel to limit deflection

Introduction

Paul GrovesNetwork Cair for Tunneling

Atkins Middle East & India

Eric ChuiDivisional Director

Atkins China

Tiffany ChanPrincipal Engineer

Atkins China

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during adit mining and allow removal of lining segments to complete the connection. The annex shafts were drilled and mining commenced from the shaft towards the bored tunnel.

Ground conditionsIn general, the adits are located in weak interbedded calcareous sandstone (CSS) that is overlain by calcareous sand (CS) and marine deposits (MD). Sandstone, although classified as rock, has strength and stiffness properties equivalent to very densely compacted soil or extremely to very weak rock material. Its fractures are close (60 to 200mm) to medium spaced (200 to 600mm), with some poorly cemented carbonate silt and sand layers noted.

The permeability of these materials is around 10-5 to 10-6 m/s; however,

structural defects such as solution cavities, joints and geological faulting could provide a pathway that would significantly increase groundwater flow. The groundwater level along the tunnel alignment is around 2 to 6m below ground level.

Geotechnical risks associated with adit miningBased on the ground investigation information and observations during station / shaft excavation, the following key risks were identified:

• Local face instability in adit headings;• Excessive ground movement impacts

on existing structures; and• Excessive groundwater ingress and

piping (ie. running or flowing sand).

A) Local face instability in adit headingsWhen fully drained, sandstone and cemented sandstone material are likely to behave as firm, competent ground without noticeable movement in the adit heading face or unsupported arch. Stability analysis using the wedge failure approach indicated that the excavation face was stable and achieved the required factor of safety. Hence, it was considered unnecessary for the adit to be pre-supported with forepoling.

B) Ground movement impactsGround movement due to stress redistribution in the ground will occur around the advancing adit heading. However, we considered that the likelihood of large movement was low as the adits are relatively short compared to their depth. Provided that instability does not occur, settlement effects will be quite local in the area above the short adits. Having said that, local instability (eg. raveling or block failure) related to mining in drained ground is always a risk, and needs to be minimised and controlled during the work by conventional mining measures based on observational approaches; however, this is not covered in this paper.

C) Groundwater ingress and pipingWhen mining in sandstone below the water table, there is always a risk of significant groundwater ingress and piping, also known as running or flowing sand conditions. A comparable situation is illustrated in Figure 3, where excessive water inflow and possible piping have occurred in cemented sand material due to excessive hydraulic gradients and highly permeable ground.

Based on the ground investigation data and site observations during the excavation of the shafts and tunnel boring in the same area, we considered that piping and groundwater ingress were major risks of adit mining. Piping failure can be significant because it can lead to flooding, severe ground loss and possible instability, which can endanger the workforce. The phenomenon of piping occurring along a possibly weak permeable layer of cohesionless material is illustrated in Figure 4. Measures to reduce these risks through a structured approach are essential and discussed in the following sections.

Drain Pipe from tunnel drainage

Bore tunnel segment (TOP) Segment joint (TOP)

Vent Opening

Escapeway

Frame

DRIVEDIRECTION

3700

7500

Frame

Figure 1. Typical section of adits

Figure 2. Layout of adits for annex 3

annex 3 footprint

annex 3 entrance

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Tunnelling113 Adit mining in high permeability, interbedded

sandstone, Red Line, Dubai Metro

Risk mitigation measuresThe following measures were proposed to reduce the risks due to excessive groundwater ingress and piping:

• Deep well dewatering;• Use of a slurry wall or jet grouting

from the ground surface to form a hydraulic cut-off wall;

• Horizontal gravity drains from annex shafts; and

• Other measures such as local drainage pipes, probing and grouting from the excavation face where necessary.

We considered deep well dewatering and horizontal drains to be the principal measures whereas the cut-off wall was of secondary importance because its efficiency is difficult to ascertain. The arrangement of the deep well and cut-off wall at annex 2 is shown in Figure 5.

Other measures that can be used just prior to or during adit mining are considered to be contingency measures against the possible partial success of dewatering and ground treatment measures.

Analytical approach and resultsTo investigate the effectiveness of deep well dewatering to reduce the pressure head at the adit level, analyses were carried out using the computer program SEEP / W. The general approach is summarised below.

Stage 1: Axisymmetrical and transient modelling – to adjust the parameters and pump rate to simulate a three-dimensional (3D) effect with two-dimensional (2D) models and to predict the time required for dewatering to reach a steady state.

Stage 2: Initial seepage and piping risk assessment – to investigate piping risk based on the current dewatering scheme and assumed permeability of the cut-off wall.

Stage 3: Back analysis – to calibrate the model and replicate the results of the pumping trials.

Stage 4: Design of risk mitigation measures – to design final measures to mitigate piping risk based on information obtained from stage 3.

The staged analyses are described in detail below with a discussion of the results.

Stage 1 – Axisymmetrical and transient modelsBefore setting up the seepage models for actual layout, two calibration analyses were undertaken, as described below.

A. Axisymmetrical seepage analysis. Well dewatering is a 3D problem with different boundary conditions in the two horizontal directions. An axisymmetrical approach was adopted to simulate the deep well dewatering effect to obtain the water drawdown and hydraulic pressure. A general case of seepage into a deep well in two dimensions was then undertaken to replicate a similar dewatering pattern by adjusting the pumping rate. Figure 6 shows results from the axisymmetrical model.

B. Transient seepage analysis. Transient analysis was carried out to evaluate the rate of groundwater drawdown, and hence the change of the hydraulic head at the adit face at different time intervals. As indicated in the analysis, approximately six days were required to achieve steady state flow for the given ground conditions.

Stage 2 – Initial seepage and piping risk assessmentBased on the stage 1 findings, 2D models were set up at each annex to give an initial idea of the piping risk and a range of factors of safety against piping.

The critical hydraulic gradient is based on the suggestion of Kiya (2000):

Ic = 0.179 e, 0.046 DR (1)

where DR is the relative density. For dense sand, the relative density is about

Figure 3. Example of piping failure

Figure 4. Significant water ingress from the strata bed

Figure 5. Plan of a deep well at annex 2

Annex Structure 2

escape way

pw1

5250 5250 6000 6000

pw3pw2

vent opening

diaphragm wall annex structure

dewatering wells(typ)

bored tunnel

110

65% and therefore the critical hydraulic gradient is 3.6.

Slurry walls were constructed around the adits of annex 1 and because of site constraints, jet grouting walls were used as hydraulic cut-offs for annexes 2 and 3. To model the possibility of the cut-off wall being leaky and how that might affect the amount of groundwater ingress and piping, the cut-off wall was modelled with smeared permeability.

The results of the initial analyses suggested:

• Deep well pumping significantly reduces the hydraulic pressure head around the adit excavation and hence the piping risk. See Figure 7;

• With the use of piping wells, the groundwater flow was reduced by 35% for annex 2 and 67% for annex 3; and

• Piping may be a problem at the invert level.

However, the analyses were based on homogeneous soil materials with permeability of one order of magnitude. The permeability of actual ground conditions can vary, which can lead to higher groundwater inflow, and a single permeable layer can entirely change the situation. It was suggested, therefore, that trials of a deep well pumping and horizontal probe drains be carried out and the results adopted to calibrate the seepage models.

Stage 3 – Back analysis to replicate the pumping trialsTwo sets of trial tests were carried out.

A. Deep wells. Flow meters were connected to each of the three pumping wells and three observation wells were installed to measure the groundwater drawdown. The procedure was as follows:

a. Turn all three pumps on and monitor the water level and pumping rate until a steady state is reached.

b. Cease pumping until the groundwater table recovers its natural level.

c. Turn on only pump 2 and monitor the water level and pumping rate until a steady state is reached.

d. Repeat step c for pumps 1 and 3.

Example results of the deep well pumping trial for annex 2 are given in Figure 8.

B. Probe drain pipes from segmental lining. Probes were drilled and drain pipes were installed from the bored tunnel at each adit location, and each pipe was equipped with a packer with a tap. The water pressure was measured using a pressure gauge connected to the drains

(when the tap was closed) and the flow rate from the drains was measured by the number of buckets filled per minute (when the tap was open). The trial procedure was as follows:

a. Measure the pressure head with all pumps off.

b. Measure the flow rate from each of the drains opened one by one with all pumps off.

c. Measure the pressure head with all pumps operating.

d. Measure the flow rate from each of the drains opened one by one and with all pumps operating in the steady-state mode.

Back analysis was carried out to replicate the trial results by adjusting the following parameters and assumptions:

• The permeability of the jet grout wall, which represents a partially leaky cut-off wall;

• The soil permeability; and• The pumping rate of the deep wells.

Figure 9. shows the model replicating the trial test results from deep wells and drain pipes.

Stage 4: Design of final risk mitigation measuresThe revised model, which replicated the trial test results, indicated that the hydraulic gradient might be too high to achieve a minimum factor of safety against piping of 1.5.

Therefore, in addition to the deep well pumping within the jet grout cut-off walls, a series of horizontal drains below the level of the adit inverts were proposed. The layout is given in Figure 10 and installation Figure 11.

New analytical models were undertaken, taking into account the modified parameters to reflect the trial test results, adit excavation sequence and provision of the proposed horizontal drains installed from the annex shafts.

Three scenarios were studied: with horizontal drains only; with deep wells only; and with both horizontal drains and deep wells.

Figure 8. Analysis of the pressure head contour for annex 2

Figure 6. Axisymmetrical model

Pressure Head Couture

Distance (m)

Elev

atio

n (m

DM

D)

5

10

15

20

Figure 7. Analysis of the pressure head contour for annex 2

0

10

12

2

2

4

4

6

8

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Tunnelling

The adit excavation was simulated by specifying that the pressure head at the adit face equal zero. The inflow of groundwater was obtained by the “flux section” around the opened face.

Phased excavation was modelled in the SEEW analysis, taking into account the construction time that would likely be required for adit mining using the heading and benching method, as illustrated in Figure 12.

The hydraulic gradient for various cases was estimated based on the SEEPW models for comparison with the critical hydraulic gradient; the factor of safety against piping and the inflow rate from

the excavation faces were also estimated from the models. The results are summarised in Tables 1 and 2.

By comparing the results of the analysis for the case with deep wells only and the case with both horizontal drains and deep wells, it is clear that dewatering around the adit via horizontal drains can significantly reduce the piping risk and initial inflow rate for adit excavation.

Verification from additional field dataPrior to the start of excavation, as planned, drain pipes were installed,

forming part of the proposed horizontal drain holes, from the annex shaft diaphragm wall for each adit, each with a packer with a tap. The flow rate from the drains was measured based on the number of buckets filled per minute (when tap is open).

Based on the observable inflows and drawdown, further seepage back analyses were carried out to verify the design approach. The results are summarised in Table 3.

The seepage analysis generally gives a drain inflow less than that of the field measurement. The ratio between the field data and analysis results is in a range from 1.2 to 4.5, which is considered an acceptable range when compared with the possible range of permeability of sandstone, which is from 10-5 to 10-6 m/s (at least 10 times). The variation is probably due to the following:

• Variation in ground permeability.• The 3D effect. The model carried out

in the direction along the adits shows that the flow in the third direction could contribute more than 20% of the total flow.

• The uncertainty and localised effect of permeability.

• The inflow affected by the degree of leakage of the jet grout wall.

ConclusionAlthough the groundwater table is above the adit crown level, the use of preinstalled horizontal drainage pipes below the adit inverts reduces pore water pressure around adit openings, resulting in the reduction of seepage ingress and the maintenance of relatively low hydraulic gradients compared with the critical gradient for piping failure.

Further back analyses based on observations were made after horizontal drain installation and immediately prior to adit break-in. These confirmed the applicability of the design approach and that the models, based on back

Annex 3

600

SSL – 14.30

Figure 10. Layout of drain pipes installed below the adit invert of annex 3

Figure 11. Installation of drain pipes installed below the adit invert of annex 3

Figure 9. Back analysis model for annex 2

PW1PW2PW3

5.9655e-005

113 Adit mining in high permeability, interbedded sandstone, Red Line, Dubai Metro

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Analysis During excavation

Groundwater inflow into adit

(L/s) Annex 3 Escapeway Adit

With horizontal drains only

H 0.66 B & I 0.32

With deep wells only H 0.44

B & I 0.17

With both horizontal drains and deep wells

H 0.21 B & I 0.23

Annex 3 Ventilation Adit

With horizontal drains only

H 1.25 B & I 0.38

With deep wells only H 0.86

B & I 0.14

With both horizontal drains and deep wells

H 0.25 B & I 0.26

Table 2. Summary of groundwater inflow for annex 3

analysis-calibrated models, were in good agreement with the actual cases.

The analytical approach described in this paper, involving predictions, field trials, back-analysis verifications and further calibrated analyses, helped us to gain a better understanding of hydraulic behaviour during adit excavation and how it can be affected by various risk mitigation measures related to piping and excessive water ingress.

The Red Line adits have been safely mined and are fully lined.

AcknowledgementThis paper was presented at the International Symposium on Rock Mechanics “Rock Characterization, Modelling and Engineering Design Methods” held on 19-22 May 2009 at The University of Hong Kong.

ReferencesKiya, H 2000. Study on the Experimental Evaluation method for Sandy Tunnel Face, Journal of the Japan Society of Engineering Geology.

Analysis During excavation

Safety factor against piping at

inverts Escapeway Adit

With horizontal drains only

H 12 B & I 7.2

With deep wells only H 1.3

B & I 1.3

With both horizontal drains and deep wells

H 12 B & I 6.0

Ventilation Adit

With horizontal drains only

H 4.5 B & I 4.5

With deep wells only H 2.3

B & I 1.8

With both horizontal drains and deep wells

H 12 B & I 4.5

Table 1. Summary of piping assessment for annex 3

Case Description Inflow into adit (L / s) RatioSEEP / W

1 Drains at adit level only 0.5 1.85 3.7

2 Drains at adit level Drains at raft level

0.36 0.35

0.41 1.56

1.1 4.5

3 Drains at raft level only 0.4 1.8 4.5

4 Drains at raft level Drains at top level

0.34 0.57

1.53 0.88

4.5 1.5

Table 3. Observed inflows

Figure 12. SEEPW model for heading excavation of annex 3

2.4818e-0040

1

2

3

4

56

2.0805e-004

3.97

67e-

005

3.37

55e-

005

3.15

83e-

005

3.01

77e-

005

3.13

81e-

005

3.86

38e-

005

3.43

32e-

005

2.67

02e-

005

2.76

52e-

005

3.39

17e-

005

8.23

13e-

005

5.69

92e-

005