MONITORING DURING EXCAVATION OF

ROUTE 8 EAGLE’S NEST TUNNEL

Tim Leung1, Peter Poon

2, Bruce Cunningham

3 and Joseph Lo

4

Abstract

Route 8 Eagle’s Nest Tunnels have been constructed to provide dualthree-lane carriageway tunnels. The main tunnels are 2.1 km long and of 19 metres span with emergency pedestrian cross-passages of approximate 4 metres span at every 100 metres. A secondary 400 metres long adit tunnel connects to the main tunnel overhead ventilation galleries with a fan building adjacent to Tai Po Road. The main tunnels also have ventilation buildings at the North and South portals. The tunnels are bored in the rock mass of granitic origin by drill and blast method. There are existing Water Services Department facilities, and other existing structures and slopes that are close to the tunnel alignment and within the zone of influence of the blast vibration. In this case, the instruments have been installed both within the tunnels to monitor the ground behavior, and above the tunnels to monitor the effects of tunnel construction at the ground surface and the sensitive areas. An accurate, systematic and continuous monitoring is absolutely necessary during the excavation to ensure stability of the tunnels. Extensive convergence monitoring and water flow monitoring have been carried out within the tunnels. There are over 400 instruments that require regular monitoring outside the tunnel. These include settlement points, piezometers, vibration monitoring points as well as inclinometers and multiple point extensometers. Overall managing the instruments and monitoring of the excavation posted a difficult and challenging job for excavation of the large span tunnels, with an excavation section of over 180 m2.

INTRODUCTION

Route 8 in Hong Kong was the proposed major trunk road between Chep Lap Kok and Sha Tin, via Tsing Yi and Lai Chi Kok. The section between Chek Lap Kok and Tsing Yi has already been opened to traffic. When completed the Engle’s Nest Tunnel will provide relief to the other heavily loaded links between the Northeast New Territories and urban Kowloon. The location and layout of Lai Chi Kok and Sha Tin section of Route 8 is shown in Figure 1.The Kowloon section includes the slip roads from Lai Chi Kok leading to the Eagle’s Nest Tunnel approach road to the Eagle’s Nest South Portal and the Eagle’s Nest Tunnel itself which extents up to the location of the Toll Plaza. Northwards of the toll plaza is the Sha Tin Heights Tunnels.

The slip road from Lai Chi Kok is mainly on viaducts connecting to a fill embankment at Butterfly Valley. The embankments stretch towards extensive cut slope formation for the approach road leading to a two 2.1 kilometres section of tunnel. Eagle’s Nest Tunnel is designed as a dual three-lane carriageway.

1 Resident Engineer, Maunsell AECOM - Maunsell Hyder Joint Venture 2 Chief Resident Engineer, Maunsell AECOM - Maunsell Hyder Joint Venture 3 Senior Resident Engineer, Maunsell AECOM - Maunsell Hyder Joint Venture 4 Executive Director, Maunsell AECOM - Maunsell Geotechnical Services Ltd.

Figure 1. Route 8 Eagle’s Nest Tunnel

The HK$1.83 billion contract for the Route 8 - Eagle’s Nest Tunnel and Associated Works was awarded by Highways Department (HyD) of Hong Kong Special Administrative Region of the People’s Republic of China (HKSAR) to Leighton-Kumagai Joint Venture (LKJV) in September 2003. Maunsell Hyder Joint Venture (MHJV) is the design and construction consultant.

The North Portal tunnel section and South Portal tunnel widened section are of 17 m and 19 m span respectively and with a secondary 400 metres long adit tunnel for ventilation (see Figure 2). The excavation cross-sectional areas are about 140 m2, 180m2 and 45m2 for normal, widened and typical adit tunnels respectively. Two tunnels were driven curved and parallel on plan from the South Portal widened tunnel at road level of +67 mPD towards the mid-point of the tunnel at road level of +73 mPD and then towards the North Portal at road level of +67 mPD, with a minimum ground cover to the tunnel about 10m and a maximum ground cover to the tunnel about 180m.

A significant portion of the main tunnels are close to Kowloon Reservoir group which comprises the Kowloon Reservoir and the adjoining Kowloon Byewash Reservoir. The tunnel South Portal is under the Water Services Department (WSD) Cheung Sha Wan water treatment plant. The tunnel North Portal is bored above WSD Bend B tunnel. The tunnel sections are bored in rock mass of granitic origin. Figure 3 shows the WSD facilities and other existing structures close to the tunnel alignment and within the zone of influence of the blast vibration.

This paper describes the groundwater measurement, monitoring results of ground response and vibrations from blasting during tunnel construction. The roles of specialists in monitoring of the tunnel excavation are also described.

Typical and Widened Tunnel Section Junction of main tunnel and adit tunnel

Figure 2. Typical Tunnel Section andExcavation Photo of Junction of Main Tunnel and Adit Tunnel

Figure 3. WSD Facilities and Other Structures Close to the Tunnel

GEOLOGICAL AND HYDROLOGEOLOGICAL CONDITIONS

The tunnels are located in an area of mainly in slightly/moderately granitic rocks with some faults through the tunnel sections. The general geological sections of the tunneling area as inferred from the results of the ground investigation are shown in Figure 4. The geological conditions found during excavation of the tunnels were as expected and prediction by site investigation. A particular tunnel section near the south portal is overlain by a mantle of saprolitic soils (completely/highly decomposed granite) weathered from the parent rock. The saprolite is covered for the most part by a thin moderate layer of residual soils/colluvium. The band of weathered granite was found approximately 8 m to 14 m thick and the weathering pattern closely followed the inferred rockhead profile. The residual soils and colluvium were found to be between 1 m to 15 m thick. This posted a difficult and challenging job for the excavation and initial ground support as well as monitoring for the tunnels incorporating a 19m span in an area of minimum ground cover of about 10m of saprolitic soils consisting completely decomposed granite and historic fill at chainage about Ch. 1123 of southbound tunnel (see Figures 5a and 5b).

Southbound tunnel

South

Portal

North

Portal

Adit tunnel

Figure 4. Longitudinal Geological Section of Main Tunnel

Figure 5a. 19 m Span Widened Tunnel Figure 5b. Cross-section of Low Ground

Cover to Widened Tunnel Section

Site Hydrology

As mentioned, a significant portion of the region serves as catchment for the existing Kowloon Reservoir group which comprises the Kowloon Reservoir and the adjoining Byewash Reservoir. It is considered that most of the reservoir capacity is fed by the intercepted run-off from the uphill tributary catchment via the existing lined catchwater and the direct catchment uphill of the basin, see Figure 1. The existing catchwater is one of the main feeders for the reservoir and treatment stations. The site is currently covered by dense vegetation, which would normally mean considerable infiltration rates during precipitation. However, the gradients of the existing terrain and the streamcourses would suggest high surface flow velocities, which would result in relatively larger run-off hence reduced infiltration potential.

The groundwater contours were developed using “Surfer” software and adopting the “Kriging” method in generating the plots using piezometric data. The plots of piezometric data from September 2000 and July 2001 are shown in Figures 6a and 6b respectively.

Figure 6a. Plan showing developed groundwater contours (September 2000)

Figure 6b. Plan showing developed groundwater contours (July 2001)

In the design stage, the groundwater contours plotted in Figures 6a and 6b indicated the general gradient to be dipping northeasterly and west northerly and the groundwater mainly occurred within the rock mass. The groundwater contours developed from the piezometric data shows the groundwater gradients generally conform with the topography. Dry season water levels were considered to pose a more critical condition since the relatively lower groundwater tables had less recharge potential from the ground around the reservoir hence increasing the likelihood of water being drawn from the reservoir. The models presented were therefore based on a dry season baseline.

Tunnel Excavation

The principal objective in the design of tunnel support during excavation is to help the rock mass to support itself. Systematic temporary supports consisting of an integrated systematic rock reinforcement of fully grouted dowels and shotcrete that effectively acts together with the rock mass to limit rock deformation, and therefore bring the surrounding rock mass to a self-supporting and stable state with an adequate safety during construction until the permanent tunnel lining is installed. One of the most useful aspects of the Q-system is the support selection chart in which the tunnel support requirements are presented in a function of rock mass quality and ratio of the tunnel span to ESR (Excavation Support Ratio) that represents the safety of an excavation. Analytical methods were used to assess the stability of tunnel excavation and determine the requirement of all support classes, which were encountered during excavation of the varying conditions of hard, weak and mixed ground.

An assessment of the internal stability of the tunnel excavation in weak or mixed ground was made using a finite element program, Phase2 (Roscience, 2001). A simplified model in two dimensions was established to simulate the complicated three dimensional behaviour of staging excavation and ground relaxation. The Mohr-Coulomb constitutive model was used to present the plastic behavior of rock mass and roof canopy. Four elements of temporary support have been included in the analyses such as, rock dowels, pile roof canopy, steel arch ribs and fibre reinforced shotcrete.

Due to the relatively large spans of the tunnels, the excavations were undertaken by the heading and bench method, so that supports was installed immediately after exposure thereby not allowing significant convergence to occur. For example a two stage excavation strategy of South Portal southbound tunnel in a thin rock cover condition is firstly a top heading (maximum 8m height) with one row pile roof canopy, lower bench staging excavation and steel ribs of UC 203mm x 203mm x 46 kg/m with 200mm thick fibre reinforced shotcrete. The predicted ground responses at chainage Ch. 1123 of southbound tunnel are shown in Figure 7.

Vertical deflection at the tunnel crown in the order of 14 mm

Invert heave of above 8 mm at stage 1 top heading excavation

Figure 7. Numerical simulation of staging excavation

INSTRUMENTATION MONITORING

The rock tunnel sections were excavated by conventional blasting method using cartridge and bulk emulsion explosives and non-electric detonators. In the poor and extremely poor rock masses, and where blasting and bulk explosive was not practical on the basis of high levels of blasting vibration in the poor ground, mechanical excavation by stages was carried out, with

supports installed close to the face for limiting ground deformations and convergence, and ensuring that ground movements would not exceed the preset limits.

Monitoring and the facility to upgrade support in the light of any adverse or unexpected ground conditions were also essential parts of the design and construction process. Monitoring and instrumentation along the alignment of the tunnel were established on the basis of the detailed design. Site monitoring during the whole construction process was controlled by relating the derived figures to alert, action and alarm levels. The instruments were installed both within the tunnels to monitor the ground behavior, and above the tunnels, particularly near the portals, in close proximity to the WSD facilities and the existing buildings and slopes, in order to monitor the effects of tunnel excavation by blasting. Prior to the commencement of construction, instrumentation at some critical locations was installed to establish baseline readings to monitor the ground movements and vibration during tunnelling. Over 400 instruments were installed outside the tunnel and over 2000 monitoring points were installed inside the tunnel for monitoring of tunnel construction. The total number of each type of instruments installed is shown in Table 1. Instruments installed outside the tunnel are shown in Figure 8.

Table 1. Number of instruments installed for monitoring of tunnel construction

Instrument type Total number

Standpipe piezometers 35

Vibrating wire piezometers 10

Vertical inclinometers 1

Horizontal inclinometers 2

Multiple-points borehole extensometers 5

Ground settlement markers 224

Structure settlement marker 99

Structure tiltmeter 5

Utility monitoring points 3

Telltale 5

Vibrograph in borehole 7

Vibrograph 36

Air overpressure monitoring 3

Flow meter with V-notch weirs 43

Convergence monitoring 2000+ *

* Multiple arrays of convergence monitoring points at each blasted profile after advance by blasting.

Standpipe piezometers and vibrating wire piezometers at selected locations were installed to monitor the groundwater levels and to prove the performance of the temporary drawdown. The major underground monitoring consists of convergence measurements, groundwater measurements and vibrations from blasting operations.

As the instrumentation monitoring was numerous in quantity, a database was developed for storage all the instrumentation monitoring data. This also allowed data trends for individual instruments to be reviewed. This assisted in the operation of the alert, action, alarm system and also demonstrated when the instrument response was stabilized. The monitoring frequency could then be reduced and bring focus on the sensitive areas.

Groundwater Measurements

Groundwater levels were obtained over two seasonal durations from the standpipes and piezometers and have been analysed in conjunction with meteorological conditions over the seasonal durations. Hence, the groundwater contours were developed.

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8.

In

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men

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pla

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All significant ground water ingress was monitored and the inflows have been measured in segments of 100m in length over the full length of the tunnels. Measurements obtained are expressed in litres/min/100m for each designated segment. Groundwater levels have been obtained from standpipes/piezometers installed in boreholes. Initial readings and past records have formed the basis of a baseline data set. If any monitored groundwater levels for any geotechnical feature should exceed its ‘action level’ for any of the three days immediately preceding a proposed blast, then appropriate action at the tunnel face was taken, such as grouting of the rock mass.

Figure 9. Plan showing developed groundwater contours (during excavation and

construction of tunnel)

Figure 9 plots the piezometic data obtained when the permanent tunnel concrete linings are almost in place. The phreatic profile is found slightly altered with the tendency for the gradient to lower towards the toe of the tunnel portal formations.

During the actual excavation the ground water inflows encountered were with only two exceptions within the specified limits. Those two exceptions were treated by pre-grouting at the excavated face to reduce the inflows below 36 litres/min on any 100-metre length of tunnel. Drawdown from the Kowloon and Kowloon Byewash reservoirs was not encountered during the excavation or concrete permanent lining operation, and water inflows to the tunnel ground water drainage system are low (see Figure 10a and 10b respectively).

Convergence and Deformation Measurements

The tunnel excavation profile tends to close under the geostatic stresses of the surrounding rock mass and, depending on the ground characteristics, this effect may cause either reduction of the excavated area or collapse. Thus, a basic requirement for the excavation of a successful tunnel is the control of section closure or of its instabilities, with the establishment of a detailed monitoring system to record tunnel deformation during the excavation and indicate the need for additional measures if convergence exceeds acceptable preset levels.

Figure 10a. Charges of water storage in

reservoirs

Figure 10b. Groundwater inflow

measurement

The tunnel support shells react to the stresses of the surrounding rock mass tending to reduce the excavated opening and this reaction induces a time-dependent deformation of the various sections. An attempt to explain the relationship between ground and lining requires the detailed recording of their interaction and the analysis of the monitoring data in terms of time-dependency, tunneling method adjustments and local conditions. The geodetic observation of tunnel deformation provides data concerning the behaviour of the excavated area and permits the understanding of the kinematics of the support shell.

Within the tunnels, convergence monitoring during excavation has been carried out using 3 or 5 point arrays, and the monitoring locations and frequency were determined according to the excavation stages and the support class. The least ground cover section at chainage about Ch. 1123 of southbound tunnel, the crown settlement after top heading excavation was increased up to about 15mm and converged after bench excavation (see Figure 11).Comparison measured ground response showed a reasonable correlation with predictions. In addition, two horizontal inclinometers were installed above each shoulder of southbound tunnel at a distance of 2.5 metres in length of 40 metres from the south portal face and the multiple-points borehole extensometers were installed immediately about the tunnel crown to monitor the ground deformation particularly for this tunnel section with low ground cover.

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17/02/2005

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Date

Crown S

ettlement (m

m)

CH. 1120

Top heading

excavation

Bottom bench

excavation

Figure 11. Measured deformation of tunnel crown at Ch. 1120 of southbound tunnel

Vibrations from Blasting in Tunnel

The blast-induced ground vibrations were monitored using vibrographs at the sensitive receivers in the vicinity of the blasts. Preliminary instrument locations are shown on site wide

monitoring and instrumentation layout plans. The locations and numbers of instruments shown on the site wide layout plans were a general regime of instrumentation, and during the actual blasting operations, the positioning of the surface mounted instruments was reviewed on a “blast by blast” basis to ensure that vibration monitoring was applied directly between the blast location and the adjacent to respective sensitive receiver on the surface.

For sensitive receivers that were underground such as the High Island Water Tunnel bend ‘B’, which was 30 meters under the ENT North Portal, blast monitoring instrumentation was installed in boreholes sunk to the same depth as the deep sensitive receiver being monitored.

Instruments on particularly sensitive receivers such as water retaining structures were commissioned before the blasting face was less than 50 meters from the sensitive structure and maintained operational until the blasting face had moved 50 meters from the structure. Vibration from each blast was reviewed and propagation constants for the rock matrix were constantly reviewed in conjunction with vibration levels sustained. Air overpressure measurements were also recorded and hence overhead and airblast was kept limited.

In addition to the blast vibration monitoring, pre-construction condition surveys, and regular weekly surveys were carried out by the Independent Blast Consultant (IBC) employed by the Contractor in order to check for damage due to blast-induced vibrations. The regular surveys included the monitoring of tell-tales and crack-meters installed on the sensitive receivers. Particular attention was placed on the identification of new cracks, the opening up of old cracks, or the initiation of tension cracks. A record of each subsequent survey was prepared and submitted to the Engineer. Pre-construction and subsequent condition surveys for features within the WSD Special Protection Zone were carried out in WSD’s presence and were also submitted to the WSD’s Consultants. In addition the scope of the IBC included:-

(a) recommendation for continuance or halt to blasting works at that particular development face;

(b) recommendation for amendment/adherence of the propagation constants for determination of allowable charge weights; and

(c) recommendation for the amendment/adherence of the current blasting methodology.

The review discussed was certified by the IBC prior to submission to the Commissioner of Mines and the Engineer for approval. Blasting at that particular development face proceeded after the Engineer’s approval was given to the Contractor’s blasting designs.

Action levels due to blast vibrations assigned following the IBC’s assessment were subject to the approval of Commissioner of Mines and the Engineer. Additional approval was required from the WSD’s Consultants for blasting works within the WSD Special Protection Zone.

For any portion of the blasting works falling within 50 metres of the WSD Special Protection Zone, the Contractor submitted Special Blasting Designs that make full allowance to comply with the Contract requirements. Special Blasting Designs were fully supported by calculations, dimensioned sketches, documentary evidence and all means necessary to demonstrate that there was no risk of damage to the Water Supplies Department facilities. Special Blasting Designs provided the following as a minimum:

(a) layout, diameters and lengths of loaded and unloaded holes (including details of opening cut);

(b) quantity and type of explosive and stemming (including initiators); (c) explosives ratio; (d) firing sequence including delay types and timing;

(e) charge weight per delay – distance – anticipated PPV and similar relationships, developed for each of the WSD facilities; and

(f) information on rock strata

SPECIALISTS

Specialists play an important role in monitoring of the tunnel excavation. These specialists include engineering geology and geotechnical engineering construction staff, blasting engineer, Registered Shotfirer, Independent Checking Engineer (ICE) and Independent Blasting Consultant (IBC) appointed by the Contractor, Resident Site Staff (RSS) and the specialists in the design office appointed by the Employer and the specialists consultants appointed by the WSD.

Different Types of Specialists and Their Roles

The above specialists are categorized below in terms of the level of technical skill and experience required for them to perform their duties.

ICE shall check the Contractor’s works and then submit to the Engineer for the following areas of the works prior to execution on site:-

(a) initial supports and ground stabilization for tunnel excavation and corresponding instrumentation scheme;

(b) tunnel excavation and groundwater control proposal;(c) certify face mapping, grouting and day to day monitoring of the works before

submission of these documents to the Engineer;(d) any other remedial work proposals as instructed by the Engineer or any

adjustment in the Contractor’s method of working to suit the probing water inflow and monitoring results; and

(e) pre-construction condition surveys and subsequent periodic surveys for the duration of the Works. The ICE shall certify the results of the surveys before submission to the Engineer.

IBC whose role is the same as that of an ICE to undertake the following:- (a) review and certify blasting designs including trial blast programmes and other

special measures that may be required for sensitive areas; (b) attend initial and all subsequent conditions surveys and certify the results; (c) participate in supervision of drilling and charging and give final approval to blast; (d) review and certify blast remedial and contingency measures; (e) review and certify protection works; and (f) review pre-blast and post-blast records and monitoring instruments to

independently monitor the blasting

The Contractor submitted a list of the personnel nominated for the tunnelling works as required in the contract and employed qualified personnel and carry out all the certification and recurrent training in accordance with the Factories and Industrial Undertakings (Confined Spaces) Regulation.

RSS are essential in any large civil engineering contract to ensure that the construction procedures and quality are in accordance with that specified in the contract drawings and specifications.

WSD’s Consultants to ensure the tunnelling works to be carried out have complied with

the requirements for any part of the works falling within the WSD Special Protection Zones (WSDSPZ).

DISCUSSION

Two main tunnels of approximate 2.1 km long and of maximum 19 m span, pedestrian cross-passages of approximate 4 m span at every 100 m centres and a 400 m long ventilation adit tunnel were successfully excavated. A tightly controlled excavation and comprehensive monitoring scheme was implemented during tunnel construction. Ground movements and groundwater measurement during the excavation showed reasonable correlation to the estimation.

Through out the tunneling works, all parties especially the Contractor take on a significant role of protecting the WSD facilities and all other structures close to the tunnel from many risks and threats posed by drill and blast method for the large span tunnels excavation. It was therefore considered that the specialists played an important role in monitoring of tunnel excavation.

The management of such comprehensive monitoring data and continuously reviewing the data from time to time in the tight time schedule of tunnel construction work is an extremely critical event. Experienced that extensive resources and time input are essential, in order to ensure that there are adequate safety margin or the need for mitigation measures. If unstable ground condition encountered, the people would always question that is it moving? what is the direction of movement? what about the magnitude of movement? any groundwater drawdown? etc. It is therefore advisable to enforce in the contract to develop a database in order to process data and produce graphical plots daily, desirably after each production blast, for checking against the predicted.

REFERENCES

YANG W.W., LEUNG Man-Tim and LO Joseph (2003). Temporary supports for tunnels of

trunk road T7 in metasediment ground. Proceedings of the 23rd HKIE Geotechnical Division Annual Seminar, pp. 296 – 303.

Roscience (2001). Phase2: Finite element program for calculating stresses and estimating

support around underground excavations, 1998-2001 Rocscience Inc.

ACKNOWLEDGEMENTS

This paper is published with the kind permission of the HyD of HKSAR who the authors would like to thank. The views expressed in this paper are those of the authors and not of the HyD.