swissMon – An approach to 4D Monitoring of Tunnels in Urban Environments

Christian Meyer, Stephan Schütz

terra monitoring ltd., Zurich (Switzerland)

1 Introduction

Swiss Railways (SBB) have been constructing the Cross-City-Link in Zurich since 2007.

The heart of this project is the new underground station being constructed 16m below

the existing tracks and the river Sihl. From 2014, trains will be able to approach and

leave through the new 11.2m diameter twin track, mainly TBM-drilled, Weinberg tunnel.

It passes under the existing station, the River Limmat and parts of downtown Zurich and

continues for five kilometres to Oerlikon. Tunnelling projects within urban areas, such as

this, need special supervisory measures to meet safety requirements. A broad

monitoring project, using state of the art sensors and digital data communication, has

been deployed to identify potential hazards and mitigate their impact.

This paper introduces swissMon, the web based monitoring platform deployed

throughout the project to automatically gather, analyse and display over 390’000

datasets generated by geotechnical and geodetical sensors every day. By way of

example, 3D-Surface Monitoring using Total Stations and tShape Inplace Deformation

Monitoring (two of the core sensor technologies used with swissMon) are presented.

SwissMon has now been in use for 4 consecutive years at Zurich Central Station,

handling large data traffic 24 hours a day, 7 days a week, under tough construction site

conditions and with an exceptional success.

2 The Project

Swiss Railway Company (SBB) have commissioned a 2 billion Euro infrastructure

project, the Cross-City-Link, Zurich. Starting at Altstetten, the 9.6 km railway link will

pass below ground through Zurich Central Station and will continue as far as Oerlikon.

Zurich Central Station is at the core of the Swiss railway network, where commuter flow

is constantly increasing. Up to 300’000 passengers travel through Zurich Central Station

each day. SBB estimate that, the number of daily passengers can be expected to

increase to 500’000 by 2020. These passenger numbers cannot be handled using

standard supporting measures, such as optimising train flow rates, as they will exceed

the station’s capacity limits. Based on this knowledge, a decision was taken to

significantly extend the existing infrastructure.

Figure 1: Overview Cross-city link Zurich [1]

The heart of the Cross-City-Link is the second underground through-station,

Löwenstrasse. To the West, tracks pass across two new bridges (“Letzigrabenbrücke”

and “Kohledreieckbrücke”) to reach Zürich Altstetten. To the East, the “Weinbergtunnel”

connects Zurich’s Central Station with Oerlikon. The two new bridges between the

Central station and Altstetten will help to reduce congestion on the tracks to the West of

the station. The “Weinbergtunnel” will significantly increase the station’s capacity

towards the East. From 2014, trains using the new “Durchgangsbahnhof Löwenstrasse”

will be able to pass directly through the Central Station without having to change

direction.

The new “Löwenstrasse” through-station is being built 16 metres below the Central

Station’s tracks (platforms 4 to 9) and directly under the Sihl riverbed. As the existing

station is already operating close to its full capacity, SBB could not allow the

construction works to affect the existing train flow rates. Due to this, one of the most

important project prerequisites was that the construction method of the new station

should minimise its impact on the existing traffic. Therefore, it was necessary to choose

a top-down construction method for the buried structure.

Figure 2: Cross Section and Longitudinal Section showing “Löwenstrasse” Station [1]

Heading east, trains will leave the through-station on two tracks, entering the

“Weinbergtunnel”. The first part of the tunnel passes under the 150 year old landmark

Central Station building. Due to the complexity of the construction, this part of the tunnel

had to be drilled using a manually driven boring machine for approximately 220m. The

remainder of the 5km tunnel was drilled using a Herrenknecht convertible S-451

Mixshield (an 11.2m diameter TBM). Starting at Oerlikon, the TBM ran through Molasse

rock in an open mode for the first 4.1Km. During the final 245m the machine faced

unconsolidated soft rock and had to be converted to closed mode in order to pass under

the Limmat River. On the 22nd November 2010 the successful breakthrough of the TBM

into the target shaft was celebrated, completing the first important part of the new rail

link, which will be opened in 2014.

3 swissMon – Web-based Realtime Monitoring Platform

Tunnelling projects within urban areas have a special requirement for supervisory

measures. As they are constructed near to existing critical infrastructure, including

residential and commercial sites, any kind of incident can have a profound impact on

building structures, human safety and commercial logistics, such as traffic flows.

Furthermore, engineers can face unforeseeable problems and delays due to

archaeological sites and historic infrastructure, which might have been installed without

documentation.

In order to identify potential hazards at a very early stage and mitigate their impact, a

broad monitoring project using state of the art sensors and digital data communication

has been installed for the Cross-City-Link scheme. It is one of the world’s largest

monitoring projects, requiring a large variety of geodetic and geotechnical sensors.

These include:

Automatic Measurements Manual Measurements

- 80 Total Stations covering more than 3000 3D-Targets

- 850 Hydrostatic Settlement Cells - 50 Inclination Sensors - 30 Inplace-Inclinometers - 5 Systems for Water Quality Control - Piezometers - Achor Force Cells - Straingauges - Meteorological Station etc.

- Manual levelling covering more than 1000 targets

- Inclinometer measurements - Manual inclination measurements - Sliding Deformeter measurements - Rod Extensometer measurements - Chemical measurements etc.

The automatic measurements have to be carried out every 30 to 60 minutes

continuously throughout the project.

Additionally, the customer requested online access to the real-time measurement

results. Based on these demands and considering estimated data rates of up to 500’000

datasets per day, a decision was taken to develop swissMon, a cutting edge monitoring

platform. The structure of swissMon is completely modular and can therefore be scaled

to every kind of project and project size. It provides interfaces for all important

geodetical and geotechnical sensors. Due to its modern architecture, new sensors and

features can easily be integrated so long as a digital interface is available. For analog

sensors, a/d-conversion has to be performed by external hardware.

Figure 3: Modular Architecture of swissMon

The SwissMon Monitoring-System is divided into three basic units:

Sensor Unit (tMon):

All on-site sensors which perform automatic measurements are connected to a Node-

Computer. The Nodes are strategically placed near the sensors in order to reach as

many of them as possible. The Nodes take control of the sensors, automatically taking

the measurements within defined time periods and analysing the field data. The

validated datasets are temporarily stored on the tMon-Node and are regularly

transmitted to the database Unit via internet or internal network connection. Internet

connection can either be by telephone earth line or in special cases through a mobile

telephone network with a high bandwith. tMon already provides basic alarming features

by performing simple trigger tests. This makes it possible to provide near real-time

alarming on the site using sirens or flashlights.

Database Unit (tLis):

This unit stores all datasets measured within the project. Datasets can also be updated

with additional information and manually measured datasets. This unit is able to perform

complex triggering tests, which can be individually adapted to project requirements. In

the event that a trigger level is breached, tLis can automatically issue alarms to single

persons or groups of persons through email messages, SMS, facsimiles, pager calls or

voice calls. It can also confirm whether an alarm messages reached the required

recipients. The database unit runs on a server, which is located outside the project

vicinity. It is continuously backed up to ensure all data is secure. It also features a

watchdog functionality to automatically check if all attached tMon-Nodes are still

working.

Web Unit (tWeb):

All data is presented to an online user in a numerical or graphical format. All

stakeholders can access the data at any time by using their preferred Web Browser. By

using their personal login they have access to all necessary information to make

important decisions. tWeb features the following functionality:

Visualisation of data as timeplots, profiles, cross sections, surface plots etc.

Visualisation of trigger breaches.

Storage and access to Team Documents.

Direct access to Webcams on site.

Quicksearch of single measuring points

Selecting of targets by sections, subsections or trigger-status.

PDF- oder CSV-download of all data available

Data protection through secure personalised logins

Site engineers and site management staff are not able to address the entire vast

amount of data arriving from the comprehensive monitoring sensors each day (in

Zurich: 390’000 datasets). Allowing the ability to focus on the critical tasks in order to

make decisions at the right time, requires tWeb to provide information in a suitable and

comprehensive way to the pertinent staff.

Figure 4: tWeb Screenshot showing tShape Sensor Positions and Results

Measurement results can be visualised in tWeb in a number of ways:

Figure 5: Borehole Deformation (Inclinometer Measurements)

Figure 6: Real-Time easy to recognize Trigger Breaches

In Addition to the measured data, tWeb can also present secondary information, such

as the TBM advance, in order to facilitate data interpretation.

4 Sensors used with swissMon

The logistics required to operate a monitoring platform for a project of the size of the

Cross-City-Link, such as data communication and data processing need to be

addressed when designing the site specific system. Furthermore, the construction

methods can also provide challenges for the measuring technologies used.

Details about two of the instruments, selected from the many geodetic and

geotechnical instruments used in swissMon at the Cross-City-Link, are provided in

section 4.1 and 4.2.

4.1 Total Stations

This sensor is one of the most advanced instruments available for monitoring tasks. In

summer 2011, total stations were operating in more than 80 locations in Zurich Central

Station, monitoring 2800 Targets.

Significant planning had to be carried out to select the type and number of instruments

to use. The final solution was based on the project requirements, using the following

criteria:

Accuracy: 1 mm at 100 m for all 3 Dimensions.

Speed: 100 targets per 30 minutes in 2 faces considering an urban environment

Data transfer: Easy to use wireless data communication.

Option: Reflector-less distance measurements should be possible.

Operation: Term of Guarantee; Short term reaction for service and spare parts.

Although the operating conditions vary significantly throughout the project, taking into

account efficiency of labour and flexibility, the decision was taken to use a homogenous

fleet of instruments.

Figure 7: Benchmark Test with Total Stations in summer 2007

After a pre selection benchmark test was performed at the construction site in summer

2007 using total stations of 2 manufacturers. As a result of this test, Trimble’s S8 robotic

total station was adopted for the monitoring solution as the instrument has proven to

perform measurements at a higher speed and is more convenient for monitoring issues

than alternative options. Due to its MagDrive Servotechnology (low wear dive with a

rotational speed up to 128gon/sek) the Trimble S8 needs only 6 seconds to measure

one target. It also comes equipped with the Finelock-functionality. This enables the S8

to precisely target prisms located in a building line even over high distances, where

distance in the measuring plane is less than 20mrad. During tests the S8 was able to

automatically track prisms with a minimum distance of 25cm between each other,

located at a distance of 100m from the total station.

Figure 8: Total Stations at Zurich Cross-City-Link

To allow completely automated measurements with total stations, a special emphasis

was given to these instruments during the development of swissMon. Therefore,

SwissMon is able to control these instruments, perform least squares adjustment

(Gauss-Helmert Model) and plausibility checks, apply filters, estimate adaptive

parameters or even dynamically update reference point coordinates. It delivers

coordinates and deduced data.

4.2 tShape – In-Place Deformation Monitoring

Despite the power of total stations the Cross-City-Link project provided challenges

where other innovative measurement systems had to be introduced.

Only 245m before reaching the target shaft, the TBM drilling the Weinbergtunnel leaves

the molasse rock and proceeds into unconsolidated soft rock with only a few meters

overburden. Urban tunnelling has the special challenge requiring that ground

subsidence must be avoided. Therefore special equipment, such as Mixed Shield

TBM’s, are used to maintain the soil pressure during and after the tunnel construction. If

operated properly and the underground conditions are well known, it is possible to these

to reduce the risk of surface subsidence and voids.

During the advance of the machine, all important parameters have to be carefully

monitored. One of these parameters is the deformation (convergence) of the drilled

tunnel. As the main part of these deformations occur very shortly after face advance, it

is necessary to start measuring as early as possible.

A state of the art method to determine convergences are geodetic deformation

measurements using high precision total stations. As this method requires a free line of

sight inside the tunnel, it can hardly be used with TBM’s as they fill substantial parts of

the tunnel cross section.

At the Weinbergtunnel the TBM, together with the trailing support decks had a total

length of about 150m. Therefore, it was not possible to measure convergences during

the crucial phases using geodetical methods. As classic methods were unrealisable on

their own, tShape was introduced as an innovative measurement system for in-place

deformation monitoring.

Relying on the modularity of swissMon, tShape uses Measurand’s SAA-technology to

obtain its data. The SAA is a rope-like array of MEMS-based accelerometer sensors

and microprocessors, which fit into a small 30mm diameter casing.

Figure 9: Limited line of sight inside a Tunnel during TBM advance

Mechanically, SAA is an array of rigid segments connected by joints that permit bending

in any direction, but are stiff in torsion. Standard segment length is 305 mm, which

dictates spatial resolution. The hollow segments each contain three orthogonal MEMS

accelerometers. Every eighth segment includes a microprocessor.

Any deformation that moves the casing is accurately measured as a change in shape of

the array. The array was installed on the inner lining covering the upper 120° of the

tunnel full section.

Figure 10: tShape installation behind TBM head (left) / Staff checking installation (right)

Unlike conventional convergence measurements, it is not possible to use fixed point

control when using tShape. This is because all the measurement points within the

tShape “chain” lie within the area of deformation. This fact needs to be taken into

account when processing the data. In addition, the processing of the data needs to

include the following requirements:

Selection of the points to display in the convergence measurement output

Transformation to global (national) coordinates

User choice of transformation control points

tShape measurements are generally recorded in three dimensions. Conventional

convergence measurements are usually only recorded in two dimensions. Therefore,

one of the dimensions can be fixed when using tShape.

The fixed dimension needs to be in the plane of the tunnel cross section. The shifts or

deformations of the individual points can be determined using a 2D-Helmert

transformation.

The following formula is used:

Where:

Translation in x and y direction

Scale factor

Rotation

The transformation is carried out relative to the centre of each of the measured points.

The scale factor and rotation parameters can be calculated separately or combined

during the adjustment. A translation is calculated in each case.

The required number of transformations will vary depending upon the number of

transformation parameters. The maximum number of points is defined by the number of

available measurement points in the tShape chain.

Figure 11: Example of a plot of the transformation residuals

Weighting can be applied to the adjustment in order to modify the quality of the result.

This makes it possible to assign less influence to less reliable points in the calculation.

SwissMon determines the transformation vectors based upon the formulas given

previously. Subsequently, all the points in the measurement chain are transformed and

integrated into the national coordinate system, according to the transformation

parameters. The output data is supplied in the same format as the input data. The

residuals of the transformations are plotted in order to provide data control (see Figure

11). In addition, a log file is created, which also allows interpretation of the translation

outputs.

The system was set up as described to measure convergences during TBM advance.

The data corresponded to the predicted output. The system delivered reliable results

(see Figure 12). Any anomalies were detected and corrected with the help of the

generated log file.

Figure 12: Convergence measurement diagram from tShape data, shown in tWeb

Figure 13: Time Series showing convergences on one point during TBM advance

Even under difficult situations, tShape has shown to be a reliable system for automatic

in-place deformation measurements inside tunnels.

Figure 14: tShape installation in the Weinbergtunnel

References:

[1] Schweizerische Bundesbahnen SBB; Informationsbroschüre und Faktenblätter zu

den Abschnitten der Durchmesserlinie (www.durchmesserlinie.ch)

[2] S. Eisenegger; Alarmzeichen automatisch erkennen, BY RAIL.NOW! 2009,

Sonderpublikation der SWISS Engineering-Reihe.

[3] L. Danish; Measurand ShapeAccelArray (SAA), General description.

Authors

Dipl.-Geol. Christian Meyer

Dipl.-Ing Stephan Schütz

terra monitoring ag

Obstgartenstrasse 7, CH-8006 Zurich

meyer@terra.ch

schuetz@terra.ch

www.terra-monitoring.ch

www.terra.ch