Shani Wallis, TunnelTalk

Linking controlled excavation systems with real-time movement measurements to set triggers to activate a toolbox of mitigation measures is the goal of engineers in The Netherlands to assure promised trouble-free excavation of the North-South Metro line in Amsterdam.

The alignment of the Hubertus tunnel in The Hague is the first in The Netherlands to pass directly underneath buildings. For the engineers involved this is a testing time. It is not the first large diameter soft-ground TBM bored tunnel in the country. The Netherlands is home to a series of road and rail bored tunnels, but this is the precursor to driving TBM tunnels beneath the heart of Amsterdam for its new North-South Metro Line.

Those metro tunnels pass within meters of urban infrastructure and buildings that identify the Dutch capital and with memories of riots, ignited by the demolition of old buildings to make way for open-cut construction of the East Line in the 1970s, still raw in the minds of the general public, a trouble-free boring of tunnels for the North-South Line is an absolute must for planners and politicians.

Computer-controlled systems for achieving maximum control excavation are now built into the latest generation of closed-face TBMs. Along side this, methods of measuring the slightest of ground and building movement as a result of tunnelling have become highly sophisticated. As a direct extension, the set of mitigating measures to prevent the damaging effects of ground loss or subsidence has expanded impressively to include actions that can be applied by the TBM, within the tunnel, and those affected from the surface.

The objective of work undertaken in The Netherlands is to link all these together into a seamless fabric of responsibility; to remove the compartmentalisation of duties; to have all information available to everyone at the same time; and, perhaps controversially, create "one captain of the ship" in the TBM operator.

Development of the strategy in The Netherlands has been spearheaded by consulting engineering firm Wittenveen + Bos and more specifically by engineer Frank Kaalberg and his team of technical wizards and computer experts. The work began about five years ago when the company was engaged in the design team for the 3.8km long twin bored tunnels for the North-South Metro Line in Amsterdam. The firm was contracted to prepare the geological investigations; predict ground behaviour and potential subsidence using state-of-the-art finite element analysis; survey all the structures within the zone of influence; judge their tolerance to movement; develop a project-wide monitoring system; set movement trigger levels; and design a toolbox of applicable mitigating measures. This entire package of project services, starting at the earliest days of project planning, continuing through the full construction period, and being among the last of the project services to close out, is known collectively as the Geographical Information System (GIS).

Extensive investigations, based largely on cone penetration tests into the marine silty clay beneath Amsterdam, resulted in detailed geological sections for the metro's entire subsurface route comprising twin running tunnels, three open cut stations, and a TBM launch and reception shaft. The installed system for monitoring movement comprises 74 Robotic Total Stations (RTS), which are fully automatic theodolite reading and data collecting units, plus 7,000 prisms or monitoring points attached to buildings. RTS readings are taken automatically and in continuous cycles. The collected data - augmented with data collected from extensometers, inclinometers and other instruments installed at extra sensitive zones - is transferred wirelessly to a central processing computer that collates the information into various graphic and tabulated forms for distribution to authorised linked computers for instant real-time information delivery within one hour of the original survey. This allows a rapid response to any trigger alarms and activation of any mitigation measures required. In more sensitive reaches, where settlement predictions are high, mitigation is based on pre-excavation intervention systems such as compensation grouting.

The purpose of all this instrumentation, data compilation and delivery, and designed mitigation methods, is to complete the entire metro excavation to within ±2mm of project induced movement. Much of this control can be achieved by correct and skilled operation of a modern closed-face TBM system, the computer controls of which are today more akin to the controls of a jet airliner.

The North-South Line was to be the initial application of the GIS settlement control/damage limitation programme. However, due to various causes for delay, TBM tunnelling on the North-South Line is yet to start. The tunnelling contract has been awarded to a JV led by Züblin AG and excavation of the underground stations and the launch and reception shafts is well advanced. Tunnel excavation is programmed to start in 2009 and the two 7m diameter TBMs needed for the contract are expected to be ordered later in 2007.

In the meantime, the GIS system has been applied to the 1.5km two-lane, twin tube Hubertus urban road tunnel project in The Hague.

Fig 1. Hubertus twin tube road tunnel alignment

The Hague test run
The Hubertus Tunnel will close the northern section of the ring road around The Hague. Original plans were for an open cut operation. The alignment however passes through an exclusive residential area, beneath a city park, through an artificial sand dune built in the 1930s, under a major north-south surface road, and beneath a national military complex that will accommodate construction of new offices for the International Court of War Crimes and of the world's International Criminal Court (ICC). Before this construction can begin the northern section of the ring road must be completed.

Considering the location and the urban logistics, city planners rejected the initial open-cut design and specified instead a bored tunnel alternative. The first of the 10m diameter twin tubes runs under a narrow street and within 8m of private homes (Fig 1). The second tube is aligned only 5m, or half a TBM diameter, to the north, under the military complex property and directly beneath some of its buildings. Most of these buildings are 60 to 80 years old, built of brick, to three or four stories high, and are founded on plate footings on the area's very well compacted dune sand geology.

The project's 10.53m diameter Mixshield

Working with a single public entity as a third party to the project has been a major advantage and while buildings over the second drive to the east of the dividing surface road are to be demolished to make way for the new ICC, those to the west of the road must remain undamaged. Some are of historical importance and others house sensitive military equipment. The GIS design imposes limitations of no more than 15mm of surface settlement. Damage to buildings caused by settlement of 15mm or less will be corrected at the City's cost. Repair of damage caused by settlement of more than 15mm is at the contractor's cost. To ensure maximum control of excavation through the waterbearing compacted dunesand, the owner's design specified the use of a slurry TBM system.

The conventional design-bid-buiId tunnel contract, including extensive cut-and-cover ramps, other appurtenant works and internal M&E fittings and finishes, attracted four bids, all of which were within a 5% range and all about US$20 million below the engineer's estimate. The contract was awarded for a bid price of US$127 million in early 2004 to a JV comprising BAM CMeI, Van Hattum & Blankewoort and Wayss & Freytag which is owned by the BAM conglomerate of The Netherlands. To comply with specifications, the JV ordered a 10.53m diameter Herrenknecht slurry Mixshield and the segments of the precast segmental lining are being transported from a facility in Germany.

In the project design group, the Witteveen + Bos team is managing all the GIS monitoring and mitigation requirements. As in Amsterdam, there is a great deal of instrumentation on the Hubertus project including seven fixed RTS (Robotic Total Stations) and 14 that move with the tunnel advance. As a new development, monitoring is based on reflectorless survey, the theodolites scan the surface of structures rather than pin pointing prism reflectors.

Fig 2. Illustration of the GIS instrumentation installations

The installed system on the Hubertus project began recording surveys about six months before boring started. As tunnelling advanced, the equipment was set to survey zones of 105m long, from 70m ahead of the TBM cutterhead to 35m behind the tailshield and in a corridor of about 25m wide over the drive. Subject to setting adjustments, a survey sweep is taken and the results are made available on-line within one hour, which is twice for each 2-hour, 2m long excavation and ring build cycle.

This produces a huge amount of data that is processed by high capacity computers and fed back wirelessly via secure internet to the offices of all project parties involved and, importantly, to the TBM operator in the operators' cabin on the TBM back-up.

Captain of the ship
With real-time indicators of ground loss and surface settlement data available to the TBM operating cabin and within an hour of the readings being taken, the role of operating the machine is brought directly into the front line of settlement prevention management and responsibility. This is done by combining the data of the GIS monitoring system with the TBM operating data to provide for the safest possible advance of soft ground TBM tunnels beneath developed urban environments.

The Mixshield's plc electronic operating systems, developed by Herrenknecht's subsidiary VMT, provides data from all possible sources of operating parameters such as working pressures in the excavation chamber and forward thrust pressures etc, as well as in-tunnel surveys, guidance and navigation information, ring build sequencing, and segment erector operations. It also incorporates operating information from the slurry separation plant of the Mixshield and operating data of the annular grouting volumes and pressures.

The last major element to be combined into the data delivered to the TBM operator is the surface settlement monitoring data. This is the step accomplished on the Hubertus project and to be carried forward to the Amsterdam North-South Metro Line tunnels. Integrating GIS data with TBM operating data was a three-stage process that was explained to TunnelTalk by Joost Joustra, the GIS tunnelling representative for Witteveen + Bos on site.

Fig 3. The prisms on the buildings

The first stage was calculation of plenum pressures and annular grouting pressures expected to be needed along the complete tunnel alignment, depending on geological conditions and structures within the zone of influence all assessed in advance of excavation starting. The second important stage is constant analysis and evaluation of actual settlement performance in relation to TBM settings by the contractor's head of tunnelling. On Hubertus, this is Thomas Schubert of Wayss & Freytag. This analysis produces instruction sheets that are sent to the TBM and direct settings to be used for 5 to 50 rings at a time, depending on the TBM location in the drive. The third stage is managed by the shift engineer on the TBM who keeps an eye on the GIS data screen in the operators' cabin and is free to deviate from the instruction sheets if he finds it necessary and instructs the TBM operator accordingly. " Delivering measurement data within 1-hour of taking the readings was essential to getting high quality data of settlements as they are actually happening", said Joustra. "The availability of this kind of information and its distribution and visualisation via the internet was very important and is the innovative element of the GIS system."

In parallel, VMT is developing its own data integration system for Herrenknecht TBMs. Called the CBP (Controlled Boring Process) or tunnel construction information system, the objective is to integrate TBM operating data with data supplied by settlement monitoring data but uses a different operating software package. Early versions are being used on the Herrenknecht TBM projects in Rotterdam and Leipzig.

"The process of integrating settlement monitoring instrumentation with TBM operating data certainly does increase the level of responsibility on the TBM operation," said Paul Janssen, Project Manager of the Hubertus Tunnel for project owner, The Hague City government. "Surface settlement induced by the TBM tunnelling process is caused about 50/50 by the excavation process at the front of the TBM and by the annulus back-filling operation as the rings of segmental lining leave the tail shield at the back. Maintaining correct operating pressure in the plenum and avoiding excessive over cut by the cutterhead will prevent ground losses during excavation and grouting through the tailskin and maintaining correct annular grout volume and pressure will prevent settlement at the tail skin. Both actions are under the control of the TBM operation. As a result, the speed of excavation, the control of settlement, and the quality of the lining, is under the control of TBM operation. The skill, experience, and proficiency of the shift engineer and the ultimately of the TBM operator himself will be revealed in the record of all three."

GIS data in the operator's cabin makes the TBM operator the 'one captain of the ship' and in the front line for controlling surface settlement

So when asked, can you determine who was the TBM operator by looking at stored data, an emphatic 'yes' was the reply. The expertise of the TBM operator was explained as vital to the success of the project for both the client and the contractor.

"This is a change of paradigm", said Janssen. "The full GIS, comprising the geological site investigation, ground behaviour predictions, structural surveys, geotechnical instrumentation, monitoring systems, data collection, processing and delivery systems, accounts for some 2% our total tunnel project budget but consider what that is buying. It is nearly impossible to repair the image of a project or of tunnelling if a major problem occurs during the excavation process. For the contractor, costs of settlement problems are three fold. First he must repair any damage to the segments or any sub-standard ring builds. Secondly, he is responsible for repairing any damage to buildings caused by surface settlement beyond the contract benchmark of 15mm. Thirdly there is a hefty penalty for exceeding the settlement benchmark - a penalty that is more than the estimated cost of repairing any structural damage."

As a test run of the integrated GIS+ TBM data delivery systems, the Hubertus experience has been positive. The first 1.5km long drive was completed from July to November 2006 and the second drive broke through at the end of May 2007 after starting in February. Over the alignment of the first drive, surface settlement directly above the TBM centre line has been controlled to within 10mm and on average below 7mm. Over the second drive, settlement control has been the same. "Exceptional results", said Janssen, "and well within the 15mm contractual limit."

Fig 4. Reading of settlement as recorded by the GIS system

In responding to recorded data, a trend of 1-2mm is considered within the range of 'scatter error' (perhaps a bird alighting on a fixed reflector prism) and therefore requires no immediate response. Above this, movement of 5mm is the first trigger level, with accumulated settlement to -10mm setting off the second attention trigger. "Of course there is a significant difference between gradual movement and rapid and dramatic change", said Janssen. Any instantaneous change sets off a chain of alarms that activates emergency intervention procedures, the most immediate required by the TBM operator to shut down excavation and try to re-establish the pressure balance.

There were no emergency situations encountered on the Hubertus project, apart from a sinkhole at the end wall just before breakthrough of the second drive and under a minimum 3m to 4m of cover. ''This caused no damage and can be considered one of those incidents that can occur without warning under very shallow cover," said Joustra. "There was no surveying of the surface being undertaken at the time, and even if there had been, surveying data would not have warned of the incident. It was too immediate. For the majority of the tunnel drives however, cover above the TBM was 10-15m providing ample time for gradual or 'creep' excavation-related movement over the 105m of zone of progressive monitoring to be recorded. It was taking on average some five to seven days for the TBM to advance through a 105m zone of monitoring and the maximum settlement of 10mm took a span of 24h to develop."

"Mobilising pre-excavation mitigation measures from the surface, such as compensation grouting, is very expensive", said Frank Kaalberg for Wittenvenn + Bos. ''The decision to apply such measures depends on the value and importance of the building. It is often less expensive to rebuild a building than to apply damage prevention mitigation measures from the surface. The most cost effective method of achieving maximum settlement control is via operation of the TBM. Providing the TBM operator with all the geotechnical infonmation available, and within the shortest possible time from taking the initial surveys, is the objective of the GIS strategy."

Breakthrough of the Herrenknecht Mixshield

As with all new procedures, there is a period within which everyone becomes familiar with the new routines and responsibilities. Introducing appropriate responses to monitored settlement data is a work in progress. An incident in Amsterdam on early open-cut excavation of an entrance adit into one of the underground stations illustrated the point. Data recorded during installation of the sheet piled support walls around the adit excavation indicated that all appeared as anticipated, but as core excavation advanced, data began to log a steady indication of settlement. When this was brought to the attention of the contractor, the response was that all in the excavation was fine; that there was no indication that the excavation could be causing the settlement readings.

When the readings took a steeper fall from -18mm to -30mm over a period of a few days, the concern was that water and soil was flowing into the excavation area through a gap in the piled wall below the excavation invert. This proved to be the case when the contractor was instructed to stop and investigate the situation - but not before a superficial crack appeared in the wall of an adjacent three-story high building as a result.

The rapidly advancing capabilities of electronic systems to monitor, record, deliver and store digital infonmation is having a quiet, but profound, impact on the art of ground excavation. Ensuring that contractual practices and the attitudes of all major players evolve as quickly, to achieve truly faultless tunnel excavation is as important as bringing TBM operators up to speed with their expanding duties and responsibility.

References

TBM underway for Amsterdam Metro - TunnelTalk, Apr 2010

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