January 27, 2004

Tunneling with Laser Scanning

Laser scanning helped contractors on the Dublin Port Access Tunnel avoid a half-million-euro problem that surveying with total stations alone had failed to prevent.The Port of Dublin handles two-thirds of Ireland’s seaport trade by value. Some two million trucks travel to and from the port each year, a number expected to grow. Because of the port’s location, these trucks have to use surface roads to reach the M50, the main thoroughfare to the rest of
Ireland. In 1994 the Government decided to proceed with plans to build a 5.6km tunnel at an estimated cost of €650 million, to streamline the flow of goods and tourist traffic to and from the port. The goal is to reduce heavy goods vehicle traffic in the city center, improve the city center
environment, and reduce traffic in residential areas.

In October 2000 a consortium of Nishimatsu Construction Company Ltd., Mowlem & Company PLC, and Irishenco Construction was awarded the main design and construct contract for the project. The tunnel project includes 2.6km of bored tunnel, 1.9km of cut-and-cover, and 1.1km of surface road. The bored portion of the tunnel consists of two tubes, one running north-to-south and the other south-to-north.

Surveying with total stations failed to detect clashes

In boring the first tunnel, the north-to-south tube, Nishimatsu experienced clash problems in the 60m long x 12m diameter “adit” – tunnel-speak for horizontal entrance. Essentially the tunnel boring
machine (TBM) got stuck in the adit, and took a more than a week to free before boring operations could resume. This resulted in the loss of approximately half a million euros, and schedule delays.

This was not supposed to have happened. Before boring commenced, the adit had been surveyed using conventional total station means. However, the survey grid specified was too coarse to capture the clashes between the shotcrete-covered tunnel walls and the TBM.

After freeing the TBM and completing boring operations for the north-to-south tube, Nishimatsu decided to conduct a more thorough survey of the Fairview adit – the entrance to the south-to-north tube – before
starting boring operations on that tube.Before starting the second tunnel, the contractor asked for a laser scan survey

In June 2003 Maptech 3D Solutions was called in by Nishimatsu to provide a 3D laser scan survey of the Fairview adit for potential clashes – just two days before tunneling was due to commence. According to managing director Mark Phelan, Maptech 3D Solutions was the first surveying firm in Ireland to own a 3D laser scanner.

Maptech surveyed 100m of the adit with a MENSI GS100 scanner. For scan registration, 5 targets were observed with an SEC Robotic Total Station. Total scan time was 1 hour and 40 minutes. The point clouds were registered and georeferenced in MENSI RealWorks 4 database.

After scanning the adit with a MENSI GS100, surveyor Maptech used MENSI RealWorks software to detect clashes and identify clash coordinates

Maptech was supplied with the coordinates of the centerline for the Fairview adit, the DTA (design tunnel alignment), and the tunnel diameter specifications at 50mm intervals. From this information, the
company produced an AutoCAD model and exported this via DXF to the MENSI RealWorks database. Next, the areas of clash between the scan data and the DTA specifications were identified using the software’s 2D inspection tool, and clash coordinates were identified using the measure tool.

The clash coordinates were then supplied to the survey crew for marking and subsequent remediation. This method was particularly effective because not only were the coordinates of the clashes provided, but also the depths. The clash areas were set out with a reflectorless total station, and broken out by tunnel engineers to the identified depths. This work was completed in one day, start to finish.The deliverable was a table of clash coordinates – laser scanning used as a construction tool

The key benefit of the laser scanning method was completeness and resolution of the data set – all potential clashes could be detected using the scan method. The problems encountered with the first tunnel adit point out the risk in relying on the coarse measurements typical of traditional survey methods.

Maptech reports that the speed of data capture achieved with the MENSI GS100, and the rapid processing within RealWorks, were also important. These allowed fast delivery of the clash coordinates to the contractor, and resulted in significant cost and schedule savings over the tunnel surveyed with total stations alone.

While AutoCAD and MENSI RealWorks were the software tools used in executing the project, the project deliverable was not a CAD model, but instead a table of clash coordinates and depths that was used by construction crews to mark and remediate the clash areas. This underscores the utility of laser scanning as a construction tool.

This material is excerpted from Spar Point’s just-released publication
Capturing Existing Conditions with Terrestrial Laser Scanning: A Report on Opportunities, Challenges and Best Practices for Owners, Operators, Engineering/Construction Contractors and Surveyors of Built Assets and Civil Infrastructure

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