At 57.5’ in diameter, Big Bertha is the largest tunnelling machine built to date.
By Gavin Schrock, PLS
V1, July 20th 2013
The Seattle Tunnel project, which will provide a traffic bypass underneath two miles of downtown Seattle, got an official launch with a dedication ceremony for Big Bertha, the largest diameter tunnelling machine ever built. Folks can follow the progress of Bertha with her own twitter feed (Bertha @BerthaDigsSR99) but more seriously, the progress of Bertha will be tracked by an army of surveyors using a combination of some of some of the most precise and innovative tools and methods ever marshalled for a project of this scale.
Bertha weighs in at 7,000 tons, is 326’ long, and is 57.5’ in diameter. The team of contractors, dubbed the Seattle Tunnel Partners is led by Dragados USA, that has experience with massive tunnel project worldwide, with much of the surface and utility monitoring being performed by Soldata. The team has enlisted the services of a small army of local surveying firms and surveyors. In addition to the conventional level runs, there is a seismic team, local geotechnical firms, and a very complex and innovative system for monitoring the impacts that the tunnelling may have on historic buildings and critical utilities.
There is a type of “plough wave” effect above tunnelling activities of this scale. Key monitoring is being performed by Soldata, a company that has perfected their craft on similar tunnel projects in Barcelona, Amsterdam, and other sites worldwide. Soldata has deployed an array of robotic total stations integrated with other sensors on buildings along the tunnel path. These sensors, dubbed “Cyclops” check in with each other, perform a least squares adjustment to mitigate for any incidental movement of the mount or building, and then run automated series of observations looking for deflections of critical structures and utilities. Soldata has also pioneered a new way to use satellite-based INSAR (radar images) to perform on-going surface change detection. This is not a simple tunnelling project; the downtown Seattle corridor is subject to various types of settling and tectonic movement, plus a critical seawall will be upgraded at the same time and the aging Alaska Way Viaduct has seen its own issues; the tunnel will serve as a replacement for the viaduct.
Washington State Governor Jay Inslee takes a look at Big Bertha at the public dedication event.
“We love surveyors so much we named our state after one” was Governor Jay Inslee’s response, when I asked about his views on surveying and surveyors. The governor had operated heavy construction equipment to pay his way through college and is keenly aware of the role of surveyors in building and maintaining critical transportation infrastructure. He is also a strong supporter of wind and solar power generation development.
The two tiers of the tunnel – two lanes each way, one above the other, will be house within the inner 55’ of the tunnel. Attendees of the event were invited to put their signatures on the first curved pre-cast sections of the tunnel lining that Bertha will automatically snap into place as it progresses under the city. Bertha will reach a maximum depth of 200’ below the surface during an expected 14-24 month dig.
PSM will be running detailed features about the monitoring systems and construction surveying in future issues.