Environmental engineers: Mott MacDonald Pettit
North side engineers: Spanish consultancy Epitsa and PH McCarthy (WYG)
South side engineers: Arup and CFCSL

The N25 Waterford River Suir Bridge is proving to be a goldmine for geotechnical engineers in terms of the foundation’s micropile design and innovative tension stress testing

Although structural codes attempt to keep up with the latest developments in engineering design, enactment is naturally always one step behind. So what happens when the British Standards let you down, when the Eurocodes just won’t do, when the design you have in mind won’t fit into tidy boxes?

Like it or not, as an engineer in Ireland your expertise won’t guarantee swift approval of an innovative design by the authorities. Such was the lesson learned by the Spanish designers working on the N25 Waterford Bypass, a €600 million awarded to The Celtic Roads Group (Waterford) Ltd., a consortium formed by Spanish contractors Dragados, NTR, and Royal BAM (Ascon).

The structural engineers responsible for the cable stayed bridge design were surprised to find bureaucratic hurdles delay their proposals on how to deal with the foundations.

The proposals were submitted to the National Roads Authority at the end of the summer of 2006 and were only approved in the spring of 2007. In Spain, such approvals typically only take a couple of months.

Yet it is important to note the reason for the delay is that so many parties are involved in the process, namely the designers, geotechnical and structural checkers, contractor and subcontractor, administration and appointed consultant for the administration.

Approval took time and was only reached after everybody agreed on the principles and on the details of the proposed solution, which somewhat evolved during the process.

At the centre of the standardisation issue is an innovative micropiling system currently used in America but not in Europe, and which therefore hasn’t registered on the British Standards or Eurocodes scale. Bridge designer Miguel Astiz of Carlos Fernández Casado S.L. (CFCSL) consulting
engineers considered a number of alternatives before opting for the large diameter micropiling system.

“At the beginning we only had partial information about the soil conditions and as we got more information we arrived at the conclusion that this kind of deep and wide piling was the best option,” he says.

The soil conditions, which he describes as “chaotic” were such that design and construction became quickly interlaced, as it commonly does. The issue here is that traditional foundations couldn’t be availed of because the hard rock wouldn’t have been able to bear the load.

This article is an extract from an article in February edition of construction Engineer - Click HERE to Subscribe Today!