Low-impact reinforcement of Norrbro bridge

Published 2010-06-14 00:00:00

Foundation reinforcement using jet grouting technology is not a very common practice in Sweden. With the exception of a few projects, the method has been used mainly for tunnelling, grout curtains and in sealing supporting structures. However, under the right conditions, foundation reinforcement using jet columns can be both economically and technically advantageous.

Norrbro, which is Stockholm’s oldest bridge (1807) and the only stone-arched bridge of its kind, has been settling for 200 years, ultimately resulting in cracks in the walls and arches, and this has led in turn to water seepage and damage to the outer stone cladding. To preserve this historical structure, which is of national interest (and also houses the entrance hall to the Museum of Medieval Stockholm), the City of Stockholm is carrying out an extensive renovation project, which includes foundation reinforcement.

Foundation reinforcement contract

The Stockholm Traffic Administration Office was the principal. With Golder Associates as the planning consultant, Lemminkäinen Sverige AB performed the foundation reinforcement contract from September 2007 to April 2008. To summarize, the preparatory work consisted of core drilling through the first level of the bridge. After the core drilling and the installation of “sealing tubes,” the casings were driven to a specific depth, which was also the base of the jet column. Air drilling was not allowed, so the method selected was therefore drilling using a water-powered down-the-hole hammer from Wassara. Pre-filling was then done by using cement slurry, followed by the actual jet grouting. The end result was a jet column in highly porous esker material. The process of pre-filling and jet grouting was essential in achieving this result.

The conclusion drawn from the project is that, under certain conditions, foundation reinforcement using jet grouting is the optimum method, both technically and financially.

This article is based on a technical evaluation by Viveca Arvidsson, Golder Associates and Risto Takkinen, Lämminkäinen. Published in samhällsbyggaren Issue 1 2009.

Tags: Drilling and grouting and reinforcement