Cofferdam Failure - Geotechnical Lesson Learned

Silvana Micic, Stan Gonsalves, TaeChul Kim

Dans les comptes rendus d’articles de la conférence: GeoSt. John's 2019: 72nd Canadian Geotechnical Conference

Session: Professional Practice/Case Studies

ABSTRACT: The existing Kabinakagami River Bridge located on Highway 11, approximately 32 km west of Hearst, Ontario, was replaced with a new bridge structure in 2008-2009. During construction of the West Abutment, the cofferdam failed and the construction was delayed. After the failure, the subsequent geotechnical investigation, commissioned by the Contractor was carried out in December 2008. Based on the results of that investigation, the Contractor contended that the borehole soil data shown in the Contract Documents was significantly different than what was actually encountered in the site during the sheet pile and H-pile installation. It was claimed that the existing soil conditions had contributed to problems encountered during the cofferdam installation and to its failure. Consequently, the third-party verification of the subsurface conditions in the area of interest was requested. To verify the subsurface conditions and clarify discrepancies in data collected during pre-failure and post-failure geotechnical investigations, the additional geotechnical investigation in the vicinity of the failed cofferdam was performed in September 2009, results of which are presented in this paper. The paper also includes the assessment of the original subsurface information available for the cofferdam design and suitability of that design as well as the assessment of installation procedures and sequences applied during the construction. The geotechnical lesson learned is discussed.

RÉSUMÉ: Le pont de la rivière Kabinakagami, situé sur la route 11, à environ 32 km à l'ouest de Hearst, en Ontario, a été remplacé par un nouveau pont en 2008-2009. Lors de la construction de la culée ouest, le batardeau a échoué et la construction a géotechnique suivante, commandée par le contractant, a été réalisée en décembre 2008. Sur la base des résultats de cette enquête, le contractant a affirmé que les données relatives au sol dans les trous de forage figurant dans les documents contractuels étaient très différentes de celles réellement rencontrées dans le site lors de l'installation des palplanches et des pieux en H. Il a été affirmé que les conditions du sol existantes avaient eau et à son échec. En conséquence, la vérification par une tierce partie des conditions du sous-sol dans la zone d'intérêt a été demandée. Afin de vérifier les conditions du sous-sol et de clarifier les divergences dans les données collectées au cours des enquêtes géotechniques effectuées avant et après la défaillance, les recherches géotechniques supplémentaires menées à proximité du batardeau en panne ont été effectuées en septembre 2009 et les résultats en sont présentés dans le présent document. Le document inclut également - construction. La leçon géotechnique apprise est discutée. 1 INTRDUCTION Kabinakagami River Bridge is located on Highway 11, approximately 32 km west of Hearst, Ontario. The bridge is a five-span structure with steel girders supporting a reinforced concrete deck. It is approximately 107 m long and 10 m wide. The bridge is oriented in an east-west direction and conveys one westbound lane and one eastbound lane of Highway 11 over the Kabinakagami River. It is reported that the structure was constructed in 1942, and that, the northern half of the bridge deck was replaced in 2002. The existing bridge structure is most likely supported on a combination of shallow and deep foundations. The East Abutment and Pier 4 are likely supported by shallow foundations placed on bedrock, while the West Abutment and Piers 1, 2, and 3 are likely supported by deep foundations. Highway 11 at the Kabinakagami River area is generally higher than the surrounding lands. It is built on shallow embankments with wide gravel shoulders. Figure 1 shows the existing Kabinakanagi River Bridge. In 2007, the Ministry of Transportation (MTO) planned to replace the existing bridge structure at the Kabinakagami River with a new bridge structure. The new structure was planned to be constructed to the south of the existing structure. The new bridge was a three-span bridge constructed with pre-cast pre-stressed concrete girders and a reinforced concrete deck. The new bridge was planned to be supported also by a combination of shallow and deep foundations. The East Abutment and East Pier of the replacement bridge structure was designed to be founded on spread footings resting on the underlying bedrock, while the West Abutment and West Pier would be supported on piles driven to bedrock. The bridge was designed and constructed based on the results of geotechnical investigations at the site performed in 2007 (GI 2007) which was a part of the Contract Documents. In 2008, the construction of the new bridge was commenced. However, in November 2008, during construction of the West Abutment, the cofferdam failed and the construction was delayed. After the failure, in December 2008, the Contractor commissioned additional borehole investigations (GI 2008) and based on the results,

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