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MDOT M85 Bascule Bridge Replacement

The M-85 Bascule Bridge Project involves replacement of the existing M-85 Bascule Bridge and approaches located in Detroit, Michigan.  In addition, the project includes reconfiguration of existing streets east and west of the existing M-85 Bascule Bridge.  The existing Bascule Bridge is approximately 278 feet long and about 74 feet wide.

According to MDOT’s drawings dated January 18, 1978, both pits of the existing Bascule Bridge are supported on 12-foot square caissons extending approximately 1 foot into hardpan.  The bank-to-bank clear distance of the Rouge River underneath the existing Bascule Bridge is approximately 125 feet and the bottom of the river is at elevation of about 551.5.  In addition, two utility tunnels, one 6 feet in diameter and the other 10 feet in diameter, run below the existing Bascule bridge within the current footprint.

The replacement bridge is proposed to be a three span bridge skewed approximately 13 degrees to the south of the alignment of the existing structure, about the east abutment.  The proposed two piers, approximately 200 feet apart, will be located within the Rouge River, approximately 56 feet in front of the proposed abutments.  The proposed width of the new Bascule Bridge is about 92.5 feet. 

One of the reasons for the replacement of this Bridge is the fact that the distance between the existing abutments and piers has been steadily being reduced, apparently since the initial construction of the bridge.  Somat reviewed and analyzed historical survey measurements and provided an estimate on the magnitude, direction and rate of this movement.  Somat also performed soil borings and rock coring and performed laboratory and field tests on cohesive soil samples obtained.  Finally, Somat provided geotechnical and construction options of stabilizing the soils and/or constructing a braced structure that would withstand future movements.


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