Case History: Foundations for the New Mississippi River Bridge, St. Louis

Abstract

This case history describes the foundation design and construction for the New Mississippi River Bridge, presently under construction in St. Louis, Missouri. The 1,500 ft (457.2 m) main span of the cable-stayed bridge is supported by two delta tower pylons in the river, with anchor piers on each bank. The final foundation design was completed as a part of an Alternate Technical Concept (ATC) proposal by the winning construction team, and utilized drilled shafts up to 11.5-ft (3.5-m) diameter that were socketed into very hard limestone. As part of the ATC foundation design, a full-scale, bidirectional static load test was performed to verify the design and allow use of higher resistance values. This paper presents the results of the site exploration, construction of the load test shaft, details of the load testing, and a comparison of the test results with various design parameters.

Load tests on drilled shaft foundations in moderately strong to strong limestone

“…The C value, as determined using equation (1), was 0?43 in the competent moderately strong to strong limestone. This C value is consistent with the C values that were reported in Brown (2009) and Axtell and Brown (2011) for moderately strong to strong limestone. In weathered moderately strong to strong limestone, the C value ranged from 0?11 to 0?17.…”

“…However, for Piers 11 and 12, there was a layer of lower strength rock that was approximately 1?5 m thick with an f ' r value equal to 35 MPa at a depth of 6?1 m. Four bidirectional load cells (BLCs), each with a total capacity of 213?5 MN, were used to confirm the side and end bearing resistance values and the quality of the construction methods. The average unit end bearing and unit side resistance in the rock socket were 22 and 2?1 MPa respectively (Axtell and Brown, 2011). These values were not the ultimate strength values because very small movements were measured (displacement values less than 0?4 cm in either direction corresponding to 0?1% of the diameter of the shafts).…”

“…In Axtell and Brown (2011), four 3?5 m diameter DSFs were utilized in the design and construction of the New Mississippi River Bridge located north of St. Louis, Missouri. The test shaft for these foundations was socketed 7?1 m into moderately strong limestone (f ' r .69 MPa with an average f ' r value of approximately 166 MPa).…”