<p><p><h2>Project Summary/Scope:</h2>Hollow-core bar micropiles (Type E) were used to retrofit four existing bridges. Underpinning of the foundations was necessary due to deterioration of the exposed section of the existing concrete precast piles. Limited headroom and difficult access provided challenges to retrofitting the bridge foundations. Hollow-core bars were selected to retrofit the bridge because they have faster installation rates.</p><p>Subsurface Conditions: Soft organic silt extended anywhere from 10 to 17 feet. below the river bottom for the four bridges. In most cases, the silt was underlain by stiff to very stiff clay. At Bridge 1, the silt was underlain by medium dense sand to a depth of 57 feet and stiff clay existed below this layer. The clay was underlain by a layer of dense to very dense silty sand.</p><p>The design load at each of the four bridges was 80 kips per micropile. The micropiles also included a 9.625-inch external diameter steel casing with 0.5-inch wall thickness to extend from the cap beam to the anticipated scour depth. The following chart summarizes the design of the micropiles:<br><table class='tablepress' id='tablepress-316'><thead><th><center>Bridge</th><th><center>No. of
Micropiles
</th><th><center>Bond Length
of Micropiles</th><th><center>Grout Body Diameter</th><th><center>Reinforcement Type</th></thead><tbody><tr><td ><center>1</td><td ><center>180</td><td ><center>30 ft</td><td ><center>6 in</td><td ><center>1 - 52/26 IBO Titan hollow core </td></tr><tr><td ><center>2</td><td ><center>80</td><td ><center>40 ft</td><td ><center>9 in</td><td ><center>1 - 52/26 IBO Titan hollow core </td></tr><tr><td ><center>3</td><td ><center>64</td><td ><center>40 ft</td><td ><center>7.5 in</td><td ><center>1 - MAIR51N hollow core </td></tr><tr><td ><center>4</td><td ><center>80</td><td ><center>40 ft</td><td ><center>7.5 in</td><td ><center>1 - MAIR51N hollow core </td></tr></tbody></table>All work occurred under limited headroom with difficult access and within temporary cofferdams. At each production micropile location, the steel casing was installed using the external water flush method. The hollow-core bar was then inserted and drilled to the design depth. At Bridge 1, 6-inch clay drill bits were used. At Bridges 2, 3, and 4, 7.5-inch clay drill bits were used. The hollow-core bars were fed with a lean cement grout mix with a water-cement ratio of 0.89. When the design tip elevation was reached, a final grout mix with a water-cement ratio of 0.45 was used. Drilling rates<br>typically ranged between 0.5 to 1 feet per minute.<br><h2>Performance Monitoring:</h2>The micropiles were subject to a rigorous quality control that included grout quality testing and proof testing of each production micropile. All production micropiles were proof tested up to 150% of the design load. In addition, two to three verification tests were performed at each bridge on sacrificial micropiles to two and a half times the design load. The micropiles were found to perform well.<br><h2>Project Technical Paper:</h2>Gómez, J.E., Rodriguez, J., Mikitka, J., and Robinson, H.D. (2008) “Hollow Core Bar Micropile-Design Parameters Interpreted from 404 Load Tests.” Proceedings of the 33rd Annual and 11th International Conference on Deep Foundations, 2008, New York, NY.<br><h2>Date Case History Prepared:</h2>November 2012</p></p>