Grouting Karst to Support Driven Pile Foundation for a Truss Rail Bridge

The paper describes geotechnical investigations, design, construction, and performance evaluation of a grouting program completed to support a new railroad truss bridge over an Interstate highway in Pennsylvania. A large two-span truss was required to carry a local main for the Norfolk Southern Railroad across I-76 (Schuylkill Expressway) and new ramps. The construction project was required to improve access to I-76 from US Route 202 and US Route 422 in King of Prussia Pennsylvania. A geotechnical investigation indicated limestone rock below 15 to 20 m of medium clay. The clay was not suitable to support the high loadings imposed by the railroad bridge. The limestone was found to have frequent solution cavities on the order of 0.5 to 2 m in size, to depths of more than 30 m. The design solution was to grout the upper 3 m of the variable quality rock to form a mattress of stable rock to support driven piles over the deeper poor quality rock. The design used limited mobility displacement grouting. The paper presents technical details of the conditions encountered, design approach, construction observation and grouting data, and an analysis of the grouting records to indicate the performance of the grouting.