Abstract:
The Morton Street Bulkhead is located between Morton and Christopher Streets along Route 9A and the Hudson River within Hudson River Park in Manhattan. The bulkhead was constructed between 1873 and 1877 using two types of timber pile supported retaining wall relieving platforms (Christopher Street Section Type A and Morton/Christopher Street Section Type B). During construction large lateral movements had occurred. To increase the walls lateral resistance one additional row of batter piles was added (Type B Wall). While providing added lateral resistance lateral movements for this section of the wall continued. Observations of the bulkhead were performed between 1980 and 2014 indicating gaps of varying width typically 1 to 3 inches between the bulkhead wall and granite pavement. The bulkhead alignment was distorted along the entire length from Christopher Street to Morton Street. A geotechnical investigation revealed that a soft/weak organic silty clay stratum existedon top of a downward sloping hard glacial sand layer. Limit equilibrium analyses were performed to investigate global slope stability. It was determined that the wall movements were likely caused by an inadequate foundation design on a sloping hard bottom (downslope creep). Several repair options were considered to arrest the lateral movement of the bulkhead. Based on the magnitude of the supplemental restraining force needed to stabilize the bulkhead, tiebacks installed into the glacial sand layer with vertical steel pipe piles core drilled through the existing bulkhead was deemed most effective to provide lateral support while minimizing overall costs. Construction commenced in early 2017 and was completed in December 2018. This paper presents the various technical challenges faced during design due to the condition of the historic bulkhead and difficult subsurface conditions in the marine environment.