The Washington Monument Case History

Abstract

The Washington Monument was built between 1848 and 1884 in honor of George Washington, first President of the United States of America. The Monument consists of a 169.16 m high column with a large pyramidal foundation. It was built in two periods straddling the Civil War. During the first period from 1848 to 1858, the foundation base was 24.38 m square, the column was built to a height of 55.5 m and the average pressure under the foundation reached 513 kPa. In 1879, the foundation was underpinned and the base of the foundation became a square ring with an outside dimension of 38.54 m and an inside dimension of 13.41 m. The column was completed in 1884 and created an average pressure under the foundation of 465 kPa. The original foundation rested on a medium compact sand - stiff clay mixture which was 3.76 m thick, underlain by a 8.30 m thick layer of very dense sand, gravel and clay, followed by a 11.68 m thick layer of stiff to very stiff high plasticity blue clay resting on decomposed Wissahickon schist. The underpinning brought the foundation level down to the very dense sand and gravel layer. Calculations indicate that the Monument settled about 1.3 m during the first construction period, 1848-1858. During underpinning in 1879, the measured settlement was 52 mm. During the completion of the column from 55.5 m to the full height of 169.16 m the measured settlement was an additional 63 mm. From 1884 to 1993, the Washington Monument settled an additional 55 mm in a very linear fashion with time. This remarkable case history is described in some detail including settlement and bearing capacity calculations which show that the underpinning process in 1879 likely saved the Monument from serious trouble.