STRATIGRAPHY ACROSS THE HUDSON RIVER AS REVEALED IN BORINGS FOR THE NEW NY BRIDGE REPLACING THE TAPPAN ZEE, TARRYTOWN-NYACK, NY

In 2002, ‘06 and ‘12 a total of 85 borings under the inspection of MRCE were made in the Hudson River to produce geotechnical data reports leading to the construction of the New NY Bridge to replace the Tappan Zee Bridge. Borings went as deep as El. –395’, but did not reach the bedrock (at ~El. –740’) or till, or fully penetrate the glacial lake deposits near the western 1/3 of the river. Rock recovered to the east and west had no features present that would indicate why the western rock was cut so deep.

The eastern 1/4 of the river is underlain by the Proterozoic Fordham Gneiss; above it to the west is the Mesozoic Brunswick Fm. (largely sandstone and siltstone). Bedrock is overlain by basal till. Above that, the river is largely filled with glacial Lake Hudson gray varved silt and clay with red brown silt and fine sand partings. Varved sediments are found up to El. –20’ along the shorelines. After the lake breached at the Narrows (Stanford 2010) and drained, the center of the river was scoured out to around El. –190’ and covered with a layer of sand. As relative sea level rose and estuarine conditions took over, the river sand mixed and interlayered with organic silty clay, grading upwards into an organic clay with a decreasing amount of fine sand partings and an increasing number of shells and higher water content. Between El. –30’ and –60’, layers of sand extend from both shores into the river, and peat layers are present along the western shore. The softest soil and most shells are found above El. –30’.

Varve thicknesses vary greatly, but tend to be thicker with depth (silt grading to clay measured ~4” thick at El. –271’) and thin upward. During construction there was an increase in the pile resistance below El. –275’, which is also the elevation of a small sand layer along the western shore. CPT borings and varve counts indicate a greater amount of silt and fine sand below, and the soil becomes denser below El. –300’.

Consolidation tests suggest that if Lake Hudson was filled with sediment up to El. –20’, then the varved soil is over-consolidated just below the scoured surface, normally consolidated in the center of the stratum and over-consolidated below El. –350’. There aren’t enough data points to be certain, but this hints at subaerial exposure and slight desiccation of the clay after the lake drained, and possible glacial loading +/or desiccation below El. –350’.