Filling a Giant Washout

As geotechnical engineers, we are not unfamiliar with filling holes. Although, we are not sure if we had ever filled a hole this size before.

Last September, Hurricane Ida wreaked havoc at the TPC River Highlands Golf Course in Cromwell, CT. Overnight, torrential surface runoff washed away the fine sand within the riverbank into the Connecticut River, leaving a “washout” measuring approximately 60 feet deep, 90 feet across the riverbank, and 75 feet into the slope. A railroad track and an underground jet fuel line, directly adjacent to hole No. 13, were undermined and left hanging in the air. Both the railroad and the fuel line had to suspend their services.

A race against time ensued. ConnDOT, who owns the railroad right-of-way, led the charge, supported by an engineering team led by H.W. Lochner and a construction team led by O&G Industries, Inc.

GEODesign served as the geotechnical engineer. We drilled two soil borings to depths of approximately 100 feet, which revealed that the subsurface soils were predominantly fine sand, highly prone to erosion. The borings were performed concurrently with efforts to reroute the jet fuel around the washout and required close coordination, given the limited work area and short project schedule.

Our analysis suggested that the heavy precipitation which preceded the landslide, possibly acting in conjunction with recently installed drainage features and the absence of engineered slope protection, likely initiated the erosion. The soil loss due to erosion along the face of the slope, along with a temporarily elevated groundwater table and saturated soil conditions, likely resulted in a rapid progressive global failure.

The Solution

The solution engineered by the design team included the following main components:

  • The slope is flattened from its original inclination of approximate 1.3H:1V to 1.5H:1V.
  • The slope is reconstructed with imported granular fill reinforced with uniaxial geogrid. Primary geogrids with lengths of 35 to 42 feet were installed at 32-inch vertical spacing. Six foot long secondary geogrids were installed in between the primary layers to improve the surface stability.
  • As surface-stabilizing vegetation would take years to grow back, a riprap layer was installed on the slope surface for erosion protection. New drainage improvements were incorporated to protect against future heavy precipitation.

The drainage improvements included a swale and a headwall along the crest of the slope, drainage pipes and manholes on the face of the slope, and a riprap-protected outfall structure at the toe.

GEODesign’s experience in slope stabilization was instrumental in the team effort to develop a solution which could be safely and quickly implemented with a restricted construction access due to the steep slope, over a short schedule. Close communication and collaboration between the engineering team with other stakeholders (i.e., ConnDOT, the railroad, the golf course, the jet fuel utility company, and construction contractors) was the key to success.

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