Preloading of Varved Clay Soils

This part of our ongoing series about building settlement and geotechnical engineering strategies to mitigate settlement. Mike Talbot is OTO’s geotechnical engineering practice lead.

I recently had the pleasure of presenting a talk on this topic at the Connecticut Society of Civil Engineers Spring 2017 Geotechnical Conference. This post provides some highlights of my talk. The entire presentation is available on OTO’s web site.

Connecticut River Valley Varved Clay (or CVVC) is a type of fined grained soil deposit that was laid down within ancestral Lake Hitchcock, which filled the Connecticut River Valley after the melting of the continental glaciers  approximately 14,000 years ago.  CVVC is characterized by alternating layers of silt and clay, and is similar in composition to other varved deposits that formed in meltwater lakes across the northern part of North America following the retreat of the continental glaciers.

CVVC deposits are soft and compressible, and significant building or embankment settlement may result from building dead or live loads or from the placement of new fill loads. Having practiced extensively in the Connecticut River Valley for approximately 25 years, OTO has wide-ranging experience in investigating CVVC sites and providing geotechnical engineering solutions to address settlement concerns.

In my talk, I presented two case studies where a soil preload was constructed to mitigate post-construction settlement.  One case study involved the new Easthampton (Massachusetts) High School and the second involved a new book depository in Hatfield, Massachusetts. Both project sites were similar in that they were underlain by greater than 50 feet of soft CVVC, and in that the new construction (a combination of fill placed to form the building pad, and building dead and live loads) resulted in an increase of greater than 1,000 psf (pounds per square foot) in the vertical effective stress within the soil profile– books can  be heavy! Without ground improvement, this load increase could have caused the maximum past pressure to be exceeded within portions of the soil profile, causing virgin consolidation to occur. Consolidation is the process where the water content of the soil decreases, without being replaced by air, so that an overall volume of the soil layer changes. During design, we estimated that up to 6 inches of total settlement and as much as 3 inches of differential settlement could occur at both sites, enough to cause  damage and loss of functionality to the planned buildings.

From Langer, William H. Map Showing Distribution and Thickness of the Principal Fine-Grained Deposits, Connecticut Valley Urban Area, Central new England. Department of the Interior, United States Geological Survey, 1979

Easthampton Site

 

CVVC Deposits Map From Langer, William H. Map Showing Distribution and Thickness of the Principal Fine-Grained Deposits, Connecticut Valley Urban Area, Central new England. Department of the Interior, United States Geological Survey, 1979

Hatfield Site

Both structures were sensitive to post-construction settlement, so it was determined that soil improvement was required to mitigate the amount of settlement. The application of a soil preload was selected as the appropriate soil improvement technique. In essence, preloading is intended to simulate the design loads of a building, in this case by stockpiling large quantities of soil on the site, so that the consolidation occurs before the building is constructed. In addition, the construction schedule was tight for both projects, so the design solution included features (wick drains, which provided additional pathways for water to be eliminated) to expedite the consolidation process. Our design solution was similar for both projects and involved the following:

  • The installation of wick drains to help remove water from the soil matrix, and speed the rate of settlement (in this case, time was reduced to about three months),
  • The placement of a preload fill, which varied from two to 11 feet high, and
  • The monitoring of settlement during construction.
Easthampton Soil Profile

A typical soil profile at the Easthampton site.

Preload settlement monitoring and the post-construction performance of both structures indicate that the preload application was successful in reducing the amount of post-construction settlement. The use of wick drains allowed the preload settlement to occur without significantly impacting the construction schedule. However, despite the similarities of the site geology and the selection of a similar design solution the amount of preload settlement varied significantly between the two sites. At the Easthampton site, the preload settlement was approximately half of what occurred at the Hatfield site. The variation is likely attributable to a high silt and sand content in the CVVC at the Easthampton site.  We have found that simple moisture content data from bulk samples (combination of both silt and clay varves) provide a good indication of this variation.

More detail is provided in the complete presentation, downloadable from the link above. If you have any questions please feel free to contact us.

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