Large areas of the Connecticut River Valley are underlain by a type of soft soil deposit known as Connecticut Valley Varved Clay (CVVC). This deposit formed at the bottom of a large glacial lake, commonly known as Lake Hitchcock, which once filled almost the entire valley from Vermont to near Long Island Sound. This lake formed following the retreat of the last glacier and drained approximately 14,000 years ago. During the warm “summer” runoff season, as the ice melted, soil particles were carried into the lake. As water drained rapidly into the lake, the sand and silt, having a larger grain size, settled out first. Fine-grained clay particles have a tendency to remain suspended much longer, particularly in turbulent water. As the cold “winter” months moved in, the waters calmed and allowed the clay particles to settle. This act of differential sedimentation created alternating layers of sand, silt and clay, also known as varves.
The word varve comes from the Swedish word varv, which can be translated to revolution, round, or layer, and is used to describe a layer of semi-annual sediment deposited by a glacial lake. In CVVC, these varves are typically composed of either dark gray or red clay, or light gray or red sand and silt. The finer grained clay particles tend to hold moisture much longer resulting in a darker coloration, whereas the sand and silt particles tend to allow moisture to escape more easily, resulting in a lighter coloration. The transition from one layer to the next is often abrupt, which is characteristic of the rapid deposition of sand and silt particles that occurred during the runoff season. The individual varves are typically between 1/16 and 1/2 inches in thickness.
An example of varved silt and clay layers.
Lake Hitchcock existed for many thousands of years, so there can be several tens of thousands of individual varves within a soil profile. The depth of the varved clay varies drastically throughout the valley, with the thickest sections exceeding 250 feet. The distribution and thickness of the clay can be seen in more detail on the provided map.
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
The largest issue presented by varved clay is consolidation. Fine-grained, cohesive clay restricts the movement of water, and therefore takes a significant amount of time to expel water and consolidate. The sand lenses give the water a path to escape and significantly reduce the initial consolidation time, but maximum consolidation can typically take up to 10 years. Depending on the size of the proposed structure, and the clay conditions at the Site, the amount of settlement can be significant and may lead to structural damages over time.
A typical characteristic of the varved silt and clay stratum is that the upper few feet tends to be desiccated, resulting in a very stiff to medium consistency. This can be advantageous, as this layer is less prone to settlement, reducing the time to consolidate under the applied load.
Several solutions exist to reduce the effects of post-construction settlement, including wick drains, preloading, and post-construction settlement monitoring. Preloading involves the temporary placement of material to compress the varved clay prior to the construction of a building. As discussed above, the sand lenses significantly reduce the time rate of consolidation within the varved silt and clay, since they allow excess pore water pressures that are generated when loads are applied to dissipate relatively quickly (allowing the soil to consolidate quickly under the applied load). Wick drains may be installed, allowing additional paths for the water to escape and further expediting the consolidation process. Post-construction monitoring helps assess the effectiveness of the pre-load (if applied). Additionally, if on-going settlement occurs after construction, preventative measures may be taken before significant damages occur.
For large structures such as tall buildings or bridges, it may not be possible to support the structure directly on the varved clay, and deep foundations may be required. In addition, slope instability may be of concern where embankments are built over the clay (such as highway or railroad embankments) or if deep excavations are made into the clay. For example, significant geotechnical studies and improvements were required for the construction of the I-91 roadway embankments.
Building on varved clay will always be a challenge. However, with modern technology and practices it is often not a question of whether it can be done, rather a question of how much it will cost and how much time it will take. We would love to help you evaluate projects involving CVVC soils, so feel free to contact us at 413-788-6222 or www.oto-env.com.