Month: March 2018

What is Connecticut Valley Varved Clay?

March 20, 2018 Matt Nichols

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.

 

varved clay
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.

USGS 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.

 

New Book! – Manufactured Gas Plant Remediation: A Case Study

March 15, 2018 Tom Speight

Well, it’s done.

 

I’m proud and distinctly relieved to announce the publication of my book, Manufactured Gas Plant Remediation: A Case Study (2018, CRC Press).  Like any proud parent, I can’t fight the urge to talk about it.

 

So, here’s a quick introduction to what it is about. The ‘case study’ in the title refers to the entire state of Massachusetts, since this is the first state-level overview of the gas industry.

Northampton gasworks
A gasholder in Northampton, Massachusetts, one of four surviving in the state out of what were once hundreds.

 

‘Manufactured gas’ refers to several types of gas made from coal or oil during the 19th and early 20th centuries, and which was used much as we use natural gas in the present day. The term ‘natural gas’ was actually coined to distinguish gas naturally present in coal beds or oil reservoirs from gas made out of coal. Manufactured gas lit the foggy streets of Victorian England (some parts of Boston and London still have gas street lights). It also lit houses, heated uncounted numbers of kitchen stoves, and fueled innumerable industries. By the early 1900s, most cities and large towns had at least one gasworks; Massachusetts alone had roughly 100 manufactured gas plants (“MGPs”) and the second largest manufactured gas industry in the country, second only to New York).

 

On a larger scale, the gas industry also:

 

  • Played a crucial role in the development of urban areas and industries during the 19th and early 20th Centuries, since many industries sought to locate in communities where gas service was available. Where this wasn’t possible, many industrial plants would start their own private gas plants, some of which fell into disuse and were forgotten, while some expanded to serve the neighboring mill towns and in the fullness of time grew into utility plants themselves.

 

  • Became the first major example of the modern concept of a public utility, together with all the government regulations that went with it.

 

  • Launched the modern organic chemical industry, with coal tar derivatives becoming feedstocks for manufacturing aniline dyes, ammonium sulfate fertilizers, creosote, laboratory reagents, explosives, plastics and disinfectants, most notably carbolic soap (familiar to anyone who’s seen A Christmas Story as the foul-tasting red soap). Modern organic chemistry exists largely because of the numerous byproducts the manufactured gas industry provided.

 

The first half of the book reconstructs the history of the gas industry from its origins in the early 19th century through the general changeover to natural gas in the middle of the 20th century, including discussions of gas-making processes, equipment, business practices, and important persons. Some of this information is specific to Massachusetts, but the discussion of gasmaking technology is universal to the gas industry.

 

 

1915-Fall River-Chas St-Panorama-NEAGE copy
A panoramic photograph of the a gasworks in Fall River, Massachusetts, from 1915

 

Everett
A waterfront view of the New England Gas & Coke coking plant in Everett, Massachusetts, from 1899. At the time, this was the largest modern coking plant in the world, and would supply much of metropolitan Boston’s gas supply until the 1950s.

The second half of the book deals with the ‘dark side’ of this industry, namely its troublesome environmental legacy. Due to the toxicity of many gasmaking byproducts such as coal tar, sites contaminated due to gasworks operations can pose a risk to public health. The assessment, remediation, and redevelopment of coal tar sites pose a significant technical and financial challenge. This part of the book includes information on the chemical composition, origins, and hazards posed by gasworks wastes including coal tar and cyanide wastes, as well as on regulatory issues, assessment and remediation strategies, and other useful topics.

dig
An example of buried materials encountered at a former gasworks

My coauthor, Allen W. Hatheway (one of the preeminent experts on MGPs and coal tar sites, and author of several other publications), and I started the research and writing process in March 2012. At the beginning our goal was simple—to compile an inventory of all of the former manufactured gas plants in Massachusetts. As we continued with our research, however, (to paraphrase J.R.R. Tolkien) “the tale grew in the telling,” and the project eventually grew into a rather large book. This was partly because there were so many former gasworks and partly because a discussion of these sites required a vast amount of historical, technical and modern regulatory context.

 

I’ll be giving presentations on this topic at several conferences in 2018 and 2019, including the Society for Industrial Archaeology annual conference in Richmond, VA this June.

 

The book is available from Amazon or direct from the publisher.