Sediment Testing on the St. Lawrence Seaway

Way up in Northern New York, the St. Lawrence River splits the state’s North Country region and Canada, historically acting as an incredibly important resource for navigation, trade, and  recreation. Along the St. Lawrence River is the St. Lawrence Seaway, a system of locks, canals, and channels in both Canada and the U.S. that allows oceangoing vessels to travel from the Atlantic Ocean all the way to the Great Lakes.

Recently, the St. Lawrence Seaway Development Corporation (SLSDC) contracted Princeton Hydro to conduct analytical and geotechnical sampling on material they plan to dredge out of the Wiley-Dondero Canal. Before dredging, sediment and soils have to be tested to ensure their content is suitable for beneficial reuse of dredged material. In August, our Geologist, Marshall Thomas and Environmental Scientist, Pat Rose, took a trip up north to conduct soil sampling and testing at two different sites within the canal near Massena and the Eisenhower Lock, which were designated by the SLSDC. The first site was at the SLSDC Marine Base, which is a tug/mooring area directly southwest of Snell Lock. The second location was directly northeast of the Eisenhower Lock, which is also used as a mooring area. Both of these sites require dredging in order to maintain mooring access for boat traffic navigating the channel.

During this two-day sampling event, our team, which also included two licensed drillers from Atlantic Testing Laboratories, used a variety of equipment to extract the necessary samples from the riverbed. Some of the sampling equipment included:

  • Vibracoring equipment: this sampling apparatus was assembled on Atlantic Testing’s pontoon boat. To set up the vibracore, a long metal casing tube was mounted on the boat more than 10 feet in the air. The steel casing was lowered through the water approximately 17-20 feet down to the mudline. From there, the vibracore was then vibrated through the sediment for an additional 4-6 feet. For this project, vibracore samples were taken at 4 feet in 10 different locations, and at 6 feet in 3 different locations.

  • A track mounted drill rig: this rig was positioned along the shoreline to allow advancement of a standard geotechnical test boring close to existing sheet piling. Advancement of the boring was done by way of a 6-inch hollow stem auger. As the auger was advanced, it resembled a giant screw getting twisted into the ground. This drilling method allows the drilling crew to collect soil samples using a split spoon sampler, which is a 2-foot long tubular sample collection device that is split down the middle. The samplers were collected by driving the split spoon into the soil using a 140 lb drop hammer.

For our team, conducting sampling work on the St. Lawrence Seaway was a new experience, given most of our projects occur further east in the Mid-Atlantic region. The most notable difference was the hardness of the sediment. Because the St. Lawrence River sediments contain poorly sorted, dense glacial till, augering into it took a little more elbow grease than typical sediments further south do.  The St. Lawrence River is situated within a geological depression that was once occupied by glaciers. As the glaciers retreated, they were eventually replaced by the Champlain Sea, which flooded the area between 13,000 and 9,500 years ago. Later on, the continent underwent a slight uplift, ultimately creating a riverlike watercourse that we now deem the St. Lawrence River. Because it was once occupied by a glacier, this region is full of glacial deposits.

For this project, our team was tasked with collecting both geotechnical and analytical samples for physical and analytical testing. Physical testing included grain size analysis, moisture content, and Atterberg limit testing. Grain size analysis helps determine the distribution of particle sizes of the sample in order to classify the material, moisture content testing determines exactly that — how moist the sediment is, and Atterberg limits help to classify the fines content of the materials as either silt or clay. Analytical testing included heavy metals, pesticides, volatile organic compounds, and dioxins.

Our scientists were responsible for logging, testing, and providing a thorough analysis of fourteen sampling locations. The samples collected from the vibracore tubes filled with sediment were logged and spilt on-shore. In order to maintain a high level of safety due to the possible presence of contaminants, all of the sampling equipment was decontaminated. This process involves washing everything with a soapy water mixture, a methanol solution, and 10% nitric acid solution.

The samples collected at each vibrocore location were split into multiple jars for both analytical and physical testing. The physical test samples were placed into air and moisture tight glass sample jars and brought to our AASHTO accredited soils laboratory in Sicklerville, New Jersey for testing. The analytical samples were placed into airtight glass sample jars with Teflon-lined caps. These samples were then placed into an ice-filled cooler and sent to Alpha Analytical Laboratories for the necessary analytical testing.

Once all the laboratory testing was completed, a summary report was developed and presented to the client. This report was made to inform the SLSDC of the physical properties of each sediment sample tested and whether contaminants exceeded threshold concentrations as outlined in the New York State Department of Environmental Conservation (NYSDEC) Technical & Operation Guidance Series (TOGS) 5.1.9. This data will ultimately be used by the SLSDC to determine the proper method for dredging of the material and how to properly dispose of the material.

Princeton Hydro provides soil, geologic, and construction materials testing to both complement its water resources and ecological restoration projects and as a stand-alone service to clients. Our state-of-the-art Soils Testing Laboratory is AASHTO-accredited to complete a full suite of soil, rock, and construction material testing for all types of projects. For more information, go here: http://bit.ly/2IwqYfG 

Don’t Get Sunk: Everything You Need to Know About Sinkholes (Part One)

Photo by Steven Reilly/New Jersey Herald

Sinkholes are a phenomenon that tend to baffle and frighten most people. How is it possible that the ground beneath our feet could just drop? How do we know if we’re nearby a sinkhole? What should we do if we see one? How are sinkholes fixed? The mystery of the unknown around sinkholes can be quite unnerving.

Have no fear, we’ve got answers to all of those questions and more! In this two-part blog series, our experts share their knowledge and provide important information about this scary occurrence. In part one, we provide a detailed look at what a sinkhole is, three different types of sinkholes, and what causes them to form. In part two, we explore how to detect sinkholes and the steps taken to repair them.

What is a Sinkhole?

Sinkholes are a common phenomenon around the world. They result from both man-made and natural causes. Marshall Thomas, a Princeton Hydro geologist, describes sinkholes as “depressions observed from the surface, caused by dissolution of carbonate rocks.” In other words, sinkholes form when the rock below the land surface gets dissolved by water that penetrates the surface and continues to move downward, further into the subsurface.

Most common in areas with “karst terrain,” or types of rocks that can easily be dissolved by groundwater, sinkholes can go undetected for years until the space underneath the surface gets too big or enough of the surface soil is washed away. Sometimes the holes are small, measuring a few feet wide and ten feet deep. Sometimes the holes are hundreds of miles wide and deep. However, all of them can be dangerous.

Sinkholes are found throughout the world. States like Pennsylvania, Texas, Florida, Alabama, Tennessee, and Missouri are at higher risk for sinkholes because they tend to have more soluble rocks like salt beds and domes, gypsum, limestone, and other carbonate rocks. People living in these states are recommended to have professionals look at any property they intend to buy to make sure it isn’t in an area above soluble rock.

Types of Sinkholes

Not all sinkholes are the scary, earth-falling-out-from-underneath-your-feet events. Some occur slowly over time and are very evident from the surface. Geologists classify sinkholes in three major types. Their formation is determined by the same geological processes, barring a few differences. Let’s dive in!

1. Dissolution Sinkholes

Illustration by USGSDissolution sinkholes start to form when limestone or dolomite is very close to the soil surface, usually covered by a thin layer of soil and permeable sand which washes away or is eroded. Rain and stormwater runoff gradually percolate through crevices in the rock, dissolving it. Consequently, a bowl-shaped depression slowly forms.

Sometimes, dissolution sinkholes become ponds when the depression gets lined with debris, which traps water inside. Dissolution sinkholes develop gradually and are normally not dangerous. However, the ones that become ponds can drain abruptly if water breaks through the protective bottom layer.

Fun fact: Most of Florida’s lakes are actually just large sinkholes that filled up with water!

2. Cover-Subsidence Sinkholes

Illustration by USGSThis type of sinkhole, which starts with the dissolution of the underlying carbonate bedrock, occurs where the covering sediment is permeable (water can pass through it) and contains sand. First, small pieces of sediment split into smaller pieces and fall into openings in the carbonate rock underneath the surface. With time, in a process called piping, the small particles settle into the open spaces. This continues, eventually forming a dip in the surface ranging from one inch to several feet in depth and diameter. Again, these aren’t the sinkholes movies are made about.

3. Cover-Collapse Sinkholes

Illustration by USGSThis type of sinkhole is the one making headlines and causing fear. In order for cover-collapse sinkholes to happen, the covering soil has to be cohesive, contain a lot of clay and the bedrock has to be carbonate. Similar to the cover-subsidence sinkholes, the cohesive soil erodes into a cavity in the bedrock. The difference with this is that the clay-filled top surface appears to remain intact from above. However, underneath, a hollowed out, upside down bowl shape forms. That hollowing gets bigger and bigger over time until eventually, the cavity reaches the ground surface, causing the sudden and dramatic collapse of the ground. Just like that, poof, we have a sinkhole that appears to be surprising and abrupt but really has been brewing for many years.

What Causes a Sinkhole?

Sinkholes can be natural or man-made. The most common causes of a sinkhole are changes in groundwater levels or a sudden increase in surface water.

Intensive rain events can increase the likelihood of a sinkhole collapse. Alternatively, drought, which  causes groundwater levels to significantly decrease, can also lead to a greater risk of collapse of the ground above. In a world with a greater variability in rainfall and drought events due to climate change, sinkholes may become a more common occurrence around the world.

Humans are also responsible for the formation of sinkholes. Activities like drilling, mining, construction, broken water or drain pipes, improperly compacted soil after excavation work, or even significantly heavy traffic (heavy weight on soft soil) can result in small to large sinkholes. Water from broken pipes can penetrate through mud and rocks and erode the ground underneath and cause sinkholes.

Most commonly, human-caused sinkholes are the result of:

  • Land-use practices like groundwater pumping, construction, and development
  • Changing of natural water-drainage patterns
  • Development of new water-diversion systems
  • Major land surface changes, causing substantial weight changes

In some cases, human-induced sinkholes occur when an already forming sinkhole is encountered during construction processes such as excavation for stormwater basins and foundations. Dissolution of bedrock generally occurs in geologic time-frames (thousands of years). In these cases, the excavation process has removed the covering soils, decreasing the distance between the top of the void and the ground surface.  

In other cases, voids in the bedrock are generated due to rock removal processes such as hammering and blasting. Hammering and blasting can generate fractures or cracks in the bedrock that soil can then erode into. A void in the bedrock may already exist, however, the process of removing the bedrock by hammering and/or blasting can speed up the meeting of the upside-down bowl and the surface that much quicker. One site where this happened has experienced over 35 sinkholes in 4 years.

Overall, it’s generally not a good idea to pump groundwater or do major excavation in areas that are prone to sinkholes. According to the USGS, over the last 15 years sinkhole damages have cost on average at least $300 million per year. Because there is no national tracking of sinkhole damage costs, this estimate is probably much lower than the actual cost. Being more mindful about the subsurface around us and our actions could help lower the average yearly cost in damages and even save lives.

Photo by Barbara Miller PennLive Patriot News

Stay tuned for Part Two of this blog series in which we explore we explore how to detect sinkholes and the steps taken to repair them! For more information about Princeton Hydro’s Geotechnical Engineering services, go here: http://bit.ly/PHGeotech

Special thanks to Princeton Hydro Staff Engineer Stephen Duda, Geologist Marshall Thomas, and Communications Intern Rebecca Burrell for their assistance in developing this blog series.

Sources: