Why is clay such a problem in the Ottawa Valley

Posted  by Steve Keddy on Sept 20, 2023

Leda Clay also called Quick Clay is a particular type of clay deposit that is found in the Northern Hemisphere world wide with large deposits in Russia, Norway, Sweden, Finland and North America. It is especially prevalent in the Ottawa Valley and Eastern Quebec regions. Quick Clay has the unique properties that allow it to be liquified under certain conditions often with disastrous results. 

Most recently on Sept 23,2023 a highway in Stenungsund Sweden along with 37 acres was affected by a quick clay landslide.

 In 2021 Ottawa south home construction was halted so developers could deal with Leda clay and the Rideau Street sinkhole of 2016 as well as that on Hwy 174 were in part related to the presence of quick clay.

 Locally on May 10,2010 quick clay claimed the lives of a family in Saint-JudeQuebec,
when their house suddenly tumbled into the Salvail River. The landslide was so sudden all family members died where they sat while watching an ice hockey game on television.

 By far the worst disasters in the Ottawa area are those of April 26, 1908 when 34 people lost their lives and twelve homes were taken in the middle of the night in Notre-Dame-de-la-Salette. The banks of the Lièvre River simply collapsed, sending mud, ice and freezing cold water through the river and into the town. 

 A 23 kilometer stretch of the South Nation River near Casselman is the most active landslide region in Ontario. This section sits in a 25 m valley on top of a clay bed and is active approximately every two decades. In 1971, after geotechnical studies the town of Lemieux's residents were relocated and sure enough in 1993 a large earthflow consumed 35
acres of farmland at the site of the former town .

Geological Properties of Quick Clay

At the end of the last ice age the Ottawa Valley sat at the bottom of  the Champlain Sea that had formed as the glaciers melted and fresh sea water from the Atlantic Ocean consumed the area. As the sea receded and glacial weight diminished the seabed expanded upwards with the result that today much of the Ottawa area is situated on sensitive marine deposited clays. 

The majority of these mineral clay deposits are negatively charged and electrically balanced by the positively charged sodium atoms from this ancient salt water. This bonding serves to hold the clay together but in the presence of excessive fresh water a leaching can occur that serves to undermine its strength. Strong seismic activity and even heavy loading on such subgrades has been known to act as catalysts to quick clay liquification.

Overall the Ottawa area extends from sea level to 550 feet vertically with some clay deposits being up to 200 feet thick. Ottawa is known for large variations in moisture content and this can lead to differential settlement as leaching waters diffuse sodium leaving an excess of negative charge repulsion further reducing the strength in the substrate. The reverse happens when clay dries out and shrinkage occurs which has been responsible for cracking in foundations, concrete  and asphaltic pavements. All of this can be happening many meters below the surface as deep ground waters penetrate the clay subgrade.

In the Ottawa Valley clay exists in both large and small distributions throughout as shown in the figure below. Of particular note is Cumberland Township and much of Orleans with maximum elevations approx 300 feet above the Ottawa river the clay borders entirely on its northern parts with the river.

Leda clay map for Ottawa ON

Driveway stability on Clay Subgrades

For the most part much of the effects of severe stability issues associated with Leda Clay can be mitigated by grading landscapes for fresh water drainage and by our sewer systems to direct rain and flood water away from high density areas. As well, before building approval case studies in the region have all been required to show that sever landslides have a less than 1 in 10,000 year probability of occurrence. Having said that, there is still the likelihood that tremor activity will affect buildings and infrastructure.

Tolerable ground movement will lead to acceptable 10-30 year observational damage in the form of small material interface gaps, mortar cracks,foundation openings, cracking in garage and basement floors etc. As well, observable damage to roads and driveways are expected within reasonable limits. It is simply the case that approvals would not have been issued if there existed high probability of excessive damage to structures and infrastructure.

Fluctuations in the groundwater levels, significant grade changes from hardscaping and heavy load additions can be expected to cause some property damage however slight. As well due to the high shrinkage factor of leda clay under water depletion the action of large trees in close proximity to a driveway can cause the asphalt to crack. These are easily identified as such as the pavement tends to crack in lines concentric about the tree root system.

On balance structures built on approved sites exhibiting stable leda clay formations will experience movement well within conventional construction tolerances and expected home maintenance with not generally be adversely impacted.

Maintaining Driveways Built on Clay 

Driveways built after 1976 use a driveway mix that is meant to flex with temperature and hence reduce cracking. However under severe ground movement cracking will occur. It is important to replace oils in an asphalt surface by regular application of oil based asphalt sealer as well as fill cracks completely as they occur.

If you notice any cracks that you cant see to the bottom and are thin and span for some distance you likely have deep ground shearing occurring as a result of clay shrinkage. The crevasse should be filled with dry sand so as to drop completely to the bottom of the crevasse.  Sand helps to stabilize the clay as well as fill the void. Then apply a thin layer of crack filler over the top. This will need to be repeated over the lifetime of the driveway.

If your driveway has experienced significant shifting then it may need to be rebuilt entirely. Generally after the asphalt is removed your contractor will access the base for clay content. Depending on its severity clay may have to be excavated and the base rebuilt from the subgrade up. The use of geosynthetic fabric can be used but its utility is limited only to maintaining the interface between subgrade and base material only so as to reduce the two materials from mixing together under load.  Generally this is unnecessary and simply enhancing the base with extra stone is enough since a new asphalt pad will prevent rainwater from entering the base. Alternatively and for added security a thin layer of salt below the base material will stabilize a clay subgrade against the action of fresh groundwater as noted earlier. 

Depending on its severity clay may have to be excavated and the base rebuilt from the subgrade up.

The above is a brief description of the geology of the Ottawa Valley with respect to Leda Clay or Quick Clay. Here in the region we have been dealing with clay sensitivities throughout our history here and by careful engineering practices limited adverse effects of Leda Clay sensitivity. Being aware of these things goes a long way for the home owner as well both in their understanding the basic properties of clay and as guidance in their home maintenance endeavors.

PENNER,E , (1976),Insulated road study
EDEN,W.J ,MITCHELL , (1970) The Mechanics of Landslides in Leda Clay
PATERSONGROUP (2022) Landslide Risk Assessment Proposed Residential Development
2983, 3053 and 3079 Navan Road
Ottawa, Ontario, (2022) https://webcast.ottawa.ca/plan/All_Image%20Referencing_Zoning%20Bylaw%20Amendment%20Application_Image%20Reference_2022-05-03%20-%20Landslide%20Risk%20and%20Hazard%20Assessment%20-%20D02-02-21-0090.
SHUT,L.W , WILSON, E.A (1987)The Soils of the regional municipality of Ottawa Carleton Fig 6 p15
EDEN, W. J.; CRAWFORD, C. B. (1957) Geotechnical properties of Leda clay in the Ottawa area NRC Publications Archive https://publications-cnrc.canada.ca/eng/view/ft/?id=1fd9daf3-ba72-49e8-b5d3-0a1849b533b8

Layout and text by Steve Keddy Photographs by A1 Tophat Blackcoat Corp..