Quick Clay: (Leda Clay gone bad!) Origin, Mineralogy, Chemistry and Landslides
What is Leda Clay? The fine-grained (silty clay and clayey silt) sediments that accumulated in the St. Lawrence drainage basin, under marine and brackish conditions, during and shortly after the retreat of glacial ice from the basin at the end of the last (Wisconsinan) Pleistocene glacial ice sheet. One of the sediments within which quick clay can develop.
What are quick clays? Quick clays are defined by their behaviour. At their natural water content, these sediments have: -- sensitivity >30, and -- remoulded shear strength <0.5 kpa. Sensitivity (S t ) is defined as: in-situ shear strength remoulded shear strength When disturbed, quick clays behave as a liquid. This allows very large, retrogressive flow slides to occur on landscapes that are essentially level (i.e. only a river valley cut into a level landscape).
What are Quick Clays? (Cont d) A geomorphological phenomenon that forms distinctive landscapes. A geotechnical problem: the large landslides can be very destructive. Also, major damage can occur to buildings and other structures by consolidation (settlement) under conditions of heavy loading or improved drainage.
Plasticity Plasticity is the ability for a material to be moulded and to retain the shape after the moulding force is no longer applied. Soils with phyllosilicate clays exhibit plasticity over a water content range that depends on both mineralogy and particle size. The liquid limit (WL) is defined as the water content at which the liquid/plastic transition occurs. The plastic limit (Wp) is the water content at which the plastic/brittle solid transition occurs. The plasticity index (Ip) is defined as Ip = WL - Wp
Plasticity: influence of chemistry The plasticity of a soil is influenced by: 1) the clay content: 2) the clay minerals present; 3) the cations satisfying the CEC: and 4) the salt concentration in the pore water. Smectite Illite/Chlorite Kaolinite WL 400 700 50 90 40 60 Ip (range) 100 400 30 60 20 40 For Smectite: Ip decreases as pore water salinity increases and when Ca replaces Na on CEC sites. For Illite/Chlorite/kaolinite: Ip increases as pore water salinity increases and when Ca replaces Na on CEC sites. The plasticity of swelling and non-swelling clay minerals to chemical concentration increases or decreases changes in opposite manners to one another in response the imposed chemical change.
Ullensaker landslide, Norway, December, 1953 Photo: Norwegian Geotechnical Institute, Oslo, Norway
St Jean Vianney, Quebec, 1971 Photo: National Research Council of Canada
South Nation Landslide, Ontario, 1971 Photo: National Research Council of Canada
South Nation Landslide, Ontario, 1971 Photo: J. K. Torrance
South Nation Landslide, Ontario, 1971 Photo: National Research Council of Canada
Quick clays are known to occur in: - Norway, Sweden and Finland - Canada eastern (large area) - western (small area) - United States New York and Alaska - Japan The Scandinavian and North American sediments are composed of glacially ground rock material. The Japanese sediments are composed of volcanic ash and eroded soil.
Quick clays are a product of their history: All of the sediments in which they have developed were deposited during the late stages of the last glaciation. Let us look at that history in Eastern North America.
Deglaciation 12,000 BP At 12000 BP, the ice still blocked the St Lawrence at Quebec City, holding in a fresh water lake in which varved sediments accumulated. When the ice blockage melted, the fresh water lake drained to sea level and salt water entered, by tidal action and density currents, creating the Champlain Sea (salinity ~26 g/l). Salt water sediments with flocculated structure accumulated.
Deglaciation 11,000 BP The Champlain Sea still occupied the St Lawrence- Ottawa Lowlands. Marine sediments continued to accumulate. With isostatic rebound in response to glacial unloading, the Sea became shallower and some of the sediments at its edges emerged above sea level.
Leda clay: extent The Champlain Sea occupied the basin west of Quebec City. The Laflamme Sea occupied the basin of the Saguenay River. The Goldthwaite Sea occupied the St Lawrence Valley, east of Quebec City.
Champlain Sea sediments, Carleton University, Ottawa, Ontario Photo: J.A. Donaldson
Weathered material, with nodular structure Le Coteau landslide, Gatineau, Quebec, 1971
Rotational landslides in weathered slopes Le Coteau landslide, Gatineau, Quebec, 1971
Rotational failure caused by human actions Dubeau landslide, Shawville, Quebec, 1999