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 (St) 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.
WL
Ip (range)
Smectite
400 – 700
100 – 400
Illite/Chlorite
50 – 90
30 – 60
Kaolinite
40 – 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