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Geological Hazard Resources: Leda Clay Geohazard Assessment
Introduction
The Leda Clay can be considered a metastable soil with the potential to liquefy
when disturbed.
This material underlies much of the Ottawa Gatineau area in Ottawa, Canada.
This area was occupied by the Champlain Sea at the end of the last stages of the Pleistocene (Fig 1.)
The soil is susceptible to a variety of triggers which can initiate major flow slides with catastrophic consequences.
These landslides move very rapidly, can flow considerable distances and destroy relatively flat lying land.
Over 250 of such landslides have been recorded within 60km of Ottawa.
Fig 1. Landslides in the Leda Clay of the Champlain Sea.
Task
A new hospital is proposed to be built in the Ottawa
Gatineau area where possible quick clay deposits could be encountered.
Geotechnical data is available to characterise the deposits found during a ground investigation.
The landslides are more prone to occur on steep slopes in the area and if they are triggered then they are likely to retrogress up to 500m into the land above the steep slope.
Debris will flow up to 900m from the bottom of the failed slope.
Fig. 2 shows the geology and general topography of the area.
Examine the geological and topographical map and outline the high hazard zone on the map associated with potential quick clay failure.
Fig. 2 Geological and topographical map of the hospital site.
Evaluate the following potential hospital sites for their suitability. Comment on your decision.
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Site |
Yes/No |
Comment/Reason |
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A |
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B |
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C |
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D |
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E |
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F |
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Indicate on the map the most likely location for a flow slide to occur. Justify this interpretation.
Complete the table with risk reduction strategies for dealing with the flow slide hazard
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Risk Management Strategy |
Advantages |
Disadvantages |
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From the site investigation geotechnical data below plot the following:
Clay, Silt, Sand Percentage against depth
Moisture Content, Liquid and Plastic Limits against depth
Liquidity Index and Activity against depth
Remoulded Shear Strength against depth
From the plots delineate the zone of potential weathered crust and Quick Clay.
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Depth (m) |
W (%) |
WL (%) |
WP (%) |
IP |
IL |
Activity |
Sand (%) |
Silt (%) |
Clay (%) |
SR (kPa) |
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1.4 |
33.4 |
51.5 |
23.4 |
28.1 |
0.35 |
0.61 |
0.3 |
54 |
46 |
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2.8 |
38.0 |
46.0 |
23.6 |
22.4 |
0.65 |
0.55 |
0.6 |
58 |
41 |
10.2 |
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3.3 |
34.5 |
45.3 |
22.6 |
22.7 |
0.52 |
0.52 |
0.2 |
56 |
44 |
7.2 |
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5.5 |
37.5 |
41.6 |
24.4 |
17.1 |
0.76 |
0.39 |
0.4 |
56 |
44 |
8.9 |
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5.9 |
37.9 |
44.8 |
23.5 |
21.2 |
0.68 |
0.46 |
0.4 |
54 |
46 |
7.8 |
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7.4 |
33.1 |
42.7 |
20.9 |
21.8 |
0.56 |
0.46 |
0.8 |
52 |
47 |
7.8 |
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7.8 |
35.7 |
40.3 |
19.0 |
21.4 |
0.78 |
0.45 |
0.1 |
52 |
48 |
9.3 |
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9.0 |
30.9 |
40.3 |
21.0 |
19.3 |
0.51 |
0.39 |
0.2 |
51 |
49 |
8.5 |
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10.5 |
31.0 |
34.3 |
21.7 |
12.6 |
0.74 |
0.29 |
0.4 |
56 |
44 |
5.3 |
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10.8 |
32.3 |
33.2 |
19.0 |
14.3 |
0.93 |
0.34 |
0.8 |
57 |
42 |
3.6 |
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12.1 |
30.9 |
32.4 |
18.0 |
14.3 |
0.90 |
0.35 |
1.1 |
58 |
41 |
3.3 |
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12.4 |
31.6 |
32.3 |
19.6 |
12.7 |
0.94 |
0.29 |
0.5 |
56 |
44 |
4.4 |
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13.5 |
33.1 |
35.4 |
17.3 |
18.0 |
0.87 |
0.39 |
0.4 |
54 |
46 |
5.0 |
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13.8 |
32.3 |
32.6 |
16.5 |
16.2 |
0.98 |
0.37 |
1.0 |
55 |
44 |
3.6 |
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15.0 |
33.8 |
32.6 |
19.5 |
13.1 |
1.10 |
0.32 |
0.5 |
59 |
41 |
5.5 |
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15.4 |
27.4 |
23.4 |
19.3 |
4.10 |
1.94 |
0.12 |
2.6 |
64 |
33 |
1.8 |
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18.1 |
31.7 |
33.4 |
19.0 |
14.4 |
0.88 |
0.35 |
0.8 |
58 |
41 |
2.9 |
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18.5 |
30.6 |
32.1 |
19.7 |
12.4 |
0.88 |
0.30 |
1.0 |
58 |
41 |
5.0 |
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19.6 |
25.6 |
38.0 |
20.2 |
9.80 |
0.54 |
0.26 |
2.1 |
61 |
37 |
5.1 |
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19.9 |
23.1 |
29.5 |
16.6 |
12.9 |
0.50 |
0.42 |
14.0 |
55 |
31 |
5.0 |
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23.4 |
32.3 |
30.6 |
22.0 |
10.6 |
1.16 |
0.30 |
0.5 |
65 |
35 |
2.1 |
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25.0 |
26.5 |
30.3 |
20.5 |
9.80 |
0.62 |
0.26 |
1.7 |
61 |
37 |
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26.6 |
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7.0 |
58 |
35 |
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Source Articles
Geertsema, M. and Torrance, J.K., (2004) Quick clay from the Mink Creek landslide near Terrace, British Columbia: Geotechnical properties, mineralogy, and geochemistry. Can. Geotech. J. 42: 907–918. doi: 10.1139/T05-028
Geoscape Ottawa-Gatineau Curriculum-linked
Classroom Activities. Theme Nine: Landslides
http://geoscape.nrcan.gc.ca/ottawa/curriculum_e.php
DPG (Version 2, 2012)