Landslides Classification

Transcription

1 ENVIRONMENTAL GEOTECHNICS Landslides Classification Prof. Ing. Marco Favaretti University of Padova Department of Civil, Environmental and Architectural Engineering, Via Ognissanti, 39 Padova (Italy) phone: website: 1

2 LANDSLIDE DESCRIPTIONS The term landslide is sometimes felt to be inadequate because many types of slope movement do not involve sliding. Cruden (1991) has suggested a simple definition of landslide: the movement of a mass of rock, debris, or earth down a slope. The term landslide can cover all slope movements that occur from natural or man made causes except ground subsidence. The dimensions and geometry of a landslide have been described by Varnes (1978) using the cutaway drawing shown on Figure. 2

3 Varnes (1978) 3

4 LANDSLIDE DESCRIPTIONS Subsequently, the International Association of Engineering Geologists (I.A.E.G.) created a Commission on Landslides that has produced the section and definitions shown on the following figure (IAEG, 1990). 4

5 Length: horizontal distance from top of headscarp to toe (upslope to downslope). The horizontal distance for slope length is chosen because it is a quick method of determining size from plan drawings. Width: generally, the widest dimension across the slope. 5

6 Depth: usually described as up to the maximum depth below the existing ground surface. Slope: average slope in degrees from horizontal, or an average gradient of horizontal:vertical. 6

7 LANDSLIDE CLASSIFICATION (IAEG, 1990) 1 Crown: Practically undisplaced material Adjacent to highest parts of main scarp 2 Main scarp: Steep surface on undisturbed ground at upper edge of landslide caused by movement of displaced material (13, stippled area) away from undisturbed ground; it is a visible part of surface of rupture (10) 3 Top: Highest point of contact between displaced material (13) and main scarp (2) 4 Head: Upper parts of Iandslide along contact between displaced material and main scarp (2) 7

8 LANDSLIDE CLASSIFICATION (IAEG, 1990) 5 Minor scarp: Steep surface on displaced material of landslide produced by differential movements within displaced material 6 Main body: Part of displaced material of Iandslide that overlies surface of rupture between main scarp (2) and toe of surface of rupture (11) 7 Foot: Portion of Iandslide that has moved beyond toe of surface of rupture (11) and overlies original ground surface (20) 8 Tip: Point on toe (9) farthest from top (3) of landslide 8

9 LANDSLIDE CLASSIFICATION (IAEG, 1990) 9 Toe: Lower, usually curved margin of displaced material of a Iandslide, most distant from main scarp (2) 10 Surface of rupture: Surface that forms (or that has formed) lower boundary of displaced material (13) below original ground surface (20); also termed slip surface or shear surface; it planar, can be termed slip plane or shear plane 11 Toe of surface of rupture: Intersection (usually buried) between lower part of surface of rupture (10) of a landslide and original ground surface (20) 9

10 LANDSLIDE CLASSIFICATION (IAEG, 1990) 12 Surface of separation: Part of original ground surface (20) now overlain by foot (7) of landslide 13 Displaced Material: material displaced from its original position on slope by movement in Iandslide; comprises both depleted mass (17) and accumulation (18) 14 Zone of depletion: Area of landslide within which displaced material lies below original ground surface (20) 15 Zone of accumulation: Area of landslide within which displaced material (13) lies above original ground surface (20) 16 Depletion: Volume bounded by main scarp (2), depleted mass (17), and original ground surface (20) 10

11 LANDSLIDE CLASSIFICATION (IAEG, 1990) 17 Depleted mass: Volume of displaced material (13) that overlies surface of rupture (10) but underlies original ground surface (20) 18 Accumulation: Volume of displaced material (13) that lies above original ground surface (20) 19 Flank: Undisplaced material adjacent to sides of surface of rupture; jf left and right are used, they refer to flanks as viewed from crown; otherwise use compass directions 20 Original ground: Surface of slope that existed surface before landslide took place 11

12 LANDSLIDE CLASSIFICATION In the original work by Varnes (1978), which has been updated and partly revised by Cruden and Varnes (1996), slope movements have been subdivided into six categories: 1. Falls 2. Topples 3. Slides - rotational and translational 4. Lateral spreads 5. Flows 6. Composites - combination of types 12

13 LANDSLIDE CLASSIFICATION For each of these subdivisions, the materials are grouped as either (1) rock, (2) predominantly coarse material (debris) (3) predominantly fine material (earth). Predominantly coarse is defined as having 20-80% of particles in the gravel/ boulder size (>2 mm). Sketches of various failure types (excluding composites) from Varnes (1978) are given on the following Figures with abbreviated comments. 13

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16 NATURAL AND ARTIFICIALS SLOPES The analysis of slopes takes into account a variety of factors relating to topography, geology, and material properties, often relating to whether the slope was naturally formed or engineered. Natural slopes that have been stable for many years may suddenly fail because of changes in topography, seismicity, groundwater flows, loss of strength, stress changes, and weathering. Significant uncertainty exists about the stability of a natural slope. 16

17 NATURAL AND ARTIFICIALS SLOPES Knowing that old slip surfaces exist in a natural slope makes it easier to understand and predict the slope s behavior. The shearing strength along these slip surfaces is often very low because prior movement has caused slide resistance to peak and gradually reduce to residual values. Engineered slopes may be considered in three main categories: embankments, cut slopes, and retaining walls. As these slopes are manmade less uncertainty exists about their stability. 17

18 MODES OF FAILURE Slope failures are usually due to a sudden or gradual loss of strength by the soil or to a change in geometric conditions, for example, steepening of an existing slope. 18

19 MODES OF FAILURE 19

20 Geologic Factors Controlling Shape of Potential Failure Surface GEOLOGIC CONDITIONS POTENTIAL FAILURE SURFACE Cohesionless soils Residual or colluvial soils over shallow rock Stiff fissured clays and marine shales within the upper, highly weathered zone Translational with small depth/length ratio Sliding block Interbedded dipping rock or soil Faulted or slickensided material Intact stiff to hard cohesive soil on steep slopes Single planar surface Sliding blocks in rocky masses Weathered interbedded sedimentary rocks Clay shales and stiff fissured clays Stratified soils Sidehill fills over colluvium Thick residual and colluvial soil layers Soft marine clays and shales Soft to firm cohesive soils Multiple planar surfaces Circular or cylindrical shape 20

21 MODES OF FAILURE As most soils are generally heterogeneous, noncircular surfaces, consisting of a combination of planar and curved sections, are most likely. Often, retrogressive failures consisting of multiple curved surfaces can occur in layered soils. Such failures are typical where the first slip tends to oversteepen the slope, which then leads to additional failures. 21

22 Falls Free fall Topple 22

23 CASE HISTORIES Toppling failure Planar Sliding 23

24 Slides Rotational Slide Usually occurs in slopes consisting of homogeneous materials Translational Slide Usually occurs in shallow soils overlying relatively stronger materials 24

25 Slides Rock slide Earth slide Debris slide 25

26 Slides Single rotational slide Successive rotational slide Multiple rotational slide 26

27 Case History Pizzo Coppetto, Alta Valtellina (SO), settembre

28 Rotational landslide with debris-flow La Conchita, California,

29 Fort St. John, Alberta, Canada, 2001 Roto-traslational landslide Young River Landslide, Canada 29

30 South Tyrol/Alto Adige (Italy) 30

31 Colleumberto (PG), dicembre

32 Centuripe (EN), ottobre

33 Holbeck Hall, North Yorkshire,

34 Valle d Isarco (BZ) agosto

35 Lateral spreading & Debris flow Lateral spreading Debris flow 35

36 CASE HISTORIES Lateral spreading Tully Valley Landslide, Phoenix, New York,

37 CASE HISTORIES Cones of dejection and screes at the feet of Canadian Rockies 37

38 CASE HISTORIES S. Salvador, El Salvador, 2001 Debris Flow 38

39 CASE HISTORIES Creep The roots of trees are embedded in the stable soil, while the trunks follow the downward slow movement of the superficial cover. 39

40 earth-flow 40

41 earth-flow 41

42 La Conchita, California. 42

43 Val d Orcia, april

44 Creep 44

45 Calanchi, Val d Orcia, Toscana 45

46 Railway embankment UK

47 Riverside Erosion 47

48 Riverside Erosion 48

49 Road Embankment Texas

50 Bridge Abutment 50

51 Trench Excavation 51

52 Tailing dam Ungheria

53 Tailing Dams Stava TN, ITALY,