Deterministic landslide hazard assessment Case study Manizales Colombia

Deterministic landslide hazard assessment. Case study Manizales, Colombia Cees van Westen International Institute for Aerospace Survey and Earth Sciences (ITC), Enschede, The Netherlands. E-mail: westen@itc. nl Landslide case study Manizales

Objective l In this exercise, a simple slope stability model (the infinite slope model) is used to calculate safety factor maps for different conditions. l The effect of groundwater depth and seismic acceleration is evaluated using input maps of these factors for different return periods of rainfall (related to the groundwater level) and earthquakes (related to the seismic acceleration). l In ILWIS, the model is represented by a user-defined function. Different scenarios are calculated by changing the variables of this function. The model is applied on a data set of the city of Manizales, in central Colombia. Landslide case study Manizales

Shear strength / stress Shear stress = W sin / A Shear strength (Mohr-Coulomb criterion) s = c + tan = normal stress = W cos / A c = cohesion (KPa) = angle of internal friction (degrees) and c are geotechnical properties, which are measured in the laboratory using triaxial tests or shearbox tests. Landslide case study Manizales

Safety Factor The degree of slope hazard can be expressed by the Safety Factor (F) which is the ratio of the forces that make a slope fail and those that prevent a slope from failing. · F < 1 unstable slope conditions, F = 1 slope is at the point of failure, · F > 1 stable slope conditions. · Landslide case study Manizales

Infinite slope: l Conditions at crest and toe of the slope may be ignored. l Resulting forces from left and right are equal Weight of the block: g = unit weight of soil (N/m 3). Shear component of weight: Normal component of weight: Landslide case study Manizales

Infinite slope Stress = Force / area Shear component of weight: Normal component of weight: Shear stress: Normal stress: Safety factor: Landslide case study Manizales

Infinite slope & water pressure Height watertable above failure surface Weight of the water: Normal component of water weight: Pore pressure on JK: Factor of safety including pore pressure: Landslide case study Manizales

8 Landslide case study Manizales

Case study Manizales 9 Landslide case study Manizales

Manizales 3 D view 10 Landslide case study Manizales

City growth, Manizales 11 Landslide case study Manizales

Mass movement types 12 Landslide case study Manizales

Landslide activity 13 Landslide case study Manizales

Surficial materials 14 Landslide case study Manizales

Input data Landslide case study Manizales

Slope information l ILWIS Mapcalc functions work with radials and not with degrees e. g. 360 degrees = 2 * radials = 2 * 3. 14 = 6. 3832 l With some Mapcalculations in which you will use the inbuilt ILWIS functions DEGRAD( ), SIN( ), COS( ), and SQ( ), first some maps are prepared that will be frequently used in this application: · map SI, sine of slope · map CO, cosine of slope · map CO 2, squared cosine of slope Landslide case study Manizales

Maps showing relation Z/Zw (m) for different return periods During two months a year (rather dry): Gamma=14000, m=M 016 once in every 20 years: Gamma=14000, m=M 20 17 Landslide case study Manizales once a year: Gamma=14000, m=M 1 once in every 50 years Gamma=14000, m=M 50

User-defined function l The Safety Factor formula as presented above will be transformed into a user-defined function FS. This function already contains the known parameters (maps ASHT, SI, CO 2 and the known constants) but it also contains the variables Gamma and m. l The function can then be easily applied for various heights of the watertable (zw), by filling out the variables Gamma and m. This will result in a number of Safety Factor maps for specific watertable heights. l Function FS reads: (10000+((Gamma-m*10000)*ASHT*CO 2*0. 58)) / (Gamma*ASHT*SI*CO) Landslide case study Manizales

Scenarios (10000+((Gamma-m*10000)*ASHT*CO 2*0. 58)) / (Gamma*ASHT*SI*CO) · For the dry scenario, Gamma = 11000, and without water zw=0 and thus m=0 as well. To obtain a map with Safety Factors for the dry scenario (FSDRY), function FS is applied using parameters 11000 and 0. FDRY = FS(11000, 0) l For the saturated scenario, Gamma = 16000, and when all soil above the failure surface is saturated with water, zw=z thus m=1. A map with Safety Factors for the saturated scenario (FSAT) is then calculated by using function FS with parameters 16000 and 1. FSAT = FS(16000, 1) l Subsequently, function FS is applied repeatedly on the command line and Safety Factor maps are calculated for a number of watertable height scenarios. For each scenario, a value for Gamma is provided and the values for m are found in a number of existing groundwater Landslide case study Manizales

Results l Safety factors are calculated for each scenario l Also earthquake acceleration can be incorporated Landslide case study Manizales