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Contact 52 North
  


Summary

There are several methods that can be applied for the construction of an engineering geological data-base. The most used method is to interpolate between existing boreholes in an area. However, when boreholes are scarce, or the terrain is very heterogeneous, this is not a good solution. In such situations use could be made of geomorphologic information, and an engineering geological data base can be constructed by logical reasoning. The material map is calculated according to the so-called sequence method.

Three dimensional view of the city of Manizales

Theoretical background and methods

Geomorphologic map

The sequence method considers all the material sequences in the area as standard sequences. The standard sequences are a result of the statistical analysis of the sampling points, which have been obtained during field work. The eight main material types that can be found, either at or below the surface in the area around the city of Manizales, are given as records in the table below. Remark that one row in the table is one sequence.

Sequence Layer1: MAT1 Layer2: MAT2 Layer3: MAT3 Layer4: MAT4
1 Volcanic ash Residual soil Weathered rock Fresh rock
2 Residual soil Weathered rock Fresh rock Fresh rock
3 Weathered rock Fresh rock Fresh rock Fresh rock
4 Fresh rock Fresh rock Fresh rock Fresh rock
5 Colluvium Residual soil Weathered rock Fresh rock
6 Alluvial material Weathered rock Fresh rock Fresh rock
7 Normal fill Volcanic ash Residual soil Weathered rock
8 Hydraulic fill Volcanic ash Residual soil Weathered rock

In this application four different maps (MAT1-MAT4), displaying the layers 1 to 4 are constructed. The procedure for creating these maps is given schematically in the figure below. The material in each of the layers is obtained by combining five input maps: a geomorphological map, a fill thickness map, a colluvial thickness map, an ash thickness map and a slope map, and by taking into account the material in the layer above.

Scheme of the analysis

Creating the four material maps

The surface material map (MAT1) is made using conditional statements in ILWIS according to the following steps (Van Westen et al. 1994):

  1. The geomorphological map is renumbered using an attribute that contains the most probable material type for each geomorphologic unit.
  2. Those pixels which have fill material (fill>0) are classified as normal fill (7) or hydraulic fill (8), depending on the geomorphological information.
  3. Those pixels which have slope deposits (col>0) are classified as slope deposits (5), if they were not yet classified in the previous step.
  4. Those pixels that have alluvial deposits (6) are classified as such.
  5. Those pixels that have volcanic ash (ash>0) are classified as ash (1) if they had not been classified earlier.
  6. The remaining unclassified pixels are classified as residual soil (2) when the slope angle is less than 60 degrees, and as weathered rock (3) if the slope is steeper.

Based on the information from the surface layer (MAT1) and the various input maps, also the second layer (MAT2) can be modelled, using the following steps:

4 material maps

  1. Those pixels classified in MAT1 as normal fill (7) or hydraulic fill (8) and which have underlying slope deposits (col>0) are classified as slope deposits (5). If this is not the case, but they have ashes underneath (ash>0) they are classified as ashes (1), and otherwise as residual soil (2).
  2. Those pixels classified as slope deposits (5) in MAT1 are classified as residual soil (2) in MAT2 taking into account that most landslides remove the ash cover.
  3. Pixels classified as alluvial material (6) in MAT1 are classified as weathered rock (3)in MAT2.
  4. Ash (1) in MAT1 is assumed to be underlain by residual soil (2) in MAT2.
  5. Residual soil (2) in MAT1 is assumed to be underlain by weatherd rock (3) in MAT2
  6. Weathered rock (3) in MAT1 is assumed to be underlain by fresh rock (4) in MAT2.

Analogous steps are followed for the construction of material maps MAT3 and MAT4 of deeper layers.

Differentiating material types according to geology

Geological map

The maps that have been created sofar cannot be directly connected to the geological map of Manizales. In order to make this possible, the 8 material types should be subdivided on the basis of geology. One way to do this in ILWIS is to combine the maps MAT1-MAT4 with the geological map using a so-called two-dimensional table. Using this table the residual soils, weathered rocks, fresh rocks, slope deposits, normal fills and hydraulic fills can be subdivided in units related to lithology. Another way to do this in ILWIS is by using cross operations.

References

  • Van Westen, C.J., Van Duren, I, Kruse, H.M.G. and Terlien, M.T.J. (1993). GISSIZ: training package for Geographic Information Systems in Slope Instability Zonation. ITC-Publication Number 15, ITC, Enschede, The Netherlands. Volume 1: Theory, 245 pp. Volume 2: Exercises, 359 pp. 10 diskettes.
  • Van Westen, C.J., Rengers, N., Soeters, R. and Terlien, M.T.J. (1994). An engineering geological data base for mountainous terrain. Proceedings 7th International IAEG Congress, Balkema, Rotterdam. pp. 4467-4475.

For more information on this case study, contact:

C.J. van Westen
Department of Earth Systems Analysis,
International Institute for Geo-Information Science and Earth Observation (ITC),
P.O. Box 6, 7500 AA Enschede, The Netherlands.
Tel: +31 53 4874263, Fax: +31 53 4874263, e-mail: westen@itc.nl

  
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