Figure 8 Landslide frequency vs. slope angle
These observations show that the applied force on the clay due to the gravity from the earth influences the frequency of landslide a lot, because the force that is paralleled with the sliding direction due to the gravity is dependent on the angle of the slope, the variation of frequency of landslide varies with different slope angles shows that the applied force on the clay influences the frequency of landslide occurrence, and normally the higher applied force leads to higher frequency of landslide, because that the higher slope angle leads to higher frequency of landslide.
However, the frequency of landslide does not increase without limit when the angle of slope increases, which means the slope angle is not the only issue that needs consideration when talking about the applied force on the clay influencing the landslide’s frequency, the strength of the underlying material cannot be considered independently with the slope angle. And as Kirkby and Carson (Carson and Kirkby, 1972)reported already, 35 degrees is actually the critical angle of the slope, the massive and highly competent materials like granite can keep vertically without threat of potential failure.
In the meantime, the effect of slope angle depends on the rock type also, like shown in below figure, the relationship between the slope angel and the rock type is presented, the data is from the database of San Mateo County. In the graph, the proportion of each rock type exists in six slope intervals is represented with the size of the circles. It could be concluded that the nature of the underlying bedrock affects the slope’s distribution a lot. The greatest proportion of granite (gr) stands at steep slope angles, while the alluvium (Qal) shows remarkable tendency at slopes of 05 percent.
Figure 9 Slope distribution vs. rock type
The data concludes that greater potential energy is provided by steeper slopes to cause failure, which also represents the materials with higher strength. The tradeoff between increased strength and increased driving force appears to decrease the effect of angle of slope on the slopes that have angles over 15 percent. The slopes that have angles higher than 15 percent can be affected more by the rest factors that influence the susceptibility of landslide.
In conclusion, the applied force on the clay influence the frequency of landslide a lot, normally higher angle of slope induces high frequency of landslide, especially for the slopes with angle less than 15 percent; while for the slopes with angle over 15 percent, remaining factors affect the frequency of landslide also. And to dig further the influence of slope angle on frequency of landslide, the nature of the rock type should be studied at the same time, because for different types of rock, the angles of slopes changes also.
Proposed plan
Sep. 2015Oct. 2015: Literature review on the effect of clay on landslide.
Nov. 2015Jan.2016: Choose an area where landslide occursfrequently, collect relevantdata (rainfall data and soil type, frequency of landslide) through different ways, to study the effect of water content on the frequency of landslide.
Feb. 2016Apr. 2016: Collect the official data about the angles of slopes and the frequency of landslide through different ways, study the effect of angles of slopes (applied force) on the frequency of the landslide.
May 2016Jun. 2016: Write the final report to consolidate the results gotten, prepare the slides for oral defense, to rehearse for oral defense.
Reference
ALEOTTI, P. & CHOWDHURY, R. 1999. Landslide hazard assessment: summary review and new perspectives. Bulletin of Engineering Geology and the Environment, 58, 2144.
ARATTANO, M. & FRANZI, L. 2003. On the evaluation of debris flows dynamics by means of mathematical models. Natural Hazards and Earth System Science, 3, 539544.
BLANC, R. P. & CLEVELAND, G. B. 1968. Natural slope stability as related to geology, San Clemente area, Orange and San Diego Counties, California.
BRABB, E. E. 1987. Analyzing and portraying geologic and cartographic information for landuse planning, emergency response, and decision making in San Mateo County, California. Proceedings of the GIS, 87, 362374.
CARSON, M. A. & KIRKBY, M. J. 1972. Hillslope form and process, Cambridge University Press Cambridge.
CHRISTENSEN, B. T. 1992. Physical fractionation of soil and organic matter in primary particle size and density separates. Advances in soil science. Springer.
JENNINGS, C. W., STRAND, R. G., ROGERS, T. H., BOYLAN, R., MOAR, R. & SWITZER, R. 1977. Geologic map of California, Division of Mines and Geology.
LEGGET, R. F. 1939. Geology and engineering.
RADBRUCHHALL, D. H. & CROWTHER, K. 1973. Map showing areas of estimated relative amounts of landslides in California.
ROTH, R. A. 1983. Factors affecting landslidesusceptibility in San Mateo county, California. Bulletin of the Association of Engineering Geologists, 20, 353372.
