| Summary: | One devastating landslide event in northernThailand occurred in 2006 at Ban Nong Pla village, ChiangKlang highland of Nan province after, a massive amountof residual soil moved from upstream to downstream, viacreek tributaries, into a main stream after five days of unusualheavy rainfall. In this paper, the geological and engineeringproperties of residual soil derived fromsedimentaryrocks were analyzed and integrated. Geological mapping,electrical resistivity survey and test pits were carriedout along three transect lines together with systematiccollection of undisturbed and disturbed residual soilsamples. As a result, the average moisture content in soilis 24.83% with average specific gravity of 2.68,whereas theliquid limit is 44.93%, plastic limit is 29.35% and plastic indexis 15.58%. The cohesion of soil ranges between 0.096–1.196 ksc and the angle of internal friction is between 11.51and 35.78 degrees. This suggests that the toughness propertiesof soil change when moisture content increases. Resultsfrom electrical resistivity survey reveal that soil thicknessesabove the bedrock along three transects range from2 to 9 m. The soil shear strength reach the rate of high decreasesin the range of 72 to 95.6% for residual soil fromshale, siltstone and sandstone, respectively. Strength ofsoil decreaseswhen the moisture content in soil increases.Shear strength also decreases when the moisture contentchanges. Therefore, the natural soil slope in the study areawill be stable when the moisture content in soil level isequal to one, but when the moisture content between soilparticle increases, strength of soil will decrease resultingin soil strength decreasing.
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