6. Slope Creep Instability in Krajang Lor Village, Magelang Regency, Central Java, Indonesia: Inducement and Developmental Prediction

Initiation of shallow soil creep has been occurring in the volcanic soils covering hill slope in Krajang Lor Village, Salaman District, Magelang Regency, Central Java, and Indonesia. Affected area reaches to 9.08 hectare with damages to most of houses in the village. Accumulated creep results in formation of a number of 2–21 mm wide tension cracks observed on the road pavement and ground surface. Geological conditions of the area may be the source of soil creep, confirmed by discovered. In this paper, field works and laboratory experiments were carried out in order to verify geological factors causing soil creep; and a slope stability model based on Mohr-Coulomb failure criterion was developed, in order to evaluate thickness of creep zone and critical slope angle for creep occurrence and logarithmic creep models was developed, in order to predict the development of soil creep displacement. As the results, three main factors causing soil creep are: (1) poor engineering properties of the soil including high water content, low dry density and low shear strength parameters, (2) irreversibly dehydration of hydrated halloysite clay as well as swell-shrink activities of smectite clay, and (3) loadings of about 5 kN/m² as a primary trigger of creep initiation. Creep behavior of soils was significantly affected by the mineralogical compositions, dry density, and water content and stress level. The thickness of creep zone in the research area was 10.368 m and critical slope angles were 41.7° at the toe of the slope; 23.9° at the middle slope and 38.5° at the top of the slope. Soil creep displacement at the soil surface was predicted increasing to approximately 22 mm after 15 years and 23 mm after 60 years.