Study on landslides in loess slope due to infiltration

Abstract

In this thesis, the mechanism of landslides in loess slope is studied based on a field test

in association with laboratory tests and numerical modeling. The field test was carried

out in the Heifangtai Plateau in China. Heifangtai belongs to semiarid monsoonal with

scarce precipitation. Therefore, agricultural irrigation is the major source of water

infiltration for the loess slopes in Heifangtai.

The test site was selected at the crest of a steep loess slope with developed cracks,

covered by more than 40m loess layer. Ground investigation, including lithological

composition, groundwater level, soil sampling and soil permeability, was conducted. A

field monitoring program was then performed by installation of inclinometers,

piezometers, moisture probes, tensiometers and crackmeters.

The field irrigation test with instrument monitoring was conducted at the test site on

October 2009, lasting for 12 days. Based on field observation and monitoring results,

some typical failures occurred in the field test were analyzed in details, including

development of cracks, formation of sinkholes and failures on slope surface. These

failures are representative in loess regions and are potential factors of loess landslides.

A conceptual model was proposed to reflect soil failures induced by water infiltration

along the crack.

To characterize the loess behavior to infiltration, a comprehensive triaxial testing

program was conducted on trimmed loess specimens sampled in Heifangtai. The

fundamental behavior of loess was investigated following different stress paths under

both saturated and unsaturated conditions. Based on triaxial tests, the soil parameters

including mechanical properties and hydraulic properties were obtained.

The finite difference program FLAC3D was adopted in this study, the fluid model of

which was extended from saturated flow to saturated/unsaturated flow. A numerical

model was built to study the mechanism of soil failures induced by variation of water

level in the crack, using a proposed moving boundary algorithm. In the numerical

modeling, fluid-mechanical coupled analysis was conducted by solving Biot’s

consolidation equation, utilizing soil parameters obtained in the laboratory tests. The

proposed conceptual model in the field test was verified in the numerical analysis.

Subsequently, different draining rates of water in the crack were simulated to

investigate soil failures affected by the dropping of water level in the crack. Based on

the numerical analysis, the mechanism of sinkhole formation was discussed.

Finally, conclusions and recommendations for future research on loess landslides

induced by infiltration were made. It is hoped that the study on the mechanism of loess

landslides can provide a useful reference for the future research.