Process and stability of slow moving landslides

Process and stability of slow moving landslides

Sloof, E.J.

Hicks, M.A. (mentor)
Molenkamp, F. (mentor)
Brinkgreve, R.B.J. (mentor)
Kringos, N. (mentor)

Civil Engineering and Geosciences

Geotechnology

Geo-Engineering

2010-06-15

The stability of the slow moving Big Rock Mesa (BRM) landslide located in California (USA) was analysed. A detailed description of its geometry and soil structure is presented. For the stability vital shear zone and unsaturated zone were identified. The strength and permeability of the different layers that exist in the BRM were determined using computational methods and from literature data. The stability of landslides is susceptible to rainfall and changes to the groundwater level. The corresponding pressure changes cause slopes to become unstable. In dry periods the landslide is stable and in wet periods displacements by deformations occur. For the BRM landslide the rainfall data and displacements recorded with inclinometers were analysed. A water balance was made for a critical and stable period to evaluate the hydraulic flow. Besides rainwater it is likely that also water from adjacent areas flows into the BRM mass. Most of the rainwater is stored in the unsaturated zone of the BRM, which has an average thickness of about 42.5 m. Only a part of this water disappears by run-off and dewatering. The water pulse in the unsaturated zone is slowly drained by the evapotranspiration. The groundwater level profile of BRM was determined from measurements and computed with a statistical program called Surfer. The infiltration of water into the unsaturated zone reduces the suction. This suction is smallest over the vertical infiltration length of the water pulse. The water pulse propagates deeper into the unsaturated zone when rainfall increases. The reduction suction weakens the soil. Consequently, the stability of a slide increases with evapotranspiration and decreases with water infiltration. The stability of the BRM and the flow through the unsaturated zone was analysed with the finite element program Plaxis. However, due to the size of the BRM a full coupled analysis of deformations and hydraulic flow was not possible. Instead steady state calculations were performed. The flow through the unsaturated zone was compared with results from the one-dimensional flow program called SWAP. For the conditions of steady state calculation with suction the BRM is stable with a safety factor of 1.17. The safety factor of the BRM decreases slightly with increasing head on the hill side of the geometry. Suction in the unsaturated zone increases the safety factor by only 0.01. This suction decreases when water infiltrates. The calculated small influence of increasing groundwater table corresponds with inclinometer readings.

stability
landslides

http://resolver.tudelft.nl/uuid:0c792beb-f1a0-4e27-ad45-d37ab4d45887

Student theses

master thesis

(c) 2010 Sloof, E.J.