Failure Mechanisms in Unsaturated Silty Sand Slopes Triggered by Rainfall

Landslides triggered by rainfall cause significant damage to infrastructure annually and affect many lives in several parts of the world, including Switzerland. These landslides are initiated by a decrease in the effective stresses, and hence the shear strength of the soil, as a result of the increase in pore water pressure. The frequency of their occurrence is directly affected by the climatic and hydrological conditions in the region. Therefore, it is expected that the predicted rise in the number of extreme meteorological events, accompanied by the concentration of population and infrastructure in mountainous regions, will result in an increased number of casualties associated with landslides in the future. The main goal of this doctoral project was to study the effects of pore water pressure perturbations on the stability of unsaturated silty sand slopes and to investigate the mechanisms leading to the initiation and propagation of the shear deformations and eventually possible rapid mass movements. The behaviour of the test slope prior to the failure induced by the artificial rainfall event was investigated using analytical and numerical methods. The mechanical features of unsaturated soils and reinforcing effects of the vegetation were implemented in 2D and 3D limit equilibrium analysis. The possible depth of the failure surface was calculated based on these simplified models and was compared with the depth of the real failure surface in the landslide triggering experiment. The soil-bedrock interactions, in terms of the pattern of pore pressure distributions and their influence on stabilising or destabilising the slope, were studied and the results were compared to the field measurements.

Inhalt
1;Failure mechanisms in unsaturated silty sand slopes triggered by rainfall;1
2;Imprint;4
3;Dedication;5
4;Foreword;7
5;Acknowledgments;11
6;Contents;13
7;List of Figures;21
8;List of Tables;35
9;Kurzfassung;37
10;Abstract;39
11;1 Introduction;41
11.1;1.1 Motivation;41
11.2;1.2 Objectives of the project;43
11.3;1.3 Methodology and layout of the thesis;44
12;2 Literature review;47
12.1;2.1 Unsaturated soil mechanics;47
12.2;2.2 Triggering mechanisms of landslides due to rainfall;61
12.3;2.3 Previous field tests;66
12.4;2.4 Centrifuge modelling;69
12.5;2.5 Concluding remarks;73
13;3 Ruedlingen experiment field;75
13.1;3.1 Test slope;75
13.2;3.2 Characterisation;80
13.3;3.3 Field instrumentation;87
14;4 Ruedlingen soil characterisation;109
14.1;4.1 Grain size distribution and Atterberg limits;110
14.2;4.2 Volumetric properties of the soil in saturated and unsaturated conditions;110
14.3;4.3 Shear properties of Ruedlingen soil;116
14.4;4.4 Hydraulic properties of Ruedlingen soil;128
14.5;4.5 Compaction test;139
15;5 Slope monitoring and landslide triggering experiments;141
15.1;5.1 Slope monitoring experiment (October 2008);141
15.2;5.2 Slope monitoring under natural atmospheric conditions;160
15.3;5.3 Landslide triggering experiment;163
15.4;5.4 Comparison between the slope monitoring and landslide triggering experiments;185
16;6 Physical modelling in a geotechnical drum centrifuge;189
16.1;6.1 Scaling laws;190
16.2;6.2 The basic concept of the climate chamber;193
16.3;6.3 Rain simulator and the tool table;194
16.4;6.4 Bedrock;196
16.5;6.5 Instrumentation and monitoring;197
16.6;6.6 Centrifuge tests;201
16.7;6.7 Effect of bedrock shape on the stability of slopes;206
16.8;6.8 Drainage into the bedrock with buttress (test T10_1);210
16.9;6.9 Discussion: the effects of bedrock shape and drainage on slope stability;212
16.10;6.10 Exfiltration from bedrock;215
16.11;6.11 Effect of vegetation on slope stabilisation;218
16.12;6.12 Static liquefaction;228
16.13;6.13 Summary;232
17;7 Analytical and numerical simulations;233
17.1;7.1 Analytical modelling (based on Askarinejad et al., 2012b);233
17.2;7.2 Uncoupled 2D simulations of Ruedlingen experiments (based on Bischof, 2010 and Askarinejad et al., 2012b);240
17.3;7.3 Coupled hydro-mechanical simulations of the behaviour of slope during rainfall;247
17.4;7.4 Conclusions of the hydro-mechanical simulations;266
18;8 Summary, conclusions and outlook;269
18.1;8.1 Summary and conclusions;269
18.2;8.2 Suggestions for future work;274
19;9 References;277
20;Appendix A: Pixel-based slope monitoring;287
21;Appendix B: Centrifuge model preparation;291
22;Appendix C: Discharge charts and location of the nozzles;295
23;Appendix D: Direct shear test on vegetated soil;305
24;Appendix E: Viscous pore fluid and its effects on Ruedlingen soil;315
25;Appendix F: Calibration charts for the field sensors;323
26;Appendix G: DPL Results for Ruedlingen test slope (Askarinejad & Kienzler, 2008);325
27;Appendix H: Field measurements;337
28;Appendix I : Results of numerical simulations;355
29;Appendix J: Data from the geological borehole (Based on Brönnimann, 2011);361