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The mechanics of secondary landslides from earthquakes: implications for slope engineering and landslide hazard.

Dr Bill Murphy (SEE), Dr Mark Hildyard (SEE)

Project partner(s): Dr Ranjan Dahal (Tribhuvan University of Kathmandu)

Contact email: w.murphy@leeds.ac.uk

Large earthquakes have the ability to trigger landslides over extensive areas. This frequently results in significant loss of life as well as the disruption of infrastructure, which hampers the ability to move emergency relief.  Such challenges were clear following the MW=7.8 Gorkha (Nepal) earthquake in April 2015 where landslides resulted in significant loss of life with the destruction of Langtang Village and relief access was problematic.  Marc et al (2015) noted after a strong earthquakes the rate of landslide activity remains higher for about 5 years but the mechanism of how this occurs is unclear. The consequences of earthquake-induced softening of slopes was evident during the MW=6.6 earthquake in Afghanistan on 10th April 2016 when subsequent rainfall triggered landslides that closed the Karakoram Highway in Gilgit-Baltistan region of Pakistan resulting in difficulties in food transport in an area which remains one of the poorest regions of Pakistan.  Therefore, in order to evaluate middle-lo ng term landslide processes and the changing engineering properties of rock masses in seismic zones, it is essential to understand the mechanics of these rock masses. Field evidence suggests that rock masses show significant disturbance with dilation of joints and small amounts of strain in slopes (see figure 1) causing a significant decrease in mobilised strength in rock masses. This is analogous to the damage caused to rock masses by blasting resulting in an excavation damaged zone. This is characterised by a significant loss of inter-block friction resulting in decreases in the mobilised strength of c. 16-20%. Such mechanisms have been confirmed at the laboratory scale by Lia et al (2001). Therefore, there is a significant geotechnical issue surrounding the degree of ground disturbance following strong earthquakes and it has a significant impact on landslide hazard and risk to infrastructure during subsequent seismic events, such as aftershocks, or during periods of intense rainfall. Given the limited understanding of the interaction between propagating waves in the shallow subsurface in slopes the damage to the rock mass this remains impossible to quantify. The aim of this project, is to understand that process.

Figure 1: Ground cracking associated with rock mass disturbance and incipient landsliding induced by the (left) 2011 Mw=6.3 Christchurch Earthquake in New Zealand and (right) the 1999 MW=7.6 Chi Chi earthquake in Taiwan.

Objectives:

The overall aim of the project is to understand a fundamental mechanical process in seismic zones that influences landslide hazard, slope engineering and landscape evolution. The project will involve elements of field and laboratory data collection, and numerical modelling. The relative emphasis of the different components of this project will be determined by the interests and skills of the candidate. The four broad objectives are:

1.    Field identification of rock mass disturbance and the engineering properties of rock masses;

2.    Establish an inventory of landslides induced in slopes subjected to strong shaking during the 2015 Gorkha Earthquake for model validation purposes.

3.    Use field based observations to derive realistic inputs for fractured rock masses to numerical models of slopes to evaluate modelled impacts on the rock mass.

Making an impact

Understanding the role of seismic waves on rock mass properties has two potential impacts. The primary aim of this project is to evaluate the impacts on the mechanics of rock masses. This has a role in understanding engineering performance of slopes after strong shaking or sudden stress relief (e.g. other landslides). This also has relevance in surface excavation of engineered slopes and the prediction of disturbance from wave propagation The role of shaking as a preparatory factor influencing landform evolution will help to explain the role of dynamic, transient events i.e. earthquakes as a major contribution to the geomorphology of mountain belts.

Training

The student will carry out field data collection under the direction of Bill Murphy and Ranjan Dahal in Nepal and numerical modelling with the supervision of Dr Mark Hildyard. The student will gain expertise in rock mechanics modelling using tools such as the industry code FLAC and the seismic modelling code WAVE, and will be encouraged to attend the Landslide Risk Assessment and Mitigation (LARAM) in Salerno.

Student profile

The student should have a background in geological sciences, geophysics, civil or mining engineering.  Geoscientists should have evidence of strong numeracy skills in their background while applicants from an engineering background should be able to demonstrate at least a basic understanding of geology.

References

Lia, N., Chen, W., Zhang, P. & Swoboda. G. 2001: Technical Note: The mechanical properties and a fatigue-damage model for jointed rock masses subjected to dynamic cyclical loading. International Journal of Rock Mechanics & Mining Sciences, 38, 1071–1079.

Marc, O. Hovius, N. Meunier, P. Uchida, T., & Hayashi, S. 2015: Transient changes of landslide rates after earthquakes, Geology, 43, 883-886 doi: 10.1130/G36961.1

Related undergraduate subjects:

  • Civil engineering
  • Geological science
  • Geophysics
  • Mining engineering