Do slopes have a favourite shape? Innovative slope profiles for remediation of landslides and the extractive industry

Overview

Landslides increasingly pose a destructive hazard to people and infrastructure resulting in hundreds of deaths and billions of dollars of damage every year (Petley, 2012). In the last century, in Europe alone 16,000 people have lost their lives because of landslides (Safeland, 2012). In the period 1998-2009, 70 major landslides claimed a total of 312 lives and damaged or destroyed an extensive amount of infrastructure, including roads and houses. These major events are only a glimpse of the full impact of landslides, as the enquiry carried out by Eurogeosurvey yielded a total of 712,089 recognised mass movements recorded in Europe since World War II (Eurogeosurveys, 2010). The climate change foreseen to take place in the 21st century is expected to make landslide likelihood of occurrence progressively higher. All these data point to the need for an urgent step change in the mitigation of landslide risk through both reducing exposure and improving remediation against landslides.

Key remediation methods post small landslides consist of the use of heavy engineering solutions such as anchors, nails and /or geosynthetics or the reprofiling of the slope into a straight planar profile. The second solution is preferable from an environmental and aesthetical point of view (Schor and Gray, 2008). However, the vast majority of natural slopes manifest non-linear profiles. Several geomorphological studies of landscape evolution accounting for slope erosion and deposition phenomena point to the fact that straight profiles are never stable whereas landscapes undisturbed by anthropic activities tend to evolve towards equilibrium profiles whose shape in the cross section is partly concave and partly convex (Jeldes et al., 2018).

In the last 10 years, 4 independent systematic studies on the mechanical stability of slopes (Utili and Nova, 2007; Jeldes et al., 2015; Vahedifard et al., 2016; Vo & Russell, 2017), conclude that non-linear concave slope profiles are significantly more stable than the equivalent straight profile. Profiles of the same average inclination, i.e. sharing the same crest and toe points are considered equivalent. Also Rieke-Zapp & Nearing (2005) and Jeldes et al., (2015) show that concave profiles are better to reduce slope erosion.
Recent theoretical studies have led to a software (Utili, 2016) that systematically determines the optimal profile from a mechanical stability point of view for a given average slope inclination and ground type. These profiles exhibit a significant increase of the factor of safety, up to 40%, over their equivalent straight profile as long as the ground exhibits some degree of cohesion. These findings have a number of implications. First, slopes at risk of impending failure and slow-moving landslides could be remediated by slope reprofiling alone avoiding the use of hard engineering solutions. Second, adoption of these profiles in quarries and open cast mines could allow steeper safe-excavation achieving significant saving on the amount of material to be excavated. Third, these profiles represent a hypothesis for equilibrium form of landscapes responding to topographic perturbation whether by excavation or through a natural process (e.g. river incision, earthquake, or deglaciation).

The industrial partner for this project, Aggregate Industries Ltd, is the largest firm managing quarries in the UK. End users consist of SRK consultants Ltd, which is the largest firm of consultants in the world for the design of open pit mines headquartered in Cardiff, Network Rails, HS2 and Highways Agency for their role in commissioning the construction of large cuttings and embankments.

The geomorphic significance of this work is in a confronting theory with the extensive and high resolution topographic data to understand where real landscapes do and don’t conform to the theory. This work has implications for both landscape evolution and for hazard mapping – though the possibility of identifying out of equilibrium landscape locations.
Project objectives
O1: identification of the suitable range of landslide sizes for which re-profiling is a viable method
O2: extension of the current analytical model based on limit analysis for the calculation of optimal slope profiles from 2D to 3D topographies.
O3: construction of the optimal profiles by re-profiling existing landslides and quarries in the UK.
O4: monitoring of slope performance over time by Ground based SAR, InSAR and geotechnical suite of tools.
O5: topographic analysis to test the extent to which and timescales over which landscapes evolve to fit optimal profiles

Methodology

The project design and methodology is closely structured around four key objectives which also provide a timeline for the sequence of study.
The main steps of the PhD programme include:
1) identification of the suitable range of landslide sizes. This will entail working closely with contractors to accurately estimate costs of intervention so as to identify the maximum and minimum volume of landslip for which topographic re-profiling may be a cost-effective intervention.
2) extension of the current numerical model (Wu, 2017) for the determination of the optimal slope profile to 3D topographies. In the model the formation of tension cracks (Utili, 2013) and seismic conditions (Utili & Abd, 2016) will be accounted for.
3) construction of the optimal profiles to be undertaken at a few Aggregates UK sites and other landslide sites.
4) monitoring of ground water pore pressures in the slope and erosion over time employing a suite of geotechnical tool, e.g. piezometers, and remote sensing like InSAR (with guidance from Prof Zhenhong Li at Newcastle) and terrestrial laser scanning (with guidance from Prof. Nick Rosser).
5) Topographic analysis will draw on freely available topographic data across a range of scales and resolutions and will involve applying 2- and 3-dimensional theoretical equilibrium slope forms scaled by the ridge and river networks. Global scale analysis will draw on global elevation data at 30 m resolution (e.g. SRTM 30) and processed using powerful cloud computing architecture (e.g. Google Earth Engine). Finer resolution analysis will be targeted to study areas identified to control for perturbation type (e.g. river incision, earthquake, or deglaciation), material properties (e.g. material strength and heterogeneity), and perturbation timing (e.g. varying time since glacial de-buttressing). These analyses will be targeted to take advantage of existing freely available topographic data at high resolution and will be processed using flexible desktop routines (e.g. TopoToolbox in Matlab) to enable a wider range of topographic analyses than is possible at global scale.

Project Timeline

Year 1

Year 1
i) Training in the use of the software coded in Matlab for the design of optimal slope profiles.
ii) Extension of the numerical model to 3D topographies.
iii) Inclusion of erosion resistance among the criteria to determine the optimal profile

Year 2

Year 2
i) Identification of suitable sites.
ii) Secondment at Aggregates UK to identify quarry sites and need for any geotechnical investigation to properly characterise the chosen sites.
iii) construction of a few optimal profiles by re-profiling existing slopes at risk of impending failures and quarries in the UK.
iv) Setting up of the geotechnical instrumentation for the long-term monitoring of the optimal profile slopes.
v) Local scale (fine resolution) topographic analysis and development of topographic analysis methodology.

Year 3

Year 3
i) Analysis of the monitoring data against the expected behaviour from the model in terms of mechanical stability and erosion resistance.
ii) Global scale topographic analysis in Google Earth Engine. https://earthengine.google.com/
iii) Refinement of the model in the light of the monitoring results

Year 3.5

Year 3.5
i) PhD thesis write up
ii) Scientific publications write up

Training
& Skills

The School of Engineering at Newcastle University requires each student to collect at least 60 PGRDP credits, corresponding to attendance of in-school delivered workshops, taught modules and other activities that display further engagement. The training in (a) research skills and techniques and (b) research environment are provided through four mechanisms: (i) a programme of taught modules; (ii) internal training ‘workshops’ that focus on key geographical research skills and techniques; (iii) input from supervisors; and (iv) School and academic Group seminars by visiting and internal speakers and presentations by postgraduate students themselves.
In addition to generic training offered by the University, the School also provides training through a series of in-house ‘workshops’. Engineering research postgraduates normally take the following Workshops: ‘Scientific Writing’, ‘Research Ethics (Theory)’, ‘Data Management’, ‘Time management’, ‘Document Management – Content and Layout’, ‘Introduction to Learning and Teaching’ during their first year. Also, it is envisaged that the student will undertake from 2 to 4 taught modules depending on the academic background of the appointed student of the MSc in ‘Engineering Geology’ and ‘Mapping and Geospatial Data Science’ . Modules particularly relevant for the project are ‘Geotechnical design’, ‘Geomechanics’, ‘Observation processes and analysis’, ‘Geohazards and Deformation of the Earth’. Most of these modules are delivered in one intensive week so well suited for PhD students.
Research training continues through the second and third years, and is based around a number of themes: (i) Recognition and validation of problems; (ii) Demonstration of original, independent and critical thinking, and the ability to develop theoretical concepts; (iii) Knowledge of recent advances within the research field and in related areas; (iv) Understanding relevant research methodologies and techniques and their appropriate application within the research field; (v) Ability to analyse and critically evaluate findings and those of others; and (vi) Summarising, documenting, reporting and reflecting on progress.
Bespoke technical training will also be provided by the research supervisors (numerical and analytical modelling of slope instabilities, geomechanical principles for the design of slope profiles, use of the National Landslide database,) and technical staff in the School of Engineering.

References & further reading

Abd, A. 2017 Geosynthetic-reinforced and unreinforced soil slopes subject to cracks and seismic action: stability assessment and engineered slopes. PhD thesis, Warwick.

Crosta GB., Utili S., De Blasio, Castellanza R. 2014. Reassessing rock mass properties and slope instability triggering conditions in Valles Marineris, Mars. Earth and Planetary Science Letters, 388: 329-342.

Eurogeosurveys, 2010. Annual report. Available at http://www.eurogeosurveys.org/about-us/our-products/

Guzzetti, F., Ardizzone, F., Cardinali, M., Rossi, M. and Valigi, D., 2009. Landslide volumes and landslide mobilization rates in Umbria, central Italy. Earth and Planetary Science Letters, 279(3), pp.222-229.

Jeldes, I.A, Drumm E.C., Yoder D.C. 2018. Sustainable slopes satisfying rainfall-erosion equilibrium and mechanical stability. Transactions of Am. Soc. Agricultural & Biological Engineers, 61(4): 1323-1333.

Jeldes, I.A, Drumm E.C., Yoder D.C. 2015. Design of stable concave slopes for reduced sedimentary delivery. ASCE Jnl Geotech & Geoenv Eng, 141:4040-93.

Petley, D., 2012. Global patterns of loss of life from landslides. Geology, 40(10), pp.927-930.

Safeland, 2012. Living with landslide risk in Europe; Assessment, effects of global change, and risk management strategies. Summary report, available at www.safeland-fp7.eu/

Schor, H.J., Grey, D.H. 2008 Landforming an environmental approach to hillside development mine reclamation and watershed restoration. John Wiley and Sons, New York, N.Y. (USA).

Schor, H.J., Grey, D.H. 2013 Landform/geomorphic grading for sustainable hillside developments. ASCE Congress on Stability and Performance of Slopes and Embankments III, Geo-Congress Issue 231 Geotechnical Special Publication: 1460-1471

Rieke-Zapp, D.H., Nearing, M. 2005. Slope shape effects on erosion: A lab study. SSSA J., 69:1463-71.

Utili, S., 2013. Investigation by limit analysis on the stability of slopes with cracks. Geotechnique, 63(2),140.

Utili, S., Abd, A. 2016. On the stability of fissured slopes subject to seismic action. International Journal for Numerical and Analytical Methods in Geomech, 40:785-6.

Utili, S., 2016. Software for the determination of optimal profiles for slopes and pitwalls.

Utili, S., Nova R. 2007. On the optimal profile of a slope. Soils and foundations, 47(4): 717-729.

Vahedifard, F., Shahrokhabadi, S., Leshchinsky, D. 2016. Optimal profile for concave slopes under static and seismic conditions. Canadian Geotechnical J., 53:1522-32.

Vo, T., Russell, AR. 2017. Stability charts for curvilinear slopes in unsaturated soil.Soils & Foundations, 57: 543-56.

Wu, W. 2016. A semi-analytical approach utilising limit analysis for slope stability assessment and optimal design. PhD thesis, University of Warwick.

Further Information

Prof Stefano Utili
email: stefano.utili@newcastle.ac.uk
Telephone: +44 (0)19120 86414
webpage: https://www.ncl.ac.uk/engineering/staff/profile/stefanoutili.html#background

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