Design of road cuttings in lower-income countries

Abstract

Roads are critical to the economic and social development of a region. When roads are constructed, they often require excavation into the ground which can result in a road cutting; a slope adjacent to the road which is inclined at a steeper angle than the natural topography. Road cutting failures can be hugely costly and disruptive when slope debris blocks the road and collides with infrastructure. They are most prevalent in regions of hilly topography and those that experience periods of heavy rain. However, natural susceptibility to failure is hugely exacerbated where there is poor design and construction of road cuttings and slope stabilisation (measures or protocol to improve the stability of a slope). This situation is most common in low and lower-middle income countries (LIC/LMICs) where there can be lack of resources, weak design standards and challenging environments. The aim of this thesis is to gain an understanding for the issues resulting in the poor design of road cuttings in LIC/LMICs and use the findings to motivate the development of practitioner-focused tools to improve the design and planning of stable road cuttings in LIC/LMICs. An understanding of the issues involved in road cutting design and slope stabilisation in LIC/LMICs is developed through conducting interviews with stakeholders involved in road cutting design and carrying out a field study to assess cutting stabilisation measures along roads in a LIC/LMIC, using Nepal as a case study. The first tool that is presented in this thesis is a methodology to develop user-friendly and rigorous road cutting design guide lines. This was motivated by the findings that there is a lack of expertise in government engineers and unrigorous existing road cutting guidelines in Nepal. The guidelines are de veloped according to commonly used failure criteria (i.e. Generalised-Hoek-Brown for rock and Mohr-Coulomb for soil) and stability analysis method (Limit Equilibrium Method, LEM). The upslope geometric boundary conditions and slope seepage are considered. Ac cessible descriptions to characterise the slope material and geometry are included in the guidelines to improve their ease of use by practitioners. The methodology is showcased by producing new guidelines for the design of road cuttings in Nepal and Ethiopia. The guidelines are developed to be used by practitioners for the preliminary design of road cuttings or to evaluate the stability of an existing cutting. This thesis also presents a methodology to determine the frequency of road cutting fail ures and time varying failure probability to aid road project planning and a cost-analysis methodology to justify the choice of road cutting stabilisation measures. These tools were motivated by the finding that there is poor communication between stakeholders, a gen eral lack of prioritisation of slope stabilisation in road projects, and no commonly used methodology for cost analyses. To determine a frequency of failure and time varying fail ure probability, firstly, multiple LEM stability analyses are performed with varied rock strength parameters for different groundwater levels imposed at the upslope boundary of a slope stability model. A phreatic surface time series is then generated by performing the hillslope-storage Boussinesq 1-D model. The outputs of these analyses are then coupled to determine a time dependent probability of failure and a frequency of slope failure. This methodology is utilised in the cost-benefit-analysis methodology presented in this thesis to determine the frequency of failures of different slope stabilisation measures. The direct costs associated with the failure of a road cutting (remediation of the slope and debris clearance) along with the initial investment costs and maintenance of slope stabilisation are combined to estimate a cost per annum of a stabilisation measure. This methodology is performed on a road cutting in Nepal to evaluate cost of different stabilisation measures commonly implemented along roads in Nepal. One of the most commonly implemented stabilisation measures, a mortared masonry wall, was found to have the highest cost per annum in this analysis whilst an anchoring system, with high initial costs, was found to have the lowest cost per annum. This methodology can be used by practitioners to de termine the most appropriate approach for a road cutting and justify this choice to other stakeholders