Rockfall hazard mitigation on infrastructures in volcanic slopes using computer-modelled ditches

Highlights

• Block density, hardness, stiffness, roundness and size show a direct correlation with the rockfall stop-distance.

• Triangular ditches with steeper foreslope gradient present higher retention capacity and better road safety than deep flat-bottom ditches.

• Block accumulation shows bimodal distribution for hard rock slopes, being unimodal for soft lithotypes.

• The ditch design charts proposed optimize the dimensions of rockfall catchment areas of previous studies.

• These defence ditches offer economic and environmental advantages compared to other structural solutions.

Abstract

Rockfalls on transport infrastructures are a serious hazard to users and many resources are invested in rock slope maintenance, stabilization, and protective measures. In volcanic territories, the risk of rock instabilities and rockfalls is very high due to the rugged natural slopes and origin of rock masses. With the aim of determining the influence of the geometric and material-related properties affecting rockfall motion and the effectiveness of catchment area design criteria, this study applies a computer simulation model considering 150 different slope configurations and ditch geometries, 4 types of materials and 9 size and shape combinations of falling rocks. A statistical analysis of the simulated rock stop-distances was performed. Results show that density, hardness, roundness and size are material properties directly correlated with the rockfall stop-distance. However, block accumulation distribution differs with the rock hardness. Furthermore, practical application design charts are proposed for infrastructure planning and design tasks. These offer the ditch dimensions depending on the relation between the optimal stop-distance and the cumulative percentage retained along the trajectory, complying with specific retention requirements, and optimize the dimensions of previous studies. A triangular ditch of foreslope steepness 14° offered better retention capacity and road safety than a deep flat-bottom ditch. These rockfall protection areas constitute non-structural defence measures of reduced environmental impact and cost in volcanic territories.

Introduction

In volcanic territories, such as many island regions with this geological origin, the risk of rock instability and rockfalls on transport infrastructures is prominent because of both topographical and lithological factors. The rugged natural relief makes it necessary to design roads and railways with limited width and steep adjacent slopes. The lithological characteristics, origin of rock masses, and even seismicity result in rock slopes with abundant potentially unstable blocks.

Rockfall protection does not have a single, clear solution. There is a wide range of possible situations that require specific treatment and engineering [1]. The use of catchment areas to reduce the hazardous consequences of rockfalls on transport infrastructures is a simple, economic and effective measure [2], [3], [4], [5]. It also means low environmental impact and easy maintenance. In fact, this is a competitive solution compared to stabilization structures (mesh, bolts, anchors) or defence constructions (dynamic rockfall barriers, retaining walls, fences, tunnels), that usually require important financial investment [4]. Catchment areas are therefore an ideal method for protection of infrastructures in developing countries or with limited economic resources.

Ritchie (1963) [2] identified the characteristics of rockfall motion and proposed a graphic design chart and tables to determine the minimum depth and width of catchment ditches according to slope height and gradient, establishing the impact distance of a rockfall as a function of the slope height and steepness. This author proposed a deep flat-bottomed ditch (up to 2 m) of variable width, connected to the roadway by a constant foreslope (1.25H/1V). This graphic chart and its version modified by the FHWA [6] represented a significant step forward in highway and railway protection design (Fig. 1). However, Ritchiés model is now seen to have some limitations: (a) it does not provide a cost criterion allowing for choice of the most suitable capacity of block retention for each slope section; (b) it offers results for only one geometry (trapezoidal ditch); and (c) this deep and steeply-sloped ditch design makes it difficult for vehicles to return to the roadway safely as well as difficult maintenance of roadway margins.

After Ritchiés research, some authors have evaluated the mechanics of rockfalls [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. The Oregon Department of Transportation (ODOT) carried out on site experimental work between 1992 and 1994, gathering data from three types of catchment areas with different gradients (1H/0V or flat, 6H/1V and 4H/1V). To validate this work, blocks from different slope heights (40, 60 and 80 feet) over a constant gradient (0.25H/1V) were rolled out [17]. The results provided an estimation of rockfall frequency, quantified the probability of blocks reaching the road, and verified the retention capacity of catchment areas. In 2001, the ODOT and the FHWA evaluated other configurations of the slope-ditch system. They rolled 11,250 blocks of different sizes over different slope gradients (0.25H/1V; 0.5H/1V; 0.75H/1V and 1H/1V) and from differing heights (40, 60 and 80 feet). In this occasion, three kinds of triangular ditches were evaluated (1H/0V; 6H/1V and 4H/1V). The results allowed new design charts to be drawn up [3].

A more economical and practical approach using numerous numerical simulation tools have been developed based on the rockfall motion equations and interactions between the blocks and the slope [18], [19], [20], [21], [22], [23]. Pantelidis (2010) [4] used “RocFall” computer program [21] to develop adapted graphic charts for catchment areas based on the Ritchie ditch: deep flat bottom, covered by a gravel layer and with vegetation coverage at the edges. His research was based on the results of 100 rocks falling over hard rock slope with a catchment area at the base. Moreover, Ref. [24] considered the use of additional structures (fences and concrete walls).

Consequently, the catchment areas have not followed standardized design criteria. Those designed by using empirical design charts may not be optimized and some might present unsafe conditions for road traffic. Moreover, there are no standard specifications for computer-designed catchment areas at present. As a result, there are many types of ditches —some oversized and others with low efficiency— which has led to higher costs and higher environmental impact. Thus, new design criteria that are more rational and quantitative must be found to solve this problem.

This study offers a useful tool to optimize the slope-bench-ditch system design, permitting easy evaluation of its retention capacity at the planning stage or even when built, and justification of any possible improvements. The criteria applied are quantitative and are based on numerical models. Five different geometric factors were assessed to determine the stop-distance of rockfalls: shape (F_b) and size (S_b) of the blocks, slope height (H_t), slope gradient (α_t), and foreslope steepness of the catchment ditch (α_d) [Fig. 2]. Both gradients (slope and ditch) are also expressed as a relation between the triangle sides (H/V). Moreover, other material-related factors such as density and hardness were also considered. This produces a wide combination of possible values in these inputs, generating multiple arrangements and output data.

The results obtained allow the estimation of the frequency of rock accumulation at different distances, quantify the probability of these blocks reaching the roadway and verify the retention capacity of the proposed catchment areas. These may be designed using the practical graphic charts produced in this study. These rockfall protection ditches constitute defence measures with a reduced environmental impact and much lower cost compared to other structural solutions. Furthermore, the interest and opportunity of this topic acquires special relevance in order to save costs at the planning stage of construction projects and also in the maintenance of transport infrastructures.