Rural road networks in low-income countries (LICs) and low/middle-income countries (LMICs) are critical for economic and social well-being. However, they are mostly unpaved, are of poor average condition, can be impassable after periods of rain and have high user costs. There is therefore a need to identify low-cost, proven sustainable solutions for rural roads in these countries. To this end, the UK Department for International Development commissioned a systematic review to identify and appraise technologies appropriate for low-volume rural roads that have enabled improved and sustainable rural access in LICs and LMICs. Its findings are summarised in this paper. The review found that there is an evidence base of engineering-related technologies (primarily associated with the use of materials and design philosophies) that could be used to improve the performance of gravel or earth rural roads and that proper construction and appropriate maintenance are vital for the technologies to be sustainable in physical terms. However, the review argues that, since there are few empirical studies demonstrating the sustainability of rural road technologies, expert knowledge is needed to support the implementation of its findings.
1. Background
It is estimated that around a billion of the world's population do not have reliable all-season road access and, as a result, social and economic development is substantially constrained (Lebo and Schelling, 2001). In particular, rural communities in low-income countries (LICs) and low/middle-income countries (LMICs) rely completely on access to low-volume rural roads (LVRRs) for the pursuit of social interaction, access to schools, health facilities, the workplace, markets and basic needs such as clean water (Akpan, 2014; Knox et al., 2013). However, the vast majority of these roads are unpaved (typically more than 90%) and suffer from inadequate maintenance.
Predominantly, LVRRs in LICs and LMICs are earth or are made from gravel. Earth surfaces are normally unable to provide all-season access in many regions, and both of these low-cost surface types require regular routine maintenance of camber and drainage systems. Gravel surfaces also require regular periodic maintenance to replace gravel loss, which can be extreme in many environments, and is relatively expensive compared with the road's initial cost. Replacing lost gravel, which is a finite resource, can be unsustainable. As a result, LVRRs in LICs and LMICs are often in poor condition, can be impassable after periods of rain and have high road-user costs. Climate change is exacerbating this situation as many regions in LICs and LMICs are experiencing more extreme weather events.
The UK Department for International Development (DFID) commissioned a systematic review of the literature with the aim of identifying and appraising technologies appropriate for LVRRs that have enabled improved and sustainable low-volume rural access in LICs and LMICs. This paper describes the results of the review and discusses how expert knowledge can be included within the process to enable the findings to be adequately applied.
2. Systematic review
Often used to inform policy and practice, a systematic review is a critical appraisal and synthesis of research findings carried out using explicit, systematic and transparent methods (Gough et al., 2013). The systematic review described in this paper addressed the following questions.
What is the evidence supporting the technology selection for LVRRs in LICs and LMICs?
What evidence is there to support the sustainability of different rural road technologies?
2.1 Definitions
The systematic review followed a search protocol based on the following definitions. The World Bank's definition of countries was used to identify LICs and LMICs (WB, 2015). LVRRs were considered to be roads with an annual average daily traffic (AADT) of up to 300 motor vehicles per day (mvpd) and a design cumulative traffic load of less than 0·5 million equivalent standard axles (ESAs).
Technology for LVRRs was considered to be associated with the planning and building of new roads, providing all-season access through upgrading existing earth and gravel roads, and carrying out maintenance. Technology therefore was taken to include
resources (materials, labour, equipment, capital/credit)
management tools (e.g. for economic appraisal, planning, computer software)
design, construction and maintenance methods.
The review focused on studies reporting a range of outcomes associated with the implementation of LVRR technology. A technology was considered to be sustainable if it had ensured the capability of an LVRR to perform to its planned, designed and constructed standards, with the available financial and physical resources, using local operational arrangements and in the local environment.
2.2 The search approach adopted
The review questions lent themselves to an unbiased aggregation approach to identify studies that demonstrate the sustainable use of technology in different contexts (Gough et al., 2013). The strategy tried to identify longitudinal studies that had been carried out over a significant part of the life cycle of an LVRR. The sources used were websites of organisations involved in the road sector, bibliographic databases, internet search engines, hard copies of books and journals, reference lists and professional reports. A systematic review software application (EPPI-Reviewer 4) facilitated the review and was used for screening, coding, analysing and storing retrieved documents (Thomas et al., 2010). The search process is summarised in Figure 1.
2.3 Weight of evidence: assessing the quality of studies
A weight of evidence (WoE) framework was used to assess the quality and relevance of the included studies in three categories – soundness of the study, appropriateness of study design for answering the review question and relevance of the study focus to the review (see Table 1). The studies considered were required to achieve a high rating in at least two categories and a medium rating in the third.
Weight of evidence (Gough et al., 2013)
| Weight of evidence | |
|---|---|
| Soundness | |
| |
| Appropriateness of study design | |
| |
| Relevance of the study focus | |
|
2.4 Synthesis of evidence
The data were synthesised, using narrative methods, to indicate the sustainability of the technology as a function of the parameters that affect road pavement performance, namely geometry, structural design, maintenance history, traffic composition and natural environment (i.e. climate, soil type and topography).
3. Summary of main findings
Fifteen high-quality studies were identified which provided quantitative data that could be synthesised to answer the primary review question. Fourteen of the studies were associated with the materials used for the construction of LVRRs (see Figure 2) and one was to do with the means of carrying out maintenance.
3.1 Road pavement construction types
3.1.1 Block surfaces
Two studies (Roughton, 2013; TRL, 2009) described the results of trials to assess the performance, over 2–5 years, of four types of block surfaces – fired clay bricks, concrete bricks, dressed stone and cobble stones (see Figure 3).
The extent to which block pavements may be considered a viable option for LVRR surfacing was found to be influenced by
the quality of construction
compliance with brick crushing strength specifications (20–25 MPa)
timely routine maintenance
low tensile strength of joints.
The studies showed that all of the block surfaces considered are sustainable options and are particularly suited to high-rainfall (> 2000 mm/year) and weak-subgrade environments (i.e. California bearing ratio (CBR) less than 10%). The most durable surfaces were found to be dressed stone and cobble, which could also be considered to be the most sustainable when the stones can be locally sourced and shaped. Fired clay and concrete bricks are less sustainable since energy is required for their production. However, dressed stone/cobble surfaces have a high roughness and therefore may not be suitable in cases where it is important to minimise road-user costs. The high roughness of these surfaces was also found to encourage motorcyclists and cyclists to use the road shoulder, causing edge wear that, over time, may cause water ingress into the pavement structure and thus accelerate deterioration.
3.1.2 Concrete road surfaces
Concrete road surfaces appear to provide an advantage over most other surfaces since they have low roughness when constructed properly and require little maintenance other than to the joints. However, they have high initial construction costs and are therefore mainly appropriate when all-season access with low road-user costs is required.
Two studies (Roughton, 2013; TRL, 2009) reported trials of concrete LVRRs in Vietnam, Cambodia and Lao PDR. Both studies described trials of bamboo-reinforced and non-reinforced concrete slabs, one described the use of a concrete geocell and the other assessed steel-reinforced concrete. The studies suggest that all four types of concrete surface performed satisfactorily in all environments considered. However, the performance of the surfaces in terms of roughness was shown to be directly related to the quality of construction. Inter-slab joints required maintenance after 2–3 years of operation. Steel-reinforced concrete pavements were found to perform only marginally better on weak subgrades compared with non-reinforced options and, as steel is a finite resource and relatively expensive, it may be surmised that steel is not a sustainable (or necessary) option.
Both studies showed that concrete sections reinforced with bamboo performed at least as well as those without reinforcement although bamboo-reinforced slabs were found to be more expensive to manufacture, in part due to the requirement to treat them chemically to prevent deterioration. However, a detailed supporting analysis carried out as part of one study (Roughton, 2013) found that the bamboo disintegrated over time and did not therefore provide reinforcement in the long term.
3.1.3 Sealed surfaces
Sealing roads helps to prevent moisture ingress and provides a satisfactory all-season running surface that can reduce road-user costs through lower road roughness. As a consequence, seals can allow for the use of weaker (marginal) materials within the road pavement or a reduced thickness of more competent materials.
Ten studies assessed the performance of a variety of seals (see Table 2) and established that sealed LVRRs are sustainable from a durability point of view. Many of the roads examined were found to be performing adequately after the expected lifetime of the seal despite little or no maintenance, overloading in a number of cases and the use of materials below the recommended standards. One study (Pinard, 2011) suggested that this might indicate excessive over-design.
Sealed road studies
| Reference | Technology | Trial | Environment | Outcome: positive (+) or negative (−) sustainability |
|---|---|---|---|---|
| Bhasin et al. (1987) | Within sealed and unsealed roads | 5-year monitoring of trial sections; India | Average annual rainfall: <1000 mm/year AADT: < 300 | (+) Seals render Kankar sustainable as base/sub-base |
| Gourley and Greening (1999) | Seals for quartzitic, lateritic, calcareous gravels and sand road/sub-bases | Back-analysis of roads; Botswana, Malawi and Zimbabwe | Climate: Weinert N-values of ∼2 < N < 5 Design traffic: 0·05–0·8 million ESAs Subgrade: CBR 15–30% | (+) Seals enable natural gravel road base materials to be used successfully |
| Lionjanga et al. (1987) | Locally available calcrete bases and sub-bases in sealed LVRRs | 7-year study of 9 × 100 m sections; Botswana | Rainfall: 200–800 mm/year, during experiment AADT: 180–260 mvpd, during experiment Gradient: 1 in 6 | (+) Calcretes as road bases and sub-bases (−) Stabilised materials |
| Newill et al. (1987) | Seals for volcanic cinder gravel bases and sub-bases | 7·5-year trial road sections; Ethiopia | Rainfall: ∼750 mm/year AADT: 150–200 mvpd | (+) Surface dressed sections (−) Unsealed gravel roads |
| Pinard (2011) | Two-layered sealed, upgraded gravel LVRRs | Back-analysis of two- and three-layer roads; Malawi | Rainfall: 600–1200 mm/year Traffic: ∼300 mvpd/0·5 million ESAs | (+) Two-layer pavement system for upgrading gravel roads |
| Rolt et al. (2013) | Marginal materials | Back-analysis of low-volume sealed road up to 10 years old; Mozambique | Rainfall: 532–1288 mm/year AADT: 166–993 mvpd | (+) Structural condition (+−) Functional condition |
| Roughton (2013) | Double Otta, single Otta with sand and sand seals | 5-year trial of road sections; Lao PDR | Rainfall: 1300–1500 mm/year Traffic: 126 000 ESAs Gradients: flat (0–3%) to steep (10%–15%) | (+) Otta seals with routine maintenance and periodic resealing (5 years for single seal, 10 years for double seal) |
| Sahoo et al. (2014) | Thin (20 mm) bituminous surfacing of LVRRs | 6·5-year monitoring roads built to Indian Road Congress specifications | Rainfall: 1435–2252 mm/year AADT: 68–281 commercial vehicles per day Subgrade: 41–119 MPa | (+) Assuming maintenance after 5 years, 84% of sections performing satisfactorily |
| TRL (2009) | Thin flexible seals: double stone chip/sand bitumen emulsions; triple bitumen surfacing; penetration macadam; Otta seal; dry/water bound macadam; single and double bitumen stone chip seal (DBST); armoured gravel | 24–37-month monitoring of roads built to dynamic cone penetrometer design method; Vietnam and Cambodia | Rainfall: 1400–3000 mm/year Traffic: 1000–330 000 ESAs Gradients: flat to steep | (+) Penetration macadam and bitumen DBST emulsion most durable options |
| Wason and Oli (1982) | Moorum road bases/sub-bases in sealed roads | 20 sections monitored four times in 16 years; India | Rainfall: < 1000 mm/year Traffic: 100–150 carts and 10–15 heavy vehicles per day | (+) Sealed surfacing enables use of moorum as base/sub-base |
The review found that the extra thickness of surfacing material provided by double surface dressings and Cape seals makes them the most durable of the seals considered in a wide range of environments, particularly where gradients are steep, albeit at the expense of higher initial construction costs. In low-rainfall environments where low road roughness is a requirement (e.g. to transport agricultural goods to market undamaged), an emulsion sand seal may be an appropriate solution as it provides a satisfactory running surface and can be produced and maintained using locally available resources at low cost. Where annual rainfall is in excess of 2000 mm/year and low road roughness is required, bitumen, macadam-based seals or emulsion seals with stone chips may be a more appropriate choice. However, it should be noted that most seals require the use of scarce material resources and, with the exception of emulsion stone chip seals, necessitate the use of mechanical equipment for construction and maintenance.
3.1.3.1 Construction and maintenance requirements of sealed roads
Poor construction quality was found to greatly affect the performance of both the seal and the road pavement. The studies reported that contractors who had developed prior experience of seals through the construction of trial sections performed better than contractors without such experience.
Seals deteriorate over time and it is therefore vital that timely routine maintenance is carried out to fix edge breaks, patch potholes and seal cracks. Routine maintenance prevents water ingress into the road structure and therefore prevents softening of the subgrade and possible premature failure. Routine maintenance notwithstanding, periodic re-sealing is required after approximately 5 years (for single seals) and 10 years (for double surface dressed seals) depending on the environmental conditions and, to a lesser extent, on the cumulative traffic loading.
3.1.3.2 Base and sub-base performance
The performance of seals can be influenced by the base, sub-base or subgrade performance. Lionjanga et al. (1987), for example, demonstrated a link between reflection cracking in seals and lime- or cement-stabilised bases.
3.1.3.3 Shoulder sealing
Four studies (Newill et al., 1987; Rolt et al., 2013; Roughton, 2013; TRL, 2009) demonstrated the benefit of sealing the shoulders of LVRRs, enabling a more stable moisture content regime to be maintained under the road pavement, particularly during periods of high rainfall. Sealed shoulders, in conjunction with an adequately designed road drainage system, facilitate the movement of moisture away from the wheel track, thereby preventing softening of load-bearing fine-grained subgrades and hence inhibiting accelerated road deterioration.
3.1.4 Bases and sub-bases
The ten studies listed in Table 2 also reported the performance of low-volume rural sealed roads constructed with a variety of bases and sub-bases. The studies found that, in general, LVRRs performed satisfactorily from a functional and structural point of view provided that the road was sealed, designed appropriately and well-constructed. Without periodic maintenance, roads with a single surface dressing started to show signs of significant deterioration after approximately 5 years (i.e. when resealing would be expected).
3.1.4.1 Unconventional pavement designs
Many road design procedures used in LICs suggest three-layer road pavement systems to adequately carry traffic loads experienced on high-volume roads. These designs have been adopted for LVRRs despite their low traffic volumes. However, two slice-in-time studies by Rolt et al. (2013) and Pinard (2011) investigated the performance of a number of two-layered sealed LVRRs founded on relatively strong subgrades (i.e. CBR ≥ 30%). The studies showed that, in many environments, two-layer designs perform satisfactorily (and beyond their design life in many cases), reducing construction costs by 166–233%.
3.1.4.2 Stabilisation
In three of the studies (Lionjanga et al., 1987; Newill et al., 1987; Wason and Oli, 1982), the performance of marginal materials was found to be enhanced by chemical stabilisation (with lime and/or cement) provided that the stabilisation had been applied according to appropriate standards. Newill et al. (1987) also demonstrated that the behaviour of marginal materials with inappropriate grading characteristics could be enhanced by mechanical stabilisation with fines. Lionjanga et al. (1987) found that roads with bases made of stabilised calcretes did not perform satisfactorily. This was attributed to the lack of a stabilisation reaction in the calcrete and the consequent instability of the bases under traffic loads.
3.1.5 Engineered earth roads (EERs)
Although EERs provide the majority of access routes for most communities in LICs/LMICs, only one high-quality study was identified. This slice-in-time study by Rolt et al. (2008) assessed the performance of a large number of existing EERs in Cambodia. The different environments considered by the study were of limited variety in comparison with those that occur in all LICs/LMICs and therefore the study may be considered of limited applicability. Nevertheless, the study showed that a wide range of soils can be used to provide an adequate surface for motorised traffic of up to 50 vehicles per day (vpd) and higher (particularly if heavy trucks are absent) and in climates with rainfall of up to 2000 mm/year.
3.1.6 Construction and maintenance requirements of EERs
Taking into account the findings from the study of Rolt et al. (2008), the generic prerequisites for the sustainable use of EERs include the following.
In situ soil soaked strength of CBR ≥ 15%, although findings from research in non-LICs (Ahlvin and Hammitt, 1975) suggest that lower strengths (CBR > 8%) may be sufficient albeit with higher maintenance requirements. Figure 4 shows the in situ assessment of the strength of an EER.
Adequate design, with emphasis on drainage including sufficient camber, side, turnout and cross-drainage arrangements.
Regular routine maintenance of camber and drainage.
A longitudinal gradient, which progressively increases surface erosion and maintenance requirements, limited to below 6%.
3.1.7 Gravel roads
For many years, natural gravel has been the commonly accepted solution for providing all-season rural access in developing regions. However, limited and depleted sources of gravel, life cycle costs and maintenance and environmental sustainability issues have prompted reconsideration of its use.
The included studies (Table 3) examined the performance of gravel roads in terms of road condition and the amount of gravel loss as a function of a number of factors, including gravel type and the environment. Although technically feasible for a wide range of situations, the sustainability of natural gravel surfacing was shown to be vitally dependent on a range of influential factors. The factors include
achieving appropriate specifications relating to particle grading, plasticity and strength
restricting application to roads carrying traffic of less than 200 vpd
restricting application to environments with a rainfall of less than 2000 mm/year and longitudinal gradients of less than 6%
ensuring the provision and maintenance of adequate camber and run-off arrangements through side drains, turnout (mitre) drains and cross-drainage
timely provision of re-gravelling to replace material losses.
Gravel road studies
| Authors | Technology | Trial | Environment | Outcome: positive (+) or negative (−) sustainability |
|---|---|---|---|---|
| Bhasin et al. (1987) | Within sealed and unsealed roads | 5-year monitoring of trial sections; India | Average annual rainfall: < 1000 mm/year AADT: < 300 | (+−) Sustainable within a sealed road pavement only |
| Cook and Petts (2005) | Gravel roads | Back-analysis of roads; Vietnam | Rainfall: 800–4000 mm/year AADT: < 200 Gradient: > 6% | (−) Gravel roads unsustainable |
| Hodges et al. (1975) | Lateritic, coral, volcanic, quartzitic, gravel | 2-year monitoring of existing roads; Kenya | Rainfall: 400–2000 mm/year AADT: 25–200 Gradient: 0–5·5% | (−) Gravel roads unsustainable |
| Jones (1984b) | Lateritic, coral, volcanic, quartzitic, gravel | 2–4-year monitoring of existing roads; Kenya | Rainfall: 500–2000 mm/year Annual traffic: 1533–81 760 | (−) Sustainable for 50–200 vpd depending on gravel |
| Newill et al. (1987) | Volcanic cinder gravel surfaces | 2-year monitoring of sections; Ethiopia | Rainfall: ∼750 mm/year AADT: 150–200 vpd | (−) Gravel roads unsustainable |
| Roughton (2013) | Gravel control sections within a wider study of alternative surfacing | 5-year monitoring of trial sections; Lao PDR | Rainfall: ∼1500 mm/year Traffic: 126 000 ESAs Gradient: flat (0–3%) to steep (10–15%) | (+−) Sustainable when sealed and providing that resealing can take place periodically |
| TRL (2009) | Gravel control sections within a wider study of alternative surfacing | 24–37-month monitoring of trial sections; Vietnam and Cambodia | Rainfall: 1400–3000 mm/year Traffic 1000–330 000 ESAs Gradient: flat to steep | (+−) Sustainable when sealed and providing that resealing can take place periodically |
3.2 Road maintenance approaches
3.2.1 The impact of road maintenance approaches on LVRR sustainability
High-income countries aim to mechanise maintenance because labour is expensive and productivity can nearly always be increased using technology. However, in LICs and LMICs, heavy plant and its operation are significantly more expensive than readily available labour. In addition, heavy plant (and replacement parts) are mainly imported and are therefore problematic to maintain. The maintenance of unpaved LVRRs thus offers scope for both intermediate equipment and labour.
The effectiveness of three different maintenance approaches (heavy equipment, intermediate equipment or labour-intensive methods) was reported by Jones (1984a). This study showed that intermediate technologies that were less expensive in terms of capital expenditure (e.g. tractor-towed graders (Figure 5)) and more labour-intensive methods could be considered at least as sustainable as heavy equipment (e.g. a mechanical grader). However, to achieve similar results it was found that labour-intensive technology requires adequate supervision and less expensive technologies require more frequent maintenance cycles.
4. Discussion
The review showed that the large majority of the technologies identified may be considered to be sustainable from an engineering point of view provided that their design, construction and maintenance are robust. However, the behaviour of roads over time is complex and can be affected by a number of factors, including the environment in which they operate, the design specification to which they are built, the way they are constructed, the quality of construction, the behaviour of their constituent materials and the frequency and effectiveness of their maintenance regimes. Furthermore, the behaviour of an individual component within a road is also influenced by the performance of other components. Therefore, since LVRRs deteriorate over time, monitoring programmes lasting at least until a component of the road has reached the limits of its physical life are required to properly assess durability. During this time, the performance of the road and the environment should also be periodically recorded. Since many sealed LVRRs are designed to last in the region of 20 years, with perhaps the application of two or three planned periodic maintenance treatments in that time, it may be argued that such experiments should take place over at least two planned maintenance cycles so that the effect of maintenance may be established and life cycle costs determined. Clearly, these types of experiments are costly and problematic to undertake.
This review found only one study that was carried out over such a period (Wason and Oli, 1982), although road condition data were captured on only four occasions. Five other studies reporting experiments to determine the performance of LVRRs and their components were carried out over 5–7·5 years and one other study described an experiment lasting 2–3 years (see Tables 2 and 3). However, none of these studies captured the impacts of maintenance. Three other slice-in-time studies (Gourley and Greening, 1999; Pinard, 2011; Rolt et al., 2013) captured the performance of roads at different stages of their life cycles and considered the impacts of maintenance. Five studies indicated the environmental conditions in which gravel and/or earth roads may perform satisfactorily; one of these took into account the effects of maintenance (Jones, 1984a).
Considering that the vast majority of road transport routes in developing regions are still EERs and many communities rely on these for access, there appears to be a lack of research into EERs.
To this end and to address the above issues in an integrated fashion, Table 4 provides a framework to assess the sustainability of the technologies against some key indicators. The average score provided was calculated assuming that each of the key indicators has the same weighting and should therefore only be considered a guide to the sustainability of any of the technologies in a given context or environment. To use the approach in practice, a weighting should be applied to each indicator to reflect the particular environment at hand. For example, in a region that has a relatively strong subgrade and where it is necessary to have roads of low roughness (e.g. to minimise the cost of transporting crops that are easily damaged by rough roads), a lower weighting could be given to the indicator ‘Suitability for use on weak subgrades’ and a relatively higher one to that associated with ‘Achieved road serviceability’. A variety of approaches that can utilise expert opinion to determine weighting factors can be used (e.g. Saaty, 1980).
Trial technologies against some key markers (after Roughton, 2013; TRL, 2009): 1 = advantage; 2 = possible advantage; 3 = neutral; 4 = disadvantage. An average value of less than 3 may suggest that the technology may be considered to be sustainable when used appropriately
| Technology | Measure of sustainability | Average score | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Local/marginal material | Labour based | Simplicity of construction | Maintainability | Suitability in high rainfall environs | Suitability for use on weak subgrades | Small-contractor suitability | Local economy advantages | Resistance to axle overloading | Initial cost | Possible whole-life cost advantage | Environmental impact | Achieved road serviceability | ||
| Sealed surfaces | ||||||||||||||
| Emulsion sand seals | 1 | 1 | 1 | 3 | 4 | 3 | 1 | 2 | 3 | 1 | 3 | 2 | 2 | 2·1 |
| Emulsion stone chip seals | 3 | 1 | 2 | 2 | 2 | 3 | 1 | 2 | 3 | 2 | 2 | 2 | 2 | 2·1 |
| Sealed dry-bound macadam | 3 | 3 | 2 | 1 | 2 | 2 | 2 | 3 | 2 | 2 | 2 | 3 | 2 | 2·2 |
| Sealed water-bound macadam | 3 | 3 | 2 | 1 | 2 | 3 | 2 | 3 | 2 | 2 | 2 | 3 | 2 | 2·3 |
| Hot-bitumen stone chip seals | 3 | 2 | 2 | 2 | 2 | 3 | 2 | 2 | 3 | 2 | 2 | 3 | 2 | 2·3 |
| Penetration macadam | 3 | 3 | 2 | 2 | 2 | 3 | 2 | 2 | 3 | 2 | 2 | 4 | 2 | 2·5 |
| Unsealed water-bound macadam | 2 | 2 | 2 | 4 | 4 | 3 | 2 | 3 | 3 | 1 | 4 | 3 | 4 | 2·8 |
| Otta seals | 4 | 2 | 2 | 2 | 2 | 2 | 2 | 3 | 2 | 2 | 2 | 3 | 2 | 2·3 |
| Sealed armoured gravel | 2 | 3 | 2 | 2 | 2 | 3 | 2 | 3 | 4 | 2 | 2 | 2 | 2 | 2·4 |
| Block surfaces | ||||||||||||||
| Dressed stone/cobbles | 1 | 1 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 3 | 2 | 1 | 4 | 1·6 |
| Fired clay bricks | 1 | 1 | 2 | 2 | 3 | 1 | 1 | 1 | 2 | 2 | 2 | 2 | 3 | 1·8 |
| Concrete bricks | 2 | 1 | 2 | 2 | 3 | 1 | 1 | 2 | 1 | 3 | 2 | 1 | 2 | 1·8 |
| Concrete surfaces | ||||||||||||||
| Steel-reinforced concrete | 2 | 2 | 3 | 1 | 1 | 1 | 2 | 3 | 1 | 4 | 2 | 3 | 1 | 2·0 |
| Bamboo-reinforced concrete | 2 | 2 | 2 | 1 | 1 | 2 | 1 | 2 | 1 | 4 | 2 | 2 | 1 | 1·8 |
| Non-reinforced concrete | 2 | 2 | 2 | 1 | 1 | 2 | 1 | 2 | 1 | 4 | 2 | 2 | 1 | 1·8 |
| Bases | ||||||||||||||
| Lime-stabilised base/sub-base | 1 | 2 | 3 | 3 | 3 | 3 | 1 | 2 | 2 | 2 | 2 | 3 | 3 | 2·3 |
| Cement-stabilised base/sub-base | 1 | 2 | 3 | 3 | 2 | 3 | 1 | 2 | 2 | 2 | 2 | 2 | 3 | 2·2 |
| Emulsion-stabilised sub-base | 1 | 4 | 4 | 2 | 2 | 3 | 4 | 3 | 2 | 3 | 2 | 4 | 3 | 2·8 |
| Two-layer pavement | 1 | 3 | 1 | 3 | 2 | 2 | 1 | 2 | 3 | 1 | 3 | 3 | 2 | 2·1 |
| Unsealed natural gravel | 1 | 3 | 1 | 4 | 4 | 4 | 1 | 2 | 3 | 1 | 3 | 3 | 3 | 2·5 |
| Engineered earth road | 1 | 2 | 1 | 3 | 4 | 3 | 1 | 2 | 3 | 1 | 3 | 1 | 3 | 2·2 |
There were also other issues associated with assessing sustainability. For example, none of the studies considered in this review were able to address whether the trialled technologies were economically, socially and environmentally sustainable. Several studies, however, recognised the complexity of these issues (e.g. Roughton, 2013; TRL, 2009). Therefore, studies are required that will build on the concepts presented, such as environmentally optimised design (TRL, 2009). Other studies are required that consider, at the strategic level, the economic, environmental and social sustainability of a variety of rural road design, construction, maintenance and rehabilitation options. In the light of climate change, studies should also be carried out to examine how predicted changes in the climate and, in particular, the occurrence of extreme weather events may influence both strategy and design choices of LVRRs.
5. Road institutions context
It is important to note that the selection of a sustainable technology is related not just to the technology itself, but also to the institutional structure of the road administration in which the technology is implemented. For example, sustainability depends on the country context and on the parallel interventions that might be put in place, such as training of local engineers and contractors to make the chosen technology work as effectively as possible.
Selecting a sustainable technology needs to be viewed from the perspective of the decision maker who triggers the construction of a new road or agrees to provide funds for maintenance. This could be a donor agency, a road fund, the ministry of finance, local government, or another responsible ministry. Sustainability therefore needs to be considered in the context of the way road organisations are managed. Examples include the following.
- (a)
Roads managed and financed by a traditional district council. Typically, there is an acute shortage of qualified technical staff; work is often done by force account, but may also be carried out by local contractors. Design and supervision is problematic and therefore sustainable LVRR technology should be
- (i)
simple and easy to implement with little supervision
- (ii)
robust enough to remain serviceable without regular routine and periodic maintenance
- (iii)
inexpensive and use locally available materials
- (iv)
capable of having routine maintenance carried out by local villagers with minimal training.
- (i)
- (b)
Roads managed and financed by a central government roads department. Typically, a special-purpose rural roads department exists to manage roads. Such organisations are usually given reasonable budgets for maintenance and employ technically qualified staff including private-sector contractors. Sustainable technology in this context
- (i)
could use advanced engineering technology
- (ii)
should not require close supervision for construction and maintenance
- (iii)
should be robust enough to perform adequately under irregular (and often underfunded) maintenance.
- (i)
- (c)
Contracted-out management and maintenance. In this situation, the roads are managed and maintained by consultants and contractors working as agents for local road agencies. Within a competitive enabling environment, all road works are audited. In such cases, sustainable technology can use cutting-edge engineering technology because the work is properly designed, the contractor is effectively supervised and all work is subject to a detailed financial audit.
6. Other factors not present in the studies
At the outset of the review it was hoped that studies would be found that could provide evidence of the use of non-engineering-driven technologies to facilitate, for example
the development of policies and strategies
the appraisal of investment in rural road technologies – economic tools such as HDM-4 have been used on behalf of road agencies and donor organisations to assess the economic benefits of high-volume roads; such tools perform life cycle analyses based on agency and road-user costs but, in the LVRR context, it would be necessary to utilise the findings from these tools within a multi-criteria approach that may capture the wider socio-economic benefits more satisfactorily (e.g. Ortiz-Garcıa et al., 2005).
the management of construction and maintenance of LVRRs.
7. Conclusions
There is a sufficient evidence base of technologies that can be used to improve upon the functional and structural performance of earth or gravel LVRRs in LICs/LMICs. These technologies may be considered to be sustainable in engineering terms in specific environments. However, strictly from the evidence of the review alone, it is not possible to suggest that these technologies are financially, economically, operationally, environmentally or socially sustainable in all environments. However, the evidence suggests that well-designed roads using available resources, under good construction supervision and subject to appropriate maintenance practice will yield a sustainable road from a wide variety of materials in a wide variety of environments. Furthermore, the selection of sustainable technologies to suit any particular environment may be inferred from the body of existing research based on sound criteria and subsequent analysis drawing on the wealth of existing experience.
It is important to stress that the findings of this review should be considered in a holistic manner by taking into account not only the explicit engineering-driven knowledge extracted but also the wider social, economic and institutional environment of the road sector. By so doing, the value of the review may be maximised and the findings can be transferred to other contexts. This can be achieved with the use of engineering judgement and expertise that will aim at identifying the commonalities of the prevailing conditions. In particular, the sustainability of the technology should be examined following a methodology such as that suggested in this paper based on appropriately defined measures and associated weighting factors that sufficiently reflect the conditions in the environments concerned.
8. Recommendations for policy and practice
- (a)
To consider revising LVRR design standards so that they are performance-based rather than specification-based. This would encourage further innovation, including
- (i)
two-layered LVRR pavement designs
- (ii)
the use of marginal materials
- (iii)
the use of alternative surface materials
- (iv)
the development of environmentally optimised design
- (v)
feasible technology options
- (vi)
the development of more effective quality assurance regimes.
- (i)
- (b)
To encourage whole-life cost approaches that adequately consider construction and maintenance costs, vehicle operating, accident and environmental costs, and whole-life socio-economic benefits.
- (c)
To provide a facilitating environment that ensures that LVRRs are constructed to the given design, materials and construction standards.
- (d)
To introduce effective asset management practices.
- (e)
To consider institutional and political parallel interventions that may be in existence or put in place (e.g. the training of local engineers, contracts) to make the chosen technology work as effectively as possible.
Acknowledgements
The team from the EPPI-Centre is acknowledged with gratitude for its assistance throughout the review process. In particular, the continual guidance and comments provided by Ms Kelly Dickson throughout the review stage were invaluable. The patient assistance of Mr Jeff Brunton in helping the review team make best use of the EPPI-Reviewer software is also gratefully acknowledged. The review team are also grateful for the guidance and advice provided by the project's advisory group. The systematic review was funded by the DFID and the valuable advice provided by DFID's Mrs Liz Jones and Mr Suman Baidya throughout the review process is also noted with gratitude.
