The commonplace technology for bridges is either concrete or steel. These techniques require a high input of industrial materials and a low amount of labour. However, in rural Tanzania, it is very expensive to use industrial materials, while labour is cheap.

The stone arch technology capitalises on this fact by using readily available local materials in a labour intensive process. This leads on average to a reduction of 80 to 85% in construction costs, and a 50 to 80% reduction in carbon emission due to lower use of cement and reinforcement steel, when compared with conventional construction techniques.

Masonry bridges also avoid some of the disadvantages of conventional alternatives, such as the use of steel or concrete culverts. These frequently break during rough transport over bad roads from industrial centres and, because they are relatively light, they run the risk of being washed away during floods. In addition, such prefabricated elements are sometimes stolen. People dig them out and try to resell them to make a bit of extra money.

Stone arch bridges have been built for more than 2000 years and some of the early structures are still used today. The Fabricius bridge in Rome for example has two arches spanning 24m each and dates from 62 BC. Both the design and execution rely on simple rules of thumb that have been tried and tested through time. This avoids the need for complicated design and precision sensitive execution of steel and concrete bridges.

The stone arch technology provides a cost saving of 80% compared to concrete or steel bridges and can be implemented by a public tender procedure. Alternatively, the village and Tanzanian Rural Road Agency (TARURA) can be involved with distributed responsibilities: the village provides stones and sand, TARURA provides supervision and Enabel provides technical support. On top of the cost benefit, the approach creates ownership, builds capacity and empowers local leadership.

An average annual TARURA budget can finance 1 concrete bridge via a public tender procedure. The same budget can instead be used to finance 5 stone arch bridges via public tendering or 10 stone arch bridges by involving the local community.

Enabel has written a construction manual and translated it to Swahili so it can be used by TARURA.

Thanks to close collaboration with TARURA officials during the project, the technique attracts more and more interest. So far, it has already been promoted by the national director of TARURA. Three arch bridges are currently under construction without Enabel support and outside of Enabel’s intervention area, which is currently limited to the North-Eastern Kigoma region. Within the Kigoma region, TARURA has also started the construction of two stone arch bridges outside of the Enabel project.