Mizingani Sea Wall, Tanzania

Aurecon provides engineering consultancy services for the construction of the Mizingani Sea Wall and Associated Promenade in Zanzibar

“The Mizingani Sea Wall is a once in a life time type of project,” said Pieter van Heerden, Aurecon Associate.

Zanzibar’s tourism industry is one of the fastest growing in the world and as a result, hospitality and tourism have become the top income generators for the island, outpacing even the lucrative agricultural export industry. Forming part of the Stone Town Historic Conservation Plan, the Zanzibar Urban Services Project (ZUSP) and the Stone Town Conservation and Development Authority (STCDA) aims to develop and upgrade selected urban infrastructure within the Zanzibar Municipal Council, and to enhance the physical environment at public locations within Stone Town. One of these projects is the Mizingani Sea Wall and Associated Promenade, aimed to create a public seafront promenade along Mizingani Road in Stone Town.

Aurecon carried out the environmental and social impact assessment, developed an environmental and social impact management plan, and the facilitation of a public participation process. The company was also appointed as lead consultant for both phases of the project to provide engineering consultancy services such as Design and Tender Documentation during Phase IIA and full time site supervision, environmental control, Crack surveys & Monitoring on heritage buildings, construction and project management and engineering design support during Phase IIB, the construction of the sea wall and associated promenade.

The project comprised the construction of 340 running meters of vertical seawall, with a five meter-wide reclamation extended out of its existing boundaries; refurbishment of underground infrastructure (including potable water, sewer, storm water drainage, electrical and telecommunication lines); rehabilitation of sinkholes and upgrading of the existing Mizingani road; introduction of traffic calming measures and the creation of a pedestrian promenade, including landscaping, street lighting and street furniture.

Maintaining the site’s integrity as a heritage site

The project team faced numerous challenges in terms of maintaining the site’s integrity as a heritage site, as well as the environmental challenges associated with a coastal seafront site subject to periodic tidal inundation.

The project team made use of highly specialised marine concrete (40MPa) for the foundation of the wall (marine concrete mixes are much stronger, denser and corrosion resistant). Furthermore, given the challenging location and equipment available to the appointed contractor, they were unable to perform batch mixing according to weight, mixing had to be done by volume, with hand labour and a bucket system used to measure material, which was ultimately mixed with drum mixers on site.

Due to the heritage of the surrounding buildings and their weak structural integrity, it was specified that no vibratory construction methods may be used, as this could jeopardise the buildings. This meant the typical construction methods such as sheet piling to deal with marine conditions and ease marine concrete construction could not be used. A less popular, age-old design was used to overcome these restrictions.

During the construction of the road, the team encountered various unforeseen heritage elements which had to be dealt with in accordance to the STCDA heritage guidelines. These were old steel cannon balls that were dug up and various underground stone canals running from the Palace Museum. These were preserved for future archaeological studies and safe keeping.

Incorporating the original look and feel of Stone Town

Given that the sea wall is located on a United Nations Educational, Scientific and Cultural Organisation (UNESCO) world heritage site, the design team had to consider UNESCO standards and needed to incorporate the original look and feel of Stone Town (colonial streets, stone-type benches, sandstone or stone-like paving) into the design of the wall and promenade for it to blend in with the surroundings.

However, the original wall was built using a combination of sandstone and coral, but sandstone isn’t available on the island anymore and the team could not harvest coral for construction due to the environmental impact it would have, as well as being less durable.

Therefore, the project team had to specify special aggregate to be imported together with local available aggregate to be used in the various concrete mix designs as an alternative solution. Several revised mixes and sample batches were tested to ensure that the final product adhered to the requirements of marine concrete and now accommodated local sand and aggregates as much as possible in an effort to save construction costs.

Sometimes referred to as the “Gateway to Zanzibar”, Mizingani Road boasts some of the oldest buildings in Stone Town. The improvement of the seafront in Zanzibar’s Stone Town has encouraged safer vehicular movement; minimised uncontrolled parking; and preserved and enhanced the traditional seafront setting and its historic and architectural context ultimately creating a revitalised communal space for the local community and travelers alike.

Additional designs have been commissioned and are completed to further extend this heritage project by upgrading Bunyan Tree Square and the walkway connecting the new promenade with the Ferry Terminal, Zanzibar’s most used transport note linking the island with mainland Tanzania and surrounding islands.

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