The Zanzibar president, Dr. Hussein Ali Mwinyi, has identified investment in transport infrastructure as one of his core priorities to accelerate economic and social development in Zanzibar’s Unguja and Pemba islands. Zanzibar’s vibrant tapestry of cultures and coveted status as a UNESCO World Heritage Site present unique challenges and opportunities for large-scale infrastructure interventions. Here, we offer insights from the field, illustrating SMEC’s commitment to design excellence in sensitive environments.

Narrow streetscape

The narrow streets of Zanzibar and their proximity to residential structures have required an adaptable approach to road rehabilitation. Heavy mechanised excavation, compaction and paver operations would easily disturb adjacent historical buildings and compromise existing foundations. Increased disturbance from noise and vibrations also introduced increased environmental risk.

Light compaction techniques

Given these challenges, it was not feasible to construct a flexible pavement base, as recommended in the Tanzania pavement design manual. To address the limitations of the space-constrained working environment and proximity of residential structures, SMEC recommended the adoption of rigid pavements. The construction process for rigid pavement reduced ground vibration, whilst also distributing the loads over a broader area, lessening the impact on surrounding foundations.

In Stone Town, narrow streets function primarily as feeder roads leading to residences. Here, SMEC was able to simplify the pavement design by modifying the sub-base treatment, using rockfill blinded with a sand layer. This method could be undertaken using hand compaction, rockfill blinded with a sand layer. To compensate for lower stiffness of rockfill, 180mm reinforced concrete pavement is specified for enhanced durability.

Corrosive soft soil conditions and integrated geotechnical design

A robust analysis of sub-surface geotechnical conditions has been critical in safeguarding the structural integrity and longevity of Zanzibar’s first ever flyovers set to be constructed at Amani and Mwanakwerekwe. The variable ground conditions presented challenges for sizing of foundation piles. The foundation piles are 900 millimeters wide and will be up to 35 meters in depth. Geotechnical modelling and rigorous testing, including test piles for both Flyovers, were conducted to ensure validation of on-site conditions, offering assurance for the subsequent construction phases.

The team has also needed to incorporate corrosion protection measures due to the high acid content of the soil. To protect flyover substructures from corrosive soil conditions, SMEC is innovating by specifying fly-ash concrete for improved strength and durability and to reduce permeability, therefore protecting structural components.

SMEC undertook detailed modelling to optimise the size and locations of the span supports and abutments. This provided efficiencies in material costs and helped to minimise disruption to traffic and surrounding property.

Accelerated construction programme

An accelerated program adopted by the contractor China Civil Engineering Construction Corporation (CCECC) has required the team to rethink traditional design delivery cycles. SMEC’s on-site presence, including a client-facing office in Zanzibar, has enabled prompt response to design clarifications whilst fostering a more collaborative culture. To speed up construction, the Flyovers superstructure is made-up of the precast beam and slab method. The beams will be pre-cast at CCECC’s construction yard and lifted into place.

The design team is supported by specialists dispersed across South Africa, Namibia, Kenya, and Tanzania – supported by centralized collaboration in Autodesk Civil 3D. Advanced modelling in Autodesk Civil 3D has allowed SMEC to overlay structural and drainage elements for a unified design that surpasses existing benchmarks and design codes.

Leveraging software like Rubicon for lifetime analysis and integrating data into Civil 3D has enabled precise layer thickness specifications, ensuring longevity and durability. Expanding the level of design detail at earlier stages has helped mitigate delays. For the client, the benefits include speed, reduced costs, and an enhanced ability to manage stakeholder expectations.

The complexity in the design and speed of construction has meant that regular interface has been required with the Owner’s Engineer. Open dialogue has resulted in strong collaboration across the team, working together to develop robust design review and contractor RoI processes. On-the-ground support, combined with ongoing stakeholder engagement and detailed design specifications, helps to foster a seamless, goal-oriented approach even as unforeseen challenges arise. During construction of the flyovers SMEC was able to provide feedback on method statements and testing regiments which has de-risked construction.

International standards and local context

In pavement design classes SMEC looked at East Africa standards as a precedent to inform axle load testing but the level of detail in our design is completely new for Zanzibar and Tanzania. The team has drawn on international experts with knowledge of best practice whilst researching international standards to understand what can be applied to the local context. We have assessed Eurocode, British and American standards to inform some of our assumptions but we have relied on local knowledge and engineers to document and interpret this in our specification.

Inclusive design and cultural legacy

Rainwater tunnels, streetlights, and inclusive lanes will all be a part of the road works.  The roads, which will mainly lead to rural areas, will be designed to accommodate pedestrians, bicycles, and people with disabilities.

Transport infrastructure can do more than merely moving people and goods; it can become a canvas for cultural expression. Integrating local aesthetics and values deepens community connection with vital infrastructure projects. On the Zanzibar project, SMEC is collaborating with local architects to develop bespoke architectural features inspired by the island’s rich heritage. Situated on major roundabouts and interchanges, these monuments will provide both symbolic and functional value.

Looking ahead

The Zanzibar Urban Roads project demonstrates SMEC’s commitment to delivering world-class solutions that honour and preserve cherished historical settings. From reimagining urban construction methodologies to ensuring structural integrity with cutting-edge material specifications, SMEC combines technical expertise with local knowledge and unwavering collaboration to ensure enduring positive outcomes for communities. Balancing technical excellence with environmental considerations, cultural preservation, and efficient collaboration, SMEC sets a new standard for responsible and impactful engineering in the built environment.