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1st Geosynthetic Bridge in Kenya

Kenya is considered the “Gateway to East Africa” with its sea port located in the coastal city of Mombasa. The causeway in Changamwe leading into the port is notoriously known for its bumper-to-bumper traffic. There are only limited entries to the port and trucks are regularly queuing for hours, sometimes days to drop off vital cargo for export. This causes several delays and leaves CFS’s cut off due to heavy traffic.

One such CFS, located adjacent to the port, strategically procured a parcel of land on the opposite side of Tudor Creek with the aim to create a new private causeway bypassing the traffic. The new causeway would cross over the creek directly into their CFS. Crossing the water body meant building a bridge.

The challenges were several. The location of the proposed causeway passed directly through a toxic dump site which had sprawled uncontrolled over the last 40 years. The bridge would cross at a point where the creek channel narrowed to approx. 100m, accelerating the flow of water creating rapids and churning up silt, making safe working conditions difficult. The creek flows into the Indian Ocean making it tidal, limiting daily operations to a 6hr window in-between tides.

Accepting the challenge, GEA was appointed as a consultant to assist in the design & construction of the bridge and causeway. The outcome needed to be both time-sensitive and cost-efficient. Several feasibility studies were conducted and the most economical and fastest to construct was a completely new concept in Kenya; a geosynthetic bridge.

Saving on Material Resources

The design of the bridge was essentially a dyke with the causeway passing over the top. In cross-section the bridge has a pyramid shape, constructed in layers of fill material wrapped in geotextile and geogrid envelopes. With the exception of the raft layer at the base, the entire dyke is constructed with in-situ fill material found on site.

Fast Construction

The bridge and causeway were constructed in record time taking less than 11 months to complete. By using Geotextiles and Geogrids, reinforced concrete was eliminated. The materials are quick to deploy on site and are simply rolled out. They are versatile and can be installed in wet conditions; which was a requirement due to the site’s location in the creek channel.

A Pioneer Project

The geosynthetic bridge is the first of it’s kind in Kenya. Throughout the construction process, GEA organised several site visits for other engineers, contractors and consultants alike, to raise awareness on alternative methods of construction. Save for the box culverts, paving blocks and steel guard rails, steel and concrete are almost entirely eliminated in the civil engineering element of this infrastructure project. Since it’s construction in 2014, the geosynthetic bridge and causeway have proved to be low maintenance and sustainable project. By using the mounds of in-situ shale material on site, minimal imported fill material was required providing savings on natural resources and associated transport costs.

Geosynthetic Materials Installed

In the construction of the bridge over 32,300sqm of Secugrid 120/40R6 and 22,250sqm of Secugrid 40/40Q6 were installed in addition to 66,080sqm of F-400 nonwoven geotextile. The Causeway: 22,250sqm of Secugrid 40/40Q6 and 26,000sqm of F-34 nonwoven geotextile installed along with 520Rm HDPE slotted geopipes for subsoil drainage. 630m3 of HDPE gabion mattresses protect the embankments on either side of the bridge from erosion caused by strong tidal currents. Over 5,000sqm of 100mm geocells were installed on the slopes / causeway embankments.

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