Designing and Building in Central and East Africa

Designing a home in South Sudan

In March of 2012 I received an email from Eli Fader, a missionary in South Sudan.  Mr. Fader and his family had recently moved from Yabus to Melut (along the Nile) and are preparing to build their new home.

Eli’s Sketch-Up design

Eli had a few questions about construction.  His initial concerns were in regard to wall design and material selection.  With outrageous prices for cement ($36/bag) and the lack of gravel or stone in the region, Eli and his family were exploring alternative methods to construct their new home’s walls.  I sent him an image from a previous project of mine that used mud/dung + wire mesh for his reference.  Such a design detail can resolve cost constraints while still being resistant to termites.  We also discussed possibilities for painting and preserving the walls against heavy rains.  As our communication continued however, it became clear that the design for the foundation would be the most critical element in constructing a structurally sound home.

The unique soil type in this region of South Sudan is called “black cotton soil” and is very difficult to build on.    During the rainy seasons, the soil on the outside of buildings starts to swell as the water moves down through the earth.  In other words, the soil heaves when confronted with dry soil.   There is no bedrock within any reasonable depth in this region, so builders must build upon this flexing terrain.  It is this movement that has defeated and destroyed many structures in the region – and the foundation is where these failures begin.

Eli had received two suggestions from local engineers as to how to build a strong and appropriate foundation:

The first recommendation described a minimal foundation, “keeping the structure light.”  Each column would have its own independent footing which would support the continuous wire mesh walls.  The strategy being that there is no way to fight the battle against the rains.  Building a foundation wall to counteract the pressures is all but futile.

The second recommendation was to construct a substantial continuous brick foundation around the perimeter of the building, perhaps topped with a reinforced concrete beam just below the floor slab.

I have disagreed with both of these suggestions for different reasons.  The “keep it light” suggestion provides positives on material costs and savings.  However, conceptually floating a building upon unstable soil with independent footings will fracture the collective strength of the wall.  Localized settling and “heaving” will lead to cracks and potentially question the structural integrity of the building itself.  The second suggestion provides a common solution for foundation construction.  With such soil conditions however, it is necessary to take a step back to think about the location of the foundation and its role in relation to the walls and roof.  Placing such a critical load bearing element around the perimeter of the building – where heaving effects are the greatest – makes any foundation and wall bearing strategy inherently problematic.

Schematic Foundation Design – unstable soils

The solution that I have proposed and sent to Eli seeks to relocate the load bearing elements to the inside of the structure where soil movement is kept to a (dry) minimum.  Here I have pulled columns into the center of the home (adjusting the plan to accommodate them), and introduced a reinforced concrete “waffle foundation” scheme.  Such a series of grade beams will distribute the loads caused by soil flexing throughout the building; allowing each corner of the foundation to work together.  The walls are then relieved of their load bearing responsibilities and instead become structural connections between the waffle foundation and the roof trusses.

Upon receiving this document, Eli indicated his interest, approval, and understanding of my design logic.  I am confident that the scheme I have proposed is the best solution.  However, I still recommended he present this scheme to his local engineers and contractors that have years of experience in building in this specific region of South Sudan.  It is only with true local knowledge and experience with this project be realized.

Eli has mentioned that he hopes to start construction in November, so as of now we have plenty of time to make any necessary adjustments.

Any comments or suggestions are welcome!

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One response

  1. Why not use light construction for the superstructure sitting on a raft foundation ie a reinforced concrete slab with an edge beam all round? It seems to me that this would be simpler and more economic.

    September 29, 2012 at 10:57 am

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