WHO MOVED MY CHEESE, WHITHER WHO MOVED MY FOUNDATION?

28-May-2024

This is a discussion on the developing story of the recent collapse of the Mosjid Mutmainah Central Mosque, which occurred on Sunday 26-May-2024 around midday in Papa Ajao, Mushin, with publicly available information by an industry professional.

In summary this is a case of Foundation Engineering class 101, let us delve into it quickly.

The foundation of any structure is not the foreign object technically called the substructure which could be of concrete-filled sandcrete blocks or reinforced concrete, placed inside (or on) the ground. The foundation of a (building) structure is in reality the soil beneath and around the substructure. The substructure simply acts as a "dispatch rider or messenger" sent to deliver a parcel (the building's weight/load) to a recipient, in this case the soil. The substructure is designed and constructed to ensure a safe transfer of the building (superstructural) load to the founding soil aka ground.

In this case where the collapse of the Mosjid Mutmainah Central Mosque, situated at the intersection of Yusuph/Shotinoye streets, Papa Ajao Mushin, Lagos is being attributed to the ongoing road construction works, let us review a few of the publicly available information and draw assertions from these.

Initial thoughts seeking rent in the heads of citizens was that it was yet another incidence of collapse attributable to dilapidation or structural instability. However, when the news fully broke, both survivor video records and reports in the news media attributed the collapse to have been caused by a tracked-heavy-duty-hydraulic-excavator digging trenches for drain of an ongoing road construction project. The excavator was reported to have hit one of the walls of the mosque building thereby leading to its collapse, some other survivor video accounts claimed that the excavator pulled the building foundation. An excerpt from the PM News credited to the Permanent Secretary of LASEMA also says “Preliminary investigation carried out upon arrival at the scene revealed that an excavator clearing the road recklessly hit a mosque (a storey building) by the road side which resulted to the mosque collapsing as prayer was about to commence."

So, how plausible are the verbal claims of the populace as well as the preliminary conclusion quoted above? To answer this, further eyewitness accounts were pulled and this is shared to drive this write up - "The Papa Ajao Mosque collapse discussion came up in the office today and our I.T manager told us he moved into that area in 2004. He said the mosque was always there but was a bungalow but later they added a suspended floor. As usual, the quack way - they introduced columns and beams forgetting it was a load bearing building they built with the blockwalls not having reinforced strips and the columns they introduced having no bases.

The guy operating the excavator had removed the drain and thus the soil under the building was exposed as they could see the walls and columns. Some of the men had seen cracks on the tiles of the ground floor and ran out.

The excavator had done all that was needed at that location and was down the street, away from the mosque when the mosque came down.

He said the mosque and all those that violated setback like the shops weren't marked or touched rather it was the road and drain that the excavation focused on."

So, let us jointly analyse the latter narration and determine its plausibility from scientific and engineering basis.

Essentially with the building reported to having been built with the methodology explained above, the blockwalls of the ground floor had been conscripted to support the upper floor in perpetuity. Yes, this is despite the introduction of columns and beams which may make it appear to be a framed superstructure. With the absence of properly designed reinforced concrete substructure and also without the placing of the existing makeshift blockwall substructure at a suitable depth below the natural ground level (which standards dictate to be at least 1000mm or more) in order to allow a safe load transfer protected from natural and man-made soil processes, the existing drain inadvertently served two foundational purposes for the continued existence of the mosque structure -

1. It served as a retaining wall that retained the soil which the load bearing walls were founded upon thereby preventing a loss of the soil due to natural actions of erosion and scour.

2. The drain being rigidly in place would have also improved the resistance of the soil to shearing actions from the increased loads transmitted to the foundation from the parts added to the building. This can be likened to containment action and easily understood that a soil inside a piece of nylon (doesn't spread out infinitely) thereby supporting higher magnitude of loads than the same soil left uncontained which would spread out unconstrained.

If you are following this write up, with the explanation of the induced role the drain is playing in the scheme of the building above, your intuition would not be wrong if it leads you onto the repercussion of removing the drain.

Yes!, the removal of the drain will lead to a loss of shear strength of the founding soil and the situation would be exacerbated in geometric magnitudes where the ground water is close to the ground level, further where the drain has wastewater flowing within it and even further should a rainfall event occur within the window period of the exposure of the soil. You can call it "worse, worser, worsest" as KG kids would playfully say.

To utilise this as an opportunity to quickly educate the teeming populace on the importance of professional building appraisals, permit me to quickly add the following-

In this particular situation, you may ask what could have happened had the drain not being in place in the first place. Either of two things would have happened on a longer timeline -

1. Given that the newly introduced columns were without column bases, the founding soil under the load-bearing blockwalls would have been gradually eroded away one rainfall event at a time leading to the development of tell-tale cracks signalling distress. This could have led to a call-to-action by the property owners. The follow-on action as is widely common would have easily been ill-advised attempts at surface patching of the cracks which would not resolve the issue. This would have especially been the case where the foundation is not buried below the prevailing natural ground level, it is at stepped edges and its perimeter is also not protected with impervious finishes. In addition, being close to the drains, scouring due to consistent daily flow of wastewater would have hastened the eroding away of the founding soil.
2. The same crack development could have occurred due to a shear failure of the supporting support. Which in essence means that the carrying capacity of the soil has been exceeded and it would shear (or break) leading to cases of settlement, cracks, distress and building failure where exasperated.

The nature of manifestation of the cracks would have been the proverbial handwriting on the wall that professional structural-building inspectors would have seen and documented as part of the first steps of a systematic Building Structural Integrity Assessment or Appraisal - this is indeed a procedure far more scientific and nuanced than a robotic shooting exercise now being abused and tagged as NDT testing - to determine the underlying cause of the cracks, document its severity, monitor them for progression or dormancy as may be required including conducting of a foundation probe in a safe manner.

The synthesis of the above will birth a scientific-based conclusion on the defects, its implication and its overall effects on the continued stability of the building structure.

The above article examines only the technical component of the collapse for the purpose of knowledge sharing and learning from failures. It was neither setout to consider the regulatory, governmental, legal aspects of the matter nor does it play light of the importance of standard construction processes that may or may have not been in place as part of the road construction. All of these components have their rightful places for consideration in separate articles.

Our condolences to victims whom may have been injured by this incident and to the family of those whom may have loved lost ones to the incidence.

References:

1. https://pmnewsnigeria.com/2024/05/26/three-dead-seven-injured-as-mosque-collapses-in-lagos/

2. Google Maps street view pictures

3. Video recording accounts from survivors of the incidence shared through WhatsApp platform.

4. Pictures and video recording footage of the ground zero after the collapse as circulating on WhatsApp platform.

5. Information shared and comments made on the collapse incidence by built industry professionals on the BCPG WhatsApp groups.

A piece written by Abdulfatai Tunde-Lawal (Engr) with contributions from Engr Bola Oresanwo.

Abdulfatai is an experienced building Structural appraisal specialist and Structural Engineer based in Lagos, Nigeria. He can be reached through his SM handles - @atundelawal and on LinkedIn - www.dhirubhai.net/in/atundelawal

Whilst Bola is an experienced Structural Engineer based in Lagos and can be reached through his SM handles @bolaores and on LinkendIn - https://www.dhirubhai.net/in/bola-oresanwo-50723912

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