By Dare Olawin
The tide of history seems to have been reversed by some yet to be identified forces in Abeokuta as one of the many rocks for which the ancient city is known, and named after, came crashing down last year, crushing a family of four to death.
People found it absolutely unbelievable that rocks, which once shielded the Egba people from being killed by the Oloyo warriors in 1830 have now become 'killers' of the Egbas. In the unfortunate incident, a mother (Silifat Lawal); two of her children (Rasheedat,15, Semia,4); and a grandchild (Mariam) lost their lives.
The heavy rock had rolled off the height it had sat for hundreds of years at Iberekodo in Abeokuta North, smashing into the bedroom of the deceased who were fast asleep soon after having dinner.
However, the head of the family Ismail Lawal and two other children – Sukurat and Rofiat- survived the assault on the modest home by the deadly rock. They were just a few steps away – relaxing in the sitting room – when the tragedy struck. They were injured, but that is not without injuring them seriously.
People blamed a preceeding downpour for the incident. They said the rain appeared to have softened the base of the rock which gave way and made the rock to lose its balance.
Anybody familiar with Abeokuta would agree with me that the city is the home of rocks and stones, the most popular of which is the Olumo Rock that accommodated the Egbas then. Hence, the name, Abeokuta, that is 'under the rock'.
In major parts of the city, houses were seen built close to rocks, if not beneath rocks. Some built theirs on rocks with some other smaller rocks hanging near the roofs. This is because Egbas are of the belief that the rocks are their refuge.
They had never nursed any fear of a rock falling and killing a fowl, let alone four persons at a time. A friend of mine, who once came on a tour to Abeokuta, saw a smaller rock attached to a bigger one hanging near a building in Ago-Oko. To him, the stone was as if it were falling off from the cliff. He looked at the rock and asked: "Are you sure that stone will not fall on that building over there? He was seriously worried about the safety of the inhabitants of the ancient building. But before he could even speak further, an elderly man standing beside him retorted that “those stones hanging by the rock can never fall.” The man explained that the rocks had been like that more than a millennium ago and people had been living under them without any fear.” This settled the case.
However, the recent rock fall in Iberokodo has proven everybody wrong. What we thought could never occur can strike a million times. Indigenes of Abeokuta said it had never been reported; the more reason why some of them have started attaching spiritual meanings to the incident. Of course you know we Africans. But findings have shown that rock falls are natural phenomena.
A rock fall consists of one or maybe a few rocks that detach from the high part of a steep slope, dropping and perhaps bouncing a few times as they move very rapidly down the slope. Rock falls are very dangerous because they can occur without warning and they are not preventable. However, casualties can be prevented.
To those of us who thought rocks don’t fall, a BBC report has it that “a 23-year-old woman has died after a rock fell on her head on a beach in south Wales.” According to www.bbc.com, south Wales’ police found the woman’s body after she died in the tragic accident that occurred on Thursday, June 18, 2015. Also, a substantial rock fall was said to have occurred at Bricyard Point in Austinmer while children played nearby. It occurred on September 21, 2015, sending rocks spilling five to eight feet from the cliff face, narrowly missing the children playing nearby. These are just a few of recently reported incidents of rock falls across the globe.
We may not have witnessed it in Egba, not until recently, but that does not portend that ‘rocks don’t fall’. No one prays for a recurrence though, that must not stop the truth from being told. It is expedient at this time to let our people know that many of us are living in danger by building our houses close to rocks or almost beneath them.
Scientists have identified different causes of rock falls:
In general rockfalls happen because some event occurs which acts to reduce the strength of the rockmass. The term rockmass is used to describe the solid rock and all the bedding, joint and any other fracture planes which are spread throughout the rock and act to reduce its strength. Under normal conditions, the rockmass is strong enough to support itself (if not, it would have already collapsed until achieving a stable state!).
However if there is a change in the conditions then this can lead to failure. So what conditions can change that may cause failure?
Pore water pressure: During periods of heavy rainfall, water infiltrates the ground and can act to increase the groundwater table. It will also flow along any fracture zones or other discontinuities in the rockmass which are permeable. Normally the self weight of the rockmass (known as the in-situ stress) acts to force these discontinuities together and the roughness of their surfaces and the resultant friction acts to stop movement.
However the water is relatively incompressible and so forms a small barrier between the two surfaces which acts to reduce the strength of the interface by reducing friction and the shear strength. If the alignment of the interface in question is "bad" i.e. one along which a slip or failure would result in a block of rock falling from the slope or cliff face and the pore water pressure becomes high enough to negate the frictional resistance to sliding, then a rock failure will occur.
Fractures and discontinuities also commonly have a clay infilling. The pore water can cause these clay fills to swell (and soften) and then shrink again when they dry out. These shrink swell cycles can act to reduce the friction on the interface between two rock blocks and again increase the rick of failure and a rock slide occurring.
Weathering can also act to increase the dimensions of fractures (e.g. freeze thaw weathering and frost heave) which will reduce their strength and can act to reduce stability and may contribute towards a rockfall.
Changes in the stress state: Construction or the application of a large load on the top of the slope or cliff will increase the stress within the rock in the cliff face and this may exceed the strength of the discontinuities in certain sections leading to rockfalls. A change in the slope or cliff geometry (e.g. cutting a steeper slope into a rock face when building a road) may also lead to instability and a rockfall. This may also be caused by undercutting in coastal cliff faces due to wave action creating a wave cut notch leading to a loss of support to the overlying rockmass causing it to collapse.
Dynamic loading: The seismic waves /vibrations caused by earthquakes, explosions (from quarrying or during the blasting used during the construction of road cuttings through rock) can act to cause rock falls.
Gas overpressure: This is a very specific problem that in the past was caused by the introduction of high explosives in the construction of road cuttings (as opposed to rail cuttings which were predominantly created in the UK before the introduction of high explosives). Road cuttings are created using blasting by drilling boreholes into the rock slope at the angle you want the final slope to be and then placing explosive in these holes. The explosives would be detonated and the rockmass fractured so that it could be more easily excavated. One unexpected side effect of this process was that the high pressure gasses generated by the explosion moved along the pre-existing fractures within the rock and acted to force them wider apart making the rockmass in the resultant face of the slope weaker than it would otherwise have been and could cause rockfalls to occur in the future.
Having said this, one would now understand how important it is to educate our people on the need for them to save their precious lives by doing away from rocks as they may fall anytime without notice.
Government should expedite actions towards putting an end to rockfalls killing our people in numbers. It is true that rock falls are unpopular natural disasters. Much may not have been heard of it, but the recent one that occurred in Abeokuta is enough for us to guard against future occurrences.
The questions that will come to the minds of most readers in Abeokuta will be: Is this possible? Are they going to abandon their houses? Who will create another houses for them? Does it mean another rock will fall soon? Getting answers to all these questions might seem a bit difficult. Individuals must put their safety first in all things. Even if nobody asks us to quit these 'danger zones', we should remember how important it is to live a good life.
As for rockfall, no one knows when it would occur again and we don't pray for a recurrence. However, as a natural phenomenon, it could occur sooner or later than expected, anywhere. Those of us who still dwell on the notion that the rock fall at Iberekodo was caused by witches, wizards - perceived enemies of the victims, should wake up from their slumber. Many of us believe so much in prayers or charms as the case may be. But that must not keep us from taking all preventive measures against rock falls.
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