A perimeter wall enclosing Namirembe Guest House in Kampala collapsed two weeks ago instantly killing two guests. Heavy rain was to blame. Whereas these accidents are attributed to heavy rain and floods, there is another common underlying cause.
Weeks earlier, before the Namirembe Guest House accident, a similar incident occurred in Seguku on Entebbe Road where another perimeter wall collapsed killing seven people with five from the same family. In October 2018, another perimeter wall in Najjera, a Kampala suburb, collapsed destroying two cars.
Joseph wolekya, a civil engineer at Baruga Technical Services defines perimeter walls as free-standing walls that are not part of any building. They are constructed for purposes of providing land boundaries, privacy, and security to property. These are what we commonly refer to as wall fences.
Why walls collapse
Anthony Nakedde, a civil engineer at Infrastructure Pros, an engineering consultancy firm has an explanation as to why the perimeter walls in Namirembe, Seguku, and Kiwatule might have collapsed.
He says in all cases, heavy rain and poor construction were major contributors to the accidents. At Namirembe Guest House, he explains, the gushing water from the hill lacked channels to divert to trenches thereby hitting the wall directly. The intensity of the heavy water weakened the surrounding soils leading to destruction of the wall.
For the Seguku incident, a shallow foundation and lack of retaining wall was to blame.
“If there was a retaining wall to safely deliver water from the upper ground to a lower ground. Then, the wall wouldn’t have collapsed.”
Nakedde adds that in all cases, lack of drainage channels to divert the water led to collapse of the walls.
Another engineer, Robert Atwine, says a shallow foundation will automatically be a problem when it comes to stability; a very high wall without enough support will automatically collapse.
“Most causes of failure result from weak plinth wall, water seepage, fake material especially bricks that absorb water, poor compound drainage causing flooding near walls and they become weak. High walls with less piers, weak soils, poor construction methodology, poor mortar that is less cement-sand ratio,” He says.
Building strong perimeters
Eng. Nakedde says the first step in constructing a strong perimeter wall is to consult an architect on whether the type of wall you want is practical. The architect usually designs a perimeter wall in collaboration with a civil engineer who determines the nature of soil in place.
The second step involves setting a deep and strong foundation for the wall that will enable it withstand natural forces such as wind, heavy rain and storms. In addition, seep holes and pipes should be fitted in the foundation for easy flow of water during flooding.
Kwolekya explains that different codes of practices provide guidance on how to design and construct these free-standing walls. Unfortunately, construction of perimeter walls in Uganda is often not based on a mason’s experience with little or no consultation from civil engineers.
He says, usually, standard construction practices provide for adequate foundation depth, minimum wall thickness, movement joints, pier or columns and limit height of walls.
He also advises that adequate building materials should be used.
“When the right amount of mortar mix of cement and bricks or building blocks are used during the construction, the perimeter wall is expected to withstand the normal forces of nature provided quantity and quality control are not compromised.
Piers and columns
Piers, columns and expansion or movement joints are the main features that determine the strength of a wall fence, says Atwine.
He defines an expansion joint as one that separates brick masonry into segments to prevent cracking caused by changes in temperature, moisture expansion, elastic deformation, settlement and creep. A pier is an upright support of a wall. He stresses that piers should not exceed three metres apart.
“Expansion joints should be put in a pier for support. You will have two edges of the wall fixed. Here, an expansion joint should be put in themiddle of the wall so that one edge is free making it stable enough to resist lateral forces.”he adds
He adds that expansion joints should be in a pier not the wall.
Kwolekya says expansion joints are important during heavy winds or earthquakes. They minimise the overall impact and reduce damage, and in case of damage, limit damage to individual panels ensuring that the whole wall does not collapse.
“To get rid of or minimise cases of collapse of perimeter walls, clients should consult competent civil engineers, who will design and ensure quality control during construction. It is a small price to pay compared to the would be damage due to use of incompetent professionals,” he advises
Nakedde offers a practical example of when the Makerere University wall fence collapsed. He says the walls lacked expansion joints so when water hit the wall, whole wall collapsed. If the wall had expansion joints, only sections of it would have collapsed.
According to Wikepedia, a pier, is an upright support for a structure or superstructure such as an arch or bridge.
Sections of structural walls between openings can function as piers.
For homes on raised land like hills, a retaining wall is necessary to strengthen the ground to protect a perimeter or free-standing wall.
“Retaining walls are built for the purposes of; retaining earth, protecting embankments from erosion and in some cases, act as part of the foundation of a structure. The wall must be constructed to serve these purposes adequately and safely,” Nakedde says.
Retaining walls should be designed to resist the pressure of the soil it is retaining, avoid sliding, overturning, disintegrating and not to fail the soil its standing on. To achieve all these adequately, one has to consult an engineer.
The most important considerations for designing a retaining walls include; the height of the exposed face, the internal friction angle of the soil, materials to be used and the strength of the soil. These considerations will dictate the type of wall to build.
Types of walls The commonly known types are: gravity, cantilever, counterfort, buttress, gabion, segmental, among others.
Collapse of walls particularly when it rains result from water logging in the soil retained by the wall, making it heavier thus overwhelming the wall.
Drainage of water retained by the soil is paramount. The surface water should be intercepted and directed away from the wall and safely conveyed to a lower drain. It should not be allowed to soak into the soil. Subsurface water should also be allowed safe passage through weep holes and a drainage channel at the bottom of the wall.
Key issues to keep in mind
Emmanuel Arinaitwe, an architect says when designing a perimeter wall, key issues such as topography, strength of the soil, function of the perimeter wall and weather are considered.
“If it’s for security, the wall thickness ranges from 200- 300mm depending on the bricks used,” he says. The height of the wall should range from two and three meters depending on the risk and or quality of security in the area. Broken bottles, electrified wiring or even sharp-edged welded metal can be placed on top.