Saturday, 8 July 2017

Flooding: Turning Your Perimeter Fence into a Retaining Wall - Engr. Osaz' Enobakhare

Security and privacy are common reasons for constructing perimeter fences around a building. Some fences are simply symbolic while others are basically for defense. They also help to prevent land encroachment from neighbors which is not uncommon in this clime. However, fences can be designed to perform yet another critical function of flood control especially for buildings constructed in areas prone to flash floods, run-off waters from faulty drains, erosion, etc. This is the core functions of embankments and retaining walls including the ability to stay earth. Retaining walls hold back or fend off flood waters from entering into your compound. There are a number of designs and technologies suited for this purpose and they can be incorporated into your fence wall and not necessarily constructed separately; saving cost and space in the end.

Generally, aesthetics is valued from outside in, not necessarily inside out. Therefore the perimeter fence and gate of our houses becomes a very vital point for aesthetic reference for the buildings as a whole, being mostly the first point of contact. 

This places a burden on the designer (or architect) to allow for aesthetic considerations for perimeter fences intended to serve as retaining walls or embankments; understanding that these fence types will be under intense water pressure when the weather goes wild compared to ordinary fences. Only suitable color, texture and styles should be used for the finishes of such fence walls. Stone-based finishes are often recommended while some others prefer to use special moisture-resistant paints and coverings. There is also the structural engineering consideration which factors in the strength, stability and durability components. If the fence is beautiful but cannot stand strong against flood pressure adequately, then it may be considered good for nothing. The method of building them is slightly different from the normal ones. One striking difference is that the base for retaining walls are normally higher than those of other fence types usually extending beyond the level of projected flood heights before block walls are introduced. Some are even designed to be trapezoidal rather than rectangular in section.

Perimeter fences are normally designed either as non-load-bearing or partially-load-bearing walls but those intended for retaining walls are essentially load-bearing walls. They do not support their weights alone or small loads from security furniture but also sizable pressure forces from external flood. There are various types of materials that can be used for building special fences of this nature; the most common type is concrete. Other types include a composite of reinforced stabilized earth with bricks or blocks as well as other poly-based substitutes for concrete. Whichever material is used, there is need for it to possess sufficient strength to withstand the pressure forces of water as well as be stable enough not to deflect, crack, fail or over-settle in the process. It should also be able to perform this function throughout the life span of the building or space it was designed to serve.

In areas susceptible to flood, the sub-surface soil conditions are often poor because of the long overbearing effect of stayed water on them, so there is always a need to take cognizance of the nature of the soil during the design of the fence. Well-built solid raft base or a combination of short-bored piles and reinforced concrete strip are good options for the foundation of such walls. Reinforced stabilized earth has also been successfully used for this purpose and is said to be a cheaper option to both polymer-based fences and concrete. 

Flood control is serious business especially in rainy seasons. There is no better way to imagine the devastating effect of flood than to watch a house go partially or completely under water; destroying the building fabric and other valuable properties in the process. Although fencing alone cannot completely take care of flood on a large scale however it is known fact that the retaining wall fence type surely minimizes the effect by helping to redirect water from a property.   


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