Thursday, 9 February 2017

Dredging to Sand-fill an Estate: The Tricks and Gains - Engr. Osaz’ ENOBAKHARE

Developing housing or industrial estates close to large water bodies throw open the option of dredging to sand-fill with the aim of reclamation or just to prepare the site for the construction of buildings and infrastructure on it and/or to get sand for the actual construction. In more extensive application, sands can be dredged to stockpile and sell. Basically, for estate development the option of dredging is often weighed alongside the possibility and cost-effectiveness of the alternative approach of trucking sand from nearby sources.

Contrary to popular opinions, industry estimates reveal that for mini-estates (of 5 acres or less) located close to water bodies with shallow fill depth (not exceeding 1-metre) and where manual dredging is prohibited, the option of trucking has proven to be cheaper than mechanical/hydraulic dredging. 

Besides unlike dredging which restricts the user to the particular quality of sand deposits available in that water body at the material time, trucking also allows for the use of a variety of filling materials. However beyond these rare scenarios, mechanical/hydraulic dredging remains the best approach.

Although manual dredging is still the cheapest form of dredging, it is tedious, relatively slow and highly risky hence no longer in vogue. Mechanical/Hydraulic dredging allows for the digging up of mud, sand, gravel, pebbles, rocks and other deposit from the bottom or sidelines of the water bodies by means of a mechanically or hydraulic-driven equipment known as a dredge (or dredger). There are various types of dredgers each defined by its mode of operation. However local industry folks can easily relate with two popular types which are Cutter-Section and Plain/Jet Suction Dredgers.  The striking difference is that the former cuts into the bed, agitates it and sucks up the mixture before transferring it to the end point through a pipeline while the Suction dredger simply sucks up the sand directly and transfer.

Dredging activities requires you to obtain a license/approval which is given by the appropriate authorities after providing relevant documents. These conditions defer from state to state across Nigeria but generally a sand search is often conducted at first to determine the source, availability/quantity and quality of sand in the water body or shoreline to be dredged. This sand search result is often the first and major item developers look out for in a hydrological survey carried out pre-dredging. There is really no need attempting to dredge what is not available in required quantities? A Bathymetric survey is also carried out to determine the depth and bed ‘topography’. Both surveys are often used interchangeably to mean the same thing but they actually have their specifics. An Environmental Impact Assessment (EIA) is then carried out to determine the level of impact the proposed dredging activity will cause to the environment. 

The most suitable type of dredger to be used for a project is largely determined by the cost of purchase or leasing, the size of task to be performed, pumping requirements par efficiency as well as the nature of the sand deposits present. From observation, most small estate owners normally opt for suction dredgers of 8 – 14 inches with sand capacity averaging 350 cubic-metres of sand per hour; relatively cheaper to purchase or lease.  Larger cutter-section dredgers sizing up to 18 inches or more with sand capacity reaching 1000 cubic-metres of sand per hour is in high demand among medium and large estates’ owners. Experience shows that the rate of breakdown of jet suction dredgers on sites is relatively higher than the cutter-section type owing to their inability to withstand stresses associated with sucking up very stiff clay and rocks. But in any case, with a good dredger pumping sand constantly into your site, the heavy cost and risks associated with long-distance sand trucking is completely erased. 

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