Sand Search Survey, The Core of Sand Mining.
Sand Search simply implies exploring water bodies with the objective of locating areas with sand deposits in commercial quantity, to be exploited by dredging for reclamation of marshy terrain or stockpiling for sale to construction companies. The importance of evaluating the status of the sub-bottom of water bodies in the dredging industry cannot be emphasized enough. Geological investigation of the substrata in and around dredging sites has been trivialized in Nigeria’s dredging industry and has resulted in some cases of failed projects due to faulty sand search reports among other factors. Entrepreneurs usually make the mistake of side tracking professionals in their quest for cheap labour and end up with regrets and urge losses. Conducting a Geological, Geophysical or Geotechnical Survey, especially in a marine environment is no mean exercise and requires a scientific approach to arrive at reliable results.
The need for an accurate sand search report before acquisition of a reclaimable or stockpiling site is very important and is the first step towards carrying out a sand mining & stockpiling business. Cases of people rushing to acquire and pay for sites is common, on assumption that, sand to be mined is available almost every where in the water body. Non availability of sand in such sites after payment have been made for land, have led to huge losses and regret on the part of the client especially when a non-refundable mobilization / demobilization fee of millions of naira had been paid to a dredging company. The proper approach should have been to first ascertain availability of sand within the periphery of the proposed dredging site, the quality of such material and the distance from the points of mining to the point of discharge. Such information determines to a large extent the cost of dredging and where the report is negative, implying non availability of sand in commercial quantity and quality, negotiations for site acquisition should cease immediately. Land owners should be made to agree to a time frame for the conduct of a sand search survey before further negotiations are held. The positive result of the survey may also increase the value of such site to the advantage of the land owner.
Scientific approach for conduct of sand search surveys involves Bathymetric Mapping for depth information, Boring for sample collection and Accurate Positioning of proposed burrow pits for dredging from the right spot. More sophisticated methods include the use of a sub-bottom profiling electronic survey equipment that gives water depth and accurate sub-bottom profile of survey lines and subsequently a 2 or 3 - dimensional model of the sub-bottom of the project area. Such equipment can acoustically ‘see’ as far as 1000m below the seabed and produce traces of the sub-bottom with precise discrimination between various soil types, layer by layer.
Electronic Chirp Sub-Bottom Profiling Equipment.( Courtesy SyQwest)
Seabed profile from Electronic Equipment.
Local or manual methods include drilling by waterjet which though produces good results is however limited to shallow depths and has a singular disadvantage of not being able to discriminate between soil type layer lines. The geometry of drilling is also not adhered to by non professionals who confuse a borehole with a burrow pit and therefore do not ascertain material spread but assumes availability of sand if a borehole is successful. Volume calculations cannot be based on one point, which necessitates the drilling pattern to either be in a grid format covering the entire project area or in a burrow pit configuration using cylindrical figures. With the former approach, quantity can be calculated from sub-bottom contours while the latter approach allows volume computation by the cylindrical formular. The project area if very large is divided into blocks analogous to oil blocks in oil exploration. This of course will not be possible with out accurate mapping of the project area using land survey methods or the more recent satellite mapping technique using the Global Positioning System (GPS). The map so produced is an essential tool for sand mining project planning and implementation.
Manual Sand Search by waterjet.
Another serious source of error by non professionals is the inability to accurately position the borehole points and measure the distance between such points and the stockpiling site on land while giving considerations for slack needed for the dredger to swing. Pegs are usually placed on such points after investigations but when such pegs fall off due to wave and current movement, it becomes near impossible for such points to be accurately re-positioned. Only a scientific approach can be used to come back to such points with high precision and also determine distances. Dredging companies need to know the distances for exploitation of the sand deposits as this will enable them ascertain the length of their floating hoses needed for that particular project. Furthermore the co-ordinates of the center point of proposed underwater burrow pits must be known for accurate positioning of the dredger on the sand pit.
In conclusion therefore, a good sand search survey report must among other things provide information as to the quantity of sand available for exploitation based on mathematical formulas not guess work ; the quality of sand, usually determined by laboratory sieve analysis for particle size range of the samples ; a co-ordinated bathymetric map of the project area , not a sketch, depicting depths for smooth navigation of the dredger to the sand pit and as a base for determining changes in the sub-bottom topography after dredging ; the exact co-ordinates of the center point and radius of burrow pits; the distances of the burrow pits to the stockpiling site and a recommendation on the length of dredger ladder required for maximum mining of the deposit from the burrow pits. Professional Hydrographic Surveyors, their fees not withstanding, should be used for this exercise as a way of avoiding losses, due to inaccurate data acquisition methods. Such post operational losses are by far greater than the Surveyors fees and the report remains an asset for the client. Sand search reports determine the success or failure in sand mining and must therefore, be professionally handled as a core component of the sand mining business..