Comments /5030 Views / Thursday, 21 June 2012 00:50
A few weeks ago, based on a proposal made by the Minister for Environment, the Cabinet of Ministers agreed and informed the public that the legal requirement that permits to be obtained for the transport of sand during day as well as in the nights had been discontinued.
The action was to reduce the burden of sand prices to house builders. The move has been hailed by the public, but criticised by some of the current transporters, claiming the drop in sand prices would be minimal.
Collection of sand for the construction industry has always been a sore point between builders and environmentalists. Builders require sand, a basic material in construction that needs to be freely available at reasonable prices; the environmentalists claim that the collection of sand from river beds, the commonest source is detrimental to the environment.
This argument has led to controlling laws, licenses, permits, transport contractors, political interferences and allegations of corruption among the officials as well as police. However, for the house builder and the contractor, there were no shortages of sand in the market, but at a price.
The consumers of sand, the house builders, cement block manufacturers; small yards that manufacture concrete components as well as the majority of small contractors, all have only a vague idea of the quality of sand. The contractors consider Mahiyangana and Manampitiya sand to of superior quality and are prepared to pay a higher price, which offsets part of the long haul distance in carrying into Colombo.
The sand suppliers get their sand from persons who collect them from rivers and streams that are expected to obtain a permit from the Geological Survey and Mines Bureau and miners are given permission to collect a certain quantity of sand from the river bed. Sand collection in the streams are done manually by persons who dive into the water and scrape the bottom of the stream, or by scraping the river bed the wicker basket tied to a long stick, which are loaded into barges.
In addition there are locations of sand deposits, where the sand has collected over the ages, generally due to change of a river course or piling up of sand near sea due to wind, which are located near the sea shore. Permits for excavation of such locations are given very rarely, but illegal mining continues with the usage of machinery, mostly by persons with political connections.
With the relaxing of regulations in transport of sand, truck owners joined the band-wagon of transporting sand and prices in the market have come down. Concerned parties have warned that the increase in sand mining from river beds and the use of machinery are harmful to the environment and questioned the sensibility of relaxation of legislation.
Normally the months of May and June should bring in monsoon rains and sand prices soar. The sand dealers have made use of the abundant availability in stockpiling sand, hoping to make money during the oncoming price increase during rains. But the price escalation is unlikely due to plentiful stockpiles; there would be more sand than the market consumes.
Sand is not an independent item in house construction and reduction of sand prices will not increase consumption and house building, just as much as increase of sand prices did not reduce house construction. After the initial stockpiling, the sand suppliers will be forced to realise the reality that the market requirement is only to replace usage. The sand prices will stabilise at a lower level.
A few days ago a spokesperson for the Geological Survey and Mines Bureau informed that a large deposit of sand was discovered close to Ampara, but within a forest reserve, and they are conducting investigation to determine the suitability of sand for the construction industry.
Erosion, the source of sand
Sand is the coarse part of the soil, brought into the streams and rivers due to erosion. The finer particles of the soil are easily carried and washed away as mud, but heavier sand settles to the bottom and slowly carried away with the speed of the watercourse. The particle carrying capacity of water increases to the third power of the velocity (speed) of water, which means twice the speed of water allows transport of eight times in weight, three times the velocity gives 27 times, four times the velocity gives 64 times, etc. This explains how during major floods, streams reach the ability to uproot and carry huge trees from the river banks.
When earth particles are carried by flowing waters, on reaching flat terrain water slows down depositing sand particles on the river bed. These are the deposits for sand miners. With the building up of hydro-power stations in the upper reaches of Kelani, Mahaveli and Kalu Rivers, the sand and silt from erosion gets trapped in reservoirs and lower sections of the river are deprived of sand deposits. Meanwhile demands of the construction industry for sand continue to increase. The sand miners continue to scrape the bottom of the streams causing collapse of river banks, an environmental hazard.
Sea sand to the rescue?
How could this ever increase in demand for sand can be satisfied? The Sri Lanka Land Reclamation and Development Corporation (SLLRDC) has been selling reclaimed sea sand. During the latter part of President Chandrika Kumaratunga’s regime, a contract was awarded to a Korean contractor to construct the Katunayake expressway. The contractor intended to fill the marsh for the road bottom with sea sand, dredged the sea had stockpiled a huge quantity. After few years the contractor abandoned the contract, leaving the contract uncompleted. The Government took over the procession of stockpile of sea sand and handed it over to the SLLRDC.
Over the years, sea sand stockpile being exposed to elements, salt at the top of the pile got washed away and the SLLRDC has been selling sand for prospective house builders. SLLRDC claims the sea sand washed by the rain is sufficiently clean to be used in construction. However independent tests carried out show the chloride (due to salt) content left within the sand is over 10 times the allowable limit. Maybe SLLRDC took samples for testing from the top of the pile, which were well exposed to years of rain. But public are issued with sand from the bottom of the pile containing excess salt and chlorine.
A week ago a Government Minister proposed using sea sand for construction. If sea sand to be collected, the sand need to be dredged in the sea few km off the coast to prevent sea erosion and will have to be pumped to the shore to a pile. The stockpiled sand needs to be washed with clean water to remove the salt. Where can one get so much of fresh water for washing and at what cost?
Dredging of silted reservoirs
Meanwhile large quantities of sand and silt have been being collected over the centuries in our reservoirs, which had been washed down due to erosion. Our reservoirs small and large collect their water from streams which also bring mud and sand, which are deposited filling up the reservoirs. Irrigation Department and the CEB complain of the reduction of volume of the reservoirs and resulting shortage of water for irrigation and power generation. The consultants propose usage of heavy dredgers to pump out the mud and sand from reservoir bed, a very expensive exercise. These collected mud and sand could be utilised to meet the sand needs of the people.
Reservoirs are of two types, some with deep bed and others with shallow bed. Most of the recently built reservoirs for hydro-power are on mountainous regions have deep beds. Older reservoirs build for irrigation have flat beds. During the dry season, the water levels in shallow bed reservoirs recedes exposing deposited mud and sand. These could be excavated mechanically and washed and cleaned for their components.
For reservoirs with deeper beds, it is necessary to build a small dam (silt traps) across the stream path in a flat area upstream from the reservoir. This small dam would slow the flow of water, making the sand to collect upstream of the small dam, then spill over and continue. The sand could be collected and cleaned for usage and would give employment opportunities to locals, while keeping the reservoir free from silting. Slit traps at stream entrances could trap silt and sand from entering the reservoir. Such silt traps are already built around the Kandy Lake, but the traps are not cleaned regularly and are overflowing.
Around 5 years ago, the recovery of silt and sand from a reservoir bed, was put into practice by a Sri Lankan Mechanical Engineer, who returned to Sri Lanka, having been employed in gold mines in Zambia and in South Africa for nearly thirty years. He obtained necessary approvals and dredged Batalagoda Tank located 15 km from Kurunegala, which was heavily silted due to erosion from nearby Matale hills. With his expertise in recovery of gold from ore, he fabricated a washing plant, to separate excavated material into stones, sand and compost. He sold the recovered sand at Rs. 3,000.00 per cube and was met with a heavy demand.
His success led to proliferation of number of persons in dredging for sand, without any approvals but with political backing. During the dry season when lake waters had receded, they excavated pits in the lake bed, when the material encountered was not acceptable, moved to a different location. When rains came, waters flooded the entire lake bed. Locals who went for bathing in the lake, unaware of pits dug and left uncovered, fell into pits and two persons drowned. Environmentalists took the matter to courts and sand mining in the lake was prohibited for everyone, permits or not.
Today, the Mechanical Engineer has moved out from de-silting of tanks to production of machinery for the recovery of gems from gem bearing gravels and his machinery is very popular with gem pit owners.
The drowning of two persons was unfortunate and could have been prevented by proper, methodical removal of silted material and leveling the tank bed after excavation. Removal of silt would have increased storage capacity of the lake, provided the country with quality sand as well as compost for enriching gardens.
Sri Lanka is dotted with number of lakes which have been slowly filling up due to silting and the method could be successfully implemented if done under proper supervision. De-silting of tank beds, when properly done would yield country’s requirement of sand for construction purposes without endangering the environment.
Sri Lanka’s coastal areas, especially in the Western coastal belt, boasts of number of generally stagnant lakes, small and large, located few hundred meters away from the sea, who have got silted and become breeding ground for mosquitoes. Part of the Bolgoda Lake in Lunawa area between Galle Road and the sea was in such a condition and was dredged recently by a Chinese contractor. The material taken out was found to consist of over 85% sand, mixed with decomposed kitchen waste form housing from the area.
Material excavated would have been ideal for the recovery of sand. This material with much lower salt content than the sea sand could easily be upgraded to be suitable for construction. The dredging of coastal marshes while cleaning the environment from mosquitoes, will meet the needs of sand consumers.
Quality of sand
Quality of sand is specified under the British Standard 882:1992 for concrete works and British Standard 1199 & 1200 for Building Sands (brickwork and plaster) which specify the percentage particle sizes of the sand used for particular purpose. The particle sizes in the sand for concreting could vary from 5.00 mm to 0.150 mm and the desired distribution range for different sizes of sand particles are given in the Standard.
A very important aspect of not considered by house builders, as well as manufacturers of cement sand blocks, is the presence of clay or mud in the sand. The importance of purity of sand is highlighted in the following incident.
In the early 1970s, during the construction of Polgolla Dam across Mahaveli River, which happened to be the first major construction project in independent Sri Lanka supervised by a Foreign Consultant, the State Engineering Corporation (SEC) was the contractor. The sand for the construction was harvested from the nearby Mahaveli River. The Consultant insisted that the sand and the metal (stones) used for the concreting should be washed and cleaned. The Chairman of SEC A.N.S. Kulasinghe fabricated a washing plant and the sand and the metal were washed, stored under cover and concrete was mixed in a concrete batching plant.
To the surprise of all, the resulting concrete when tested showed a level of strength so high that it was possible to lower the cement percentage used by a heavy margin, enabling considerable saving in costs. The governing factor for cement became the minimum percentage of cement in the mix, recommended in the specification. Even so the strength of concrete so obtained was much higher than specifications required. This proved that clean sand without impurities such as mud, along with Sri Lanka’s metal (stones), which is mostly Gneiss, one of the strongest rocks in the world, can yield higher strength concrete.
Vice versa, the mud and impurities in the concrete mix can reduce the strength by a considerable value, which is compensated by addition of cement at extra cost. The usage of cleaner sand can reduce the cement usage in the construction industry, saving costs to the user and foreign exchange to the country.
Alternatives to sand
With sand becoming scarcer and prices increasing, are there any alternatives to sand? We in Sri Lanka have got so used to river sand and cannot imagine using any other. But our cement block manufacturers are already using quarry dust (fine particles resulting in mechanical crushing of stones) instead of sand for their manufacturer.
Our quarry dust is an excellent alternative for sand, but with limitations. Quarry dust contains a higher percentage of finer material, therefore in concreting, up to half the sand quantity could be replaced with quarry dust. In fact the resulting mix yields higher strength. Concrete suppliers for the construction industry in their higher strength concrete, always use a percentage of quarry dust.
But the availability of quarry dust in large quantities may not be possible, as most of the current production is already used in producing ABC (Aggregate Base Course), a major component in modern road construction.
The majority of our cement block manufacturers turn out poor quality products, not due to usage of quarry dust, but due to lack of understanding of materials they use. Insufficient cement, improper mixing, insufficient compaction are major causes. The cement blocks having made need to be kept in a shade for 24 hours for initial setting, need curing (sprinkling with water) afterwards, which is normally not done by small manufacturers, but are dried under sun; both are harmful to the strength of the cement block.
In other countries sand and metal (stones) come from other sources as well. For example in England, River Thames is continuously dredged by using dredgers to facilitate movement of ships. The recovered material consist mostly pebbles and sand. The larger pebbles are crushed and smaller ones are uses as they were and used instead of rock stones. Much smaller pebbles are crushed to recover an alternate to sand.
Sri Lankan river sand
Our rivers are comparatively short and the sand recovered from the rivers has travelled over the river bed for a shorter length, have not rounded off their edges and are contaminated with mud and silt. Sand harvested from the rivers can be divided into coarse, medium and fine. Their availability depends on the period of harvesting after the rains (floods). After a heavy rain sand would be contaminated with heavy mud, which gets washed after few days of water flow, leaving fine sand used mostly for plaster work. Dry periods yield coarse sand used mostly for concreting. Pit sand obtained from excavating old river courses yield a material heavily contaminated with dust are generally unsuitable for the construction industry unless washed.
Sri Lanka’s future requirement of sand could be easily obtained from de-silting of lakes and reservoirs and cleaning the excavated material. This would give rise to better quality sand, enabling higher strength concrete with lower cement content saving foreign exchange to the country. The extra cost of washing and cleaning would be offset by the reduced usage of cement. De-silted lakes and reservoirs with enhanced storage would enable to produce more electricity and give more water for irrigation purposes.
(The writer is a Chartered Civil Engineer graduated from Peradeniya University and has been employed in Sri Lanka and abroad. He was General Manager of State Engineering Corporation of Sri Lanka and currently employed with a Chinese construction organisation. He can be contacted on email@example.com.)
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