30 Apr COARSE & FINE AGGREGATES DEAD END: A WAY FORWARD
COARSE & FINE AGGREGATES DEAD END: A WAY FORWARD
- Nandakumar 1 & K. P. Thrivikramaji 2
- Scientist, Centre for Earth Science Studies, Akkulam, Thiruvananthapuram
- Professor Emeritus, Department of Geology, University of Kerala, Kariyavattom Campus, Thiruvananthapuram
Sand and gravel or aggregate mining is as old as the first building of brick and mortar. Aggregate, defined as the material added to cement, lime, gypsum, bitumen or other adhesive to form concrete or mortar, can be classified into two types, viz., coarse gravel and fine sand (Table 1). Addition of aggregate provides volume, stability and lower wear and tear. Broken stones, pebbles, blast furnace slag, broken clinkers, burnt shale and burnt clay (e.g., in Mullaperiyar dam) are examples of coarse aggregate. Obviously, aggregates originate directly or indirectly from the rocks and sediments of the earth’s surface.
Aggregates are truly the backbone of development of any nation and its annual consumption is yet another index of development-status, rate of growth and quality of life of the population, and hence of the nation. All constructions with cement mortar, plain and reinforced concrete (wherein aggregate constitutes @85% by volume) would not have been possible in the absence of sand and gravel. Further, construction and maintenance of a country’s infrastructure like runways, roads, rail-roads, bridges, canals, various sorts of commercial and residential buildings and such others call for large inputs of aggregates. In developed countries of the world like USA, Canada, Germany, Japan, China and others, annual consumption of sand and gravel is on the rise with rise of their GDP. Any addition to such facilities like houses, multi-storey complexes, schools, colleges, hospitals, churches, cemeteries, highways, rail roads, road and rail bridges, bus and train terminals, airport runways, harbours and so on, annually consume large volumes of coarse and fine aggregates.
Equally important is the role of mineral sands in manufacturing and other industries where in a derivative of mineral sand finds hugely significant applications (for e.g., titanium of ilmenite goes into making of airplanes and white paint). But one advantage with mineral sand is that in its absence, local or temporary shortfall, the particular metal or mineral can be imported from another country, which is untrue in respect of aggregates as the volumes involved are simply stupendous and unit costs appallingly low.
In India, the post-globalization emphasis on upgrading transportation sector, like making of 4-lane highways and 6 lane express ways, conversion of meter-gauge into broad-gauge doubling of lines, construction of new Greenfield airports and expansion of existing airports etc., is calling for huge and ready supply of good quality aggregates. Like any other mining operation (aggregates also require) mapping, assessment of volumes and grades, development, production and stockpiling to ensure delivery-on-demand to the client through a supply chain has a gestation time. One other great advantage of quarries or mines producing aggregates from glacial and river alluviam, is that no drilling or blasting is involved. Instead bulldozers, front end loaders, dragline buckets etc are used in the excavation, followed by washing, screening and stock-piling. However, in a country like India, with practically very low occurrence of fluvio-glacial deposits, hard crystalline rocks need to be quarried for manufacturing of construction rock and coarse and fine aggregates to meet country’s needs.
In what follows, various aspects of gravel and sand, like origin, formation of gravel and sand bodies, mining or quarrying of sand and impacts of mining are reviewed along with an overview of importance of sand and gravel to the nation and society.
GEOLOGIC BASIS OF AGGREGATES
For aggregates, undoubtedly the primary rock is the source and weathering is the mechanism that disintegrates readies the (parent) rock to release the durable quartz to go into sand bodies in river alluviam along with minute proportion of durable heavies. The process of Weathering attacks the rocks in a dual manner in that one is a physical modification and the other a chemical action, which jointly readies the rock for erosion by geological agents like wind, water and glaciers. Physical weathering or disintegration weakens the surface rock (Table 2) by alternate heating during the day and cooling at night, frost wedging by the pressure exerted by ice formed from water and moisture trapped in cracks and joints of rocks in colder climates, and wedging apart of blocks of rock by growing tree roots. These afford newer avenues for agents of chemical weathering (decomposition) to act on the constituent minerals.
Engines of chemical weathering are various natural chemical reagents, like, alkaline solutions, carbonic acid and humic acid. Dissolution, oxidation, hydrolysis and acid hydrolysis are the chief weathering processes. Despite their low concentrations, the constituent silicate minerals (but with the exception of quartz), are attacked steadily for very long periods of time (several 100,000s of yr) which finally transforms them to newer hydrous silicates or clays that are stabler at the earth’s surface conditions (Table 3). Process of denudation or chemical weathering is proverbially slow that we humans do not perceive any visible or even measurable change in the size, volume and appearance of rocks or the constituent minerals. Moreover, as the process of weathering is climate, relief and rock dependent, it is less logical to come up with one single number for the rate. Though, estimates of weathering rates are nearly difficult to make, some site or climate specific estimates have been made.
For e.g., in our tropical climate, with alternating wet and dry spells, most rocks and sediments are transformed to Laterite – the typical ubiquitous cover in the midland of Kerala and many parts of India. Laterite is also seen in other parts of the world, where tropicality prevails. In French Guyana, studies by a French team (Freyssinet and Probst, 1998), led them to believe in a rate of weathering of 3.0 m/Ma. But in another report from Up.Niger basin, Boeglin and Probst (1998) estimated a rate of 1.3 to 3.7 m/Ma. Menard (1966) estimated the rates of erosion for the Appalachian region, Mississippi valley and the Himalayan region in the geologic past and present time. The data is very instructive in that the unsettled Himalayan region perhaps sheds the largest volume of sediment (118.47 m3/km2/yr or 0.12 mm/yr) to the tune of 0.12 mm annually (Table 4). Therefore, availability of erodible gravelly sand is far less than the expectation of a lay person or generalist. In fact, it is this truism regarding the rate of supply of sand to the rivers that never was correctly understood or perceived by citizens outside of the geological profession. Instead, the rule of thumb happened to be that the river bed is not only an everlasting reservoir of sand, but it is automatically annually renewed on a use-it-or-loose-it basis, unaffecting the river’s physical or biological systems.
Thus erosion of weathered rock material and its transport by running water, is what creates sand and gravel deposits in the stream bed. Gravel is characteristic of up stream reaches of the drainage; while in the middle and lower reaches finer gravel and sand are very characteristic. However, unlike the terrains endowed with glacio-fluvial sediments of higher latitudes, the tropical terrains (including India) are bestowed with much less volumes of naturally washed stream bed gravel, and hence the needs for gravel or coarse aggregate all along have been met from rock-crushing-screening-units (gravel manufacturing plants) in the vicinity of quarries, or even manually breaking the rubble to required size as it used to be practiced in most parts of Kerala and chiefly employing the women day workers.
PERSPECT: INDIAN SCENARIO
Soon after independence, with the vehicle of V-year plans, India launched an ambitious program of building, very large cement and concrete structures (to the tune of several hundred thousand square meters) to house schools, colleges, hospitals, factories and office space as well as large number of dams, power stations, bridges, runways etc that consumed stupendous volumes reinforced concrete . But for the exception of certain Indian states like New Delhi, Maharashtra etc, in a vast majority of such cases, river sand was the fine aggregate while manufactured gravel (either manually or mechanically) equated with coarse aggregate. But private houses very strictly stuck to river sand. In Kerala, concrete dams (e.g., the double arch Idukki dam and Sabarigiri dam) used only manufactured coarse and fine aggregates. .
However, as cited in the foregoing, natural production of sand being a geological process is extremely slow and, rate of removal of sand from river channels more often than not out-paced the rate of supply by several hundred folds. The combination of waste water discharge (from homesteads, municipalities, waste treatment plants and industry) into one or other channel of the river net and “designed” removal of sand led to an “ecological demise” of a large number of rivers (at least in Kerala) as well as in other parts of the country.
Some of the consequences of removal of river channel sediment or sand documented Thrivikramji (1986) are summarized below. Sand borrowing affected a physical system of the river chiefly in 3 ways. Firstly, it led to fining of the texture of channel bed sediment, due to preferential removal of coarser sand fraction, resulting in a loss of spawning ground of the aquatic fauna. Secondly, abundance of finer sediment in the water column caused a decline or fall in the depth of the photic zone risking the life of primary producers. Thirdly, removal of sand resulted in deeper channels, causing disequilibrium between the channel walls and floor of river or in a higher freeboard for the channel walls with reference to channel floor leading to destabilization of the walls and slumping into the channel. Such slumps brought down and destroyed several hundred standing coconut trees over a river length of 20.0 km. and on either banks of the Neyyar (Thrivikramji, 1986), along with little or large riverfront precious land and other crops creating a new group of “landless” or “land-lost”. All these jointly caused a sharp decline of faunal diversity and population density of the riverine-aquatic-life.
Over reliance on river channel sand both in public and private constructions deprived the rivers of Kerala, from their legitimate load of sand, resulting in the transformation of physical and biological health of the river. Moreover, this in combination with effluents originating from towns and villages caused the “ecological demise” of most of the rivers of Kerala. In he Neyyar basin, sand borrowing activity reached hectic levels (i.e., 180,000 tons/yr, in CY1985), which truly was far in excess of the combined annual discharge of dissolved and suspended loads to Neyyar. Loss of income due to uprooting of standing crops like coconut palm by wall collapse or slumping, was estimated as Rs.750,000/- per annum.
India lives in the villages and the state of homes, sanitation and quality of streets, schools and health centres on the one hand and that of the irrigation systems on the other serving the population of 400,000 or more villages of India are the best indices of quality of life of majority of Indian citizens. In fact, paved streets, permanent buildings to house the families, school classes and health care facilities have not yet been fully met with. For a long while, the well-to-do, accounting for a smaller percentage of the population, lived in good houses that used masonry. River sand and to some extent river gravel, certainly met the local demand for aggregates. However, predominant portion of coarse aggregate belonged to the manufactured category, leaving the rivers to mend themselves.
Currently, the picture certainly changed (for good) drastically. Now, under the various programs of states and center, massive investments are made in the village sector and as a consequence demand for Portland cement and aggregates too skyrocketed. As per FY 2006 data, cement production in the country stood at 160 million tons, accounting for an input of 906 million tons or 515 million cm3 of aggregates. Use of “manufactured” aggregates is catching up fast in the construction sector along with hollow concrete blocks or bricks and ready-mix concrete. Recently, several large capacity rock-crushing-screening plants have come up in the various states, including Kerala associated with what are called super-quarries.
Truly with the government’s emphasis on modernizing the infrastructure in the surface transport sector, the level of aggregate and cement production is getting ready for a quantum leap. But, the rivers of India will not be in any position to meet the requirements of aggregate by the new growth centers or sectors spread across the country. Unlike cement and steel, as far as possible aggregate must be sourced locally, in order to offer a low or attractive price to the end user. For example, builders in Trivandrum dist, Kerala, due to non-availability of river sand, source it at a very high price, from places like Sri Vaikuntam (in the shore of Thamraparni Ar,) in TN – like 150 km. one way from the user site.
PROSPECTS: FUTURE DIRECTIONS
India, a large populous country (Population = one billion; area =3,166,414 km2) occupying little more than 2.0% of the land area of the earth, is developing at a relatively fast pace of over 7.0%. Majority of the Indians live in the villages, a colossus of about 400,000. But, the distressing fact is that about 1/3 of the population still lives below the poverty line. Truly in the first 4 decades after independence, no immediate attention was bestowed on the development of infrastructure of the country like modernization of roads, rail roads, airports, houses for millions etc., or securing a healthy environment in the villages.
Table 5 gives a picture of the present status of surface transport facilities in the country. All such constructions will require input of gigantic volumes of sand and gravel along with cement and steel. Mining or quarrying or manufacturing of both sand and gravel is bound to create environmental problems and this challenge can be faced up and tackled by a team of geoscientists, leaders of the society, mining company and regulators of mining. More over, with the ambitious plans in the real estate and infrastructure sectors in India, we must examine the possibility of sourcing aggregates from even the shallow seabed; mine waste storages, cinder or slag of industrial smelters and recycling of building waste etc. It is high time that construction sector buried the one-material-one-source mindset and instead one-material-multiple-sources.
In fact, new-divided 4-lane-highways, new express ways and undivided-2-lane-national highways would have consumed a large quantity of aggregate like 14.0 x 10 12 tons (or 14.0 trillion tons) only to build initially. Renewal or maintenance would demand an additional input of a lesser volume like a third of this, say once in three or four years. The story is not very different in respect of the Indian railways either. Indian railroad net work is one of the largest in the world (Table 5). In the rail road, ties and rails are placed over a track-bed of crushed stone or ballast of 4.0-6.0 cm. size and over a thickness of at least 30.0 cm. The construction of rail road system in India would have consumed a volume of 132.0 billion cubic meters or 232.0 billion tons of ballast. The renewal of track bed is one of the vital tasks of track maintenance, when older worn out stones are replaced with new ballast, needing a relatively large volume new material.
The black sand (BS) deposits are chiefly seen in the beach placers in the states of Kerala (Chavara-Ambalapuzha), Tamilnadu (Manavalakurichi) and Orissa. Being what these are, at least in Kerala, there is an ongoing controversy regarding mining of BS, Due to the high population density (side of support squareof a person =32 .0 m) of the coastal land, this valuable mineral industry is yet to get a legitimate chance to expand or to permit entry of new private players.
Interestingly, the offshore of Kerala is a vast reservoir of a silty-muddy-sand carrying fairly large percentage of black heavies. The recent Tsunami of Dec, 26, 2005, offered the smoking pistol on the abundance of BS in the offshore. Very large volumes of BS were washed over the backshore by the Tsunami wave, blocking the coastal roads for days together and withholding the entry of automobiles carrying relief supplies. JCB’s had to be brought in to clear the road pavements in order to allow entry of 4 wheeled vehicles.
In fact, the mindset of public in respect of BS mining needs change as Kerala has one of the best-known BS rich sediment of the world sitting in the offshore, a portion of which is deposited on the eroded beach-face when the waves start rebuilding of the beaches after the initial erosional phase of SW monsoon. This renewal of BS placer takes place annually in association with the SW monsoon. The seabed sand deposit is a vast reservoir of sand deposited in the inner shelf during the last 65.0 ma (Vinodkumar, 2004). According to Anthraper et al., (2005) worth of the BS in Kerala, after mining and concentration and at to day’s price is in excess of Rs.40,000 crores during the next 30 yr. or at 10 or 5% royalty a whopping 4000 or 2000 crores of rupees. A portion of this money can be used for constructing first rate township/s for re-settling the population of the coastal tract of Kollam-Alappuzha sector, who otherwise are herded to and sheltered in school buildings or similar places to escape the wrath of monsoon waves.
IMPACTS OF MINING
Aggregate mining by quarrying or open cast mining is no different from strip mining, where the overburden is initially removed to expose the valuable deposit which can be accessed with mining machinery. It is not quite true in respect of granular, unconsolidated, accumulations of sand from the modern river beds and flood plains or from deserts. Conversely, in the Indian context, when all the aggregate needs to come from quarries after crushing, sizing and washing, a degree of drilling and blasting precedes making of gravel. The important environmental concerns are the following.
- Noise pollution from the blasting, operation of large machineries in manufacturing aggregate (both fine and coarse); by the large trucks and moving in and out of the site
- Dust pollution from operations like drilling and blasting; crushing, sizing, stocking and handling; as well as from the traffic of heavy duty trucks
- Disfigurement of landscape due to the scars left behind by abandoned mines and quarries.
- Modification of ground water as well as surface water regimens and geometry of stream network, and finally
- Pollution of surface and ground waters due to unscientific disposal of used oil, grease and other petroleum products.
- Waste water with fine dust laden water from the battery of screens
- All mining activities will result in small and large scars of abandoned pits as well as some amount of stony waste.
Measures recommended for containment of negative impacts of strip mining are:
- Some of the measures suggested to contain these negative aspects of strip or open cast mining are building of a green belt or living barrier along the perimeter of the mining and operational areas. Fencing by some strip-planting of fast growing tree species are generally recommended
- Establishing a more or less continuous tree belt, around the mining and processing area to hold back considerable volumes of fine dust, as well as good deal of noise caused by the machinery. Tree strips also function as an effective barrier against air-flow or wind whipping the dust into the air.
- Sprinkling or spraying water on the truck tracks and other possible sources of dust. Vegetating with suitable species of natural turf to minimize the bare patches or parcels of loosened soil.
- Battery of water clarification tanks will create water that is suitable for wet screening. Ponding or storage of waste water in low capacity reservoirs is a process to be strictly adhered to prior to release of this water essential to lare essential to . reusable washing.
- Careful design of mine plan must be adhered to ensure avoidance of blocking of stream courses, however small they be and especially in areas dominantly with wet and humid climate.
- Large and small pits abandoned after mining needs to be restored to the original near natural state in respect of the soil horizons and vegetation, by placing the overburden back in, strictly adhering to the natural order of succession of the sediment layers. But in respect of aggregate quarries, it is nearly unfeasible due to the near total salability of practically all the material coming out of the mining operations. Further, the abandoned mine pits in crystalline rock areas, it is more impractical. On the other hand, such pits can be intelligently converted to huge water storage facilities, if required by applying some “friendly” seal to enhance water tightness of the basin. It can then be a site for water sports as well as recreation and picnicking. All it takes is an ingenuity of people in the immediate neighbor-hood to come up with practical positive uses of abandoned strip mines. The sand and gravel pits are no different in this respect.
- In respect of black-sand mining in the west coast (both Chavara-Kayamkulam tract in Kerala and Manavalakurichi in TN), operations are firstly along the beaches built by the latest monsoon wave climate, and secondly by using earth moving machinery like bulldozers, front end loaders, heavy duty scrapers/levelers and dump trucks to transport the mineral sand to the stockyard. Mining never goes below low-water-mark. Year after year blacksand placers are consistently rebuilt by the monsoon waves, However, mineral sand mining in the east coast of TN is primarily focused on the ancient coastal dunes, and hence a certain degree of disfigurement of the natural landscape is unavoidable. However, backfilling of the lows or shallow pits are with the waste from the mineral separation plants.
- Yet another way of reducing the impact on the community could be by locating lease areas outside of the region of visibility of the members of the community. In general an alliance of the leaders of the community, mining company, local and state mining regulator and geoscientists can certainly contribute to minimize and even largely forestall the negative impacts from mining and related operations.
In the state of Kerala, sand mining has been banned by law in many of the river stretches. However, a safe level of exploitation is permitted based on ‘semi-scientific’ estimations in certain river stretches at certain specific periods. This self imposed ban has created an artificial scarcity of construction grade sand and has lead to a chaotic situation with beefed up and varying costs per lorry load of river sand. If the cost of sand in Kerala is analyzed too closely the picture is alarming and may find the cost of 1 lorry load in Southern Kerala is equivalent to the cost of 2 lorry loads in Northern Kerala. It is to be noted that, the government machinery has so far failed miserably to a very large extent to propagate alternate sources of C& FA to the public while banning the river sand mining in the state Or rather the government should have shouldered the responsibility of at-least meeting a portion of the demand using alternate C& FAs. Kerala’s construction sector is therefore ‘strained’ unnecessarily and is imposing a negative impact on the overall developmental activities of the state and is leading to an un-imaginable drain of the state’s economy.
Compared to the infrastructural developmental boom happening in the rest of the country, Kerala’s mark is decimal. Kerala has to move at a rapid pace to catch up with the surrounding states. As previously emphasized, the backbone of infrastructural development is C&FA and therefore we are proposing a new state owned institutional mechanism to ensure availability, fair price, quality, equity and self reliance for the C& FA in the State of Kerala. The new institution can be the “Kerala Minor Minerals Development and Marketing Corporation” with the following overriding objectives of:
- Establishing new super quarries or quarry clusters or even subsurface mines for every district to ensure fair price and adequate supply of C&FAs with adequate environmental safe guards.
- Outsourcing of C& FAs from the surplus states internally and internationally.
- For acquiring properties under lease from the respective states.
- Separation of finer aggregate grade material from the mine over burden from within the state as well as from adjoing states.
- Probe the possibility of seabed mining for fine aggregates to partly meet a portion of the needs of the state
The KMMDMC can have Technical, Legal as well as Marketing wings.
“If you can not grow or buy it, you got to mine it”. This Chinese proverb aptly and bluntly tells us the need to mine the resources on or below or deeper below the surface in the crust. Distribution of minable minerals is far less uniform in the rocks that some continents and nations have more mineral wealth than others. Any mining activity is a special effort focusing on a target of anomalously large natural concentration of one or more minerals in the solid earth and results in a “huge” accumulation of waste. The latter will have deleterious and occasionally unpredictable impacts on the biosphere by the mediation of pedosphere and/or parts of hydrosphere. It is sort of an occupational hazard.
Yet, like any mining operation, aggregate mining can be pursued very scientifically and intelligently to provide for the ever growing needs of the society to build and maintain large infrastructural projects in the sectors like, transportation, housing, recreation and trade and commerce. For e.g., conversion of the undivided single lane National Highways of India, to divided, two and four lane highways and superhighways will initially call for nearly a trillion tons of aggregates. The requirement aggregates for the new generation airport runways and modernizations and maintenance of rail roads also point to a staggering bulk. CONSIDERING the large population base, needs of housing and commercial and recreation sectors are also bound to be very huge. With approximately 160 million tons of cement produced last year, the requirement of aggregates would be hovering around 906 or let us say 900 million tons (=511 million m3).
Scientific planning, design and execution of mines along with active co-operation of community leader, scientists and administrators, the requirements of aggregate of a district/s can be easily secured, ensuring minimal adverse impacts on the environment. But mining or gathering or aggregating black sand in the west coast (Kerala and TN) is targeting the black sand placers annually accumulating in the beaches, during the latest monsoon. Contrary to this, Garnet separation plants in the east coast utilize the mineral sand in the ancient sand dunes in the inland, and modern backshore. But refilling of the lows or scars with the waste sand to a large extent restores landscape to near natural contours.
It is high time, to establish a state owned institutional mechanism to ensure availability, fair price, quality, equity and self reliance for the C& FA in the State of Kerala.
Many of our thoughts in this regard heavily lean on a status study of the Neyyar in south Kerala, in the eightees, with funding from the MOEF, GOI. Director, CESS is also thanked.
Anthraper, BJ, et al., 2005, Black sand deposits of Kerala: A cost-benefit analysis: Abst. of Paper in “Mineral Resources of Kerala”, Feb. 2005. Trivqandrum
Boeglin,J.L, and Probst,J, 1998, Physical and chemical weathering rates and CO2 consumption in a tropical lateritic environment: the Upper Niger basin: Chem. Geol., 170,, 133-151
Freyssinet, P. and Farrah, A.S. 1998, Geochemical mass-balance and weathering rates of ultramafic schists in Amazonia, Chem.Geol., 170,113-121
Menard, HW, 1961, Some rates of regional erosion, Jour. Geology, 69, 154-161
Thrivikramji.K.P, 1986, River Metamorphosis due to Human Intervention: A Neyyar basin experience: Final Report submitted to MOEF, GOI, 153p.
Vinodkumar, N, 2003, Sedimentology of the Placer sands of Kerala coast: Unpublished Ph.D. thesis, University of Kerala, 117p
Wedephol, K.H (ed.) , 1969, “Composition and abundance of common igneous rocks” in Handbook of Geochemistry, Springer Verlag, 227-249
Table 1. Size grouping of aggregates (Encyclopedia Britanica)
|Type||Size, mm||Size, in|
|Fine||0.025 – 6.5||0.001 -0.25|
|Coarse||6.5 – 38.0||0.25 – 1.5 or larger|
Table 2 Abundance of intrusive rocks in a standard
section of upper continental crust (Wedephol,1969)
|Rock type||Abundance, %|
|Granite & Quartz monzonite||44.0|
Table. 3 Mineral proportions in plutonic igneous rocks (Wedephol,1969)
Table 4. Rates of regional erosion (Menard,1961).
Note: Geologic (1) and modern (2) deposition rates in Mississippi basin- the rates are nearly same. Deposition rates (3) in the Appalachian region in geologic past and modern day (4), Geologic deposition rate (5) in the Himalayan region and modern day rate (6)
Table 5. Road and Rail road network, India ( from website of GOI)
|Roads, Types||Length, km||Rail road||Length, km.|
|Express ways||200||Total track||62195|
|National Highways||66,590||Double track||12617|
|Major Dist. Roads||470,000|
|Rural & other||2,650,000|
 Paper presented in “Kerala Vikasana “ Seminar, Trivandrum, 2011