Post -by Gautam Shah
A Rock is a very generalized term and can include any assembly of minerals such as stones, pebbles, sand and clay materials. Rock is also defined as any large and hard mass, composed of minerals that after its extraction and size reduction becomes stone. The term Stone is applied commercially to all natural rock materials quarried or mined for construction and industrial uses. Construction stone came from hard and consolidated materials of which there are three major classes:
- Dimension stone
- Crushed or Broken stones
- Naturally formed or Weathered stone
● Boulders are pieces of stones of various compositions buffeted by movements of snow glaciers, river floods or ocean waves and storms. Boulders are of tougher mineral compositions, because natural sorting has taken place. Only the strong and stable stones have survived periods of transportation and rolling which may have taken millions of years.
● Flat boulders occur on weathering of stratified rocks. Boulders are found not only on river beds and ocean beaches but also on mountain or hill tops and at places with no apparent riverine flows. This is so because, earthquakes and movement of plates displace such deposits, as in case of Tehri Gadhwal (Himalayan range, India) regions. People have been using such stones without any further cutting or dressing for constructing buildings with catastrophic results during earthquakes (e.g. Tehri Gadhwal, Latur in Maharashtra, India). Boulders are rounded shape and have smooth surfaces. In a masonry wall composed of boulders, it is not possible to ʽbreak the jointʼ or ʽcourseʼ the masonry. Walls are usually very thick, minimum being 400mm wide. Entire masonry structure, like a fluid mass, remains unstable. During earthquakes due to lateral movements, corners are displaced, and boulders fall off destroying the structure. Since walls of boulders are thick and seemingly solid, people have a tendency to place equally heavy roof structures on them, another cause for a heavy death toll during earthquakes.
Boulders have been used as cobble stone for paving of courtyards, walkways and roads. Boulders are also used for lining of canals, waterways (though not very efficient from a hydraulics point of view), land slopes etc. Boulders are also used for garden decoration and as landscaping features.
● Pebbles are similar to boulders in formation and characteristics but of smaller size. Pebbles are used for lining with or without cementing materials, courtyards, parks, walkways, ponds etc. Pebbles are broken to form aggregates for road building and cement concrete works.
● Gravels are finer then pebbles (also formed by natural processes) are used as fillers and decorative aggregates for plasters. Gravels are also used for packing tube well bores and as a filtering media. Gravels are crushed to form high shearing sands with granular faces.
● Weathered sands are mostly composed of quartz with feldspar, mica, garnet, zircon and magnetite. A sand deposit is composed of particles of different sizes and shapes such as rounded, sub angular or angular. Most sands contain small to substantial amounts of clay particles. Sharp angular sand is more suitable for concrete work. A coarse and well-graded sand gives a better mortar. Fine sand is better for filling and reducing plasticity of soils like black cotton soils. Garden soil or river `Kanp‘ is a conglomerate of fine sand, silt and decomposed organic waste.
Boulders, pebbles, sands and clay type of materials are available mainly from surface or low depth excavations. Bedded (stratified) deposits, usually of the sedimentary variety, are next in importance for ease of extraction, although the efficiency of extraction depends on the tools available. These deposits have many advantages over boulder deposits, as size and shapes can be predetermined and formed as required during the extraction. Slabbed deposits with layers between 60 to 80 mm are the easiest to extract, and shape. Thicker layers are usually subdivided with pneumatic tools and wedges or with cutting and sawing machines Bedded deposits of suitable thickness do not require horizontal cutting, as the blocks can be lifted at the bedding plane, thus saving much time and hard work.