Rock:
Rock is a combination of different minerals. When different types of minerals are joined together, they form a rock.
Rocks are mainly of three types
1. Igneous Rocks
2. Sedimentary Rocks
3. Metamorphic Rocks
Engineering Properties of ROCKS:
Rocks have very much importance in engineering point of view. All civil engineering structures are built on rock so, engineering properties of rocks are very much important.
In the following paragraphs, engineering properties of different types of rocks are given.
Engineering Properties of IGNEOUS Rocks
The rocks formed by the molten rocks (Magma or lava) are called igneous rock.
It has two main types
• Intrusive Igneous Rocks
• Extrusive Igneous Rocks
Intrusive Rocks:
Important features of igneous rocks are following
Granular texture
Massive structure
Relatively homogeneous composition
Some times highly altered with weather
Massive igneous rocks such as batholiths may affect tunneling, mining slope stability. These rocks are also used as construction material. Tabular intrusive rocks such as dikes may create more construction problems than massive rocks because of the lack of homogeneous composition.
Extrusive Rocks:
Extrusive rocks are found in crystalline texture. The origin of these rocks are greatly influence their engineering properties. Main characteristics which influence the engineering works are followings.
Variable composition and texture.
Strength durability and permeability.
Strong unconfined compressive strength >200mpa.
Columnar Jointing.
Engineering Properties of SEDIMENTARY Rocks
The following are the important properties of sedimentary rocks in engineering point of view.
Compressive strength and deformability of sandstone is influenced by its porosity, the amount and type of cement, and matrix material, grain contact and composition. Siliceous cement is stronger than calcareous cemented sandstones. Pore water plays a significant role in the compressive strength and deformation characteristics of sandstone. It can reduce the unconfined compressive strength by 30 to 60%.Shale mineral content influences geotechnical properties; most important is the quartz-clay mineral ratio. The liquid limit of clay shale increases with increasing clay-mineral content.
Swelling properties of certain shale have proven detrimental to the integrity of engineering structures. Swelling occurs by the absorption of free water by clay minerals (montmorillonite) in the clay fraction of the shale. Highly fissured over consolidated shale have greater swelling tendencies than poorly fissured clayey shale, the fissures providing access for water.
Porosity of shale may range from slightly under 5% to just over 50%.Cemented shale are stronger and more durable than compacted shale. The elastic moduli of compaction shale range between 140 and 1400 Mpa: well cemented shale has elastic moduli in excess of 14000 Mpa.
Clay shale usually has permeability of the order 10-8 m/s to 10-12 m/s. However, sandy and silt shale and closely jointed cemented shale may have permeabilities as high as 1 x 10-6 m/s.
Grain size and Cementation influence engineering properties of carbonate sediments. Carbonate sediments can sustain high overburden pressures, and retain high porosities at considerable depth. Generally, the density of these rocks increases with age, and porosity is reduced.
Important sedimentary rocks:
Following are some important sedimentary rocks
Sandstones vary from thinly laminated micaceous types to very thickly bedded varieties. They may be cross-bedded and are invariably jointed. With the exception of shale sandstone, sandstone is not subject to rapid surface weathering. The dry density and porosity of sandstone are influenced by the amount of cement and/or matrix material occupying the pores. Usually the density of sandstone tends to increase with increasing depth below the surface.
Limestone When dolomitize, undergoes an increase in porosity of a few percent. Joints in limestone have generally been subjected to various degrees of dissolution. Sinkholes may develop where joints intersect. And these may lead to subterranean caverns. The dissolution leads to an increase in mass permeability. Enlargement of the pores enhances water circulation encouraging further solution. This s brings about an increase in stress within the remaining rock framework, which reduces the strength of the rock and leads to increasing stress corrosion.
Chalk The unconfined compressive strength of chalk ranges from moderately weak to moderately strong. The strength of chalk is reduced when saturated. The Upper Chalk from Kent is particularly deformable, typical values of Young’s modulus being 5 X 103 Mpa. It exhibits elastic-plastic deformation. Discontinuities govern the mass permeability of chalk. Chalk is also subject to dissolution along discontinuities.
Anhydrite is a strong rock, gypsum and potash are moderately strong, and rock salt is moderately weak. Evaporitic rocks exhibit various degrees of plastic deformation before failing. Creep may account for 20 to 60% of the strain at failure. Rock salt is most prone to creep.
Gypsum is more readily soluble than limestone. Sinkholes and caverns can develop in thick beds of gypsum. Massive anhydrite can be dissolved to produce seepage flow rates which increase in a rapidly accelerated manner. Heave is another problem associated with anhydrite. When anhydrite is hydrated to form gypsum, there is a volume increase of between 30 and 58%, which exerts pressures between 2 and 69 MPa.
Salt is even more soluble than gypsum, slumping, brecciaing and collapse structures occur in rocks overlying salt beds.
Engineering Properties of METAMORPHIC Rocks
Metamorphic rocks are divided into two categories Foliates and Non-foliates.
Foliates are composed of large amounts of micas and chlorites. These minerals have very distinct cleavage. Foliated metamorphic rocks will split along cleavage lines that are parallel to the minerals that make up the rock. Slate, as an example, will split into thin sheets.
Rock is a combination of different minerals. When different types of minerals are joined together, they form a rock.
Rocks are mainly of three types
1. Igneous Rocks
2. Sedimentary Rocks
3. Metamorphic Rocks
Engineering Properties of ROCKS:
Rocks have very much importance in engineering point of view. All civil engineering structures are built on rock so, engineering properties of rocks are very much important.
In the following paragraphs, engineering properties of different types of rocks are given.
Engineering Properties of IGNEOUS Rocks
The rocks formed by the molten rocks (Magma or lava) are called igneous rock.
It has two main types
• Intrusive Igneous Rocks
• Extrusive Igneous Rocks
Intrusive Rocks:
Important features of igneous rocks are following
Granular texture
Massive structure
Relatively homogeneous composition
Some times highly altered with weather
Massive igneous rocks such as batholiths may affect tunneling, mining slope stability. These rocks are also used as construction material. Tabular intrusive rocks such as dikes may create more construction problems than massive rocks because of the lack of homogeneous composition.
Extrusive Rocks:
Extrusive rocks are found in crystalline texture. The origin of these rocks are greatly influence their engineering properties. Main characteristics which influence the engineering works are followings.
Variable composition and texture.
Strength durability and permeability.
Strong unconfined compressive strength >200mpa.
Columnar Jointing.
Engineering Properties of SEDIMENTARY Rocks
The following are the important properties of sedimentary rocks in engineering point of view.
Compressive strength and deformability of sandstone is influenced by its porosity, the amount and type of cement, and matrix material, grain contact and composition. Siliceous cement is stronger than calcareous cemented sandstones. Pore water plays a significant role in the compressive strength and deformation characteristics of sandstone. It can reduce the unconfined compressive strength by 30 to 60%.Shale mineral content influences geotechnical properties; most important is the quartz-clay mineral ratio. The liquid limit of clay shale increases with increasing clay-mineral content.
Swelling properties of certain shale have proven detrimental to the integrity of engineering structures. Swelling occurs by the absorption of free water by clay minerals (montmorillonite) in the clay fraction of the shale. Highly fissured over consolidated shale have greater swelling tendencies than poorly fissured clayey shale, the fissures providing access for water.
Porosity of shale may range from slightly under 5% to just over 50%.Cemented shale are stronger and more durable than compacted shale. The elastic moduli of compaction shale range between 140 and 1400 Mpa: well cemented shale has elastic moduli in excess of 14000 Mpa.
Clay shale usually has permeability of the order 10-8 m/s to 10-12 m/s. However, sandy and silt shale and closely jointed cemented shale may have permeabilities as high as 1 x 10-6 m/s.
Grain size and Cementation influence engineering properties of carbonate sediments. Carbonate sediments can sustain high overburden pressures, and retain high porosities at considerable depth. Generally, the density of these rocks increases with age, and porosity is reduced.
Important sedimentary rocks:
Following are some important sedimentary rocks
Sandstones vary from thinly laminated micaceous types to very thickly bedded varieties. They may be cross-bedded and are invariably jointed. With the exception of shale sandstone, sandstone is not subject to rapid surface weathering. The dry density and porosity of sandstone are influenced by the amount of cement and/or matrix material occupying the pores. Usually the density of sandstone tends to increase with increasing depth below the surface.
Limestone When dolomitize, undergoes an increase in porosity of a few percent. Joints in limestone have generally been subjected to various degrees of dissolution. Sinkholes may develop where joints intersect. And these may lead to subterranean caverns. The dissolution leads to an increase in mass permeability. Enlargement of the pores enhances water circulation encouraging further solution. This s brings about an increase in stress within the remaining rock framework, which reduces the strength of the rock and leads to increasing stress corrosion.
Chalk The unconfined compressive strength of chalk ranges from moderately weak to moderately strong. The strength of chalk is reduced when saturated. The Upper Chalk from Kent is particularly deformable, typical values of Young’s modulus being 5 X 103 Mpa. It exhibits elastic-plastic deformation. Discontinuities govern the mass permeability of chalk. Chalk is also subject to dissolution along discontinuities.
Anhydrite is a strong rock, gypsum and potash are moderately strong, and rock salt is moderately weak. Evaporitic rocks exhibit various degrees of plastic deformation before failing. Creep may account for 20 to 60% of the strain at failure. Rock salt is most prone to creep.
Gypsum is more readily soluble than limestone. Sinkholes and caverns can develop in thick beds of gypsum. Massive anhydrite can be dissolved to produce seepage flow rates which increase in a rapidly accelerated manner. Heave is another problem associated with anhydrite. When anhydrite is hydrated to form gypsum, there is a volume increase of between 30 and 58%, which exerts pressures between 2 and 69 MPa.
Salt is even more soluble than gypsum, slumping, brecciaing and collapse structures occur in rocks overlying salt beds.
Engineering Properties of METAMORPHIC Rocks
Metamorphic rocks are divided into two categories Foliates and Non-foliates.
Foliates are composed of large amounts of micas and chlorites. These minerals have very distinct cleavage. Foliated metamorphic rocks will split along cleavage lines that are parallel to the minerals that make up the rock. Slate, as an example, will split into thin sheets.
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