Engineering Properties of Rocks | Geology Optional Notes for UPSC PDF Download

Engineering Properties of Rocks Summary

1. Introduction

• Engineering properties of rocks cover all characteristics crucial for engineering applications, whether extracted or in situ.
• Rocks are evaluated for construction material selection like building stones, road stones, or concrete aggregates.
• The properties of a natural rock bed influence its suitability for construction projects.
• The understanding and determination of rock properties are vital for the economy and safety of engineering projects.

2. Physical Properties of Rocks

• Rock properties include intact rock properties and rock mass properties.
• Intact rock properties are determined through laboratory tests on small samples.
• Common engineering properties obtained from lab tests include specific gravity, compressive strength, tensile strength, etc.
• Rock mass properties are assessed through visual examination of discontinuities within the rock mass.

3. Methods of Determining Rock Properties

• A combination of laboratory testing, empirical analysis, and field observations is used to determine engineering properties.
• Intact rock properties are derived from small sample tests, while rock mass properties are based on visual inspections.
• The methodology suggested by the International Society of Rock Mechanics is followed for evaluating rock mass properties.

4. Geological Characteristics of Rocks

• Geological features of rocks play a crucial role in determining their engineering utility.
• Understanding the geological characteristics helps in assessing the rock's behavior in construction projects.

5. General Characteristics of Rocks

• General characteristics of rocks provide insights into their overall properties and behavior in different environments.
• These characteristics help in the selection of suitable rocks for specific engineering applications.

6. Modulus Properties or Flexible Strength of Rocks

• Modulus properties of rocks, including their flexible strength, are essential considerations in engineering applications.
• Understanding these properties aids in utilizing rocks effectively in various construction projects.

7. Engineering Uses of Rocks

• Rocks have diverse engineering applications, such as in construction, infrastructure development, and landscaping.
• Knowing the engineering uses of rocks is vital for efficient and sustainable project planning.

8. Conclusion

• Understanding the engineering properties of rocks is crucial for the success and safety of construction projects.
• A geotechnical engineer plays a key role in evaluating and utilizing rocks effectively in engineering applications.

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Physical Properties of Rocks

• Building Stones Definition: A building stone is a rock suitable for use as a rough unit or a shaped block, slab, column, or sheet in engineering construction.

2.1 Crushing Strength

• Definition: Crushing strength, also known as compressive strength, is the maximum force a stone can withstand per unit area before failing.
• Testing Method: Compressive strength is determined by gradually loading standard test specimens (cubes or cylinders) until the first crack appears.
• Types of Compressive Strength:
  • Unconfined or Universal Compressive Strength: Determined without lateral support.
  • Confined or Triaxial Compressive Strength: Tested with lateral support, such as a special cell filled with pressurized liquid.
• Factors Affecting Crushing Strength:
  • Mode of Formation
  • Composition
  • Texture and Structure
  • Moisture Content
  • Extent of Weathering
• Rock Types:
  • Igneous Rocks: Characterized by high crushing strengths due to their crystalline and interlocking texture.
  • Sedimentary and Metamorphic Rocks: Lower crushing strengths due to planes of weakness like bedding planes, foliation, and cleavage.
• Sandstone: Shows lower crushing strength parallel to bedding planes compared to perpendicular loading. Other rocks like quartzite are composed of weak minerals.
• Common Building Stones: Generally have higher crushing strengths than required for typical building constructions.

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Rock Strength Properties

• Compressive Strength

  The compressive strengths of various rocks are measured in Kg/cm²:

  • Dolerite: 1500-3500
  • Basalt: 1500-3500
  • Quartzite: 1500-300
  • Granite: 1000-2500
  • Marbles: 700-2000
  • Gneisses: 500-2500
  • Sandstone: 200-2500
  • Limestone: 200-2000

• Classification by Deere and Miller

  Rocks are classified based on uniaxial compressive strength into the following grades:

  • Class A: Very high strength (>2240 Kg/cm²)
  • Class B: High strength (1120-2240 Kg/cm²)
  • Class C: Medium strength (500-1120 Kg/cm²)
  • Class D/E: Low to very low strength (200-500 Kg/cm² and <200 Kg/cm²)

• Transverse Strength

  Transverse strength refers to a stone's ability to withstand bending loads:

  • It is determined by the modulus of rupture, which is calculated using the formula R = 3WI/Lbd
  • Transverse strength is typically 1/20th to 1/10th of the compressive strength of the stone.

• Shear Strength

  Shear strength measures a stone's resistance to shear stresses:

  • Shear strength is calculated as P/2A, where P is the load at failure and A is the area of the specimen's cross-section.
  • Common building stones usually have shear strengths ranging from 70 to 140 kg/cm².

• Tensile Strength

  Tensile strength indicates a rock's ability to withstand breakage:

  • Tensile strength is the force per unit area required to break a material.
  • It can be measured directly or indirectly, with direct methods involving means to prevent bending during testing.

Geology Concepts Overview

• Tensile Strength Testing

  Tensile strength is a crucial property of rocks, but it's often challenging to measure directly. Instead, the Brazilian test, an indirect method, is commonly used. In this test, a cylinder is loaded in such a way that it ruptures along its diameter. By gradually increasing the load until fracture, the load at rupture (P) is determined. The transverse strength (Ts) can then be calculated using the formula: 2P / (μDL).

• Porosity in Rocks

  Porosity in rocks refers to the presence of pore spaces within a rock, influenced by factors like grain shape, size, and packing. A higher porosity indicates more pore space relative to the total rock volume. For example, rocks with interlocking crystals and abundant cementing materials tend to have low porosity. In contrast, rocks with spherical or rounded grains, or uneven distribution of cementing material, exhibit higher porosity. Porosity affects properties like fluid absorption and density, with lower porosity generally indicating higher compressive strength.

• Absorption Value

  The absorption value of a stone reflects its ability to absorb moisture when saturated. It is usually expressed as a percentage of the original dry weight of the stone. The absorption value can be calculated using the formula: (Ws - Wo) / W2.

• Permeability of Rocks

  Permeability measures a rock's ability to transmit water. Rocks like sandstones and limestones may have high absorption values, indicating significant water transmission capacity. However, selecting highly porous stones for construction can be problematic, as water within pores weakens the rock and makes it susceptible to frost damage, especially in cold and humid climates.

Density

• Density is the weight per unit volume of a substance. In the case of rocks, it's not just the solid mineral matter that determines the total volume of a specimen. Rocks can also contain pores or open spaces, which may be empty, partially filled, or completely filled with water.
• Three types of density are distinguished in rocks:
  • Dry density: Weight per unit volume of a completely dried rock specimen, including the volume of pore spaces.
  • Bulk density: Weight per unit volume of a rock sample with natural moisture content where pores are only partially filled with water.
  • Saturated density: Density of saturated rocks or weight per unit volume of a rock with all pores completely filled with water.
• True density is the weight per unit volume of the mineral matter (without pores and water) of which a rock is made up. In engineering calculations, bulk density is commonly used.
• Examples of bulk density values for some common building stones are:
  • Granite: 2.7 g/cm³
  • Basalt: 2.9 g/cm³
  • Sandstone: 2.6 g/cm³
  • Limestone: 2.2 to 2.6 g/cm³

Abrasive Resistance

• Abrasive resistance refers to the resistance a stone offers to rubbing action. It is crucial for stones used in situations involving rubbing by natural or artificial causes.
• Examples of such situations include stones used in paving along roads or facing stones in buildings in arid regions exposed to strong sand-laden winds.
• Situations demanding stones with high abrasive resistance and uniform composition to ensure even wear are highlighted.
• Stones like granite, composed of multiple minerals, may initially look attractive but can get pitted or disfigured due to unequal wear of different mineral components over time.

Frost and Fire Resistance

• Many building stones are prone to quick disintegration when exposed to frost formation or heating.
• Frost causes disintegration by water expansion within rock pores, while heating can lead to unequal expansion in different mineral components, causing disintegration.
• Rocks found porous and weak, like limestone and sandstones, are easily deteriorated in cold, humid climates due to frost action.
• Unequal expansion during heating and cooling processes can cause heavy stones, including granites, to crumble under stress.

Fire Resistance of Stones

• Fire resistance of stones is crucial, especially in applications near stoves, heating areas, and furnace walls.
• Compact and massive sandstones and quartzites are suitable for use in fire-prone and high-temperature environments.

Methods of Determining Rock Properties

• In-situ testing: This involves testing rock properties directly at the site during field exploration, providing real-time data.
• Laboratory testing: Rock properties can also be analyzed through controlled laboratory experiments for more detailed assessments.
• Back-analysis based on site performance data: This method involves studying past performance data to infer rock properties.

In-Situ Field Testing

• Geotechnical Engineers conduct in-situ field testing to determine soil and rock parameters under natural conditions.
• This type of testing is particularly useful for projects where obtaining representative samples for lab testing is challenging, such as with soft clays or loose sands below the water table.

Types of In-Situ Borehole Tests

• Correlation Tests: These tests help correlate field data with rock properties for better understanding.
• Strength and Deformation Tests: Assessments like Cone Penetrometer Test (CPT) and Standard Penetration Test (SPT) evaluate the strength and deformation characteristics of rocks.
• Permeability Tests: Tests such as Pump Tests and Water Pressure Tests determine the permeability of rocks.

Laboratory Testing of Rock

• Rock strength is typically measured through laboratory testing, providing insights into how rocks behave under different stress conditions.
• Rocks and soils generally exhibit higher compression strength compared to tensile strength.

By understanding these methods of determining rock properties and conducting tests, engineers and geologists can make informed decisions regarding the use of rocks in various applications.

Uni-axial Unconfined Compression Test

• In this test, a cylindrical rock core is loaded axially until it fails.

Tri-axial Confined Compression Test

• A cylindrical rock core is placed in a cell, subjected to confining pressure by hydraulic oil through a thin impermeable membrane, and loaded axially to failure.

Tests for Tensile Strength of Rock

• Direct Pull Test: A cylindrical rock core sample is anchored at both ends and stretched.
• Brazilian Test: A thin disk is loaded across the diameter until it splits.
• Beam Flexure Test: A thin slab of rock is loaded vertically when supported at three or four points along its length.

Back-analysis based on site performance data

• Back-analysis is a quantitative approach to adjust soil or rock properties to match measurable site performance.
• It helps determine engineering properties of soil or rock, often used in geotechnical failures.
• Back-analysis methods include Back-Analysis of Slopes, Soil Settlement, Foundations, and Numerical Modeling.

Geological Characteristics of Rocks

Mineralogical Composition

• Rocks are comprised of minerals, with properties dependent on their nature and composition.
• Rocks rich in silica (SiO2), like Quartzites, are generally stronger. Examples include Fresh Quartzite, Sandstone, and Granite.
• Carbonate rocks exhibit varying properties and should be tested before use in construction due to minerals like mica, gypsum, sulphides, tremolite, flint, chert, and clays, which can weaken the rock.

Texture and Structure of Rocks

• Texture in rocks refers to the size, shape, and arrangement of mineral compounds.
• Structure, on the other hand, pertains to the overall development of large-scale features in the rock mass.
• Rocks can be classified as coarse-grained, medium-grained, or fine-grained.
• Fine-grained equigranular rocks are superior as building materials compared to coarse-grained and inequigranular rocks.
• Structural features like bedding planes, foliations, cleavage, joints, and flow structures are crucial considerations.

Resistance to Weathering (Durability)

• Durability of a stone is influenced by its composition, texture, structure, and the environmental conditions it faces.
• For instance, a stone may endure well indoors for centuries but deteriorate rapidly when exposed to external weathering agents.
• Granite remains relatively unaffected over long periods, while limestone can weather quickly in industrial areas due to pollutants.
• Engineers and town planners must consider the durability of rocks based on their intended use and environment.
• Durability testing involves exposing stone samples to Sodium sulphate to assess their disintegration resistance.

General Characteristics of Rocks

• Aside from engineering and geological properties, other factors like cost and color play roles in rock selection.

Cost Consideration

• Cost is a crucial factor in selecting building stones, influenced by factors like availability, accessibility, and workability.
• Even if a stone is suitable in terms of properties, it might be deemed impractical if it is too costly to transport or work with.
• Good quality stones may be limited in supply, making them expensive due to transportation costs.
• Workability refers to how easily and economically a stone can be extracted from its source.

Rocks Modulus Properties

Introduction

• When selecting rocks for construction, factors like hardness, strength, and color play crucial roles.
• Harder and stronger rocks are more expensive due to their durability.
• Color is significant for rocks used in visible constructions like residential or official buildings.
• Granites are light-colored, Basalt are dark, and Sandstone comes in lighter shades.

Modulus Properties of Rocks

• The modulus properties refer to the elasticity or flexible strength of rocks.
• Rocks deform under loads but recover their shape when the loads are removed.
• Hook's law states that stress is directly proportional to strain in elastic substances.
• Modulus of elasticity (Young's modulus) is expressed as Q/E = E.

Testing Modulus Properties

• Modulus properties are tested by loading specimens under compression or tension.
• Strain gauges measure the deformation parallel to the stress direction.
• Specimens break when they reach their ultimate deformation limit.
• Rocks vary in elastic constants based on composition, texture, and structure.

Categories based on Modulus of Elasticity

• Quasi-Elastic Rocks: Show a nearly straight stress-strain relationship till failure, including rocks like Syenites, Diorites, and Basalts.
• Semi-Elastic Rocks: Have some porosity and minor structure discontinuities, with E values ranging between 4x10^5 to 6x10^5 kg/cm2.

Important Building Stones

• Granites:
  • Granites are commonly used as building stones and are known for their high crushing strength, low absorption values, low porosity, interlocking texture, variety of colors, and ability to be polished perfectly.
  • They are abundant in India, particularly in the Archean rock formations of peninsular India, which mainly consist of Gneisses and Granites.
• Sandstone:
  • Sandstones with closely interlocking and angular grains, free from structural defects, are extensively used as building stones.
  • Varieties like ferruginous and calcareous sandstones are not suitable for exterior work, especially in industrial areas.
  • Argillaceous sandstones are generally weak, while those with siliceous cement are strong and easily workable.
  • In India, vast reserves of sandstones suitable for construction are found, particularly in the Vindhyan and Gondwana systems.
  • Vindhyan sandstones, fine-grained and available in various colors, are economically feasible and widely used, especially in regions like Delhi and Agra.
  • Gondwana formations in India also yield high-quality sandstones, with examples like the fine-grained sandstones of Cuttack, known as Athgarh sandstones, used in famous temples like those in Jaganathpuri.
• Limestone (7.3):
  • Limestones are commonly used as building stones primarily due to their high crushing strength rather than their physical properties. Their strength can vary significantly, ranging from below 300 kg/cm2 to over 1500 kg/cm2.
  • However, caution should be taken when using limestones as facing stones, especially in environments where they may come into contact with sulphuric acid vapors or salt crystals, as these can lead to the disintegration of the rock's surface.
  • Indian occurrences of limestones are found in various geological formations across the country, such as the Cuddapah system, Bijawar, Kondalite, Aravalli groups, Vindhyan system, and hill limestone in northern India.
• Marbles (7.4):
  • Marbles, metamorphic rocks, are utilized for both structural and decorative purposes due to their diverse textures, colors, and composition.
  • They are known for their crushing strength, making them suitable for various applications. Marbles are often chosen for decorative use because of their ability to be polished to a brilliant shine and their attractive colors.
  • In India, commercial marbles are predominantly sourced from the crystalline formations of Rajasthan, including Makrana (white and pink), Kharva (green and yellow), and Kishengarh and Jaipur (black and dense marbles).
• Slate (7.5):
  • Slate, another metamorphic rock, is characterized by its perfect cleavage, making it unsuitable for use as a building stone but ideal for purposes such as paving and roofing.
• Conclusion (7.6):
  • Rocks play a crucial role in construction, offering strength, aesthetics, and cost-effectiveness to structures.
  • Engineers must carefully consider the properties of naturally occurring rocks to ensure compliance with safety standards. This involves a combination of laboratory testing, empirical analysis, and field observations.
  • Engineering properties of rocks are essential for civil engineering projects, aiding in the selection of suitable materials for construction and structural integrity.