Engineering Properties of Rocks | Geology Optional for UPSC PDF Download

Engineering Properties of Rocks

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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:

• 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, 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.
• 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 refers to the presence of pore spaces within a rock, influenced by factors like grain shape, size, and packing.
• Higher porosity: More pore space relative to total rock volume.
• Lower porosity: Indicates higher compressive strength.

Absorption Value

• Reflects the ability of a stone to absorb moisture when saturated.
• Calculated as: (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.
• Selecting highly porous stones for construction can be problematic.

Density

Types of Density

• Dry Density: Weight per unit volume of a completely dried rock specimen.
• Bulk Density: Weight per unit volume of a rock sample with natural moisture content.
• Saturated Density: Density of saturated rocks, where all pores are completely filled with water.

Examples of Bulk Density Values

• Granite: 2.7 g/cm³
• Basalt: 2.9 g/cm³
• Sandstone: 2.6 g/cm³
• Limestone: 2.2 to 2.6 g/cm³

Resistance Properties

Abrasive Resistance

• Refers to the resistance a stone offers to rubbing action.
• Crucial for paving stones or facing stones in arid regions.

Frost and Fire Resistance

• Frost: Causes disintegration by water expansion within rock pores.
• Fire: Unequal expansion during heating can cause disintegration.

Fire Resistance of Stones

• Compact and massive sandstones and quartzites are suitable for high-temperature environments.

Methods of Determining Rock Properties

In-situ Field Testing

• Correlation Tests: Help correlate field data with rock properties.

• Strength and Deformation Tests: Cone Penetrometer Test (CPT) and Standard Penetration Test (SPT).

• Permeability Tests: Pump Tests and Water Pressure Tests.

Laboratory Testing

• Rock strength is analyzed through controlled laboratory experiments.

Back-analysis Based on Site Performance Data

• Adjusts soil or rock properties to match measurable site performance.

Geological Characteristics of Rocks

Mineralogical Composition

• Rocks are comprised of minerals, with properties dependent on their nature and composition.
• Silica-rich rocks: Stronger, e.g., Quartzites, Sandstone, Granite.

Texture and Structure of Rocks

• Texture: Size, shape, and arrangement of mineral compounds.
• Structure: Large-scale features like bedding planes, foliations, cleavage, and joints.

Resistance to Weathering (Durability)

• Granite remains unaffected over long periods.
• Limestone weathers quickly in industrial areas.

General Characteristics of Rocks

Cost Consideration

• Influenced by availability, accessibility, and workability.

• Good quality stones may be expensive due to limited supply.

Modulus Properties

• Refers to the elasticity or flexible strength of rocks.

Categories:

• Quasi-Elastic Rocks: Syenites, Diorites, Basalts.
• Semi-Elastic Rocks: E values between 4x10^5 to 6x10^5 kg/cm².

Important Building Stones

Granites

• High crushing strength, low absorption values, low porosity.
• Found abundantly in India.

Sandstone

• Closely interlocking grains free from structural defects.

Limestone

• High crushing strength; caution required in environments with sulphuric acid vapors.

Marbles

• High crushing strength; suitable for decorative purposes.

Slate

• Ideal for paving and roofing; not suitable as a building stone.

Conclusion

• Rocks are crucial for construction, offering strength, aesthetics, and cost-effectiveness.
• Engineers must carefully evaluate the properties of rocks through testing and analysis to ensure safety and efficiency in construction projects.