Geological Foundations: Rocks, Water, and Site Stability
Mineralogical and Structural Features of Common Rocks
Granite
Mineralogy of Granite
The essential minerals of granite are K-feldspar and quartz. Common accessory minerals include mica or hornblende.
Texture and Structure of Granite
The texture of granite varies from fine-grained to very coarse-grained. Equigranular texture is common in granite, though some varieties may show a porphyritic texture. Granite typically exhibits a massive structure.
Mode of Occurrence of Granite
Granites commonly occur as major intrusive bodies such as batholiths and stocks. Many granites are considered to be the result of crystallization from melts at relatively low temperatures.
Sandstone
Mineralogy of Sandstone
Quartz is the chief mineral constituent of sandstone. Small amounts of feldspar, mica, garnet, and other minerals may also occur.
Texture and Structure of Sandstone
Sandstone is composed almost entirely of well-sorted, angular to rounded sand grains, with grain sizes ranging from 0.1 mm to 2 mm. Common structures of sandstone include stratification, current bedding, ripple marks, and rain prints.
Mode of Occurrence of Sandstone
Sandstone occurs as stratified layers within sedimentary basins, formed through the deposition of sand grains followed by compaction and cementation.
Gabbro
Mineralogy of Gabbro
Essential minerals in gabbro are calcic plagioclase, pyroxene, magnetite, and olivine. Accessory minerals such as biotite, hornblende, and ilmenite are also present.
Texture and Structure of Gabbro
Gabbro typically has a coarse-grained, equigranular texture and exhibits a massive, non-foliated structure.
Mode of Occurrence of Gabbro
Gabbro occurs in the form of intrusive igneous bodies.
Marble
Mineralogy of Marble
Marble is composed mainly of calcite. Dolomite may be present as an accessory mineral.
Texture and Structure of Marble
Marble exhibits a coarse-grained, granulose texture.
Mode of Occurrence of Marble
Marbles are formed as a result of the metamorphism of limestones.
Dam Construction and Site Selection
Understanding Dams
Dams are barriers constructed across rivers to store water. They are built primarily to control floods, for irrigation, for generating electricity, and for supplying water to cities and industries.
Types of Dams
- Gravity Dam: A massive structure of concrete or masonry that stands by its own weight.
- Arch Dam: An arch-shaped structure of a single concrete wall with the convex side facing upstream.
- Buttress Dam: Features buttresses constructed on the downstream side to support an upstream deck of reinforced concrete.
- Earth Dam: Constructed mainly of soil or earth. These dams offer an advantage as they can be built on earth or poor rock conditions.
Strategic Site Selection for Dams
Site selection for dam construction is crucial to minimize dam failures and significantly reduce construction costs. The following factors should be considered when selecting a dam site:
- Narrow River Valley: A narrow river valley requires only a smaller dam, leading to lower construction costs.
- Bedrock at Shallow Depths: For safety and stability, a dam must rest on strong and stable bedrock. If such bedrock is near the surface or at shallow depths, foundation costs will be lower.
- Competent Rocks for Safe Foundation: The presence of igneous and metamorphic rocks at the selected site provides a strong and durable foundation. However, weak sedimentary rocks, such as shales, poorly cemented sandstones, and cavernous limestones, are undesirable for foundation purposes.
Geology of Underground Water
Understanding the origin and flow of underground water is fundamental in geology:
- Meteoric Water: Water obtained from atmospheric precipitation.
- Plutonic Water: Water of magmatic origin.
- Connate Water: Water that is trapped in sedimentary rocks during their formation.
Factors Affecting Underground Water Flow
- Porosity: The measure of the amount of void spaces present within a rock. It represents the capacity of rocks to store and transmit fluids. Sedimentary rocks are generally more porous compared to igneous and metamorphic rocks.
- Permeability: The property of a rock that allows water or solutions to pass through it. Rocks possessing this property are called permeable rocks (e.g., sandstone), while those that do not allow fluid passage are known as impermeable rocks (e.g., shale).
Selection of Construction Materials
To select appropriate rock material for construction, the following properties are commonly examined:
- Mineral Composition: Rocks are aggregates of minerals. If the mineral constituents of a rock are hard, free from cleavage, and resistant to weathering, the rock is likely to be strong and durable. Rocks rich in weak minerals such as micas, chlorite, talc, or feldspar are generally not durable.
- Texture: Fine-grained rocks are generally denser and stronger than coarse-grained rocks. Fine-grained rocks have tightly packed mineral grains with less pore space, creating a more compact structure compared to coarse-grained rocks.
- Structure: Structures like stratification, lamination, foliation, and cleavage should be checked and oriented parallel to the plane of weakness rather than vertically.
- Porosity: A less porous rock is generally more durable and stronger, and therefore preferred for construction purposes.
- Permeability: Permeable rocks are considered detrimental because they can cause water seepage, potentially leading to structural damage. Thus, impermeable rocks are preferred for construction purposes.
- Durability: The capacity of a rock to retain its original size, strength, and appearance over a long period. It is directly related to its mineral composition and texture.
- Heat Resistance: Rocks may get damaged when heated to high temperatures and then cooled. This is particularly true if they are rich in minerals like calcite and feldspar. Examples of low heat-resistant rocks include shale, while marble is an example of a high heat-resistant rock.
Soil Erosion: Causes and Prevention
Soil erosion is the removal of the topsoil layer due to natural forces or human activities.
Causes of Soil Erosion
- Deforestation: Cutting trees reduces root systems that hold soil in place.
- Water Movement: Forces like rainfall, surface runoff, rivers, and streams can wash away topsoil and erode riverbanks.
- Overgrazing: Livestock overgrazing destroys vegetation, exposing soil to erosion.
- Urbanization and Construction: Land clearing for buildings and roads makes soil vulnerable to erosion. It also reduces water infiltration, disturbs soil structure, and leads to soil fertility loss and pollution.
- Mining Activities: Mining operations can loosen soil, leading to increased erosion.
Preventive Measures for Soil Erosion
- Reforestation and Afforestation: Planting trees strengthens soil stability and prevents erosion.
- Terracing: Building terraces reduces the speed of water flow on slopes.
- Retaining Walls: Construction of retaining walls on steep slopes stabilizes the soil.
- Crop Rotation: Practicing crop rotation prevents nutrient depletion and maintains soil health.
Landslide: Causes and Prevention
Causes of Landslides
- Water: The lack of support in front and lubrication behind makes water a key factor in landslides.
- Slope: Steeper slopes have greater instability, making them more susceptible to landslides.
- Nature of Rocks: Unconsolidated sediments like clay, sand, and gravel cannot stand on slopes greater than 35°. Slippery rocks like shale and mica schist are also unstable on steep slopes.
- Structure of Rocks: Landslides often occur on hill slopes with joints, fractures, shear zones, and bedding planes.
Prevention of Landslides
- Afforestation and Reforestation: Planting trees stabilizes slopes, preventing landslides.
- Constructing Retaining Walls: Provides lateral support to unstable slopes.
- Slope Grading: Reduces steepness and decreases susceptibility to landslides.
- Proper Drainage Systems: Redirects water flow away from slopes.
- Sealing Cracks: Prevents water from entering soil or rock layers.