Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Basic Principles of Sub-structure Construction
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Basic Principles of Sub-structure Construction
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Subject: Block laying, Brick laying & Concrete Works
Class: Senior Secondary 2
Term: 1st Term
Week: 4
Theme: Building Construction
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Students should be able to:Identify properties of soiland distinguish betweenexcavation.. Define the bearingcapacity of soil.
Building Construction of the trench walls, especially in unstable soil conditions or deep excavations.
2. Utility Identification: Before commencing excavation, identifying and marking the location of all existing underground services (e.g., water pipes, electrical cables, gas lines, sewage pipes) to prevent accidental damage, which can lead to serious hazards.
Commentary: This question highlights the practical and safety aspects of excavation, which are crucial for "carrying out substructure work" safely.
5. Independent Practice (Questions Only)
1. Define the term "bearing capacity of soil."
2. List five (5) distinct properties of soil that are crucial for foundation design.
3. Explain why proper compaction of soil beneath a foundation is important.
4. Identify and briefly describe three (3) different types of excavation commonly used in sub-structure construction.
5. A builder intends to construct a fence using a strip foundation. Which type of excavation would be most appropriate for this task, and why?
6. Discuss how a high water table on a building site can negatively impact the soil's bearing capacity.
7. What is the main reason for excavating to a specific depth, rather than just digging shallow foundations?
8. Imagine a construction site in a marshy, riverine area (e.g., parts of Port Harcourt). What challenges related to soil properties would you expect, and how might these influence the foundation approach?
6. Evaluation and Assessment
A. Formative Assessment: Class Discussion: Observe student participation and understanding during group activities and Q&A sessions.
Quick Check: Ask students to provide short definitions or examples of concepts discussed (e.g., "What is permeability?").
Observation: Assess students' ability to identify soil types from samples or diagrams during Activity
1. B. Summative Assessment (Aligned to Evaluation Guide):
1. List five (5) properties of soil that are important considerations for sub-structure construction. (5 marks)
Marking Scheme: 1 mark for each correctly identified property (e.g., Bearing Capacity, Compressibility, Permeability, Shear Strength, Plasticity, Moisture Content, Cohesion, Angle of Internal Friction, Density).
2. Describe the step-by-step process of carrying out a trench excavation for a typical strip foundation for a two-bedroom bungalow, ensuring safety measures are considered. (10 marks)
Marking Scheme: Site Preparation: Clearing, setting out/marking the trench lines (2 marks)
Excavation Process: Digging to required depth and width, removal of excavated spoil (3 marks)
Depth and Bearing Stratum: Reaching adequate bearing stratum, checking for stability (2 marks)
Safety Measures: Mentioning at least two relevant safety measures such as shoring, sloping, warning signs, proper access, checking for underground services (3 marks)
3. Define soil bearing capacity and explain why it is the most critical soil property to consider when designing a building foundation. (5 marks)
Marking Scheme: Correct definition of bearing capacity (2 marks) * Explanation of its criticality (e.g., directly determines if soil can support structure, prevents settlement/failure, ensures structural integrity and safety) (3 marks)
7. Real-life Applications / Integration
1. Preventing Building Collapse: The knowledge of soil properties and bearing capacity is directly applicable to preventing building collapses, a recurring tragedy in some Nigerian cities. By understanding how different soil types behave under load, engineers and builders can design appropriate foundations, ensuring the safety and longevity of structures. For instance, in areas with soft, clayey soils (e.g., parts of Lagos or the Niger Delta), deeper foundations or piles are often required to transfer loads to stronger strata, contrasting with stable lateritic soil where shallow strip foundations might suffice.
2. Economic Impact of Proper Foundation Work: Local builders and contractors who apply these principles correctly save money in the long run by avoiding costly repairs due to foundation failure. This also builds their reputation and contributes to the sustainable development of the built environment in Nigerian communities. Understanding excavation types helps optimize resources – using manual labour for smaller, precise trenches in rural settings vs. mechanical excavators for large commercial projects in urban centres.
3. Community Development and Infrastructure: Sub-structure principles extend beyond buildings to vital infrastructure like roads, bridges, and drainage systems. Knowledge of soil stability and excavation techniques is essential for constructing durable infrastructure that can withstand Nigeria's diverse geographical conditions (e.g., erosion control in areas with highly permeable soils, stable road bases in Basic Principles of Sub-structure Construction Term: 1st Term Week: 1 ---
1. Overview and Learning Objectives This topic introduces students to the fundamental concepts of sub-structure construction, a critical phase in any building project. Understanding sub-structure principles is essential for ensuring the stability, durability, and safety of structures, directly impacting the integrity of homes, schools, and commercial buildings across Nigeria. Failure to correctly execute sub-structure work can lead to structural defects, differential settlement, and even building collapse, a serious concern in the Nigerian built environment. Upon completion of this lesson, students will be able to: Identify and describe various properties of soil relevant to construction. Define excavation and differentiate between its common types used in sub-structure work. Define the bearing capacity of soil and explain its significance in foundation design. Connect these principles to real-world construction practices and safety standards in Nigeria.
2. Key Concepts and Explanations A. Sub-structure Construction The sub-structure refers to the part of a building that is constructed below the ground level. Its primary function is to safely transfer the entire load of the building (dead loads, live loads, wind loads, etc.) to the underlying soil or rock stratum.
Components of Sub-structure: Foundations: The most critical part, designed to distribute the building load over a larger area to prevent excessive settlement.
Footings: The lowest part of the foundation, directly in contact with the soil.
Ground Beams/Ties: Connect various footings, provide rigidity, and support ground floor slabs.
Retaining Walls: Used to hold back soil or earth, often found in basements or on sloping sites.
Ground Floor Slab: The concrete slab forming the floor of the lowest storey, typically resting on compacted fill or directly on the soil. B. Properties of Soil Relevant to Construction Soil, being the natural material upon which foundations rest, exhibits various physical and mechanical properties that dictate its suitability as a foundation material. Understanding these properties is crucial for selecting appropriate foundation types and ensuring structural stability.
1. Bearing Capacity: Definition: The maximum average contact pressure between the foundation and the soil which should not produce shear failure in the soil and/or excessive settlement of the structure. Simply put, it is the maximum load per unit area that the soil can safely support without failing or settling excessively.
Units: Typically measured in kN/m2 (kiloNewtons per square metre) or kg/cm2 (kilograms per square centimetre).
Factors Affecting Bearing Capacity: Type of Soil: Granular soils (sands, gravels) generally have higher bearing capacity than cohesive soils (clays, silts) when dry, but can lose significant bearing capacity when saturated. Lateritic soils, common in Nigeria, can offer good bearing capacity when compacted and dry.
Density/Compaction: Denser and well-compacted soils have higher bearing capacity.
Moisture Content: Water significantly reduces the bearing capacity of most soils, especially clays (due to increased pore water pressure and reduced shear strength).
Depth of Foundation: Bearing capacity generally increases with the depth of the foundation due to increased confinement.
Size and Shape of Foundation: Larger and deeper foundations distribute load over a wider area, potentially increasing effective bearing capacity.
Presence of Water Table: A high water table reduces effective stress and thus bearing capacity.
2. Compressibility: Definition: The tendency of soil to decrease in volume under compressive stress. Highly compressible soils (e.g., soft clays, organic soils) can lead to significant settlement of structures, which can cause cracking and structural damage.
3. Permeability: Definition: The ability of water to flow through the soil. High permeability (e.g., sands, gravels) can lead to drainage issues, erosion, and potential for rapid changes in moisture content. Low permeability (e.g., clays) can lead to water logging and pore water pressure build-up.
4. Shear Strength: Definition: The resistance of soil to deformation by shear stress. It is crucial for slope stability, retaining wall design, and foundation stability against lateral forces. It's influenced by cohesion (for clays) and angle of internal friction (for sands).
5. Cohesion: Definition: The attractive forces between soil particles, characteristic of fine-grained soils like clay, making them "stick together." Sands are generally non-cohesive. issues, erosion, and potential for rapid changes in moisture content. Low permeability (e.g., clays) can lead to water logging and pore water pressure build-up.
4. Shear Strength: Definition: The resistance of soil to deformation by shear stress. It is crucial for slope stability, retaining wall design, and foundation stability against lateral forces. It's influenced by cohesion (for clays) and angle of internal friction (for sands).
5. Cohesion: Definition: The attractive forces between soil particles, characteristic of fine-grained soils like clay, making them "stick together." Sands are generally non-cohesive.
6. Angle of Internal Friction: Definition: The resistance to sliding between soil particles, characteristic of granular soils like sand and gravel.
7. Plasticity: Definition: The ability of a fine-grained soil (like clay) to be deformed without cracking or crumbling, retaining its shape upon removal of the deforming force. Measured by Plastic Limit and Liquid Limit. Soils with high plasticity can undergo significant volume changes with varying moisture content (shrinkage/swelling), which is detrimental to foundations.
8. Moisture Content: Definition: The amount of water present in the soil. It directly influences bearing capacity, compressibility, and plasticity.
Nigerian Context - Common Soil Types: Lateritic Soil: Very common across Nigeria, often reddish-brown. When dry and well-compacted, it can have good bearing capacity.
However, some laterites can be clayey and susceptible to shrink-swell issues when exposed to water.
Sandy Soil: Found extensively, especially in coastal and northern regions. Good drainage, but can be susceptible to erosion and liquefaction under seismic activity (though less common in Nigeria). Bearing capacity is good when dense.
Clayey Soil: Abundant in various regions, particularly riverine and marshy areas (e.g., Niger Delta, parts of Lagos). Prone to high compressibility, low bearing capacity when wet, and significant shrink-swell potential, posing challenges for foundation design.
Silty Soil: Intermediate properties between sand and clay. Can be problematic when wet, prone to frost heave in colder climates (less relevant in Nigeria but still a property).
C. Excavation Definition: The process of removing earth, rock, or other materials from a site to form a trench, pit, or other depression in the ground. In sub-structure construction, it primarily creates space for foundations, basements, utility trenches, and other below-ground elements.
Importance of Excavation: To reach a stable, undisturbed soil layer with adequate bearing capacity for the foundation. To accommodate the various components of the sub-structure (footings, ground beams, basement walls). To provide necessary space for construction activities below ground level. To facilitate the installation of underground services (water pipes, electrical conduits, sewage lines). Types of Excavation (Distinguishing between them):
1. Trench Excavation: Purpose: To create long, narrow, and relatively shallow channels for strip foundations (e.g., for block walls), pipe laying, or electrical cables.
Characteristics: Typically continuous along the line of load-bearing walls. Dimensions are usually width (equal to or slightly wider than the footing) and depth (to suitable bearing stratum).
Method: Can be manual (pickaxe, shovel) or mechanical (trencher, excavator with narrow bucket).
2. Pit Excavation: Purpose: To create individual, deeper holes for isolated pad foundations (e.g., for columns), pile caps, or small utility pits.
Characteristics: Discrete, often square or rectangular pits at specific points where concentrated loads are applied.
Method: Manual or mechanical, depending on size and depth.
3. Bulk/Mass Excavation: Purpose: To remove large volumes of earth over a wide area, typically for basements, large raft foundations, site levelling, or underground car parks.
Characteristics: Covers a significant footprint of the building, creating a large, open depression.
Method: Primarily mechanical (bulldozers, excavators, loaders) due to the large volume of material.
Safety Precautions during Excavation: Shoring/Sloping: Supporting the sides of excavations (especially deep ones) to prevent collapse. Shoring uses timber or steel members; sloping involves cutting the sides at a safe angle.
Water Management: Pumping out groundwater, preventing surface water ingress.
Warning Signs & Barriers: Marking excavation zones, especially in public areas.
Access/Egress: Safe means for workers to enter and exit trenches/pits.
Utility Identification: Locating existing underground services before digging.
Competent Supervision: