Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Drainage Installation
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Drainage Installation
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Subject: Building Construction
Class: Senior Secondary 2
Term: 3rd Term
Week: 3
Theme: Building Services
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Set out and excavate trenches for in stallations Determine the gradient of fall of the trench. Select types of drainage pipe for different jobs Lay drainage pipescorrectly. State methods of testing drainage.
Setting Out: This is the process of accurately transferring the design from architectural drawings onto the ground. For drainage, it involves marking the exact position, alignment, depth, and gradient of the proposed trenches.
Tools for Setting Out and Excavation: Measuring Tape: For measuring lengths and distances.
Pegs/Stakes: Wooden or metal pins used to mark points on the ground.
String/Line: Stretched between pegs to define the exact line of the trench.
Spirit Level: For checking horizontal levels over short distances.
Builder's Square: For setting out right angles.
Dumpy Level/Automatic Level (and Staff): For establishing and transferring levels accurately over longer distances, crucial for gradient determination.
Shovel/Spade: For digging and removing soil.
Pickaxe: For breaking hard ground or compacted soil.
Mattock: Similar to a pickaxe, often with a broad cutting blade on one side.
Wheelbarrow: For transporting excavated soil.
Rammer/Compactor: For compacting the trench bed and backfill.
Procedure for Setting Out Trenches: Establish Control Points: Identify fixed points (e.g., corners of a building, existing survey pegs) from which all measurements will originate.
Mark Trench Lines: Using a measuring tape, pegs, and string line, mark the centerline and width of the proposed trench on the ground according to the drainage plan.
Determine Depth and Gradient: This involves using a dumpy level/automatic level, sight rails, and boning rods to establish the required invert levels (bottom inside of the pipe) at various points along the trench.
Excavation: Once marked, excavation begins using appropriate hand tools (shovels, pickaxes) or mechanical excavators for larger projects.
Trench Width: Should be sufficient to allow for pipe laying, jointing, and compaction of bedding material (usually pipe diameter + 150-300mm on each side).
Trench Depth: Must accommodate the pipe, its bedding, and the required cover depth (minimum 600-900mm in trafficked areas, less in non-trafficked areas, as per local regulations) while maintaining the specified gradient.
Battering/Shoring: For deep trenches or unstable soil, the sides may need to be sloped (battered) or supported with timber/steel shoring to prevent collapse, a critical safety measure in Nigerian construction sites.
Bedding: The trench bottom must be trimmed to the correct level and gradient, then a compacted bed of granular material (e.g., sand, gravel, minimum 100-150mm thick) is laid to support the pipes evenly.
Gradient (or Fall): This refers to the slope given to a drainage pipe or trench to ensure wastewater flows under gravity, carrying solids effectively without silting or excessive scour. It is expressed as a ratio (e.g., 1:60, 1:80, 1:100), meaning for every X units of horizontal length, there is 1 unit of vertical drop.
Importance: A sufficient gradient ensures self-cleansing velocity (typically 0.75-0.9 m/s) to prevent solids from settling and causing blockages. Too steep a gradient can cause water to run ahead of solids, leading to blockages; too shallow causes silting.
Common Gradients in Nigeria: For 100mm diameter pipes: Minimum 1:80 to 1:
1
0
0. For 150mm diameter pipes: Minimum 1:100 to 1:
1
5
0. Larger pipes can have shallower gradients.
Methods of Determining Gradient:
1. Sight Rail and Boning Rod Method (Most Common for Manual Trenching): Principle: This method uses a horizontal "line of sight" established by sight rails (horizontal timber or metal rails) and specially cut "boning rods" to ensure the trench bottom follows the desired gradient.
Procedure: Step 1: Establish Datum and Reference Point: Select a stable datum (e.g., a known benchmark or an arbitrary level like 100.000m). Mark the starting invert level (IL) of the pipe at the first inspection chamber (IC1) or connection point.
Step 2: Calculate Required Fall: Determine the total fall needed for the trench length (L) based on the specified gradient (e.g., for a 30m trench at 1:100, total fall = 30m / 100 = 0.30m or 300mm).
Step 3: Calculate Invert Levels: Calculate the invert level at the end of the trench (IC2) by adding the total fall to the starting invert level (IL at IC2 = IL at IC1 + Total Fall).
Step 4: Set Up Sight Rails: At IC1, erect two vertical posts and fix a horizontal timber rail (sight rail) across them. The top of this sight rail should be at a convenient, constant height (e.g., 1.5m) above the datum. Let this height be 'H'.
Calculate the boning rod length at IC1: `Boning Rod Length_IC1 = H - (IL_IC1 + Bedding Thickness)`. At IC2, erect similar posts and fix a sight rail. This sight rail will be parallel to the desired pipe gradient. To determine the height of the sight rail at IC2, calculate the boning rod length at IC2: `Boning Rod Length_IC2 = H - (IL_IC2 + Bedding Thickness)`. The top of the sight rail at IC2 should be set so that when a boning rod of `Boning Rod Length_IC2` is placed at the pipe invert, its top aligns with the top of the sight rail. A simpler, common method is to set a level sight rail across the entire trench using a dumpy level. Then calculate boning rod lengths for each section of the trench. The boning rod length will increase as the pipe deepens. Worked Example (Simplified Boning Rod using Level Sight Rail): A 20m long trench for a 100mm diameter pipe needs a gradient of 1:
8
0. The pipe's invert level (IL) at the start (Point A) is 0.80m below ground level. A sight rail is set up horizontally (level) at 1.50m above ground level.
Step 1: Calculate Total Fall: Fall = Trench Length / Gradient = 20m / 80 = 0.25m (or 250mm).
Step 2: Determine IL at End (Point B): IL_B = IL_A + Fall = 0.80m + 0.25m = 1.05m below ground level.
Step 3: Calculate Boning Rod Length at Point A: Depth of pipe invert below sight rail = Height of Sight Rail (from datum) - Invert Level (from datum). Assuming ground level is datum for simplicity: Boning Rod Length_A = (Sight Rail height above ground) - (IL_A below ground) = 1.50m - 0.80m = 0.70m.
Step 4: Calculate Boning Rod Length at Point B: Boning Rod Length_B = (Sight Rail height above ground) - (IL_B below ground) = 1.50m - 1.05m = 0.45m.
Step 5: Excavation and Checking: Excavators dig until a boning rod (cut to the calculated length for that specific point) placed on the of Sight Rail (from datum) - Invert Level (from datum). Assuming ground level is datum for simplicity: Boning Rod Length_A = (Sight Rail height above ground) - (IL_A below ground) = 1.50m - 0.80m = 0.70m.
Step 4: Calculate Boning Rod Length at Point B: Boning Rod Length_B = (Sight Rail height above ground) - (IL_B below ground) = 1.50m - 1.05m = 0.45m.
Step 5: Excavation and Checking: Excavators dig until a boning rod (cut to the calculated length for that specific point) placed on the trench bed at the pipe's invert level aligns its top edge with the line of sight of the level sight rail. This ensures the trench bottom has the correct gradient.
2. Dumpy Level/Automatic Level and Staff: For more precise work or longer trenches, a dumpy level is used to take readings on a levelling staff placed at intervals along the trench.
Procedure: Set up the dumpy level at a convenient position. Take a "back sight" reading on a known point (benchmark). Calculate the Height of Instrument (HI = Benchmark RL + Back Sight). Calculate the required staff reading at each point along the trench to achieve the desired invert level and gradient (Staff Reading = HI - Desired Invert Level). * Excavators dig until the staff reading at the trench bed (or pipe bedding) matches the calculated reading. The choice of drainage pipe depends on factors such as intended use (foul water, storm water), location (underground, exposed), required strength, chemical resistance, jointing method, and cost.
Vitrified Clay Pipes (VCP): Description: Made from selected clays, fired at high temperatures to achieve a glass-like finish, making them impervious and chemically resistant.
Properties: Very strong in compression, resistant to chemical attack (acids, alkalis found in sewage), durable, rigid.
Uses: Widely used for underground foul water and storm water drainage.
Jointing: Traditionally spigot and socket joints with cement mortar or proprietary rubber rings/bituminous compounds. Unplasticized Polyvinyl Chloride (uPVC)
Pipes: Description: Lightweight plastic pipes, increasingly popular due to ease of installation.
Properties: Lightweight, corrosion-resistant, smooth bore (good flow characteristics), flexible (can tolerate minor ground movement), easy to cut and join.
Uses: Very common for both above-ground (soil and vent stacks) and underground (foul water and storm water) drainage in residential and commercial buildings.
Jointing: Solvent-weld joints (for smaller diameter, above-ground), push-fit rubber ring seal joints (for underground, allowing for thermal movement and slight misalignment).
Cast Iron (CI)
Pipes: Description: Heavy-duty pipes made from cast iron.
Properties: Very strong, durable, resistant to impact and abrasion, fire-resistant. Heavier and more expensive than uPV
C. Uses: Historically used for underground drainage and soil stacks; now mostly replaced by uPVC due to cost and weight, but still used where very high strength or fire resistance is critical (e.g., deep installations, heavy-duty industrial applications, internal stacks in multi-storey buildings).
Jointing: Spigot and socket joints, traditionally sealed with molten lead and caulked yarn, now more commonly with rubber gasket push-fit systems or mechanical couplings.
Concrete Pipes: Description: Pipes made from reinforced or unreinforced concrete.
Properties: Very strong, durable, suitable for large diameters.
Uses: Primarily for large diameter storm water drains, culverts, and main sewers where high volume flow and structural strength are required. Less common for domestic foul water.
Jointing: Spigot and socket joints sealed with cement mortar or rubber gaskets.
Asbestos Cement (AC)
Pipes (Caution): Description: Made from asbestos fibers and cement. (NOTE: Due to severe health risks associated with asbestos, AC pipes are no longer manufactured and their use is highly discouraged.
However, they may be encountered in older installations in Nigeria).
Properties: Lightweight, corrosion-resistant, relatively strong.
Uses: Historically used for storm water and foul water drainage.
Jointing: Typically with rubber ring joints.
Selection Criteria Summary: Cost: uPVC is generally most economical for domestic use.
Strength/Durability: Cast Iron and Concrete for heavy loads/deep burial. VCP is robust.
Corrosion Resistance: VCP and uPVC are excellent.
Ease of Installation: uPVC is lightweight and easiest to handle.
Intended Use: Foul water (VCP, uPVC, CI), storm water (VCP, uPVC, Concrete).
Public Health and Environmental Sanitation: Proper drainage installation is paramount for preventing the spread of waterborne diseases (e.g., cholera, typhoid, dysentery) prevalent in many Nigerian communities. By ensuring efficient removal of wastewater and storm water, stagnant water bodies (breeding grounds for mosquitoes causing malaria) are eliminated. This directly improves community health and living conditions, especially in densely populated urban areas and rural settlements. Students can understand their role in contributing to a healthier Nigeria.
Job Creation and Entrepreneurship: Knowledge of drainage installation directly translates into employable skills within the vast Nigerian construction industry. Students can become skilled plumbers, pipe-layers, site supervisors, or even establish their own plumbing and drainage contracting businesses. This topic integrates practical skills with economic empowerment, vital for Nigerian youth seeking vocational careers. For example, a student could start a business offering drainage solutions for new building projects or existing homes experiencing drainage issues in their local community.
Flood Mitigation and Property Protection: Effective storm water drainage systems are crucial in Nigeria, a country prone to heavy rainfall and seasonal flooding, particularly in coastal and low-lying areas. Properly laid and tested drainage pipes prevent water accumulation around building foundations, safeguarding structures from dampness, erosion, and collapse. Students learn how their work contributes to resilient infrastructure and protecting property investments, directly addressing a common challenge faced by homeowners and developers in Nigeria.