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Discuss, using notes and freehand sketches, three functional requirements of a below ground drainage system to ensure the safe removal of sewage from a domestic dwelling - Leaving Cert Construction Studies - Question 4 - 2010
Question 4
Discuss, using notes and freehand sketches, three functional requirements of a below ground drainage system to ensure the safe removal of sewage from a domestic dwel... show full transcript
Worked Solution & Example Answer:Discuss, using notes and freehand sketches, three functional requirements of a below ground drainage system to ensure the safe removal of sewage from a domestic dwelling - Leaving Cert Construction Studies - Question 4 - 2010
Step 1
Discuss, using notes and freehand sketches, three functional requirements of a below ground drainage system.
Answer
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Watertightness: The drainage system must be watertight to prevent the leakage of sewage, which could lead to environmental contamination. Proper sealing of joints and connections ensures that no effluent escapes the system.
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Self-Cleaning: The system should have the ability to self-clean, allowing for the removal of sediment and waste without manual intervention. This prevents blockages and ensures the efficient flow of sewage.
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Access and Maintenance: There should be adequate access provided through manholes for maintenance and inspection. The design should allow for easy entry to clear blockages or to perform necessary repairs, thus promoting longevity and reliability.
Step 2
Using notes and freehand sketches, show the typical construction details through the manhole from the foundation to the manhole cover.
Answer
Construction Details of a Manhole
- Foundation: A 150 mm thick mass concrete base is essential to support the structure.
- Branch Pipes: These should be connected obliquely to allow free flow. They must discharge into the channel at or above the level of the horizontal diameter.
- Benchings: Channels and branches must be benched to at least the top of the outgoing pipe, and be smoothly rounded off.
- Materials: Use mass concrete or high-density concrete block, rendered inside and out for durability.
- Dimensions: Typical dimensional specifications for a manhole are about 1200 mm x 750 mm x 840 mm (internal sizes).
- Access: Ensure removable ventilating covers of durable materials like cast iron or pre-cast concrete, with steps for manholes deeper than 1 m.
Step 3
Using notes and freehand sketches, describe in detail one test that may be carried out to ensure that the below ground drainage system is watertight.
Answer
Water Test for Watertightness
- Preparation: Seal off all drainage outlets and plug the end of the pipe.
- Fill: Fill the drain with water to a height of at least 1.5 m above the invert level (the lowest point in the pipe).
- Hold: Maintain this water level for 2 hours to allow for absorption and escape of air bubbles.
- Leakage Measurement: After 30 minutes, check for any decrease in water level. If the leakage exceeds 0.5 liters per minute per 100 mm diameter pipe, the system fails the test.