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Lab / experiment Grade 6 Engineering & Design

Testing Three Pipe Bedding Materials for Oskett Lane

Students load a model pipe on soil, sand, and pea gravel, then measure settling, drainage, and soil movement to pick the bedding a fictional water main job needs.

Objectives

  1. Students will measure how far a loaded model pipe settles into three bedding materials.
  2. Students will time drainage and measure how far surface soil moves into each bedding.
  3. Students will select the bedding that satisfies all three job rules and propose a fix for one that fails.

Vocabulary

Key words: bedding, settlement, drainage, fines, trade-off

Teacher overview

A buried water pipe does not rest on the bottom of the trench. It sits on a layer of chosen material called bedding, which spreads the weight of the soil above it and lets water drain away from the pipe. Crews choose that material, and the choice pulls in two directions at once: loose open material drains quickly but lets fine soil wash down into its gaps, while tight material holds soil out but drains slowly and lets the pipe settle. In this lab students test three common bedding materials against three rules written for a fictional water main job on Oskett Lane.

Testing Three Pipe Bedding Materials for Oskett Lane — Grade 6 Engineering & Design
Name: ____________________________ Date: ______________

Procedure

  • Wash your hands after handling soil, and do not drink any water used in this lab. Keep the soup cans away from the table edge so they cannot fall.
  • Fill the first tub with garden soil to a depth of about 6 cm. Fill the second with sand and the third with pea gravel to the same depth. Smooth each surface flat with the ruler.
  • Tape the ruler upright at the side of the first tub so the millimeter marks face you.
  • Lay the pipe on the smoothed surface. Read the height of the top of the pipe against the ruler and write it down.
  • Stack the four cans gently on the pipe. Wait 60 seconds, then read the height of the top of the pipe again. Subtract to find how far the pipe settled, in millimeters.
  • Lift the cans off, smooth the surface again, and run the settling test a second time so you have two trials for that material.
  • Repeat the settling test twice in the sand tub and twice in the pea gravel tub.
  • Run the drainage test next in each of the three tubs. Remove the pipe. Pour 200 mL of water slowly onto one end of the material and start the stopwatch. Stop it when no standing water is left on the surface. If water still stands at 300 seconds, write down 300 and stop.
  • Run the soil movement test last. Spread a 2 cm layer of garden soil across the top of the sand, then do the same on the pea gravel. Pour 200 mL of water over each. The garden soil tub is not set up this way, because its surface layer and its bedding are the same material and no boundary would show, so record that row as no movement.
  • Wait 2 minutes. Dig a small trench with the spoon and measure how far the dark soil has moved down into the material below it, in millimeters.
  • Empty the tubs into a bucket, rinse them, wipe the table, and wash your hands.

Data & Analysis

Record your own results in the table below.

Bedding Settling trial 1 (mm) Settling trial 2 (mm) Average (mm) Drain time (s) Soil moved in (mm)
Garden soil
Sand
Pea gravel

The Oskett Lane job in this lab is fictional, and so are its three rules. Rule 1: the pipe may settle no more than 6 mm on average. Rule 2: the bedding must drain in 90 seconds or less. Rule 3: surface soil may move no more than 3 mm down into the bedding.

Sample data from one class run. Garden soil settled 8 mm and 10 mm, drained in 240 seconds, and let soil move 0 mm. Sand settled 4 mm and 6 mm, drained in 60 seconds, and let soil move 1 mm. Pea gravel settled 2 mm and 2 mm, drained in 15 seconds, and let soil move 6 mm.

  1. Garden soil settled 8 mm in one trial and 10 mm in the other. What is the average settling?
  2. Sand settled 4 mm in one trial and 6 mm in the other. What is the average settling?
  3. Garden soil averaged 9 mm and sand averaged 5 mm. How many millimeters less did the sand settle?
  4. Rule 1 allows an average of 6 mm at most. The three averages were 9 mm, 5 mm, and 2 mm. How many of the three beddings pass Rule 1?
  5. Rule 2 allows 90 seconds at most. The three drain times were 240 seconds, 60 seconds, and 15 seconds. How many of the three beddings pass Rule 2?
  6. Rule 3 allows 3 mm of soil movement at most. The three readings were 0 mm, 1 mm, and 6 mm. How many of the three beddings pass Rule 3?
  7. Garden soil gave 9 mm, 240 seconds, and 0 mm. Sand gave 5 mm, 60 seconds, and 1 mm. Pea gravel gave 2 mm, 15 seconds, and 6 mm. Which single bedding passes all three rules?
  8. Garden soil drained in 240 seconds and sand drained in 60 seconds. How many times faster did the sand drain?
  9. Pea gravel drained fastest, at 15 seconds, and it also let soil move 6 mm into it. Explain how those two results are connected.
  10. Propose one change that would let pea gravel pass Rule 3 while keeping its fast drainage. Say which result in the table made you choose that change.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 9 mm.
    8 plus 10 is 18, and 18 divided by 2 is 9.
  2. Answer: 5 mm.
    4 plus 6 is 10, and 10 divided by 2 is 5.
  3. Answer: 4 mm.
    9 mm minus 5 mm leaves 4 mm.
  4. Answer: 2 beddings.
    Sand at 5 mm and pea gravel at 2 mm are both at or under 6 mm. Garden soil at 9 mm is over.
  5. Answer: 2 beddings.
    Sand at 60 seconds and pea gravel at 15 seconds are both at or under 90 seconds. Garden soil at 240 seconds is far over.
  6. Answer: 2 beddings.
    Garden soil at 0 mm and sand at 1 mm are both at or under 3 mm. Pea gravel at 6 mm is over.
  7. Answer: Sand.
    Sand settles 5 mm, which clears Rule 1, drains in 60 seconds, which clears Rule 2, and lets soil move only 1 mm, which clears Rule 3. Garden soil fails Rules 1 and 2, and pea gravel fails Rule 3.
  8. Answer: 4 times.
    240 seconds divided by 60 seconds is 4.
  9. Answer: The large gaps that let water through also let soil through.
    Accept any answer connecting the size of the spaces between the stones to both results: wide gaps drain quickly and give fine soil room to wash down into them. Accept students who add that soil filling those gaps would slow the drainage later.
  10. Answer: Answers vary.
    Accept any change that keeps the gravel but blocks soil from entering, such as wrapping the gravel in landscape fabric, laying a thin sand layer over the gravel, or capping the trench with packed soil. Strong answers cite the 6 mm soil movement reading as the result that needs fixing, and note that sand's 1 mm reading shows a finer material can hold soil back.

Differentiation

Support

Set up the three tubs before class and run the garden soil trials as a whole-class demonstration, so these groups begin at the sand tub with one row of the table already filled. Give them a ruler with the first 10 millimeters shaded to make the settling reading easier.

Extension

Have groups build and test the fix they proposed in item 10, measure the drain time and the soil movement again, and report whether the fixed gravel now passes all three rules.

ELL supports

Provide a labeled cross-section drawing of a trench showing the pipe, the bedding below it, and the soil above it, with an arrow for water draining away. Post the words "bedding," "settle," "drain," and "gap" with a small picture beside each. Allow items 9 and 10 to be answered with a labeled sketch and one sentence.

Extensions

Have students watch for an open utility trench in their community, sketch what they can see from the sidewalk, and write a short paragraph on whether the material under the pipe looks like the native soil or like something trucked in.