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Lab / experiment Grade 6 Engineering & Design

Pellham Flats Well Screen Gravel Pack Design Lab

Students build three model well screens with different gravel pack depths, time how fast each drains, and measure how much sand each lets through before choosing one.

Objectives

  1. Students will build three model well screens that differ only in the depth of the gravel pack.
  2. Students will measure drain time in seconds and settled sand depth in millimeters for each model.
  3. Students will choose the design that meets both a sand limit and a drain-time limit.

Vocabulary

Aquifer
An aquifer is an underground layer of rock, sand, or gravel that holds water in the tiny spaces between its grains. Wells reach down into aquifers to bring water up to the surface.

Also: well screen, gravel pack, drain time, trade-off

Teacher overview

Pellham Flats is a fictional farm town that draws its drinking water from a sandy aquifer, and all of its design rules in this lab were invented for the investigation. A real well is finished with a slotted screen, and clean gravel is poured around that screen to hold back the fine sand while letting water through. This lab asks a testable question: how does the depth of the gravel pack change both the amount of sand that gets through and the time the water takes to drain? Students build three models that differ only in gravel depth, collect two measurements from each, and then pick the design that meets the town's two rules.

Pellham Flats Well Screen Gravel Pack Design Lab — Grade 6 Engineering & Design
Name: ____________________________ Date: ______________

Procedure

  • Put on safety goggles before you begin. An adult should cut the tops off the bottles. Do not drink or taste any water in this lab.
  • Turn the three cut bottles upside down. Cover the neck of each one with a square of window screen and hold it in place with a rubber band. This screen stands for the slotted well screen.
  • Label the bottles A, B, and C. Set each one neck down in its own clear collecting cup.
  • Leave bottle A empty. This is the model with no gravel pack, and it is your control.
  • Pour gravel into bottle B until the gravel is 2 cm deep. Measure the depth with your ruler.
  • Pour gravel into bottle C until the gravel is 4 cm deep. Measure the depth the same way.
  • Make the sandy water. Stir 2 tablespoons of play sand into 300 mL of water in the measuring cup. Stir it again right before each pour so the sand does not settle.
  • Pour 300 mL of sandy water into bottle A. Start the stopwatch as you begin to pour. Stop it when the last water drips out. Write the drain time in seconds.
  • Repeat the pour for bottle B and then for bottle C, using 300 mL of freshly stirred sandy water each time.
  • Let the three collecting cups sit still for 5 minutes so the sand settles.
  • Hold the ruler against the side of each cup and measure the depth of settled sand in millimeters. Write each depth in the data table.
  • Rinse the cups and gravel in the sink when you are done. Wipe up spills right away so the floor does not get slippery.

Data & Analysis

Record your own measurements in the table. If your test is not finished, use the sample data printed below the table.

Screen Gravel pack depth Drain time (seconds) Settled sand (mm)
A 0 cm
B 2 cm
C 4 cm

Sample data from one trial run: Screen A drained in 15 seconds and left 10 mm of settled sand. Screen B drained in 30 seconds and left 4 mm. Screen C drained in 48 seconds and left 1 mm.

  1. Using the sample data, how many more seconds did Screen C take to drain than Screen A?
  2. Using the sample data, how many fewer millimeters of sand settled from Screen C than from Screen A?
  3. Screen A let 10 mm of sand through and Screen B let 4 mm through. What percent of Screen A's sand did Screen B let through?
  4. What percent of Screen A's sand did Screen C hold back?
  5. Pellham Flats has two rules for a well screen. It may let no more than 5 mm of sand settle, and it must drain in 40 seconds or less. Using the sample data, which screen meets both rules?
  6. Screen C drained in 48 seconds. How many seconds over the 40-second rule is that?
  7. Look at the pattern in the sample data. Describe what happens to the drain time as the gravel pack gets deeper.
  8. Design a fourth screen that might beat all three. Say what gravel depth you would use and why you think it would work better.

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: 33 seconds. Screen C took 48 seconds and Screen A took 15 seconds, so 48 - 15 = 33 seconds more.
  2. Answer: 9 mm. Screen A left 10 mm of sand and Screen C left 1 mm, so 10 - 1 = 9 mm less.
  3. Answer: 40 percent. Screen B let 4 mm through out of Screen A's 10 mm, and 4 / 10 = 0.40.
  4. Answer: 90 percent. Screen C let only 1 mm through, so it held back 10 - 1 = 9 mm out of 10 mm, and 9 / 10 = 0.90.
  5. Answer: Screen B. It left 4 mm of sand, which is under the 5 mm limit, and it drained in 30 seconds, which is under the 40-second limit. Screen A fails the sand rule at 10 mm and Screen C fails the time rule at 48 seconds.
  6. Answer: 8 seconds. The rule allows 40 seconds and Screen C took 48 seconds, so 48 - 40 = 8 seconds over.
  7. Answer: The drain time gets longer as the gravel pack gets deeper. Accept any answer that reports the increase from 15 to 30 to 48 seconds, or that says a deeper pack slows the water down because the water has farther to travel through small spaces.
  8. Answer: Answers vary. Accept any gravel depth between 2 cm and 4 cm with a reason that connects a deeper pack to cleaner water and a shallower pack to a faster drain, such as trying 3 cm to keep the sand near 2 mm while staying under 40 seconds. Also accept a design that keeps a 2 cm pack and changes the gravel size, as long as the student explains the trade-off.

Differentiation

Support

Pre-build the three bottle models before class so these students begin at the pouring step, and hand them a data table with the gravel depths already filled in. Provide a ruler with the first 10 millimeters shaded so the settled sand measurement is easier to read.

Extension

Have students run a second trial of the design they proposed in analysis item 8, using the same 300 mL of sandy water, and then write a short comparison of their design's two measurements against the sample data for Screens B and C.

ELL supports

Provide a photograph sequence of the build with one short caption per step, and label a cutaway drawing with the words "screen," "gravel pack," "sand," and "drain." Allow students to answer items 7 and 8 with a labeled sketch plus a single sentence.

Extensions

Have students find out whether their own community draws any of its drinking water from wells, using a public water quality report, and write a short paragraph on why a well in sandy ground needs a gravel pack while a well drilled into solid rock may not.