Classroom activity Grade 10 Engineering & Design

Hobbleton Ridge Percolation Stations: Sizing a Drainfield

A grade 10 station rotation sizing a soil absorption field for a fictional rural school, from a percolation test through trench layout, reserve area, and tank volume.

Grade: Grade 10
Subject: Engineering & Design
Material: Classroom activity
Time: 50 min
Prep: Moderate prep
Materials: Four printed station cards, Printed Student Handout, one per student, Graph paper, one sheet per team, Calculators, Timer

Standards

HSG-MG.A.3 Common Core Math

Apply geometric methods to solve design problems (e.g., designing an object or structure to satisfy physical constraints or minimize cost).

How this material aligns: Students lay out trenches, spacing, and reserve area as a geometry problem constrained by a fixed parcel and a required absorption area.

All materials for HSG-MG.A.3 →

HSN-Q.A.1 Common Core Math

Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays.

How this material aligns: Stations carry minutes per inch, gallons per day, and gallons per day per square foot through a chain ending in trench length.

All materials for HSN-Q.A.1 →

HS-ETS1-2 NGSS

Design a solution to a complex real-world problem by breaking it down into smaller, more manageable problems that can be solved through engineering.

How this material aligns: The four stations break the drainfield design into solvable pieces: questions 1 and 2 set the soil's application rate, question 3 sizes the absorption area, questions 4 through 8 lay out the trenches and the reserve area, and questions 9 through 12 fit that layout to the parcel and the well setback.

All materials for HS-ETS1-2 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-09-03.

Objectives

Students will convert a percolation test observation into a percolation rate and read the matching soil application rate.
Students will size a soil absorption field and lay out its trenches within a fixed parcel.
Students will check a proposed layout against a required reserve area and a well setback.

Vocabulary

Key words: percolation rate, application rate, absorption field, lateral, reserve area, setback, biomat

Teacher overview

A building beyond the reach of a sewer sends its wastewater to a tank and then to a field of gravel trenches, where a layer of soil bacteria called the biomat and the unsaturated soil beneath it do the biological treatment that a plant would otherwise do in an aeration basin. This rotation takes teams of three through four stations that size such a system for a fictional school at Hobbleton Ridge, beginning with a percolation test and ending with a layout drawn on graph paper. The station order follows the order a designer actually works in, since the soil test sets the application rate, the application rate sets the area, and only then can trenches be laid out against setbacks and a reserve area. A misconception worth naming before the rotation is that a bigger tank solves a soil problem, yet the tank only settles solids, and slow soil requires more trench area no matter how the tank is sized.

Activity Directions

Before class, print the four station cards from the Student Handout, one per station, and post them around the room with a sheet of graph paper at Station 3. Arrange the class into teams of three. Set a timer for nine-minute intervals.
Setup and framing, 6 minutes. Give each student a handout. Explain the fictional Hobbleton Ridge situation: a rural school with no sewer connection, a design flow set by its enrollment, and a parcel of limited size. Sketch the system on the board as three parts, which are the septic tank, the distribution box, and the trenches, and name the biomat as the biological layer that forms where effluent meets soil.
Station rotation, 36 minutes. Teams rotate on the timer through four stations, spending nine minutes at each and answering that station's numbered questions on the handout. Station 3 requires a sketch on graph paper showing the trench layout at a scale the team chooses and labels, which the team carries with them to Station 4.
Circulate and probe. At Station 2, check that teams read the application rate from the table rather than reusing the percolation rate as a rate of application. At Station 3, check that the land width accounts for the width of the outer trenches and not only the spacing between centers.
Formative check, 3 minutes. After the last rotation, ask teams to hold up their answer to question 3. If fewer than three quarters of the class has 2,250 square feet, rework questions 1 through 3 on the board.
Debrief, 5 minutes. Ask two closing questions. First, what would the school have to do differently if the parcel were half its size? Second, why does the county require a reserve area that stays empty for the life of the system?

Hobbleton Ridge Percolation Stations: Sizing a Drainfield — Grade 10 Engineering & Design

Name: ____________________________ Date: ______________

Student Handout

Hobbleton Ridge Community School is a fictional site created for this rotation. Its design wastewater flow is 1,800 gallons per day. Answer each station's questions before you rotate.

Station 1. The percolation test. A test hole was presoaked and then refilled. The water level dropped 6 inches in 24 minutes.

Find the percolation rate in minutes per inch. Divide the elapsed minutes by the drop in inches.

Station 2. The application rate and the area. The county uses the table below to set a soil application rate from the percolation rate. If your team has not yet visited Station 1, use a percolation rate of 4.0 minutes per inch.

Percolation rate (minutes per inch)	Application rate (gallons per day per square foot)
1 to 5	0.80
6 to 15	0.60
16 to 30	0.45
31 to 60	0.20

Read the application rate that matches your percolation rate from question 1.
Find the absorption area the school needs. Divide the 1,800 gallon per day design flow by the application rate from question 2.

Station 3. Laying out the trenches. Trenches are 3 feet wide. They are spaced 9 feet apart, center to center. The design uses 5 equal laterals. Sketch the layout on graph paper as you work. If your team has not yet visited Station 2, use an absorption area of 2,250 square feet.

Find the total trench length required. Divide the absorption area from question 3 by the 3-foot trench width.
Find the length of each of the 5 equal laterals.
Find the total land width the 5 laterals occupy, measured from the outer edge of the first trench to the outer edge of the last. Use the 9-foot spacing and the 3-foot trench width.
Find the ground footprint of the primary field. Multiply the lateral length from question 5 by the land width from question 6.
The county requires a reserve area equal to the primary field, held empty for a future replacement. Find the total footprint the school must set aside.

Station 4. Fitting the site. The usable part of the parcel is a rectangle 210 feet long and 120 feet wide. The nearest trench would sit 140 feet from the school's drinking water well, and the county requires at least 100 feet of separation. The septic tank must hold 2 days of design flow. If your team has not yet visited Station 3, use a total footprint of 11,700 square feet in question 10.

Find the area of the usable rectangle.
Find the percent of the usable rectangle that the total footprint from question 8 would occupy.
Find the working volume the septic tank needs, in gallons, at 2 days of design flow.
By how many feet does the 140-foot separation exceed the 100-foot well setback?
Suppose a second test hole on the same parcel gives a percolation rate of 35 minutes per inch. Using the table at Station 2, find the absorption area that soil would require for the same 1,800 gallon per day flow.
Explain why slower soil requires a larger absorption field even though the design flow has not changed.

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

Answer Key

Answer: 4.0 minutes per inch.
Dividing the 24 minutes by the 6-inch drop gives 4.0 minutes per inch, which is a fast-draining soil.
Answer: 0.80 gallons per day per square foot.
A percolation rate of 4.0 minutes per inch falls in the table's 1 to 5 row, which carries an application rate of 0.80 gallons per day per square foot.
Answer: 2,250 square feet.
Dividing the 1,800 gallon per day design flow by 0.80 gallons per day per square foot gives 2,250 square feet of absorption area.
Answer: 750 feet.
Dividing the 2,250 square feet of absorption area by the 3-foot trench width gives 750 feet of total trench length.
Answer: 150 feet.
Dividing the 750 feet of total trench length among 5 equal laterals gives 150 feet each.
Answer: 39 feet.
The 5 trench centers span 4 spaces of 9 feet, which is 36 feet. Adding the 3-foot trench width so that the measurement runs from outer edge to outer edge gives 39 feet.
Answer: 5,850 square feet.
Multiplying the 150-foot lateral length by the 39-foot land width gives a primary field footprint of 5,850 square feet.
Answer: 11,700 square feet.
The reserve area equals the primary field, so the school must set aside 5,850 plus 5,850, which is 11,700 square feet.
Answer: 25,200 square feet.
Multiplying 210 feet by 120 feet gives 25,200 square feet of usable parcel.
Answer: about 46.4 percent.
Dividing 11,700 by 25,200 gives 0.4643, which is about 46.4 percent of the usable rectangle. The system fits with room left for the building, the drive, and the tank.
Answer: 3,600 gallons.
Multiplying the 1,800 gallon per day design flow by 2 days gives a working volume of 3,600 gallons.
Answer: 40 feet.
Subtracting the 100-foot required setback from the 140-foot separation leaves 40 feet of margin.
Answer: 9,000 square feet.
A percolation rate of 35 minutes per inch falls in the table's 31 to 60 row, which carries an application rate of 0.20 gallons per day per square foot, and 1,800 divided by 0.20 gives 9,000 square feet.
Answer: Slow soil accepts fewer gallons per square foot each day.
Accept any answer explaining that the application rate falls as the percolation rate rises, so the same daily flow has to be spread across more area to be absorbed and treated. Strong answers add that the biomat and the soil below it need time and oxygen to break down the effluent, and that flooding a slow soil pushes untreated water to the surface or sideways instead of down.

Differentiation

Support

Give these teams a handout with questions 1 and 2 answered, so their first independent work is the division at question 3. Provide a pre-drawn grid at Station 3 with the five trenches already outlined, leaving only the dimensions to be labeled, and a formula card stating that area equals flow divided by application rate.

Extension

Ask these teams to redesign the fictional Hobbleton Ridge layout for the 35 minute per inch soil from question 13 and determine whether that field would still fit on the 210 by 120 foot parcel with a full reserve area. Have them state the maximum design flow the parcel could support in that slower soil.

ELL supports

Post a labeled cross-section of a trench showing the pipe, the gravel, the biomat layer, and the soil below, with arrows showing water moving downward. Provide a term strip pairing percolation with how fast water soaks in, application rate with gallons allowed per square foot, lateral with one trench line, and reserve with the spare area held for later.

Extensions

Have teams find their own county's percolation rate table and compare its application rates with the fictional table used here, then recalculate question 3 with the local values. Teams can also research what a designer does when a site fails the percolation test entirely, including mound systems and pressure-dosed sand filters, and add a fifth station describing one of those alternatives.