Sanitation System (C-42) · Lesson 3 of 6

Sizing a Seepage Pit: Sidewall Area and Block Count

"Sizing a Seepage Pit: Sidewall Area and Block Count" is lesson 3 of 6 in the Sanitation System (C-42) Learning Hub. It opens with "A Pit Absorbs Through Its Sidewall" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. A Pit Absorbs Through Its Sidewall
  2. Why Percolation Testing Matters
  3. Pit Geometry Rules
  4. Lining the Pit
  5. Multiple Pits and Spacing

The worked example in the course applies this lesson to a realistic job in 7 steps, from "Sidewall area per foot of depth" through "Layout checks".

What you'll be able to do

  • Explain the main idea of "Sizing a Seepage Pit: Sidewall Area and Block Count" in plain language you can use on the exam.
  • Apply this step with confidence: "Pit Geometry Rules".
  • Apply this step with confidence: "Lining the Pit".
  • Apply this step with confidence: "Multiple Pits and Spacing".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Seepage Pit Sizing: Two 6-Foot PitsSEEPAGE PIT, SECTION (not to scale)gradeinlet (vented)arch 18 into 4 ft deep6 in gravelbrick or block liningporous sidewallbelow inlet = 13 ft6 ft dia.13 ft(min 10)water table: keep pit 10 ft or more above itSIZE THE PITSTable H 201.1(2), Sections H 301.1(4), H 7011,200 / 100 x 40 = 480 sq ft6 ft pit: π x 6 = 18.85 sq ft per ftTwo pits: 240 sq ft each240 / 18.85 = 12.73 ft, use 13 ft13 ft is over the 10 ft minimumLINING COUNT (4 in rows, 14 per row)13 ft x 12 in / 4 in = 39 rows39 rows x 14 = 546 blocks per pit2 pits = 1,092 blocksPits at least 12 ft apart (Table H 101.8)Two 6-ft pits, 13 ft of porous sidewall below the inlet eachArea provided 2 x 13 x 18.85 = 490 sq ft, required 480 sq ft546 blocks per pit, 1,092 blocks total2025 CPC Appendix H: Sections H 301.1(4), H 701.3, H 701.6, H 701.4 and Table H 201.1(2)CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: A Pit Absorbs Through Its Sidewall

Section H 301.1(4) of 2025 CPC Appendix H says the effective absorption area of a seepage pit is the excavated sidewall area below the inlet, not counting any hardpan, rock, clay, or other impervious formation. The required area still comes from the tank capacity or flow (whichever is greater) and the soil factor in Table H 201.1(2), the same as for a leach field. That means a pit that passes through a clay layer only gets credit for the porous layers.

Step 2: Why Percolation Testing Matters

Section H 401.1 says seepage pit sizes shall be computed by percolation tests unless the AHJ approves using Table H 201.1(2). Tight soil needs more sidewall: a clay site asks for 90 or 120 square feet per 100 gallons instead of 40, so the pits must be deeper or there must be more of them. The CSLB C-42 study guide poses this same question, and the answer is "deeper or more pits might need to be installed."

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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