Soil Morphology Versus a Fast Percolation Test
A site evaluator visits a 1.1-acre parcel for a 4-bedroom home. Two soil pits are dug. Pit A: 0–9 in sandy loam; 9–28 in clay loam with 30% redoximorphic concentrations beginning at 15 in; 28–60 in gravelly sand. Pit B: 0–11 in loam; 11–40 in silty clay with mottles beginning at 12 in; bedrock at 44 in. Percolation tests in the upper horizons give 35 min/in in Pit A and 95 min/in in Pit B. The designer proposes a conventional trench system with 24-in separation to the limiting condition, using the 35 min/in rate, and siting it on the steeper part of the lot where the slope is 13%. The homeowner wants the largest possible drainfield area near the deck. A stream is 90 ft downslope.
1. Determine the limiting condition for each pit and whether the proposed 24-in separation is available.
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The limiting condition is the seasonal high water table shown by the mottles, not the bedrock and not the percolation result. In Pit A, mottles begin at 15 in; any below-grade trench bottom can have at most 15 in of separation, so 24 in is impossible without raising the system. In Pit B, mottles begin at 12 in, so 24 in is also impossible; bedrock at 44 in is deeper and is not controlling. Therefore the proposed conventional 24-in separation is not available in either pit on natural grade. The design must change, not simply cite the 35 min/in rate.
2. Explain why using the 35 min/in result is not a valid basis for sizing the system.
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The 35 min/in test was performed in the upper sandy loam, above the clay loam and above the mottles. The restrictive horizon is the clay loam with mottles; it is the layer that will receive and transmit effluent and it is seasonally saturated. Using the fastest, shallowest test overstates the soil's capacity. Percolation results must be correlated with soil morphology and the most restrictive layer. The 95 min/in result in Pit B is more conservative and closer to the restrictive condition, but neither test replaces the separation analysis.
3. Recommend a site-planning approach that addresses the slope, stream, and soil limitations.
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Move the soil treatment area away from the 13% slope and verify the stream setback under the local code; 90 ft may or may not comply. Prefer the area with deeper gravelly sand if separation can be achieved with a raised or mounded system, but do not design on the 35 min/in rate alone. Require additional pits and a design by a qualified designer. Use pressure distribution and a raised bed or mound, or an advanced treatment unit only if the code allows reduced separation. Keep the system out of the steep area and away from fill placed for the deck.