Drainage Failure at a New Courtyard
A 2,500 sf courtyard was installed last fall. The finished grade slopes toward the building at about 1 percent, and ponding appears within 24 hours after a 1-inch storm, especially along a 40-foot foundation wall. The soil is a clay loam with a measured infiltration rate of 0.1 in/hr. Two downspouts discharge onto the courtyard. A 4-inch perforated French drain runs along the wall in gravel but has no filter fabric, and its outlet is buried. The client reports soggy soil and a musty smell near the wall. The building foundation is 18 inches below the adjacent patio.
1. Diagnose the likely causes of the ponding and rank them.
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The primary cause is grading: 1% toward the building is not positive drainage away from the foundation. A 1-inch storm on 2,500 sf is about 208 cf, or about 1,558 gallons. At 0.1 in/hr infiltration, the soil can take only about 21 cf/hr (0.1/12 × 2,500), so ponding is expected for many hours even with perfect grading. The two downspouts add concentrated water, and the filterless French drain is likely clogged with fines; its buried outlet gives water no place to go. Rank: first, reversed grade and downspouts; second, low-infiltration clay; third, failed drain and buried outlet.
2. What grading and drainage corrections would you specify?
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Regrade the courtyard to a minimum 2% away from the building, with a shallow swale or valley leading to a legal discharge point. Extend the downspouts in tightline to the discharge, not onto the patio. Do not rely on a French drain as the primary solution in 0.1 in/hr clay; if a subsurface drain is used, install a properly sized perforated pipe in washed gravel with filter fabric and a daylighted, screened outlet, plus an overflow path. Keep the finished grade at least 6–8 inches below the siding and maintain foundation exposure. Verify that the outlet is not buried and that the swale has capacity for the design storm.
3. How should soil preparation and plant selection respond to the drainage problem?
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Amend the planting soil with compost and avoid tilling or working it when wet. Plant water-tolerant species in the low swale and drier species on the high side. Do not raise soil over the foundation or bury weep holes. Because the soil drains slowly, use plants that tolerate periodic wet feet near the low point and keep irrigation separate from the drainage fix. Monitor for musty odors and correct the moisture source before replanting; otherwise root rot is likely.