Rain Gardens and Tree Pits on Compacted Fill
A 0.35 ha clinic landscape is scheduled for handover in 14 days. The site slopes 6–9% toward a low corner where water stands for 24–36 hours after a 25 mm storm. A geotechnical memo notes 0.6–1.1 m of compacted fill over stiff clay, but gives no infiltration rate. The planting plan shows 38 shade trees, two rain gardens at the low corner, and no visible drainage structures. Existing sugar maples on the adjacent lot have thin crowns and yellowing between veins. Spot tests show pH 7.4, organic matter 1.3%, and penetrometer resistance 2.1 MPa at 200 mm. The client wants a low-maintenance, native plant palette and refuses curb inlets or a surface swale. The contractor must submit planting and drainage details before ordering stock. The general contractor asks whether the rain gardens can simply be excavated into the existing soil and whether the trees can be planted in 300 mm deep pits with imported topsoil.
1. What site investigations and design checks should be completed before confirming the rain-garden and tree-planting details? Prioritize them.
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Start with a utility locate and confirm a legal overflow outlet; without an outlet the rain gardens are ponds. Then test infiltration at the exact bed footprints and at the fill-clay interface, not just rely on the geotech memo. Verify seasonal high water table, fill depth, and whether the contributing area exceeds the beds' capacity. If infiltration is very low, redesign with underdrains, an amended planting soil, and a surface overflow; do not excavate into clay and call it a rain garden. For trees, 300 mm pits in 2.1 MPa fill are inadequate: they create a small rooting volume and a bathtub. Require wider planting areas, decompaction, root-flare placement, and species tolerant of wet feet and high pH. Sequence: investigations, redesign, client approval, then order stock.
2. The contractor proposes 300 mm deep tree pits with imported topsoil and no drainage. Evaluate this proposal and state what you would require instead.
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Reject the proposal as written. A 300 mm deep pit in compacted fill is shallower than a typical root ball and limits roots to the imported soil; the interface can perch water, especially on clay. Imported topsoil does not fix compaction. Require planting areas at least two to three times the root-ball width, loosened to 300–400 mm where feasible, with the root flare at grade and no soil over the flare. Use a coarse, stable planting soil that matches the native texture, not a high-organic mix that shrinks. Provide drainage only if the test shows perched water; otherwise deep loosening and a slight mound may suffice. Select species for high pH and periodic wetness. Inspect pits before planting and reject saturated or smeared excavations.
3. What should the contractor tell the client about the no-visible-drainage goal and the 14-day handover?
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Explain that the no-visible-drainage goal conflicts with the ponding evidence and the clay fill. Offer concealed solutions: underdrained rain gardens with a raised overflow, a buried outlet to an approved discharge point, or a planted swale designed as a landscape feature. Present cost and maintenance. Because handover is 14 days away, do not install 38 trees before drainage and planting details are approved; order only after approval or use a phased planting. Document the client's decision and any accepted risk in writing. A 25 mm storm producing 24–36 hours of ponding is a design signal, not a cosmetic issue.