Polyhouse Siting and Structural Review on a Sloping Black-Cotton Plot
An NGO is helping a farmer at Junnar, Pune district, build a 4,000 m² naturally ventilated gable polyhouse for tomato. The plot slopes about 3% from west to east; the soil is black cotton with high clay that shrinks and swells. A seasonal stream runs 30 m east of the plot, and an 11 kV overhead line runs along the south boundary. A row of 8 m tall eucalyptus trees stands 10 m west of the proposed site. Prevailing winds are from the west and south-west during the monsoon, with occasional gusts above 80 km/h. The contractor has quoted a 4 m gutter height, 30 cm deep foundations, an east–west ridge orientation, and no perimeter drain. The farmer wants to plant in June, before the monsoon. The NGO has asked for a technical review before the design is approved and the first payment is released.
1. List the site constraints that can affect this polyhouse and state, for each, a practical mitigation that should be included in the design.
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Treat the site as a set of separate risks:
| Constraint | Why it matters | Design response |
|---|---|---|
| 3% west-to-east slope | Runoff from the west can enter the house; cut-and-fill creates uneven foundation | Bench the platform, set plinth 30–45 cm above surrounding grade, grade the platform away from the house at 0.5–1%, and put a cut-off drain on the uphill (west) side |
| Black cotton soil | High shrink–swell and poor drainage; shallow footings move and crack | Remove at least 30–45 cm of topsoil, replace with murrum/gravel, use pad or raft footings founded below the active zone (about 0.9–1.2 m), and provide subsurface drainage |
| Stream 30 m east | Flood and erosion risk in monsoon | Keep a vegetated buffer, raise plinth above local flood level, divert runoff around the site, and do not narrow the stream |
| 11 kV line on south boundary | Statutory clearance and safety | Keep the house, workers and irrigation equipment outside the statutory clearance; consult the DISCOM; earth metal structures |
| 8 m eucalyptus 10 m west | Shade, root competition, turbulence | Remove or shift the trees; if retained, keep them at least 1.5 times their height away and install a root barrier |
| 80 km/h gusts | Uplift and racking | Design for wind load, anchor cladding, brace bays, and use a windbreak at a distance, not touching the house |
Reasoning: the biggest irreversible errors are foundation and drainage; fix them before ordering the structure. The trees and the power line are layout constraints, not details to settle later.
2. Should the ridge be east–west or north–south? Justify using light, temperature and wind considerations for this location.
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Choose a north–south ridge. At about 19°N the sun is high for most of the year; a north–south ridge distributes light more evenly across the beds through the day and avoids the intense noon radiation load on a large east–west roof slope. East–west would give slightly more winter light but a hotter, more uneven crop in April–May, when this region already exceeds 40°C.
Wind: prevailing west/south-west means an east–west ridge would present a large gable end to the wind, increasing pressure and racking on the end frame. A north–south ridge presents a sidewall; the west side is windward and the east side leeward, so roll-up side vents on both sides plus roof vents give cross-ventilation without the gable acting as a sail. Keep the ridge within a few degrees of true north–south; do not trade a 20–30° swing just to follow the slope. Fix drainage by benching, not by rotating the house.
Operational point: with north–south, open the west-side vent first in the morning and use the east-side vent later; in strong wind, use the leeward vent only to avoid direct gust entry.
3. The contractor's 30 cm foundation and 4 m gutter are questioned. Give your judgement on foundation depth, plinth, drainage and wind bracing.
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30 cm is not a foundation on black cotton soil. The active zone can extend 0.9–1.2 m; a 30 cm footing sits in soil that moves with wetting and drying, so cracks and frame distortion are likely. Use pad or raft footings below the active zone, a reinforced perimeter beam, and a 30–45 cm plinth above grade. The 4 m gutter is workable for tomato but low for this climate: it reduces air volume and puts the crop close to the hot roof. Increase to at least 4.5 m, preferably 5 m, if the quotation allows.
Drainage is missing and must be added: cut-off drain on the west, perimeter drain, platform graded away, gutter slope about 0.5% to downpipes, and subsurface drainage in the house. Without it, the black cotton soil will stay saturated after monsoon and the foundation will move.
Wind bracing: diagonal bracing in end and intermediate bays, cross-ties, base plates anchored to the footings, and cladding fixed with washers/clips rated for the design wind speed. Also specify 40–50 mesh insect net on vents, a double entry door, and a proper anchoring detail for the eucalyptus removal or root barrier. Refuse to approve the design until foundation, drainage and bracing are corrected.