Hillside Mixed-Use Site in Busay, Cebu: Cut-and-Fill, Access Grade, and a Rain Tree Cluster
A 4.2-hectare hillside parcel in Barangay Busay, Cebu City rises from a gully at the southeast corner to a ridge along the national road, with an average slope of 14% and one natural bench at 8%. The developer wants a three-level podium parking structure on the ridge for visibility from the highway, a 1,200 m² events lawn on the lowest, flattest ground, and a 6.0 m-wide two-way access road from the national road. Five mature rain trees (Samanea saman), 18–22 m across, stand on the ridge; a 60 m length of 1.8 m stone retaining wall along the downslope property line is valued by the adjacent homeowners. Geotechnical logs show 1.5 m of residual soil over weathered shale, with groundwater 6 m below the gully floor. The construction budget is fixed at PHP 180 million, and the client has already sold event bookings for the lawn. The project architect asks you to confirm the site organization before schematic design closes.
1. Evaluate the client's proposed placement of the podium parking on the ridge and the lawn on the lowest ground. What site organization would you recommend, and why?
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The proposal inverts the site's logic on both counts.
Podium on the ridge. A three-level podium at the high point requires a deep cut into the ridge and a tall fill batter or retaining structure to bring the access road up to it. That means exporting or importing large volumes, and it puts the largest impervious footprint at the head of the catchment, concentrating runoff onto everything below. The ridgeline is also where the rain trees stand, so the podium and its fire access would compact or sever roots.
Lawn at the low point. The gully is the receiving point for all site drainage. A lawn there stays saturated for days after rain, compacts under event loads, and loses bookings - a direct revenue risk the client has not priced in.
Recommended organization. Embed the parking into the mid-slope as a split-level structure with two faces exposed, so it is visible from the road without a ridge cut and needs no long fill. Put the events lawn on the natural bench, regraded to 1.5–2% with a crown and a perimeter swale. Keep the gully as a vegetated drainage corridor and overland flow path, not as program space. Confirm the arrangement with a mass-haul diagram: the goal is a cut-fill balance on site, because haulage, not earthmoving, will break the PHP 180 million budget.
2. Two access options exist: (A) a direct 9% approach through the rain tree cluster, 140 m long; (B) a switchback at 7.5%, 200 m long, crossing the upper gully on a short culvert. Recommend one and state the engineering and arboricultural conditions that attach to your recommendation.
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Recommend Option B, subject to conditions.
Grading and safety. Nine percent is at or beyond the practical limit for a two-way access road serving fire trucks and delivery vehicles, especially where a vertical curve meets the national road. At 7.5% the switchback sits inside a workable range, gives room for a landing at the turn, and provides a safer exit sightline. The extra 60 m of pavement is a small cost against regrading a failed approach later.
Trees. Option A runs through the cluster. Compaction and grade change inside the root zone - commonly taken as a radius of about 12 times trunk diameter - will kill or destabilize trees that the client cannot replace at that size. Option B keeps the pavement outside that zone.
Conditions. (1) A tree protection plan with fencing at the protection-zone line, no storage or washout inside it, and no grade change greater than about 150 mm within the zone. (2) A culvert sized for the upper gully design flow, with an energy dissipater at the outlet and a maintenance access. (3) Guardrail or barrier at the outside of the switchback curve. (4) A geotechnical confirmation of cut slope angles in the weathered shale.
3. What measures would you require to protect the existing 1.8 m stone retaining wall and the downslope neighbours, during construction and permanently?
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The wall is unreinforced masonry relying on passive earth pressure and its own weight; it has survived because the hillside above it has never been loaded or saturated. Both conditions will change.
During construction:
- No stockpiling, equipment tracking, or spoil placement within a distance equal to the wall height behind it.
- Intercept and divert upslope runoff before any clearing, using a temporary swale and silt fence above the work area; water is what fails old walls.
- Monitor the wall (crack gauges, survey pins) before, during, and after earthworks, with a stated trigger for stopping work.
- Underpin or shore the wall if any excavation approaches its foundation.
Permanent design:
- A subsurface drainage system behind the wall - filter drain with geotextile and a free-discharging weep path - so hydrostatic pressure never builds.
- Keep the finished grade behind the wall at or below existing level; any raise adds load directly.
- A planted buffer above the wall using deep-rooted but non-heaving species, plus a maintenance easement for inspection.
- A registered drainage easement and a written maintenance obligation, because the wall's long-term performance depends on someone cleaning the drain.