Howick Fuel Retail Centre: Civil & Stormwater Design

Unicon Construction appointed Ant Consult for the civil design of a new Shell-branded filling station and convenience centre in Howick — an 8,730 m² corner plot on Zeederberg Road and the R103, carrying a four-pump forecourt, four underground fuel tanks, a store, a drive-through and a car wash.

Three things made it awkward.

The site falls east to west at about 6.7%, against a 5% maximum workable gradient across a forecourt, and the platform had to tie into fixed road levels at both frontages.

Sealing the site would take it from roughly 25% hard surface to 75%, pushing a sharper runoff peak into a municipal stormwater line already running through the low corner.

And four underground tanks in saturated ground want to float.

Earthworks: balance the cut, don't cart it away

We graded the site east to west at 1%, with a 5% ramp tying into the R103 and a secondary north–south fall to cut fill volumes and drive surface water to the south-western low point — where the municipal stormwater and sewer connections already are. The difference is absorbed in batters and a retaining structure on the western edge.

Unadjusted model volumes came out at 2,422 m³ cut against 1,363 m³ fill. A net cut of roughly 1,059 m³.

Rather than bill that as spoil, we used the excess to build the attenuation dam embankment in the same corner, removing most of the cart-away from the contract.

The tradeoff is worth naming: these are unadjusted model volumes, so the contractor carries the bulking and shrinkage risk when pricing.

Stormwater: 128 m³ required, 130 m³ provided

Runoff is calculated by the rational method on a 1:50 year, 30 minute storm at 142 mm/h, from Cedara — the nearest long-record station representative of the site.

Msunduzi Municipality attenuation reduction factors are the applicable local standard, but Msunduzi's design intensity is 165 mm/h against Merrivale's 142 mm/h. Applied unadjusted they over-size the storage.

We adapted them for the lower intensity — 56 m² per m³ hard, 285 m² per m³ soft — and documented the adaptation so the authority can check it.

That gives 128.2 m³ of attenuation required. We provided 130 m³, split across two facilities:

Splitting it rather than enlarging the dam keeps the embankment small enough to build from site cut, and holds volume up-catchment where it takes load off the pipe network in short, sharp storms.

Pre-development peak discharge is 74.2 ℓ/s; post-development, attenuated, it is 2.81 ℓ/s. The development reduces peak load on the municipal system rather than adding to it.

Conveyance is deliberately surface-led: runoff moves by grading and kerb-side channels to catchpits, and is only piped once it has to be. Less trench on a site congested with fuel and electrical services means less excavation and fewer level clashes.

Water and sanitation: two meters, not one

Potable water runs as a ring main — no dead legs, no stagnation. Reticulation below 110 mm is HDPE PE 100 PN 10, for joint integrity under a trafficked forecourt and tolerance of differential settlement in made ground.

Potable and fire water get separate meters. A single meter sized to pass fire flow sits far below its accurate range during normal trading, and the site is billed on badly under-registered consumption for the twenty years it never has a fire. Splitting them costs one extra installation and fixes the measurement.

Sanitation is 110 mm at 1% throughout, with drop manholes where the fall demands it rather than steepened runs. Holding 1% keeps invert depths shallow on a site already tight on cover.

Forecourt and car wash runoff is contaminated and never enters the clean stormwater system.

Separator sizing by the nominal size method gave 7.2 ℓ/s; we specified a Rocla Ecoline NG10 rated at 10 ℓ/s. The margin is deliberate — separator performance falls off quickly past the rated flow, and this is the next standard unit up.

One rule is printed on every drawing: sanitation and stormwater levels take precedence over water, fuel and ducting at every clash. Gravity systems have no flexibility on gradient. Pressure and pull-through systems can be adjusted at no cost.

Fuel tank civils: the empty tank is the design case

Four underground tanks — two 46,000 ℓ unleaded, one 46,000 ℓ diesel, one 56,000 ℓ diesel.

The governing load case is not the full tank. It is the empty tank in a high water table, where displaced water weighs far more than the steel around it. Restrain it inadequately and it lifts, breaks its connections, and destroys the forecourt above.

Rather than rely on hold-down straps at discrete points, we cast a 150 mm slab across the whole tank farm and place the layerworks on top, so the entire weight of the fill above acts as restraint.

At a factor of safety of 1.5 on buoyancy, the 56,000 ℓ tank governs and needs 593 mm of compacted fill over the base slab. Rounded to 600 mm, delivered as 450 mm G6 and 150 mm C3 at 95% MOD AASHTO, under the surface slab.

Fourteen construction drawings at R00, a full design report setting out the calculations and the assumptions behind them, and a priced Bill of Quantities.

Two items are carried openly as unresolved: available water pressure at the municipal connection is unconfirmed, and pavement design sits with the Pavement Engineer. Neither stops construction starting, and each has a stated point beyond which it must close out.