Piled foundations with caps, ground beams and plinths

How does Piled foundations with caps, ground beams and plinths work, step by step?

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   Step 1: Take the layout from the hazardous-area decision

   The foundation layout is not drawn first. The designer, the technology supplier and the regulator between them settle which parts of the plant are treated as hazardous and how process, electrical and occupied areas are separated from one another, because hydrogen behaves differently from the fuels most industrial sites are used to. Those decisions fix where the electrolysis packages, the electrical compound, the gas treatment, the storage and the control building can sit relative to each other and to the boundary. Only then does foundation design begin. On most projects the civils designer treats that layout as fixed input and raises any clash back through the technology supplier rather than moving equipment to suit the groundworks.

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   Step 2: Investigate ground that has an industrial past

   The land these plants occupy has usually been used before. Ground investigation on most projects has to find the competent stratum, establish groundwater and any tidal influence, and identify contamination, buried obstructions, old foundations, tanks, culverts and abandoned services left by the previous occupier. Made ground on former industrial land is variable across short distances, so the investigation is closed up wherever the fill changes. The results settle the pile type - driven, bored or continuous flight auger - and they also settle how much of the programme has to be spent removing what is already there. Obstruction clearance ahead of piling is a common and commonly underestimated item.

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   Step 3: Fix one site grid and hold everything to it

   A single primary control network is established across the whole site and every structure is set out from it. Secondary control is established in each area and checked back to the primary network rather than chained forward from the last structure. The specification sets position and level tolerances for pile caps, for plinth tops and for holding-down bolts, and the tightest of those normally comes from the technology supplier rather than from the civils designer. On most projects the survey record is a live document, updated as each element is completed, because the mechanical contractor will design pipe spools and cable routes off the as-built survey.

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   Step 4: Build the working platform and install the piles

   Piling rigs are heavy and top-loaded and they stand on the weakest ground on the project. The platform is a designed and certified structure, laid on a proven formation, inspected while the works run and re-certified when rig traffic damages it. Piles are then installed to the designer's depths and set-out, with an installation record kept for every pile. On a contaminated former industrial site the arisings are a waste stream in their own right, segregated, tested and disposed of under the site waste management plan. Integrity and load testing follow the specification and are agreed before the rig arrives rather than negotiated after a poor result.

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   Step 5: Cast caps and ground beams as one framework

   Caps are cast to the piles, and ground beams tie them into a framework that spreads load and holds the geometry of the plant. Reinforcement is heavy, congested and full of cast-in items, so on most projects the fixing sequence is planned around the items that have to be positioned accurately rather than around the steel. Earthing conductors, service ducts, drainage penetrations and cable-route sleeves are all built in at this stage. Cutting a hole afterwards through a cap on a plant that handles hydrogen is not a small matter, and the designer will normally refuse it.

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   Step 6: Set holding-down bolts to the supplier drawing

   The bolt groups are the interface between the civils and everything that follows. On most projects they are set in fabricated templates or cast-in frames, surveyed before the pour, surveyed again after it, and protected until the package arrives. The dimensions come from the technology supplier's certified drawings, and any revision to those drawings is treated as a hold point on the pour. Bolt threads are covered and the projection is checked, because a damaged thread found on the day of a lift stops a delivery that may have been scheduled for months.

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   Step 7: Form plinths, kerbs and containment to the process design

   Plinths raise equipment clear of the surrounding surface, kerbs and upstands direct spillage and washdown where the drainage design wants it, and containment structures are formed where the design calls for them. The falls, the joint positions and the surface finish all matter more here than on ordinary industrial civils, because the drainage design is one of the mechanisms by which the plant is kept safe. The designer specifies the arrangement; the civils contractor builds it and proves the falls by survey rather than by eye.

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   Step 8: Survey, release and hand over area by area

   Each foundation area is surveyed as built, checked against the supplier tolerances and formally released to the mechanical and electrical contractor before their package is delivered. Release is a documented step with the survey record, the concrete test results and the outstanding-items list attached. On most projects areas are handed over in the sequence the delivery programme demands, so the civils contractor may hand over four separated islands of the site before the ground between them is finished. Completed foundations are then protected, because bolt groups and plinth tops damaged by site traffic are a common and entirely avoidable cause of delay.