Piled foundations with caps and ground beams

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

1. 1

   Step 1: Establish what is actually under the plot

   These sites have a history and the history is in the ground. The investigation covers the fill and its variability, the depth to a competent bearing stratum, the water level and its tidal response where the site is near water, and contamination, because on a former industrial site the arisings are a disposal problem before they are a spoil heap. Obstruction surveys matter as much as the boreholes: old foundations, basements, tanks, culverts and abandoned services are normal here, and a pile that refuses on a buried mass concrete block is a delay to the erection programme, not just to the rig. The designer sets the foundation strategy from that picture and the specialist contractor prices the risk of what the probes did not find.

2. 2

   Step 2: Fix the pile type and the layout against the plant layout

   The pile type follows the ground, the loads and the setting. Displacement types suit granular fill where noise and vibration can be tolerated; bored and augered types suit sites where obstructions, neighbours or existing operating plant rule out driving. The layout follows the plant layout, not the other way round: every vessel, machine, rack leg and building gets its group, and the groups are set out against the same plant grid the mechanical designer works from. On most projects a single shared setting-out control network is established at this point and used by every trade for the rest of the project, because two networks on a process plant eventually produce two versions of where a column is.

3. 3

   Step 3: Clear obstructions and build the piling platform

   Obstructions are located and removed or worked around before the rig arrives, and the platform is designed and certified in writing as a structure in its own right - piling rigs are tall, heavy and top-loaded, and on a reclaimed site they stand on the worst ground on the project. Access routes, crane stand areas and the later erection platform are all considered together at this stage, because on these projects the same ground has to serve the piling rig now and a very large crawler crane later, and it is cheaper to build it once to the higher standard.

4. 4

   Step 4: Install the piles to the agreed criteria

   Preliminary piles are installed and tested first where the specification calls for them, and the production criteria come from those results. Each pile is then installed to the agreed depth or set, with the rig record kept pile by pile - position, depth, and the installation parameters the specification lists. Piles that refuse short, that come up out of tolerance, or that behave unlike their neighbours are reported to the designer rather than absorbed on site. On a plant site with a hundred separate foundations, the pattern across a group tells you more than any single record.

5. 5

   Step 5: Trim, cap and tie the plot together

   Pile heads are cut down to level, the reinforcement is exposed and cleaned, and the caps are formed, reinforced and poured. Ground beams follow, spanning between the caps and tying the caps into a connected system. Reinforcement congestion at the cap and beam junctions is a real buildability problem here and is normally resolved by the designer on a drawing before it is discovered by a steel fixer at seven in the morning. Concrete for these elements is placed to a pour sequence agreed in advance, because a plant site does not have room for everything to be poured at once and the sequence has to leave access for the next pour.

6. 6

   Step 6: Set the interfaces the mechanical package will use

   The value of a foundation on a process plant is measured at its interface. Holding-down bolt assemblies, pockets, base plates and levelling arrangements are set to the tolerances the specification gives, checked before the pour and checked again after it, and their positions are recorded against the shared control network. Anything the equipment supplier has fixed - a bolt circle, a plinth level, an orientation - is treated as fixed. The recurring failure on these projects is not a weak foundation. It is a foundation in the wrong place by a few tens of millimetres, discovered when a module is hanging on a crane.

7. 7

   Step 7: Cure, survey and release area by area

   Concrete is cured and protected, then each foundation is surveyed and compared against the model. Out-of-tolerance items are resolved with the designer and the equipment supplier before the area is released, because the resolution may be a shim, a re-drill, a change to the module or a re-pour, and only one of those is quick. Areas are handed over formally in the sequence the erection programme needs, with a record of what was released and when.

8. 8

   Step 8: Protect the finished work through the mechanical campaign

   Once the foundations are released the site fills with cranes, transport, scaffold and hundreds of people. Bolt threads are protected, pockets are covered, edges are protected from tracked plant, and the survey control is protected from being knocked. On most projects the civils contractor stays on site through erection to maintain the foundations, the platform and the temporary works, because the plot is in continuous heavy use for months and left alone it degrades quickly.