What are piled foun­da­tions?

If stan­dard strip or trench fill foun­da­tions aren’t suit­able for your site, deep piled ones may be needed in­stead. Tim Do­herty takes a closer look at how these work

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On sites where stan­dard strip or trench fill foun­da­tions aren’t ap­pro­pri­ate, pil­ing may be the best so­lu­tion

If you have ever won­dered how tower build­ings stand up, then part of the answer could be piled foun­da­tions. For in­stance, the world’s tallest build­ing, the Burj Khal­ifa in Dubai (829m high) used 45,000 cu­bic me­tres of con­crete to con­struct 192 piles, each buried 50m into the ground. These columns are driven into the earth to trans­fer the weight of the build­ing through less sta­ble sub soils un­til suf­fi­cient weight bear­ing strata is en­coun­tered.

Whilst self built homes in the UK are un­likely to be more than three to four storeys, the same prin­ci­ples can ap­ply, mean­ing piled foun­da­tions are of­ten the ideal en­gi­neered so­lu­tion.

When are piled foun­da­tions needed?

You’ll need a qual­i­fied as­sess­ment or a de­tailed soil in­ves­ti­ga­tion to let you know ex­actly what to ex­pect in terms of the best foun­da­tion so­lu­tion for the ground on your site. Gen­er­ally speak­ing, the type of land con­di­tions that could give rise to pil­ing in­clude: Un­con­sol­i­dated filled ma­te­ri­als, ar­ti­fi­cially im­ported, which are very dif­fi­cult to prop­erly com­press. Nat­u­rally formed and highly com­press­ible sub­soils, like peat. Low bear­ing ca­pac­ity sub­soils which could in­clude wet clay or loose and un­sta­ble sand.

High water ta­bles that are ca­pa­ble of flush­ing sub­soils, which leads to the risk of ero­sion and/or sink holes. Sub­soils with a propen­sity for vol­ume change, such as highly shrink­able clay.

How do they work?

If you are un­lucky enough to find one of the above con­di­tions on your site, then your first port of call should be a civil or struc­tural en­gi­neer. Your build­ing might have been de­signed with or­di­nary strip or trench fill foun­da­tions in mind, but the line loads and build­ing weight are easy for the en­gi­neer to cal­cu­late from your ar­chi­tec­tural plans. How these are then trans­ferred into the ground will be based on a de­signed num­ber of piles, their depths, widths (di­am­e­ter), type, spe­cific lay­out and how they are all ul­ti­mately con­nected to­gether.

For do­mes­tic projects there are essen­tially two dif­fer­ent types of piles. Those that trans­fer load di­rectly to solid strata are end bear­ing piles. Where solid strata can­not be re­lied upon, there’s a type that trans­fers load through shear strength (the over­all sur­face area of pile and sub­soil con­tact un­der­ground), known as fric­tion piles. Your en­gi­neer’s job is to de­liver the most ap­pro­pri­ate de­signed so­lu­tion based on your soil con­di­tions us­ing the most prac­ti­cal in­stal­la­tion tech­nique in terms of how much it costs. Ac­ces­si­bil­ity and the prox­im­ity of neigh­bour­ing build­ings will also play an im­por­tant part in the de­sign.

Re­place­ment or bored piles

Piles can be ei­ther bored or driven into the ground. Bored types phys­i­cally re­move the sub­soil spoil, cre­at­ing a cylin­dri­cal tube into which con­crete can be poured, so the tech­nique is ef­fec­tively re­plac­ing sub­soil in favour of con­crete. It is of­ten used on sites that are in close prox­im­ity to other build­ings as the ex­ca­va­tion process is more con­trolled, not so noisy and causes much less ground vi­bra­tion.

Bor­ing is fre­quently used for fric­tion piles as this method suits co­he­sive and firm soils. How­ever, it’s an ex­pen­sive op­tion be­cause of the var­i­ous on-site pro­cesses in­volved. Fric­tion piles could need tem­po­rary cas­ings, ex­ca­va­tion, man­age­ment of the spoil, con­crete pour­ing, re­bar cage pro­duc­tion, in­ser­tion etc, plus all of the associated equip­ment re­quired for han­dling each process.

In terms of ex­ca­va­tion meth­ods for bored re­place­ment pil­ing, there are fur­ther choices to make. The first op­tion is ro­tary drilling with a wide bladed auger head, which can re­move the spoil as the drill pen­e­trates the ground in a twist­ing mo­tion (think of a flat bladed cork screw). The sec­ond would be a per­cus­sion borer that uses a ham­mer ac­tion to drive a tube with a cut­ting blade through the ground, which then fills with spoil for in­ter­mit­tent ex­trac­tion. The third is a flight auger, which has a con­tin­u­ous helical cut­ting blade that re­moves the full depth of the pile spoil in one process. All ex­ca­va­tions may need tem­po­rary tube lin­ings to help keep the pile ex­ca­va­tion from in­wardly col­laps­ing (de­pend­ing on sub­soil type) un­til the con­crete is poured. These lin­ings would be re­moved be­fore the con­crete has cured.

Once the ex­ca­va­tion is com­plete, con­crete is poured into the pile and then (usu­ally) a re­in­force­ment cage is driven into the wet con­crete be­fore any tube lin­ings are then with­drawn. Al­ter­na­tively, the re­bar cage could be in­serted be­fore the con­crete is poured in, but with a suit­able slump and/or vi­bra­tion to en­sure all po­ten­tial voids are prop­erly filled.

The flight auger tech­nique can also in­clude grout in­jec­tion, where the con­tin­u­ous auger blade has a hol­low tube down its core through which the con­crete can be poured. Once the flight auger has reached its depth, con­crete is poured as the auger (and spoil) is with­drawn all in one process, which may re­move the need for any tem­po­rary pile cas­ings.

Where pile di­am­e­ters are gen­er­ally quite wide, some­times a ben­tonite slurry will be used (ba­si­cally a mix­ture of clay and other semi-hard­en­ers) in or­der to tem­po­rar­ily pro­tect the pile shaft sides from col­lapse un­til the con­crete can be poured.

Dis­place­ment or driven piles

The al­ter­na­tive to bor­ing would be dis­place­ment piles, which are driven into the ground with hy­draulic rams and ham­mers. There is no ex­ca­vated ma­te­rial to man­age and in­stead the pile dis­places the sub­soil as it trav­els be­low ground. Some pros con­sider this to be a cheaper op­tion and with guar­an­teed re­sults, as the piles are man­u­fac­tured off-site to pre­de­ter­mined lev­els of qual­ity. For shal­low depths, piles could be tim­ber (much of Venice is founded on tim­ber piles) but con­crete or steel are more likely, cer­tainly for deeper depths.

Con­crete piles are man­u­fac­tured in spe­cific lengths (gen­er­ally up to around 10m), with the bot­tom one fit­ted with a spiked cap in con­crete or steel. Sec­tions are then jointed to­gether with a splic­ing col­lar un­til the full pre­de­ter­mined depth is reached.

Al­ter­na­tively, some depths may be de­ter­mined as suf­fi­cient enough when the ef­fort re­quired to drive the pile be­comes too hard, con­firm­ing that the bear­ing ca­pac­ity of the sub­soil had been reached.

The piles are driven by per­cus­sion ac­tion, which sets the speed and fre­quency of weighted blows to the pile top (this must be pro­tected by a hel­met to pre­serve its in­tegrity).

Steel piles come in a va­ri­ety of sizes; some are cylin­dri­cal and these will usu­ally be filled with con­crete. Oth­ers could be H sec­tion (like a universal col­umn) or of a box shape.

Pile caps, ring beams & floors

What­ever pile type has been se­lected, it will need to be in­te­grated into the build­ing’s su­per­struc­ture at ground floor level. Caps are built at the top of each pile in re­in­forced con­crete, which are then tied to­gether via a se­ries of re­in­forced con­crete ring beams. These then sup­port ei­ther an in­te­grated re­in­forced con­crete slab or a sus­pended floor of one kind or an­other. On slop­ing ground the pile caps might end up sit­ting quite high out of the ground and this re­sul­tant void will then be ven­ti­lated just like any other sus­pended floor.

How much will it cost?

There are no stan­dard costs for piled foun­da­tions as the price is al­ways cal­cu­lated on a case-by-case ba­sis, and must be cal­cu­lated ac­cord­ing to spe­cific de­tails and re­quire­ments pro­vided by the en­gi­neer. How­ever, deep 3m trench fill foun­da­tions can be very ex­pen­sive and pil­ing is of­ten con­sid­ered a se­ri­ous al­ter­na­tive at these depths for both en­gi­neer­ing and fi­nan­cial rea­sons.

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