Large diameters piles
According to the method of execution we differentiate between:
- drilled piles in bentonite mud or with polymers;
- dry drilled piles;
- drilled piles with temporary or permanent casing.
Depending on the bore hole diameter we differentiate between:
- medium diameter piles: from 400 to 800 mm;
- pali di grande diametro: da 800 a 2.000 mm.
The equipment used is:
- rotating rigs composed of a support unit (tracked crane or tower crane mounted on a truck) and the rotary table. The drilling is in rotation with a telescopic shaft, which can be "friction-driven" for loose ground or clay or "locked" for medium to hard compact ground. The telescopic shafts currently on the market allow good depths to be reached (even 60.00 to 80.00 m) thanks to the constituents of the steel used, allowing the torque characteristics necessary to prevent breakage of the elements, above all when drilling in compact ground.
The tools used are generally buckets, augers and core barrel. In special cases, such as with the presence of boulders or rocky layers, also percussion type tools are used such as rock cutters or grabs or the same tools with bits in widia.
In particular types of ground such as gravel, sand or silt, to prevent the collapse of the bore hole walls it is necessary to use temporary steel casing, inserted using the drill itself or hydraulic vibrators operated by a crawler crane or using bentonite mud or biodegradable polymers (the latter are necessary in the case of particular restrictions regarding pollution of water-bearing strata or disposal of the resulting material). The use of bentonite mud requires adequate filtration in the case of very sandy ground, with the use of special "desanders".
- grab rigs composed of a crawler crane with a lattice girder arm which operates a shaft-sinking grab. The equipment is completed by a temporary or permanent steel casing, operated by a column-turning grab or by a vibrator carried by the same excavating crane or another service crane.
Construction materials and elements
- metal reinforcement: formed by prefabricated cages in Fe B 44 k steel, their transverse reinforcement rods are on the outside of the longitudinal rods either in the form of a continuous spiral or rings. The reinforcement cages are provided with suitable spacers in order to guarantee true positioning of the reinforcement and sufficient covering of the rods. The joint between two adjacent reinforcement cages is bound together overlapped.
- concrete: normally concrete has a water to cement ratio of W/C< 0.5 with minimum R’ck equal to 25 MPa .
Means of execution
- drilling: firstly a 3 to 5 m temporary casing is positioned called the upper shaft. In the case of excavation with bentonite mud the mud level must be maintained close to the level of the work and in any case at a level of at least 1 m above the water table.
- positioning of the metal reinforcement: of the above type, paying attention not to place the cages on the floor of the excavation.
- casting the concrete : cusing the above method through a suitable shoot or tremie pipe from the base upwards. The tremie pipes are fitted with a loading hopper at the upper end. Attention must be given to stop its footing 25 to 50 cm from the bottom of the excavation.
- checks during the work: are the checks of the laying and preparation, checks of the excavation procedure, checks of the drilling mud, checks of the reinforcement rods, checks of the fresh concrete, checks of the concrete casting.
- final checks: are the checks of load resistance (load tests), non-destructive tests of pile integrity (tests of mechanical tolerance), non-destructive tests of pile integrity (ultrasound tests).