Technológie

Deep foundations

Bored piles and barrettes, CFA –piles

Construction methods for bored pile vary considerably:

• Cased piles,
• Piles excavated under drilling fluid, including barrettes,
• Continuous-flight auger piles (CFA piles).

Sometimes it is possible to bore open-hole in the dry if the soil conditions permit.

The method chosen depend on a number of factors including ground conditions, depth and the equipment available:

• auger, mounted on telescopic kelly for discontinuous drilling in a casing or under drilling fluid,
• reverse circulation drilling rig with air lift and casing for continuous pile drilling,
• a continuous flight auger drill for continuous pile drilling,
• down–the hole- (DTH) hammer with direct or reversed circulation with or without casing for continuous pile drilling.

The diameters of the drilled piles commonly range from 300 mm to 3 m or more with use special drills.

Depths of 100 m are also becoming relatively common for inland sites.

Enlarged pile bases are formed using mechanical or hydraulic under ream tools to construct base diameters of up to 4.5 m.

Bored piles are usually of reinforced cast-in-situ concrete. The reinforcement can also be a precast concrete section or a steel section to build retaining wall such as soldier pile walls or as pre-founded (plunge) columns.

Except under very specific conditions, concrete is placed in the wet, using a tremie tube which is either:

• independent and placed after the reinforcement cage has been installed.
• independent and placed prior to installing a precast reinforced concrete or steel section,
• incorporated into the drilling tool, in which case the reinforcement is placed after concreting as in the case of continuous-flight auger piles.

The compressive strength of the concrete is usually between C20/25 and C30/37 (cylinder/cube)

Design priciples

The design is of Eurocode Standards. The geotechnical design is following:

• EN1997-1 Eurocode 7 - Part 1: Geotechnical design –General rules
• InEN1997-2 Eurocode 7 - Part 2 Geotechnical Calculations. Site survey and testing.

Execution

The execution is following STN EN 1536: Execution of special geotechnical work. Bored piles.

Checking of Construction and Testing of Piles

The usual request of clients and designers are non-destructive Pile integrity test (PIT) and Load testing.

Load testing for deep foundation can be divided into static tests and dynamic tests:

• Static load tests are those involving compressive axial loads, tensile loads or lateral loads.
• Dynamic type tests can also be used for determining the load bearing capacity of the foundation, but these are high-energy tests and are used mainly for driven piles.