The CFA technique involves drilling pile holes using continuous flight auger and combines advantages of driven cast-in-situ piles without soil excavation with versatility of bored piles. This drilling method allows constructing piles in a wide variety of soils, dry or water-logged, loose or cohesive as well as in soft rock formations, fuff, limestones, sandstone, etc. No shocks or vibrations are induced when the work is performed. The equipment is fully sound- proofed in accordance with law requirements, which makes it possible to operate in city centres.
1. Drilling with auger string to target depth
2. Extraction of auger string with simultaneous injection of pressurized concrete
3. Insertion of reinforcement cage using vibratory driver
4. Pile with protruding reinforcing bars
Execution of the piles without soil compression allows to work close to the residential buildings. The reduced volume of soil brought up to surface by the flight auger means less expenses for disposal of excavated material, which has to be disposed. The range of diameters from 450 to 1200 mm and depths up to 32 m, allows to overcome problems related to design and execution of bored piles.
Delta Drilling Company applies CFA technique and carries out drilling in complex soil conditions owing to drilling rigs which allow to increase the penetration rate depending on the soil type and its resistance properties. The company uses Bauer (Germany) and Soilmec (Italy) drilling rigs
Drilling is carried out using hollow stem auger with starter bit at the lower part fitted with 2 coils and cutting tool. A special plug fitted with o-ring and fixed by steel chain to prevent the soil from entering the hollow stem of auger. This plug is removed when injecting the concrete. CFA piles are drilled with partial soil excavation and simultaneous lateral soil compression that increases the final load bearing capacity of a pile. The CFA equipment is mounted on the rig and mainly consists of a continuous flight auger, the length of which depends on the required pile depth.
CFA Pile Formation
When the auger reaches targeted depth, concrete is poured by concrete pump, which is connected by hoses with swivel on the extended auger leader. Pressure generated by the pump while the concrete passes through hollow stem auger and the concrete is poured into the borehole. At the same time the auger is extracted with or without rotation leaving voids in the borehole.
The concrete used for the CFA piling is generally prepared with cement, gravel aggregates of fine round gravel (5 to 20mm size) and sand. The cement contents vary from 350 to 450 kg/m3 with water/cement ratio equal to about 0.45. The slump should be maintained between 19 and 21 cm. Depending on operating conditions the use of concrete additives may be suggested.
CFA Pile Reinforcement
CFA piling method requires reinforcement to be inserted in the shaft with a vibro-driver the after the auger is extracted and concrete is poured in, but no vibro-driver is needed for 16-18 m reinforcement cages. Construction of CFA piles involves installation of a reinforcement cage in the fresh concrete. The reinforcement cage must be fabricated as per design specification. Outer diameter should be smaller than the standard auger diameter to ensure sufficient isolation. Plastic centralizers are attached along the entire cage length. Cone-shaped cage bottom is obtained by using last ring with smaller diameter than the standard one. Today it is possible to install 30-meter long reinforcement cages if concrete of proper quality is used.
The following parameters can be determined and evaluated based on the equipment used for construction of augercast piles: auger rotational speed and torque, rate of drilling (auger penetration into the soil), concrete injection and auger extraction.
In the past the quality of constructed piles was controlled by site supervisors who were guided by codes and standards for borehole drilling. Now the time at the jobsites is monitored with automated systems. Although these systems can provide valuable data, they cannot replace skilled professionals. Automated quality control techniques are based on grout (or concrete mix) volume and pressure measurement. Parameters that can be measured with standard automated systems:
1) time, depth and pressure in the hydraulic system while drilling;
2) time, pressure, volume and grout(concrete mix) injection pressure while concrete is poured into the borehole.
During CFA piling various diagrams of real-time processes are displayed on the monitor in the drilling rig ( this enables to make adjustments immediately if required). Thus each pile has an electronic data sheet.