According to this technology, casing is advanced by rotary head thorough the casing clamps and simultaneously jacked. Casing consists of a series of rigidly connected sections. As the casing is advanced, the soil inside it is removed by an auger and then a new section is added. Auger is fixed to the end of the Kelly-bar that expands as the borehole is deepened.

Process starts with the advancing drilling of the borehole with casing with a reinforced tip (core bit). The core bit serves for drilling of soil and concrete of primary piles for construction of secant bored piles. The casing in inserted into the soil with a rotary head or the casing oscillator to a depth of 1,5-2,0 m. Further by using a telescopic Kelly bar with a short auger the casing is cleared from soil.

Operations on borehole drilling and soil extraction are repeated every 1.5-2.0 m of casing penetration. When the project depth drilling tool is extracted from the casing, drilling cuttings are removed from the hole, reinforcement cage is installed and fixed and the borehole is filled with tremie method.

Construction sequence

1. Погружение инвентарной трубы с помощью вращателя и трубовкручивающего стола. Формирование скважины под защитой обсадной трубы опережающей проходкой. 2. Погружение арматурного каркаса. Монтаж бетонолитной трубы. Заполнение скважины бетоном. 3. Извлечение обсадной трубы. 4.	Готовая буронабивная свая (формирование оголовка)
1. Погружение инвентарной трубы с помощью вращателя и трубовкручивающего стола. Формирование скважины под защитой обсадной трубы опережающей проходкой. 2. Погружение арматурного каркаса. Монтаж бетонолитной трубы. Заполнение скважины бетоном. 3. Извлечение обсадной трубы. 4. Готовая буронабивная свая (формирование оголовка)

Equipment

To construct piles with this technique Delta Drilling Company uses Soilmec SR-60, SR-65 and SR-80C drilling rigs, which can be optionally equipped with the casing oscillator.

Efficiency of Use

Although this techinique has low productivity it  is used due to following:

  • possibility of pile construction  in different types of soil;
  • absence of dynamic and vibrational impacts on soil during the installation of protective  constructions near existing buildings;
  • modern equipment of the drilling rig allowing to control drilling process, drilling and extract boulders;
  • in the course of drilling direct control of compliance of the characteristics of a soil put in the project is exercised;
  • besides, it allows to avoid  their vypor in a well at the expense of a soil plug and an excessive pressure in a well by means of water;
  • reliable protection of borehole walls against a collapse;
  • filling of a well through a tremic pipe that excludes formation of pins and defects of pile shafts;
  • high quality of filling a well with concrete;
  • possibility of pile base formation which allows to use the bearing ability of a pile.