As known, Turkey is located on an earthquake belt on active fault lines. The greatest damage and loss of life is caused by structures on weak and soft soil layers during an earthquake. Therefore, it is necessary to precisely investigate the ground conditions for the structures to be built in such regions. The results should be interpreted and appropriately evaluated by the experts.
The ground should be improved or deep foundation techniques should be used for buildings to be raised on weak soil type zones which might be weak to bear loads from the structure or deform and settle more than the allowed calculation values.
The most common soil improvement method for buildings to be constructed on thick alluvial beds is creating a web of Jet Grout columns in the soil. Eventhough, the jet grouting method can be successfully applied on aggregated soil types. It does not work well on the clay soils or high rise structures. Additionally, jet grout method by itself might be insufficient for lateral earthquake forces and the shear forces caused by earthquake on the foundation surface.
For such soil types floating pile technique used as the most rational and economical method for transferring loads to deeper and stronger soil layers. However, since the loads can only be transferred by the surface tension of the pile, the pile diameters increase and become uneconomical especially for heavier structures.
BST Engineering developed the Jet Grout Coated Pile technique to address the surface tension insufficiency of the piles in weak soil types and lower the construction costs. Jet grout coated pile method combines the CFA pile method with the Jet grouting method by applying jet grout around the pile. The pile in the core has the characteristics of the CFA method. Meanwhile the surrounding Jet grout coating increases the surface tension while improving overall soil strength which in return decreases the required pile diameter. This new method developed by BST engineering significantly lowers the construction costs.
JET GROUT COATED PILE PROCEDURE
(Figure 1 a) The auger designed to pump the concrete from the hollow core as well as the jet grouting nozzles around the edges. This design feature allows the concrete and the grout to pump through the transmission with torque and pressure to the auger head with high pressure. Auger head can be attached with various diameters of double, triple or quadruple nozzles and/or placed with Jet 1, Jet 2 or Jet 3 systems as well.
Auger is drilled in to required depth with pressure. A simple and larger jet grout coating can be applied during drilling if demanded.
(Figure 1 b) The concrete is pumped in and/or jet grouting is applied, once reached to required depth. At this stage the expandable cap is pushed out to pump concrete to the cavity opened by the auger.
(Figure 1 c) The auger is withdrawn with rotation while concrete is pumped from the hollow shaft and jet grouting applied from the drill head once the required depth is reached. This procedure creates a composite pile of concrete core and grout coating. The auger head can be pushed in while pumping concrete and grout to enlarge the pile groove nodes at desired depth. If this procedure is done without pushing the drill stern down a large diameter jet grout coated pile node occurs. (Figure 1 e)
(Figure 1 d) Once auger drill removed from the soil a rebar cage is lowered to required depth to finalize the procedure.
As known, the pile foundations behave as monoblock foundation under proper conditions. With that being said, pile foundations can transfer structural loads to the foundation by surface tension as monoblock foundations. It is allowed by EuroCode 7 (EN 1997-1:2009, BI 7.6.2.1 (4) and DIN 1054:2010, 7.6.2.1) to calculate pile groups as a unified pile block. Additionally, in the USA monoblock foundation characteristic is being assured by various injection methods (concrete, silica injections) for soil improvement of remaining soil between piles for the high rise structures build on pile foundations.
Surface tension and footage are increased on Jet grout coated pile method used foundations which noticeably increases the load capacity while decreasing the settlement and lateral deformations.
Jet grout coated pile method also reduces the construction costs of earthquake resistant structures designed on weak soil types. Also all equipment and machinery designed and manufactured in Turkey.
The load tests conducted on same soil conditions both CFA method and the Jet Grout Coated Pile method with following results;
Soil Analysis:
0-1.5 meter: Sand with large gravel granules
1.5-4.5 meter: Gravel, sand, silt (Sieve 22)
4.5-16 meter: Fine sand layer, silty clayey sand (Sieve11)
Ground water level at 4.5 meters.
First pile: CFA pile; 12 meter long, Ø35 diameter (Piling date: 24.08.2012, load testing date: 18.09.2012)
Second pile: JGCP pile; 12 meter long, Ø35 diameter concrete core, 30kg/mt grout pumped with 450 bar pressure (Piling date: 24.08.2012, load testing date: 18.09.2012)
Third pile: JGCP pile; 12 meter long, Ø35 diameter concrete core, 60kg/mt grout pumped with 450 bar pressure (Piling date: 17.09.2012, load testing date: 26.09.2012)
COMPARISION TEST
As shown above chart the BST Engineering’s JGCP method used 30kg/m grout coating 1.75 times and the JGCP method used 60kg/m grout coating 2.5 times stronger than the standard CFA method.
After testing the piles were dug out for further inspection of the cut sections and soil intrusions. The pile cut section inspections were revealed that the reinforced concrete core, soilcrete (concrete and soil mixture) coating and an external grout and soil mixture coat was cured and intact. Furthermore, the pile hole had grout stems rooted into the soil.
The tests show that the jet grout coated pile method is a successful and superior technique compared to the standard CFA method. BST Engineering continues to research and develop of this method for mass production of the equipment and machinery.
