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Working piles (cast in situ piles)


Various methods used to construct cast in-situ piles. In general, the concept is the same; however, there is a minor difference between these methods. Bored piles diameter ranges from 600 mm to 3000 mm, and the bored pile depth can reach up to 70 m. This provides the designer with great freedom in customizing foundation design according to site condition and optimizing the cost to produce the economical design.







cast in-situ piles method of construction

  • Casing punching

Casing depth can vary depends on site condition and soil properties. Length of the casing should be increased in case of loose oil. Using bentonite can help in preventing soil collapse. The casing length will be reduced. If bentonite used. 
The primary function of the casing is to prevent soil collapse in the event of loose soil and absent of soil supporting liquid. Also, it will enable the land surveyor to verify pile location and providing guidance for drilling machine. Installing of casing involve drilling a shallow hole of same casing diameter to facilitate inserting casing using vibration hammer.
Figure 1



Casing size should be slightly larger than the pile diameter. The casing should be free from distortion. In case of the long casing, joints shall be welded properly and smoothly. Furthermore, all residual and encrustation concrete shall be removed before casing installation.



  • Setting out

Coordination of casing should be checked by a land surveyor and compared with pile coordination. The casing location should be within 75 mm from the original pile location. Otherwise, the casing shall be lifted again and rectified.



  • Boring 

The boring process shall proceed after verifying casing location. Drilling can be performed using a rotary rig or suitable equipment. The drilled shaft length should be at least equal to pile depth. Depth of drilling can be calculated using the following equation

Depth of drilling=casing top level-toe level

Figure 2



  • Steel cage fabrication and lowering

Steel cage shall be fabricated, as shown in the drawing. The steel shall be from any rust. In case rust formed. It shall be removed thoroughly. Concrete cover block shall be used to maintain the required cover. Concrete used in the making cover blocks shall have a similar strength of concrete used for the pile. Thereby spalling and deterioration of pile concrete prevented. PVC sleeves can be used to confine projecting steel above cut off level to ease concrete breaking for the next stage. Overlap length and location shall be as in the drawing.

Figure 3


The duration between lowering steel cage and completion of boring should be reduced to the minimum, and as a good practice, it is preferred to clean the borehole before lowering cage. Two cranes must be involved in the lifting process to minimize the distortion of the steel cage during lifting. 

Figure 4


The final step is the steel cage lowering inside the borehole. Steel cage will be hooked to the casing. Steel top level should be checked by a land surveyor.

Figure 5




  • Casting of pile 

The duration between pile casting and lowering of steel cage should be minimized to avoid soil sedimentation. Tremie pipe shall be used for pile concreting. Tremie shall be free from concrete residual, oil, grease, or any other material to avoid concrete contamination. The length of tremie should be at least equal to borehole length. During casting, tremie must be embedded inside concrete all time to prevent segregation of concrete or mixing with mud.


Figure 6


  • Casing removing 

After completion of casting. Casing shall be removed when the concrete is fresh. Delaying of casing removal can induce adverse effects such as concrete cracks. If removal of casing postponed for a long time, then there is a high possibility pile will removed with casing due to the adhesive of concrete to the casing. In this case, it is recommended to leave the casing without removal and test the pile for the static compression test to verify pile capacity. Thereby it is essential to remove in the casing when concrete fresh. Any delay can result in serious problems.



    Stabilizing liquids

Stabilizing liquid such as polymer used in the event of encountering weak strata. The polymer will increase soil stability, enhance soil shear strength, and reduce erosion. The polymer should be mixed thoroughly with water. The polymer should be tested in a test borehole to verify the liquid performance. Polymer shall be poured to drilled shaft before encountering water table to avoid caving. Polymer shall not in contact with soil for more than 12 hours.


Polymer properties 
Property

At time of slurry
Introduction
In hole at time of
concreting
Test method

Density
995-1018kg/m3
(Fresh water)
1000-1018kg/m3
(Fresh water)
Mud Balance
(API 13B-sec1)
Viscosity
(minimum)
45sec/.95liter
45sec/.95liter
Marshal funnel
PH

8-10
8-10
pH paper/pH meter
(API 13B-Sec6)
MAX sand content
(1% by volume)
1
1
Sand Screen set
(API 13B-Sec4)





 Limitation for cast in-situ piles


Minimum clearance


Sufficient clearance shall be maintained between cast in-situ piles during the construction process. The distance should be more than 3 diameters of the pile. Figure no:7 showing a group of piles and the minimum distance between two piles. In the event of a lesser clearance, one pile shall be bored and cast first. Then the second pile shall be drilled and cast after a sufficient time to allow concrete of the first pile to harden and achieve enough strength (recommended 7 days).
Figure 7


Proceeding with the next pile without waiting will induce adverse effects to the previous pile such as cracking and concrete spalling. If the piles bored at the same time, there is a possibility of flowing concrete between two piles due to soil collapse and forming of the cavity at the soil. Thereby it is strongly recommended to provide adequate waiting period before drilling the next piles to avoid damaging of the previous pile.



Figure 8

 Tremie pipe

Tremie pipe internal diameter shall be more than 150 mm for 20 mm maximum size of aggregate in concrete mix; therefore, the concrete will flow through tremie smoothly. For small piles, tremie pipe diameter shall be selected carefully to avoid clashing or difficulties of inserting tremie pipe during the casting process. Cover blocks and steel cage with any attachment such as sonic tubes shall be considered.





Figure 9










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