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Working piles (cast in situ piles)

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Various Methods Used in Constructing Cast In-Situ Piles

The general concept of cast in-situ pile construction remains the same across various methods, with minor differences. Bored pile diameters typically range from 600 mm to 3000 mm, with depths reaching up to 70 m. This provides designers with significant flexibility to customize foundation designs according to site conditions and optimize costs for an economical solution.



cast in-situ piles method of construction:

Casing Installation

The depth of casing varies based on site conditions and soil properties. In the presence of loose soil, the casing length should be increased. The use of bentonite can help prevent soil collapse, reducing the required casing length.

The primary function of the casing is to prevent soil collapse in loose soil conditions or when soil-supporting liquids are absent. It also allows land surveyors to verify pile locations and provides guidance for drilling machines. Casing installation involves drilling a shallow hole with the same diameter as the casing to facilitate insertion using a vibration hammer.

Figure 1



The casing diameter should be slightly larger than the pile diameter and free from distortion. For long casings, joints must be properly and smoothly welded. Before casing installation, any residual or encrusted concrete must be removed.\

Setting Out

The casing location should be verified by a land surveyor and compared with the pile coordinates. The casing must be within 75 mm of the original pile location. If not, it must be lifted and repositioned.

Boring

Boring should commence only after verifying the casing location. Drilling is performed using a rotary rig or other suitable equipment. The drilled shaft depth must be at least equal to the pile depth, calculated as follows

Depth of Drilling = Casing Top Level - Toe Level


Figure 2



Steel Cage Fabrication and Lowering

The steel cage should be fabricated as per the design drawings. The steel must be free from rust, and any rust formation should be thoroughly removed. Concrete cover blocks should be used to maintain the required cover, with the concrete strength of the cover blocks matching that of the pile to prevent spalling or deterioration.

PVC sleeves can be used to confine steel projecting above the cut-off level, facilitating easier concrete breaking in later stages. Overlap length and location must comply with the design specifications.


Figure 3


To minimize distortion, two cranes should be used when lifting the steel cage. The borehole should also be cleaned before lowering the cage. The cage will be hooked to the casing, and the top steel level must be checked by a land surveyor.

Figure 4



Figure 5



Pile Casting

To avoid soil sedimentation, the time between lowering the steel cage and pile casting should be minimized. Tremie pipes must be used for pile concreting and should be free from residual concrete, oil, grease, or any contaminants. The tremie length should be at least equal to the borehole depth. During casting, the tremie must remain embedded in the concrete at all times to prevent segregation or contamination.



Figure 6


Casing Removal

The casing should be removed while the concrete is still fresh. Delayed removal can cause adverse effects, such as concrete cracks or adhesion between the concrete and the casing, leading to potential pile extraction during casing removal. If casing removal is significantly delayed, it is advisable to leave the casing in place and conduct a static compression test to verify pile capacity.

Use of Stabilizing Liquids

Stabilizing liquids, such as polymer solutions, are used when weak soil strata are encountered. These liquids enhance soil stability, increase shear strength, and reduce erosion. The polymer must be thoroughly mixed with water and tested in a borehole before application to ensure effectiveness. It should be introduced into the drilled shaft before reaching the water table to prevent soil caving. Additionally, polymer should not remain in contact with the 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)





Limitations of Cast In-Situ Piles

Minimum Clearance

Adequate clearance must be maintained between adjacent cast in-situ piles during construction. The minimum distance should be at least three times the pile diameter. If a smaller clearance is unavoidable, one pile should be bored and cast first, and the adjacent pile should only be drilled and cast after allowing sufficient curing time (typically seven days).

Drilling adjacent piles too soon can cause damage, including cracking and spalling of the first pile. If multiple piles are bored simultaneously, concrete may flow between them due to soil collapse, forming cavities. Therefore, an adequate waiting period is crucial to prevent damage to existing piles
Figure 7






Tremie Pipe Considerations

The internal diameter of the tremie pipe should be at least 150 mm for a concrete mix with a maximum aggregate size of 20 mm. This ensures a smooth concrete flow. For small-diameter piles, the tremie pipe size must be selected carefully to avoid difficulties during insertion. Additionally, any attachments to the steel cage, such as sonic tubes, must be considered during planning.







Figure 9











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