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Deciding on the termination point of piles during the construction process is a critical task. The actual pile termination level may increase or decrease and is finalized during the execution of the drilling process.
Pile penetration ratio and chisel energy level are two methods that are used to decide the termination depth of a pile. Through these methods, an optimized length can be specified for the pile under consideration. Hence, these two methods are used for establishing similarity in soil strata.
What are the Termination Criteria of Pile at Construction Site?
1. Pile Penetration Ratio (PPR) for Rotary Piling
The pile penetration ratio (PPR) is used to ensure that the desired rock layer is reached during the construction process. The actual pile termination point is determined based on the energy requirement of the rotary piling rig for a specified PPR.
The PPR reflects the energy in ton-meter required to advance the pile bore of 1 m2 cross-sectional area by 1 cm, as per IRC: 78-2014. With the use of rotary piling, Ganpule and Gupte modified energy concept to derive the following equation for the determination of pile termination point:
PPR=(2⋅π⋅N⋅T⋅t)/(A⋅d) Equation 1
PPR: Pile penetration ratio, kN m/m2/ cm
T: Torque corresponding to RPM, kN.m
t: Time duration in minutes, normally 10 min
A: Pile cross-section area, m2
D: Pile penetration, cm
As the resistance to rock cutting increases, RPM decreases, and Kelly bar pressure is required to be increased, i.e., increasing the torque. This is reflected in the torque- RPM curve, as shown in Fig. 1 and Fig. 2. Table-1 shows typical PPR observed at a particular construction site.
Piling manufacturers may furnish the torque-RPM curve exclusive for the machine used. Torque meters can be placed on the control panels of hydraulic rigs to measure the torque.
Table-1: Typical PPR rates for rotary auger piling rig, with Torque 180 kN m and RPM 10
|Pile penetration ratio (PPR) (kN m/m2/cm)||Type of strata||Rate of penetration (cm/min)|
|750-1500||Highly weathered rock||20-40|
|1500-3000||Moderately weathered rock||10-20|
|300-6000||Slightly weathered breccia||5-15|
2. Chiseling Energy Concept
Datye and Karandikar established the chiseling energy concept for the termination of pile depth. Use the following expression to estimate the response of founding rock strata in terms of Energy Level:
E=(W⋅h⋅n⋅N)/(A⋅d) Equation 2
E: Chisel energy level, kN.m/m2/cm
W: Weight of chisel, kN
h: Fall of the chisel in meters given in every blow (m)
n: Reduction factor, 0.8–1.0 based on likely drag on chisel due to slurry, submerged weight of chisel in high groundwater, winch rope friction, and type of muck (clayey or sandy), based on rock type.
N: No. of blows in the stipulated duration, usually half an hour
A: Nominal pile cross-sectional area, m2
D: Penetration into the rock strata in that stipulated duration, cm
The chisel energy response data can be plotted depth-wise side by side with the nearby bore log and hours of chiseling, as in Fig. 3. The pile termination zone can be decided to optimize the chiseling effort further while also ensuring the pile capacity.
FAQs about pile termination depth in construction site
There are two equations or methods that can be used to decide the termination level of piles during execution, namely the Pile penetration ratio and chisel energy level.
The actual pile termination level may increase or decrease and is finalized during the execution of the drilling process.
The depth up to which a pile-end needs to be penetrated into sound and rock or soil layer is called pile socket length.
Pile socket length is equal to the diameter of the pile times one.