Understanding the Optimal Termination Point for Piles in Construction

Determining the ideal termination point of piles in construction is a crucial aspect of the building process. This decision is refined during the drilling phase, where factors like pile penetration ratio and chisel energy level come into play. In this article, we’ll delve into these methods to shed light on how construction experts arrive at the termination point for piles.

Pile Penetration Ratio (PPR) for Rotary Piling

The pile penetration ratio (PPR) is a method employed to guarantee the reaching of the desired rock layer during construction. The actual termination point is established based on the energy requirement of the rotary piling rig for a specific PPR. Let’s explore the components and calculations involved in this process.

Pile Penetration Ratio Equation

PPR=2⋅π⋅N⋅T⋅tA⋅dPPR=A⋅d2⋅π⋅N⋅T⋅t​

• $PPR$: Pile penetration ratio, measured in kN m/m²/cm
• $N$: RPM (Rotations Per Minute)
• $T$: Torque corresponding to RPM, measured in kN.m
• $t$: Time duration in minutes, usually set at 10 minutes
• $A$: Pile cross-sectional area, measured in m²
• $D$: Pile penetration, measured in cm

As the resistance to rock cutting increases, RPM decreases, and more Kelly bar pressure is required. This increase in pressure is reflected in the torque-RPM curve. Refer to Table-1 for typical PPR rates observed at a construction site.

Torque-RPM Curve and Observation Table

Typical PPR rates for rotary auger piling rig, with Torque 180 kN m and RPM 10

Manufacturers may provide a torque-RPM curve exclusive to the machine used, and torque meters can be employed for measurement.

Chiseling Energy Concept

Another approach to determine pile termination depth is the chiseling energy concept, established by Datye and Karandikar. The following expression is used to estimate the response of founding rock strata in terms of Energy Level.

E=W⋅h⋅n⋅NA⋅dE=A⋅dW⋅h⋅n⋅N​

• $E$: Chisel energy level, measured in kN.m/m²/cm
• $W$: Weight of chisel, measured in kN
• $h$: Fall of the chisel in meters for each blow
• $n$: Reduction factor (0.8–1.0) based on various factors
• $N$: Number of blows in the stipulated duration
• $A$: Nominal pile cross-sectional area, measured in m²
• $D$: Penetration into the rock strata in the stipulated duration, measured in cm

Chisel energy response data can be plotted against depth, alongside bore log information and hours of chiseling, as depicted. This aids in optimizing chiseling efforts while ensuring the pile’s capacity.

Chisel Energy Response Plot

In conclusion, the termination point determination involves a thoughtful consideration of both Pile Penetration Ratio and Chiseling Energy concepts. These methods, along with detailed observations and measurements, contribute to the successful construction of piles with optimal termination points.