# Transfer Learning-Based Detection of Pile Defects in Low-Strain Pile Integrity Testing
## 2 Background
The depth of the pile is calculated by the arrival time of the energy. 5. _Cross-Hole Sonic Logging (CSL) [25,26]:_ This method analyzes the travel time and energy of ultrasonic pulses transmitted and received by probes inserted into access tubes installed before concrete placement. These tubes, embedded within the reinforcement cage, are filled with water to ensure proper acoustic coupling with the ultrasonic transducers. 6. _Cross-Hole Tomography (CT) [26]:_ This method is a high-resolution geophysical method used to evaluate the subsurface or structural integrity of materials, such as concrete piles. It provides detailed 2D or 3D imaging of the material's internal properties by analyzing the behavior of seismic waves as they travel between multiple boreholes. CT is similar to CSL but involves additional data collection for attenuation analysis. 7. _Single-Hole Sonic Logging [27,28]:_ Since multiple tubes cannot be installed inside smaller diameter piles, it is recommended to use crosshole sonic logging for bored piles with diameters larger than one meter, and single-hole sonic logging for smaller diameters. The method is generally restricted to detecting defects near the access tube and is typically employed only when a drilled shaft requires an integrity assessment after construction. 8. _Electromagnetic:_ This approach uses electromagnetic waves for evaluating pile integrity. Electromagnetic waves, which propagate for a long distance and are reflected at discontinuities, can be an effective tool for evaluating the integrity of reinforced earth structures. Techniques that use this approach are listed below; 9. _Borelole GPR [29]:_ In borehole radar, an antenna emits radar energy into the surrounding soil and rock, while a receiver captures the reflections generated when the radar signals encounter interfaces with varying dielectric properties. 10. _Time Domain Reflectometry (TDR) [30]:_ This method uses a TDR unit to generate and measure the electromagnetic pulses. The electromagnetic pulse is sent into the pile through an electrical wire installed alongside the main rebar in the reinforcement cage of the pile. The pulse travels through the pile, and reflections occur at material discontinuities, such as cracks, voids, or interfaces. _Electrical Resistivity Method_[31]: This method works by measuring the electrical resistivity of the pile and surrounding materials, identifying anomalies such as cracks, voids, or changes in material properties. Pile material and surrounding soil typically have distinct resistivity values. Defects cause measurable changes in resistivity. A low electrical current is injected into the pile using electrodes. The resulting voltage difference is measured at various points to calculate the resistivity of the pile and the surrounding soil. Variations in resistivity values indicate potential defects or material inconsistencies. _Induction logging_[32]: This technique involves transmitting an AC current into the ground using a source coil, while a separate receiving coil detects the response. The alternating current generates a varying primary magnetic field, which induces eddy currents in conductive materials like the soil or steel reinforcement. These eddy currents produce secondary magnetic fields that induce a voltage in the receiving coil. The amplitude of the received signal helps determine the position of the pile toe. _Thermal Integrity Profiling_[33; 34]: This test measures the released heat and the associated temperature rise due to cement hydration when concrete hardens. This allows an indirect evaluation of concrete quality over the cross-section and the pile length at an early stage. The temperature is usually measured with measuring wires that are attached to the reinforcement and embedded in concrete. Alternatively, sensors for temperature measurement can be lowered into installed ducts. _Radiometry:_ This approach uses nuclear radiation to excite the pile. It is not a preferred approach due to radiation handling and cost.