Resource
A Practical Guide to Evaluating the Quality of High-Strain Testing Signals
Release time：2025-06-21
High-strain dynamic testing relies on high-quality signals to ensure accurate pile capacity evaluation. This article summarizes key criteria for signal assessment—proportionality, consistency, zero return, and sufficient mobilization—and identifies common issues affecting data reliability. It provides practical guidance to help engineers quickly judge signal quality and improve the accuracy of analysis results.
The reliability of high-strain dynamic analysis results depends on three key factors: the testing instrument, analysis software, and personnel competency. Among these, the decisive factor is the testing personnel with solid theoretical knowledge and extensive practical experience.
Before conducting bearing capacity analysis, qualified personnel must perform a qualitative assessment and correct interpretation of the measured signals. The quality of signals is primarily reflected in whether they return to zero, maintain proportionality, show channel consistency, and whether the test is sufficiently executed.
1. Signal Stability and Repeatability
Under continuous hammer impacts, the signal should demonstrate stability and repeatability.
In practice, issues may arise such as in saturated cohesive soils around the pile, where soil strength may significantly decrease under repeated hammering. This can lead to a continuous reduction in pile bearing capacity and large variations in waveform curves.
2. Proportional Relationship Between Force and Velocity
At the initial stage, the force and velocity curves should exhibit a proportional relationship, although their peak values may not coincide.
This is governed by wave propagation theory. Before the occurrence of upward traveling waves, force and acceleration should be proportional. If there is significant soil resistance at the ground surface, or impedance variation near the sensor installation section, the peaks of force and velocity may not align.
3. Zero Return of Signals
At the final stage, both force and velocity curves should return to zero.
If the force signal does not return to zero, possible causes include:
Insufficient installation pressure, leading to relative slippage during force transmission (requiring re-tightening of expansion bolts);
Plastic deformation or cracking of concrete at the installation location, in which case the sensor must be reinstalled at a proper position.
If the velocity signal does not return to zero, it is generally considered to be caused by low-frequency acceleration components mixed into the signal.
(Field test examples of good-quality data)
4. Adequate Penetration for Full Mobilization
The penetration depth must be sufficient to fully mobilize the pile bearing capacity.
According to ASTM D4945 and international practice, the hammer impact should be sufficient to mobilize the pile capacity.
In engineering practice, a penetration of a few millimeters per blow is typically considered adequate, provided that the mobilized resistance is clearly reflected in the measured signals.
If penetration is too small, the measured bearing capacity will be lower than the ultimate value;
If penetration is too large, excessive disturbance to surrounding soil occurs, causing the analytical model to deviate significantly from actual conditions and leading to large discrepancies when compared with static load test results.
If the velocity curve returns to zero earlier than 2L/c, it indicates that the pile bearing capacity has not been fully mobilized. In such cases, the hammer weight should be increased or the drop height raised for further testing.
Common Abnormal Signals and Causes
1. The testing system is not properly grounded before the test, leading to electrical interference;
2. Sensors are not securely installed, causing oscillation interference;
3. Severe eccentric hammer impact;
4. Concrete crushing under impact;
5. Plastic deformation of concrete preventing signals from returning to zero.
Conclusion
The quality of measured data must be evaluated based on:
1.Proportionality
2.Consistency
3.Zero-return behavior
4.Sufficiency of excitation
Only measured data that accurately reflect actual conditions can produce reliable analysis results.
Invalid Conditions for Bearing Capacity Analysis
High-strain impact signals shall not be used as a basis for bearing capacity analysis under any of the following conditions:
Cracking or severe plastic deformation of concrete at the sensor installation location, causing the force curve to fail to return to zero;
Severe eccentric hammering, where the amplitude difference between force signals on both sides exceeds one times;
Thixotropic effects, where the bearing capacity of precast piles decreases under repeated hammering;
Incomplete four-channel test data.
Related Articles
A Practical Guide to Evaluating the Quality of High-Strain Testing Signals
High-strain dynamic testing relies on high-quality signals to ensure accurate pile capacity evaluation. This article summarizes key criteria for signal assessment—proportionality, consistency, zero return, and sufficient mobilization—and identifies common issues affecting data reliability. It provides practical guidance to help engineers quickly judge signal quality and improve the accuracy of analysis results.
Follow Us
3-5/F, Administration Building, Wuhan Branch of Chinese Academy of Sciences No.1 Xiaohongshan, Wuchang District, Wuhan City, Hubei Province, China
