Common Challenges in Dynamic Load Testing

Dynamic Load Testing (DLT), utilizing Pile Driving Analysis equipment (such as GPC), is a powerful tool for monitoring the installation of piles. However, like any sophisticated system, it has common issues that can affect data quality. Understanding these challenges is key to getting accurate results.

Data Quality & Sensor Issues

The most critical factor in a successful DLT test is collecting quality data. This relies heavily on the sensors. Common problems include:

• Loose Sensor Attachment: If the strain and acceleration sensors aren't firmly mounted to the pile, they will read spiky data. Sometimes, the drop-in anchors are loose (i.e, were not properly set or punched), then it is impossible to tighten the sensors. The only solution, then, is to re-drill holes and prepare new drop-in anchors.

With GPC software, the program detects loose sensors and displays warning messages on the screen. This is extremely helpful for beginners.

• Sensor Malfunctions: Damaged sensors can produce misleading data. It's vital to perform a system check before each test to ensure all sensors are working properly.

• Bending Effects: Excessive pile bending during driving can still skew the results, making it challenging to perform signal match analysis on the data. Bending is often severe when drop hammers without rigid leads are used. In this case, use 8 channels  (4 sets of GPC sensors) even on small-diameter piles (instead of conventional 4 channels), so that better average data is collected.

Field and Equipment Factors

External factors can also impact the evaluation  of your DLT data:

• Hammer Energy: An underpowered hammer may not be able to mobilize the full soil resistance, leading to an underestimation of the pile's ultimate capacity. Conversely, oversized energy can overstress and damage the pile.

• Incorrect Input Parameters: The DLT requires accurate input values for the pile's material properties (e.g., pile area, wave speed). Incorrect values can lead to significant errors in the calculated force, capacity, and energy. While some of these can be corrected in post-processing, getting them right in the field is crucial for reliable real-time analysis.

With GPC software, there is a “Wavespeed Autodetect” utility to help beginners evaluate the suitable wavespeed. However, this utility is only for Pile Monitoring (where the pile has minimal resistance on initial blows). It should not be used on the restrike test. 

• Soil and Site Conditions: For setup potential soils (e.g., cohesive) or relaxation potential soils (e.g., saturated dense silty fine sand), the soil resistances change with time,  requiring a restrike test days after initial driving.

Interpretation and Experience (Signal Matching)

Ultimately, the quality of a DLT test depends on the operator's expertise in recognizing the above issues, as well as how to identify them quickly and the methods to mitigate them. It is also important to consider that a DLT system can provide a wealth of data. Analysing the data using Signal matching analysis is a critical part of DLT analysis. Programs such as CAPWAP and AllWave are difficult to use and typically present challenges to beginners. Other programs, such as RCAPWAP, are both difficult to use and not as reliable. However, the N_GAPA program of the GPC suite offers extremely solution for beginners to obtain reliable results, yet has all the bells and whistles for the most advanced users to incorporate in complex analyses, including Radiation Damping (SK and OP), Residual Analysis (RSA), linear or non-linear static resistance development, extra toe option, side and toe plugs, additional damping for non-soil resistance, compression and tension slacks, and non-uniform pile cross sections (which can be modelled by either Pile Model or by individual impedance changes.