Methods for Inspecting Pile Tips

The condition of the tip of a bored pile directly influences the behavior of the foundation and the main structure. If the pile base has an excess of sediment or deficient concrete, it is likely to experience settlements greater than anticipated and even structural failures.

Inspecting the pile base is not a simple task, and currently, there is no precise system that allows us to accurately determine the quality of the foundation base. However, some methods and tests can help us determine, mostly qualitatively, the condition of the pile tip. Let’s review them below:

🔹 Pre-concreting inspections of the pile:
✓ Plate impact method.
✓ Ding Inspection Device.
✓ Bottom inspection with a camera.

🔹 Inspections of the completed pile:
✓ Sonic transparency.
✓ Sampling and coring.

Plate Impact Method

This system, developed by the company Arup and used for over 20 years in the UK, involves lowering a square steel plate, weighing 1 kg and measuring 100x100x20 mm, to the bottom of the excavation using a cable. The test is based on qualitatively judging the condition of the base depending on the type of impact felt when the weight touches the pile base. The impact is assigned a numbered category from 1 to 5, as shown in the table provided.

Categories 1 to 3 are acceptable, while categories 4 and 5, which indicate significant sinking of the weight, indicate possible defects in the tip. For these last two categories, it is advisable to repeat the cleaning of the pile base and perform a new contrasting test.

This method, although qualitative, is easy to use and has been successfully employed in numerous projects, especially in piles with injected tips to assess the correct injection. It has also been recently used to evaluate the cleanliness of the base of diaphragm wall panels.

You can find the complete description of the test in the document ‘ICE Specification for Piling and Embedded Retaining Walls (SPERW 2017)’.

Ding Inspection Device

The Ding Inspection Device (DID), created by John Ding, consists of an upper plate, three legs, a position sensor, and a lower plate.


Source: http://dmy-inc.com/

When the inspection device is inserted into the borehole and reaches the bottom of the pile, the three legs penetrate the existing sediment due to the weight of the apparatus, while the lightweight lower plate rests on the bottom surface. The distance between the lower plate and the position of the tips of each leg is interpreted as the thickness of the sediment deposited.

John Ding himself explains all the details of the system in the following video:

Bottom Inspection with Camera

The inspection of the pile base can also be carried out using a video camera and a light source. The success of this operation depends on the clarity of the drilling fluid and the operational limits of the camera. Generally, this method only works in a clear medium (clean water or air). In a pile executed with drilling mud, the images are usually not clear enough to draw conclusions about the condition of the excavation base.

Other more sophisticated methods use a probe with an integrated camera, where water or mud is evacuated from the device, filling the camera with gas. This provides much clearer images of the excavation. The inspection with camera method allows distinguishing anomalies and significant defects.

The lower photograph, taken from the document ‘Drilled Shafts: Construction Procedures and LRFD Design Methods’ by the American FHWA, shows a team of such characteristics (referred to as SID: ‘Shaft inspection device’).

Source: FHWA

Sonic Transparency

The cross-hole sonic logging (CSL) test involves monitoring the time it takes for a pressure wave (also known as a sonic wave) to propagate from an emitter to a receiver along the entire length of a pile. The emitter and receiver probes descend simultaneously from the top to the base of the pile, providing information about the integrity of the element.

Source: Ministerio de Fomento – Ministerio de Medio Ambiente (2006). “Recomendaciones para la ejecución e interpretación de ensayos de integridad de pilotes y pantallas in situ”

To perform the test, several tubes must be installed, attached to the reinforcement, to allow the circulation of the probes. The number of tubes required depends on the diameter of the pile but should never be less than 3. The test is carried out at least 7 days after the pile has been cast.

A complete sonic transparency test consists of all the logs obtained from the tests conducted with pairs of tubes, and it can even include a 3D modeling of the pile.

Since the tubes through which the probes circulate do not reach the bottom, this test does not provide a complete image of the base of the pile. However, it allows us to identify anomalies and significant defects. If the logs obtained during the test suggest anomalies at the base, it is advisable to perform a contrast tip sounding to confirm the possible defect and its magnitude.

Sounding and Core Extraction

Sounding with core extraction at the bottom of the pile is a direct method to determine the quality of the concrete-soil contact. It can be done through one of the sonic tubes, provided that at least one of them has a diameter of approximately 4 inches, or by drilling from the pile head. In the latter case, difficulties may arise in reaching the bottom of slender and deep elements.

In tip soundings, it is crucial to correctly select the type of drilling tool and the sounding equipment to avoid distorting the contact and thus obtain accurate conclusions about the quality of the pile tip.

Pile sounding

If the sounding reveals the presence of a defective contact, additional holes should be drilled to clean and inject the pile tip.

In conclusion, a range of inspection methods can be employed to assess the tip of a pile. Pre-concreting inspections include the plate impact method, Ding Inspection Device, and bottom inspection with a camera. These techniques help ensure the quality and integrity of the pile before it is concreted.

Once the pile is completed, inspections can be conducted using sonic transparency and sampling and coring methods. These approaches provide valuable insights into the pile’s integrity and help identify any potential issues or anomalies.

By utilizing these inspection methods, engineers and construction professionals can enhance the safety and reliability of pile foundations in various construction projects. Thorough inspections play a crucial role in maintaining the structural integrity of the piles and ensuring the overall stability of the project.

References:

  • ICE (2017). “Specification for Piling and Embedded Retaining Walls”.
  • DMY website – http://dmy-inc.com/.
  • FHWA (2010). “Drilled Shafts: Construction Procedures and LRFD Design Methods”.
  • Ministerio de Fomento – Ministerio de Medio Ambiente (2006). “Recomendaciones para la ejecución e interpretación de ensayos de integridad de pilotes y pantallas in situ”.