The key to stable pile foundation - borehole debris control
Release time:2019-01-22

The thickness of soft material or debris that covers the bearing strata is one of the most significant factors determining the load capacity of the resulting cast-in-place pile. If the debris covering the bearing stratum is too thick, it undermines the quality of the resulting concrete.  Thick debris also prevents contact between the end of the pile and the bearing stratum, which reduces the load capacity of the pile. The infiltration of water due to debris also produces buoyancy, which affects the stability of the resulting pile.



 Rules and regulations for debris thickness of different authorities



"Technical Code for Building Pile Foundations"(JGJ94-2008)&"Code for Pile Foundation of Harbour Engineering"(JTS167-4-2012)

Before casting concrete, maximum debris thickness for different piles:
End-bearing pile: 50mm
Friction pile: 100mm
Tension pile: 200mm



"Technical Specification for Construction of Highway Bridge and Culverts Culverts Culverts"(JGJ041-2000)

Debris thickness of friction pile should conform to its design specifications.
If the debris thickness is not specified
Friction pile of a diameter 1.5m: debris thickness  300mm
Friction piles of diameter  1.5m: debris thickness ≤ 500mm
Friction piles of length > 40m: debris thickness ≤ 500mm
Friction piles in poor soil conditions:  debris thickness ≤ 500mm

Debris thickness for end-bearing pile should not exceed its design specification.




"Standard for Acceptance of Bridge and Covert Works in Railway"(TB10415-2003)

Allowance of debris thickness in cast-in-place concrete piles: 
Friction piles:  300mm
End-bearing piles:  100mm


There are many reasons for over thickened debris. One of the most direct reasons is that engineers cannot accurately determine the actual thickness of debris, which leads to insufficient cleaning.

During the construction of cast-in-place piles, equipment such as hydraulic pumps is used to clear debris and remove unsuitable soil from the bearing stratum.  The debris in the borehole is cleaned twice for each construction project.

  • The first cleaning happens after hole drilling when the debris thickness is smaller than 20cm.
  • The second cleaning happens before concrete pouring when the thickness of debris is larger than 20cm for friction pile or  larger than 5cm for end-bearing pile.

Debris thickness should be measured pre- and post-cleaning

Methods of debris thickness measurement




String measurement

Engineers tie weights to a string and sink it to the bottom of the hole. Judging by the string, engineers determine the debris thickness. This method has low accuracy as it involves human judgement.



Visual feed

Engineers lower a video probe to the bottom of the borehole and judges the debris thickness using the visual feed. This method, however, is inefficient and prolongs the cleaning process, rendering it ineffective. It also endangers the life of engineers as it requires an engineer to operate close to the drilled hole.



Shaft Quantitative Inspection Device (SQUID)

Developed by Pile Dynamic Inc. (PDI), the Shaft Quantitative Inspection Device (SQUID) system is a new technology for quantitatively assessing the quality of the bottom surface of a bored pile or drilled shaft. It measures both the thickness of soft material or debris that may be covering the bearing strata, providing force and displacement in numerical and graphical form. The device can also reaffirm the bearing capacity of the bearing stratum.


 Shaft Quantitative Inspection Device (SQUID): measures debris thickness and bearing capacity of the bearing stratum, providing engineers quantitative data.


 SQUID debris thickness measurement data



 SQUID test site

SQUID borehole debris thickness and tip resistance testing sites


Foreign SQUID test site


Fujian Construction Science Academe Xiamen Branch test site




As most cast-in-place pile constructions are held underground or underwater, it poses challenges for in situ engineers to clearly observe and supervise every single aspect of the construction. It is virtually impossible to excavate and the pile after it has been made. Therefore, it is all the more important to conduct strict quality control during the pile's construction to ensure the quality of the resulting pile.


For more information, please contact Earth Products China Limited.

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