Holdfast Training Services Trial Blog

As promised in a previous blog, see below for a synopsis of the key risks, issues and recommendations from the piling on Australia 108

Analysis of issues encountered with bored piles 

The delivery of deep foundations on any large project is always on the critical path. The risks inherent to piling are substantial and failure to identify and mitigate these risks adequately can lead to significant impact on cost, schedule and performance of the piles.  Both the client and contractor have a vested interest to develop and implement an effective risk mitigation strategy to avoid such risks from being realised.  The recurring nature of some of the key issues during piling on Australia 108 signifies they did not occur by misfortune indicating that risk could have been managed better, and some issues avoided.

There are two primary requirements associated with piles: piled foundations must have both the structural capacity and geotechnical bearing capacity to safely transfer the actions from the superstructure to the ground without requiring excessive strain to develop the load capacity.

Risk and mitigation

Risks associated with piling are generally accepted as fitting four categories:

• Risk of encountering unexpected ground conditions.
• Risk to foundation performance.
• Risk to construction productivity.
• Risk of construction defects.

Mitigation strategies generally adopted:

• Geotechnical Design Report. Identification of risk and recommended design solution.  Any residual risk should be identified to ensure it can be mitigated through construction processes.
• Risk Transfer. Design-Construct contracts transfers some risk to the subcontractor.  Costs can always be passed to the subcontractor if they fail to mitigate risk appropriately, however time cannot be recouped once lost due to delay.  The attempt to reduce the overall project delay applies early pressure to a program which will often risk compromising quality.
• Testing pile performance. This verifies that piles have reached their design performance criteria.  Increasing the rate and reliability of testing procedures affords greater design resistance to the piles, increasing the redundancy they offer for the same design effect.
• Construction methodology. This must be simple and specific to mitigate residual risk from design by avoiding any ambiguity or interpretation in construction procedures.
• Technical competence. The most effective way to mitigate risk is being able to recognise early that a risk is materialising as an issue.  A timely, informed decision on appropriate action to mitigate that risk, balancing time and cost, is essential to limit the impact of that risk.

The underlying causes of materialised risk are often:

• Failure to identify the risk.
• Failure to recognise the risk was becoming an issues.
• The risk was identified but inappropriately evaluated.
• The was identified but due to either time or cost incentives, risk mitigation was not applied.

Australia 108

Foreseeable risks on Australia 108

• Difficulty drilling through the basalt in the Northern sector.
• Settlement of Coode Island Silt (clay) relative to the piles creating negative skin friction.
• Necking of the boreholes due to the soft clay collapsing and loss/contamination of polymer support fluid with ground water through gravel layers.
• Ability to core for establishment of pile in siltstone.
• Structural capacity of the pile due to high axial loads and eccentricities.

Issues encountered on Australia 108

• Out of position piles. 35% of 48 bored piles were out of position by more than the tolerance leading to significant rectification measures and redesign.
• Reduced drilling rates through basalt. The basalt encountered was harder and thicker than anticipated significantly slowing progress.
• Voided pile. The voided pile was due to broken equipment abandoned in the borehole while drilling through basalt.  This resulted in additional piles being drilled either side and significantly increasing ground works to gain access.
• Borehole collapse. Either identified during drilling which required additional drilling to correct; or during the concrete pour which risked the performance of the piles and additional testing was required to verify the pile.
• Excessive sediments. Encountered in the base of piles and required significant airlifting prior to pouring, sometimes resulting in concrete pour being delayed a day due to the remaining time on site being insufficient to pour.
• Pile cages placed too low. Cages installed at incorrect RLs required additional breaking back of piles in 80MPa concrete to locate cage followed by rectification of pile and additional welded bar to ensure correct development length of starter bars.

Recommendations

• Identify the high risk piles using probability vs impact of risk materialising. This will focus QA efforts on the right piles.
• Be specific in the construction methodology about techniques to be used to reduce any ambiguity. Ensure it is followed on site.  Measures in the construction methodology are there to mitigate risk.
• Check location of casing of bored piles after drilling before pouring. Out of tolerance piles can be evaluated prior to pouring.  Redrilling of the pile now may have less impact on the program than rectification measures to the structure later. (This might be implemented for the piles identified as high risk).
• Use of GPS in the drilling rig will give real-time information on location, depth and verticality of the borehole allowing early identification of casing shift.
• Appropriate identification and classification of soils strata, specifically rock with regard to location and strength.  On Australia 108, the basalt was not included in the design stratigraphy for the piles.
• If using polymer, slow and steady extraction rates of the drill reduces the likelihood of suction on the boreholes, reducing the effectiveness of the polymer chains used to support the borehole. If collapse is occurring in soft soils, slow the drilling and extraction rate down.
• Keep polymer levels 1-2m above the ground water level to reduce the risk of borehole collapse.
• Good polymer management is key to reducing the sediment within the polymer; especially prevalent when recycling the polymer from one borehole to another. Reducing the sediment pumped into the boreholes, reduces the need to pump out prior to pouring.
• Good polymer management is key to ensuring the polymer chains are effective. Check the length of the polymer chains dripping from the drill on extraction.  Long chains indicate an effective polymer.  Shorter chains indicate that new polymer needs to be mixed in.
• Requirement to check the RL of the pile cage prior to pouring. If necessary, the pile cages can be built up above the level of the polymer to check; or the length and laps of the cages be checked and recorded prior to installation.