Martin Larisch

Great question. Depending on your ground conditions and the quality of your bentonite fluid, the ground surrounding the pile could be subject to reductions in shear strength due to a potential bentonite layer covering your soil particles. With high quality bentonites and sandy soil conditions, such effects are less likely to have an impact. However, pile load testing can provide some evidence. If you would use bentonite fluid (or water) in swellable clays or shale formations, you should expect surface deuteriation of the clay with reduced shaft resistance compared with the undisturbed ground conditions. For clay soils, polymer systems are more suitable as they won't disturb the clay/ shale at the inside of the borehole.

As a general rule I would always be very careful to transfer loading from 'my' structure onto other structures outside my project. Such things need to be mutually agreed on, prior to any designs. I strongly recommend to get legal and commercial experts involved as early as possible to avoid any potential issues and to cover some potential future scenarios.

The process of concrete placement under water/ fluid is referred to as tremie placement. If the process is carried out correctly, no issues or defects are to be expected.

Dry placement methods shall only be carried out when the excavation is dry (< 100mm of water at the bottom of the pile bore prior to concrete placement). Direct placement with a shute is only permitted if the concrete won't segregate by hitting the reinforcement cage. I would suggest to use a discharge pipe for dry pours (e.g. lay flat tremie or steel tube)

If you refer to the concrete cover zone for bored piles, they depend on your exposure class (how aggressive is your ground and ground water). Typically, cover zones are 50mm to 75mm for bored piles. Plane installation tolerances are 75mm for onshore piles with cut-off levels within 2m below ground level. Vertical tolerances of 1 (H) in 100 (V) should be the industry standard for modern piling rigs. Reinforcement cages are normally parallel to the pile excavation. If the pile was installed at a very mild inclination of 1 in 100, the reinforcement would follow the same inclination as the cage is always parallel to the drilled hole. Consideration of pile installation tolerances is critical for piled walls (e.g. secant piled walls) and for the assessment of additional bending moments from eccentricities.

The preliminary site investigation and soil classification should be done at concept stage. If required, additional investigation should be carried out for the detailed design. The certification of the pile base is part of the drilling/ construction phase and all three activities should be carried out by geotechnical professionals. Ground investigations and laboratory testing before the detailed design stage can greatly assist to create savings in the pile design and construction phase. The knowledge of the groundwater table and it's seasonal variations id important for both, conceptual and detailed design stages.

This should be carried out during the project approval phase. Personally, I have never obtained an approval for fire Ant Management. Please check for your individual project.

Yes, certification is important, especially for the assessment of the founding material at the pile base (get a geotechnical engineer involved for certification). Typically the QA management plan should have a check sheet for every pile to ensure conformity with the design and relevant standards. AS2159-2009 and the ICE piling specifications provide valuable guidance.

I have not seen evidence that polymer fluids increase the shaft friction of piles. If there is solid evidence based on pile load testing, it might be an interesting thing to look at. However, I would be very careful with such studies as they might only apply on a project specific basis and might not work on your project. Pile load testing will provide guidance. Polymer can be used to enhance the performance of bentonite fluids, but the percentage of polymer to be added to the bentonite is low. I know that some specialist piling contractors have experimented polymer/ bentonite mixes. I know of mixes successes and I would suggest to leave it to the piling companies and support fluid suppliers to advise. The general mechanisms of excavation support and material transport are different for both fluids types. Applying them together requires in-depth understanding of the site specific ground conditions and the fluid properties.

I never came across it. I know that drilling fluid supplier have their own methods to 'block' voids with certain additives. Please enquire with the relevant support fluid suppliers.

Very good question. We could use weighted tape measures to assess the 'hardness' of the base. This is still a common method to estimate the thickness of the debris layer at the base prior to concrete placement (not cage installation). Typically the tremie process could be used, to 'flush' the base as the concrete flows down the tremie pipe and displaces the fluid from inside tremie pipe. The fluid that is pushed out of the tremie will flush a thin layer (about 50mm thick) of debris off the base. If the layer is thicker, concrete placement should not commence. However, this criterion is project specific. It is important to use the correct cleaning tools and to ensure that flat cleaning buckets are used which fit the pile diameter. don't attempt to clean a 900mm diameter bored pile with a 600mm cleaning bucket... Pumps or air lifting could also be used to clean the pile base.

In my experience, bentonite is the only fluid to be used for HDD drilling. The bentonite is used for the transport of the drill cuttings and to stabilize the excavation. The mix design of HDD bentonite is different than for deep foundation applications.

Please check with your client or local council. The environmental frameworks could be different for different councils but the general trend moves towards off-site disposal using disposal trucks (sucker trucks). Never attempt to discharge water from piling operations on site unless you have a site specific permit/ approval.

For temporary support fluids, the EFFC/ DFI guideline for drilling support fluids is a good starting point. Good resources for pile design could be AS2159 in combination with textbooks from Poulos, Tomlinson and other relevant authors. 'Australian conditions' are very variable with respect to the geology and the ground conditions must be assessed for every project. There are also online piling courses which provide a good starting point for pile design and construction; please email me for further information.

A deep foundation element or pile is usually between 3-5 times deeper than it's diameter. This ratio can change depending on your regional standards and guidance documents.