We have a failed Bolt in the Concrete foundation where the bolt insert has become free. Bolt is M20. we would like to core drill the existing hole as there is some of the concrete broke away on removal of the threaded insert which is 80mm Long and 30mm in diameter. I am looking for a solution of possible core drill the existing and replace with a new anchor bolt but unsure of what and how we could do this.
Thank you for your question, in this instance we would point you towards BS8539:2012 and their guidance regarding aborted drill holes . Extract as follows:
If the aborted hole is filled with high strength non-shrinkable mortar. If no shear or oblique tension is acting in the direction of the aborted hole, then the minimum distance between aborted and new drill hole should be:
1 x hole diameter for aborted hole depth less than 0.4 x effective embedment depth
3 x hole diameter for aborted hole depth greater than or equal to 0.4 x effective embedment depth
If shear or oblique tension is acting in the direction of the filled aborted hole, then the minimum distance should be at least 1 x effective embedment depth, or 5 x hole diameter, whichever is the greater. This might mean designing and fabricating new brackets or base plates, in which case the anchor spacing criteria of the manufacturer should be taken into account along with the effect on loadings. I can only suggest passing this back to the engineer responsible for resolving the loads acting on the fastenings so they can assess what maybe required as a remedial action. Note it is not recommended to mix post installed and cast in anchors on the same baseplate due to different stiffness's of the systems which may lead to unexpected load distributions on the anchors.