There is a general trend that toe defects in bored pile construction have become more serious. Why?

Direct coring method is commonly employed to check the quality and workmanship of bored piles. It is further divided into two main types, namely interface coring (pile/rock interface) and full coring (entire concrete pile length). In the past, the implementation of full-depth coring usually takes place near the pile centre to prevent physical conflicts with reinforcement cage. When placing concrete by tremie method, concrete pouring by tremie pipe is carried out near pile centre and soft and weak materials are usually displaced to the sides of bored piles. As a result, the pile/rock interface at pile centre should indicate the best results. However, with the recent trend of using interface coring through reservation tube attached to the reinforcement cage, there is a high chance that interface coring gives poor results as the reinforcement cage may trap soft and weak materials and they are located far from the pile centre.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.