# Numerical Analysis for Ground Subsidence Caused by Extraction Holes of Removed Piles
###### Abstract  
Around the world, and especially in Japan, the tearing down of social infrastructure, including civil engineering structures, has been increasing due to the aging of these constructions, which were built during a period of high economic growth, and a decrease in their utilization caused by a recent drop in the population. The number of existing pile foundations being pulled out has gradually risen to a higher number than that of pile foundations being newly installed. However, after the pulling-out of a pile foundation, the mechanical characteristics of the surrounding ground are of great concern due to the existence of the holes that form when the existing piles are removed (extraction holes). In this study, a three-dimensional elasto-plastic consolidation analysis was performed to examine the effect of the extraction holes of removed piles on the static properties of the surrounding ground. As examples of the results of the analysis, if an extraction hole of a removed pile is left as it is, large ground subsidence will occur near the extraction hole of removed pile, especially at the lower part of the clay layer near the hole. The greater the number of extraction holes of removed piles, the greater the compressive stress acting on the extraction holes after the pile removal. Therefore, the filler should exhibit strength early as the number of extraction holes of removed piles increases. existing pile foundation; extraction hole; finite element method; pulling-out method +
Footnote †: journal: Applied  
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Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
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structure, which also causes many problems. In addition, during the real estate transaction, the existing piles, which may not be visible but still remain in the ground, contaminate the ground and represent a hidden trap [4; 5]. Thus, the removal of these existing piles is absolutely necessary. Methods for the removal of existing piles include pulling methods and crushing methods. As crushing methods involve problems with vibrations, noise, and the environment, pulling methods are often used. However, the existing piles may have already broken in the ground or have defective joints. Therefore, problems arise when parts of existing piles are left in the ground after the pulling-out process. The pile tip chucking method [6; 7] is a pulling method that was developed to solve these problems. In this method, the entire length of an existing pile is wrapped and pulled up and out of the ground by the casing. Thus, it is possible to more reliably pull out broken piles, etc. by following this method [6]. In addition, it has been clarified that the strength of the filler used to fill in the holes left by the extraction of the existing piles can be made uniform by performing an improved airlift method using a granular material analysis by the individual element method [8; 9; 10; 11; 12; 13]. However, in the pulling method, as mentioned above, extraction holes are formed when existing piles are drawn out. If the holes are left untreated, it is thought that the earth and sand may collapse during the aerial excavation and the voids in the ground may expand and cause the ground surface to sink [14]. Therefore, it is necessary to fill the extraction holes of removed piles with a filler. With regard to fillers, the filling process was easy and inexpensive in the past, as the holes were filled with mountain sand or reclaimed sand. Recently, however, the fluidization treatment is used due to the lack of reliable filling materials and the unstable strength of these materials. The use of soil and cement/bentonite is increasing.