The size of the model and the boundary element model at initial
state are shown in Fig.2. The DOF of the initial state before
overcoring are 13824. The boundary element meshes in detail around
the conical-end (*x*_{1}=500-536.5[mm]) are shown in Fig.4.
Although the length of borehole is several-tens meters in
the practical applications, only 500[mm] part of the length of
the borehole is considered in this analysis because the released strains induced
by overcoring at far distance from the conical-end are
negligible. The length of ovecoring denoted by *L* are chosen
successively as
[mm]. The boundary element
model of the overcoring are shown for some cases, in Fig.5, 6, 7
which respectively correspond to the cases
*L*=7.3,21.9 and
36.5[mm]. The degrees of freedom of the final case (*L*=73[mm])
reached 48384. The released strains at the six measuring
points (12 elements) are shown in Fig.3.

As the numerical aspects for FMBEM analysis, some infinite series are truncated at 10 terms and the maximum number of boundary elements included in a cell are set 200.

As the initial stress field unit stress and are considered. The results of simulations by using the FMBEM are given in Fig.8 and Fig.9, which clearly show how strains develop by the progress of the overcoring.

For this geometrical shape, it may be concluded that the released
strains converge when the overcoring length reaches about 70% of
the diameter of the borehole. This fact means that the reliable data
may be obtained if the overcoring is done successfully
about the length of the borehole diameter at least.