What is the purpose of shaft grouting for friction barrettes?

For the construction of friction barrettes, some grout pipes are designed at the periphery of the barrettes. Within a short duration (e.g. 24 hours) of concreting of barrettes, the fresh concrete cover is cracked by injecting water. After that, shaft grouting is conducted where the grout travels along the interface between concrete and soil and compacts the surrounding soils which are loosened or disturbed during excavation. The hydraulic fracturing of surrounding soils by grout during the grouting operation generates planes of higher shear strength. The grout would penetrate and improve the strength of soils around the barrettes.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.