When the approach to Lime Street was made in the 1830s, most of the cutting was solid rock; but in places the rock face was not entirely stable, so in a few places dressed stone was cemented in place.
It wasn't because of 'overexcavation' because the whole lot was done by hand and you most certainly didn't dig any further than you had to. Later techniques, notably the use of explosive, often took out more rock than necessary and when this happened it would be infilled with a brick lining.
This patching process was repeated later in the 19th century when the cutting was widened to four tracks, except that this time engineering brick was used to consolidate weak points.
Over the years since, various patches have been necessary both to those 'patches' and to include new patches where the solid rock has deteriorated. It's quite a fascinating little picture of stonemasons and bricklayers working over not too far off 200 years to keep the cutting safe.
Paul