Found the cracks

After 2 days of digging all foundations is exsposed and revealed the cracks. Below is some picks showing the cracks. Not sure how to do the correct underpinning yet but will get some advice before i procede.

Pic shows the 3 cracks. The middle one shown below i think is the culprit. The outside 2 cracks is fine and i think pulled from the front section. From what i can remember the (now fired) builder started pooring concrete at the back, more or less stopped at where the front crack is then carried on at the back. The concrete have set already when he started pouring again in the front causing a weak joint. I will dig +- 400mm down alongside the foundation to check the soil before i decide how to underpin. Not sure if i should jack the front section yet.

Main crack