The images above and below were taken on a couple of larger projects i have been involved with however the majority of domestic drainage is in the great scheme of things fairly shallow, and you are unlikely to be using trench boxes and trench sheets if you are a DIYer.
There is as much danger involved in excavating the first metre (depth) of a dig as there is the next three or four, one of the main reasons is that the majority of things that are likely to spoil your day such as gas, water, electric and cable services will be within that depth.
If you have pulled up on site with an excavator, trench box and banks man you would assume that these hazards have been taken into account it is however the guy with the pick axe and wheel barrow looking to repair a defective gully pot that will not have considered where the water or gas main enters the property.
There are of course strict guidelines as to where and at what depth any service pipe, cable or duct should be buried both outside and inside of the curtilidge of any property, unfortunately the gangs that installed many of these services were paid by the yard and corners were often cut, it is not uncommon to find gas, water and even electric cables concealed within concrete hard standings or within the sub-base that supports tar-mac or flagged driveways.
In the last 15 years there has been a massive installation program with regard to cable communications beneath our highways and footpaths and to some degree these were monitored by various bodies, however once the installation entered the home owners side of the boundary anything went and we find cables inches beneath flower beds and lawns or pushed into joints between flag stones and then covered with sand and cement pointing, we even find the cables lay above ground beneath hedgerows but concealed with leaves and debris. Cutting through one of these cables will not be life threatening but the cost of renewing a section (they apparently can not be repaired so you have to renew complete lengths) will make your eyes water especially if you are not insured.
Another issue with domestic excavations is where to store the spoil, if you excavate a metre deep trench and store the spoil at the side you may as well be in a 1.5mtr deep hole when the trench sides collapse and the spoil slides in. There is an old saying used by the canal navies `dig deep, throw well back` presumably for this very reason.