No chargehands or foremen on site? Seems implausible in a busy multi-trade construction site environment.

'Supervision' is a very flexible concept and relative to the competence of those undertaking the work. Many will work with little or no supervision, but they do need to know what's to be done and how to coordinate, cooperate and communicate with others on the Project.

Managing and coordinating that work on a daily basis and to a (usually) demanding schedule is another matter entirely. As is stress mounting on that unfortunate indiviual if he truly is the only person there with a supervisory capacity.

Remember, a site manager (that being, the site representative of the principal contractor), isn't responsible for directly supervising the work of the contractors working on his/her site. The bulk of the actual supervision of the work is down to the individual subcontractors.

As an example, our policy (when i was PC) was that every contractor on site MUST have a nominated supervisor on site at all times. The actual level of trainin this supervisor had was dependent mainly on the number of guys he/she was supervising, for example...

25+ guys: SMSTS trained "senior" supervisor

5-25 guys: SSSTS trained supervisor

0-5 guys: Biggest and/or ugliest one

Those numbers are just off the top of my head but you get the idea. As for how many site managers we'd need, the actual number of people on the site wasn't necessarily the main consideration. For example, imagine Site A has 200 operatives on site split across 4 different companies. Now iamgine Site B has 100 operatives on site, split across 20 different companies. Site B has half the number of people on site, but would require much more management than Site A. 

Add into that the infinite number of variables (type of work, specialist aspects, number of buildings, site layout, logistics etc etc...) and you'll never be able to pinpoint a manager/operative ratio which will be appropriate for every job.

So far as first aid goes, 50:1 is a common ratio and there's nothing at all wrong with training up a FLT driver, labourer, gateman etc in first aid to make sure the ratio is hit. Many PCs will also insist that contractors with more than, lets say, 25 operatives must also provide first aid cover.

Hope this helps.