The agrarian collectives had considerable success despite opposition and lack of resources, as Franco had already captured lands with some of the richest natural resources.
http://en.wikipedia.org/wiki/Spanish_Civil_War

I am not really sure if this is what you want...

The idea is that this was, in effect, a "company" in the military sense, although it was composed of civilians who, in effect, had been "impressed" (or forcibly conscripted) into service, just for this: To dig, and dig, and dig... fortifications.

In fact, the "gangs" of workers were civilians, but they were commanded militarily (and by no less than a "legionario", that is, the quintessential "warrior", which meant (and this was the name of the game) that they were under military control, thus -actually at war- under military LAW. No jokes. No walking away. No idling...

As you very correctly point out, the SOLDIERS in this field are the sappers (in Spain's Spanish, the "arma de Ingenieros"), which is another kettle of fish... and who very probably were scarce enough to keep them at front (that is, under fire if required) duties, and / or for other, more specific -and requiring much more polished skills- like bridge laying, mine clearing... etc.

--------------------------------------------------
Note added at 6 hrs (2007-05-16 02:58:27 GMT)
--------------------------------------------------

Yes, that makes sense. Besides the commanding officer, there must have been some other soldiers (and NCOs), to make sure the minimum standards, and the paperwork, were properly carried out.

And, besides, this was a convenient place to keep somebody who came from the other side for a while, while he learnt the tricks and while he was being vetted, before sending him to a proper military unit. Not that every such case was handled this way (far from it).