I have a background in construction safety and casual interest in ww1 history. Trenching with modern equipment, to do things like laying pipe underground, is extremely dangerous. Even if OSHA regulations are followed to a T, accidents are still common.Men have died in trenches being dug less than 4 feet deep. The weight of soil cannot be underestimated. You will suffocate and be crushed in a matter of minutes, if not less. Rescues are overwhelmingly just recovery efforts. Seeing photos of alot of ww1 trenches, especially the British ones, they scream death trap. Was the digging a major source of casualties? Were safety precautions taken past “try to jump out if it starts collapsing”.
How many men were killed from digging itself?
From a British Army point of view, it'd be very difficult to attain that figure, given how inextricable the daily and ongoing task of digging, improving and repairing trench systems was with the actual tactical task of holding an assigned section of line. Note that I'm completely omitting tunneling from this, as a) it's not an area I can provide an answer on, and b) you seemed to be asking more about combat at ground level rather than combat 10 metres underground.
When holding the line and improving the line are viewed to be one and the same thing, the daily reports and returns on casualties from front line units back to Brigade and Divisional staffs inevitably reported enemy action in general terms, then simply listed casualties arising from the day's activities as part of the fact.
To highlight the point between the non-distinction between the difficulties of digging and also holding the line generally (be it "on the line", "in reserve" or "resting"), Richard Holmes quotes a Second Lieutenant Edwin Underhill of his experiences near Ploegsteert Wood near Ypres in 1915 (Underhill would later be promoted and die at Thiepval whilst serving with 8th Battalion Loyal North Lancs during the Battle of the Somme) -
... I took 150 men to do drainage work under the REs on the communication trenches on the left of where we were before we came out. It was an endless, hopeless task. The walls had caved in in places, and as soon as the muck was cleared out it caved in again, and it all had to be done again.... All the time we were working this morning, shells were bursting every now and then. And they were unpleasantly close too.
Basically put, if you were being shelled, and you were buried whilst digging, casualties were invariably attributed to the enemy shell that either directly (or indirectly by weaking the structure and the soil around it) buried the men doing the digging.
How often were men buried due to trenches being blown in or losing their structural integrity due to incoming artillery?
Often enough that such events litter official and unofficial unit histories of British and Dominion units. The unofficial history of the 11th Battalion AIF, "Legs Eleven" by Captain Walter C Belford, refers to either the burying of personnel or the disinterring of corpses by artillery no less than 8 times when describing the experience of the Battalion at one battle alone.
Captain Belford highlights your exact point about just how deadly a volume of earth can be when describing the Battalion's experience at being shelled for the entire day on 24 July 1916.
This shelling was constant and well directed, with many of these shells landed on the newly dug trenches, making large craters and often burying many of the troops. Some of the boys managed to struggle out, others were rescued by their mates; but many who had dug themselves only too well in were completely buried by the great upheavals of earth.
Putting aside the second type of main trench that the BEF used at the start of the war (the largely above-ground revetments that are less relevant to your question), it's also difficult to ascertain casualty figures for trenches built under the ground level because of the method used to build trenches in most tactical scenarios.
Whilst this isn't by any means the only way a "line" was established, two pretty common ways of establishing a trench line were firstly advancing to a point and digging in, with the second being prosecuting an advance and taking over a set of positions or portion of the enemy's line before going through the difficult and brutal process of integrating it into your line and segregating it from the rest of the enemy's line.
In the first method, once a unit was given an order to "Dig in", groups of soldiers in 2's and 3's would proceed to dig "posts", essentially foxholes to provide immediate cover. Once these were dug, the soldiers would proceed to dig laterally to basically join up posts into a continuous line, which would then be integrated into the existing line via the construction of saps (perpendicular trench lines) and communication trenches. Whilst the unit that is digging in is doing so, other sister units that are "in reserve" or assigned as carrying parties that are "in combat" (but were not tasked with the direct assault) are bringing up construction materials - revetments, corrugated iron, wooden boards, frames, pickets and barbed wire - anything and everything that would make the new trenchline structurally sound and defensible enough to occupy in the short term as the tactical situation required. If the situation dictated that a given section of the line would be needed on an ongoing basis, there'd be time to reinforce it as part of the regular cycle of units moving in between support, reserve and front line trenches (new or old) as they cycled through a respective tour of the line.
The second method generally involved the capture of enemy posts (or even parts of trench system that were already established), and clearing the enemy of the immediate area around the captured positions (and ensuring that any counter-attacks were beaten off). Once that was done, the exact same process to convert and integrate the new position as the first method can be undertaken (construction and improvement of interconnecting saps and communications trenches) to integrate the new line.
The reality was that integrating a new stretch of line was a pretty significant undertaking (aerial photos of some portions of the western front look more like a chessboard than a two sets of vaguely parallel trench systems), as was maintaining existing trenches which would be intermittently subject to enemy artillery and mortar fire to varying degrees of frequency and impact depending on how active the sector was at the time. It was for this reason that units in "Rest" or "Reserve" were often doing anything but resting, and were often engaged in backbreaking labour.
In either of those scenarios, the act of digging actually becomes somewhat synonymous with combat or enemy action - under those circumstances it becomes pretty understandable why the distinction of a casualty due "purely to digging a trench" doesn't really get much traction, especially when the need to structurally reinforce trenches that needed to be establish beyond a line of outposts (and indeed these looser tactical arrangements were also often used, especially when holding lines "in-depth") was already very well understood by planners and commanders from the first months of the war.
In summary
Basically it became necessary for assault forces to include carrying parties, engineers, and "troops in reserve" that would essentially work to deliver the necessary structural reinforcement to the lines as the lines were established or consolidated.
Yes, but unfortunately due to the tactical environment that these construction and maintenance efforts were undertaken in, it's nearly impossible to determine what the casualties were for men who died just by the act of digging, mainly because of the often continuous nature of the combat occurring around them whilst they dug. Certainly I've never seen British or Dominion sources make the distinction.
They sure did, Billy. Seriously, that's a subject worthy of it's own post, and one I'm regrettably not as familiar with or have resources to provide an authoritative answer. I'll let someone else answer that one if you're interested.