I might be misunderstanding the question a little, but one answer is that there was no effective way to strike deep into enemy territory to hit a barracks, nor would such action be particularly fruitful (as bombers developed later in the war barracks and bases behind the lines were attacked, but the enemy could counter with their own air defence of fighters and anti-aircraft weapons so it could be costly with limited gain).

There was in fact so little chance of breakthrough that battalions began to independently set up potato and vegetable farms a short distance behind the lines to supply fresh food, taking over abandoned farm land. The (British Army) Directorate of Agricultural Production was officially established in late 1917 to expand this endeavour. Although the surprise German offensive of Spring 1918 that smashed through the allied lines disrupted it - the farms had to be abandoned, the machinery and other equipment was shipped back to Rouen, and the men who had looked forward to spending their war working in the fields sent into the firing line.

Men were 'stored' as part of a vast decentralised system that stretched from all areas of the home country (regimental barracks, training grounds, country homes and estates commandeered for war use etc.), through occupied lands (base depots, supply depots, replacement depots and the like in France) and on towards the battle area ('hutments' 5 miles or so behind the lines, billets in villages and farms, then reserve and other trench lines back from the fighting front). There would be huge numbers of men moving in all directions by many means, and the complex logistics involved are a huge area of study, but they weren't all going in one direction at the same time like an ant trail.

On a smaller scale, and closer to the front line, movement (of troops, supplies etc.) would often be done under cover of darkness, and these movements absolutely were watched and shelled remorselessly. Even these small movements were carefully planned as they were vulnerable to attack - if the enemy knew that the line was packed with men moving in and out of a trench position they had a prime target for shelling. Here's how a single battalion would take over a fire trench (from British Trench Warfare 1917-18: A Reference Manual)

Reliefs – Going into the Line – Before taking over a trench line, the OC, adjutant and MG officer of the battalion, along with at least one officer and NCO from each company, visit the trenches by day. It is advisable for the company officer and NCO to remain in the trenches for a full 24 hours prior to the arrival of the battalion; the snipers of the relieving unit should also be in place for 24 hours before the relief takes place. Packs are taken to the trenches. Waterproof groundsheets are taken, but not blankets. Each man carries 2 sandbags tucked into his belt. All officers should have electric torches. Reliefs usually take place at night. The pace is very slow, and it is advisable for each man to hold the bayonet scabbard of the man in front [like a baby elephant at the circus]. No lights or smoking after a certain point. The platoon being relieved gets on the firestep. The reliving platoon files in behind and halts. On the word ‘Pass’, which will be given quietly and passed along, the relieved and relieving platoons change place.