Tolkien himself was involved in the battle of the Leipzig Salient and the storming of the Schwaben Redoubt at Thiepval during the Battle of the Somme in 1916. For my part, I am not aware of any recollections or memoirs from him that would suggest that he was directly involved in combat, though it's quite possible than he simply didn't like to think about it.

This certainly does not mean that Tolkien and his brother officers were not in danger: within the British Army, 12% of ordinary soldiers who served were killed, versus 17% of the officers. Among that number was the Prime Minister's son. In the trenches, officers were no less vulnerable to enemy artillery fire, while on the offensive, the fact that the platoon commander (a Second Lieutenant or Lieutenant) was expected to be first over the top as an example to his men often meant that he would be the first one to be killed. Nor did the danger decrease with seniority: the worst rank to be on the First Day on the Somme was Captain, most likely because company and battalion staff officers frequently had to expose themselves to enemy fire in an effort to reach units spread out across No Man's Land.

Not even the Generals, despite the stereotype, were safe: during the Battle of Loos in 1915, three of six Major Generals in command of the assault divisions on the first day were killed. Their instinct had been to get as far forward as possible to direct their forces at the decisive point and they were caught up in German shelling. The British Expeditionary Force actually had to issue instructions forbidding Generals to get too far forward lest attrition of senior officers became unsustainably high. This apparently only had mixed effectiveness: throughout the whole war, 232 officers of Brigadier rank and above (18.5% of the total) were killed, wounded, gassed, or captured.

Sources:

Gordon Corrigan, Mud, Blood and Poppycock

Martin Middlebrook, The First Day on the Somme

Gary Sheffield, The Chief: Douglas Haig and the British Army