Although maybe I understand the damage that normal shelling could do, it still doesn’t explain why a mixture of the shells couldn’t be used? Also, the use of shells like mustard gas shells show that they were fully capable of putting this into effect
Smoke shells were definitely used in massive quantities during the First World War, and smoke screens often played a vital role in the success of many attacks. However smoke by itself did not provide enough of a tactical advantage to breakout of the deadlock that was the trench system of the Western front.
It provided a valuable moment for infantry to rush forward across open ground and engage in combat at a closer distance, and it also was used to disorient enemy fire, but was not without its disadvantages as well.
Firstly deployment of smoke often reveals the location of where an attack will take place which would give the opposite side fair warning as to where they should fire their shells next. It also had the tendency to blind counter battery fire which was essential to knocking out machine gun posts and enemy batteries in the rear and on the flanks so that an attack could proceed.
Smoke could also inhibit unit cohesion as during the Battle of the Somme when the smoke in front of 46th Inf Division caused great difficulty in maintaining direction. British batteries also feared of hitting their own men when they could not see through the thick clouds of smoke, and thus did not fire when needed.
Wind was also a factor as just like gas, it could be blown away faster than anticipated, or back onto friendly lines obscuring accuracy.
British artillery used both smoke and shrapnel in their creeping barrage to great effect throughout 1917 - 1918 once lessons of past offensives had been put into place, whereas the Germans lagged behind in the introduction of smoke shells and primarily relied on gas.
Smoke was best used in conjunction with short pre sighted, accurate and very intense bombardments of smaller areas mixed with shrapnel shells hoping to catch the Germans by surprise. Teams of infantry would storm the position with grenades while the barrage would begin landing shells on German areas rear of the position to prevent reinforcement from quickly interfering. High explosive shells would be used in a counter battery role to suppress enemy machine guns and artillery which could stop the British attack.
Sources:
World War I Battlefield Artillery Tactics by Dale Clarke; 2014
The Field Artillery Journal - The Development of Artillery Tactics 1914 - 1918; 1922
Somme 1 July 1916 by Andrew Robertshaw; 2006
In WW1, the creeping fire tactic was used to obscure the advance of troops in no mans land
The main function of the creeping barrage was not to obscure the attacking troops, but to force the defender to stay in their dugouts, or at least at the bottom of their trenches and foxholes, unable to fire at the attacks. The barrage was not a thin curtain of fire, but initially covered the portion of no man's land where the enemy might have outposts and at least as far as the first enemy trench line. For example, in the Battle of the Somme, British creeping barrages were typically about 400 yards deep; since the German lines were usually within 350 yards of the British lines, this kept the German first trench line under fire until the final lifting of the barrage past them, just before the attackers would reach their line.
As the attackers reached the first line and, hopefully, captured it, the barrage would stop an enemy counterattack from their 2nd trench line (both it and the ground between it and the first line should be covered by the barrage).
For more on the creeping barrage and its development and use by the British, see