A week long artillery bombardment would destroy the German trench system,kill all the defenders,and allow the attacking British troops to take over the empty German defences from where they could initiate a drive on Berlin that would end the war.
The plan failed.The bombardment was largely ineffective,in many places not even cutting the German barbed wire.German troops sat out the bombardment in the safety of stollen,deep underground concrete and steel bunkers designed and built for this purpose.
As soon as the bombardment ended,the German troops were able to man their largely intact trenches and shoot down the advancing British,exposed in No Man's Land,long before they got anywhere near the German trenches. After 4 and a half months,a strip of land 20 miles long and,at its furthest penetration, 7 miles deep had been captured at the cost of 420,000 British casualties and over 200,000 French casualties.
So,the British plan was a disastrous failure.