Wikipedia:
The 240 mm Trench Mortar, or Mortier de 240 mm, was a large calibre mortar of World War I.
Service

The weapon was dismantled for transport, requiring four carts for the barrel, base, carriage and ammunition.
In action, a heavy timber platform was constructed embedded in the ground, on which the mortar base was immovably secured. The mortar carriage sat on the base and could traverse. The mortar barrel and breech were mounted on the carriage which provided elevation.
They were used in the "siege warfare" on the Western Front to destroy enemy strongpoints, bunkers and similar "hard" targets which were invulnerable to lighter mortars and field guns. The US Army handbook described it : "... the use for which it is primarily adapted is in the bombardment of strongly protected targets—dwellings, covered shelters, command posts, entrances to galleries, etc—or in the destruction of sectors of trenches, salients and the like.".[1] Their effectiveness decreased late in the war as German policy changed to a lightly held frontline, hence decreasing available targets, and they became redundant when the war of movement resumed in mid-1918.
240 mm mortar in French use

The mortar was first introduced in 1915 as the Mortier de 240 mm CT ("court de tranchee"). It was a short barreled version which fired a 192 lb (87 kg) bomb for 1,125 yards (1,030 m), using a propellant charge of 1 lb 9 oz (710 g).[2]
Its first major use was in the Champagne offensive of September 25, 1915.
This was followed later by the Mortier de 240 mm LT ("long de tranchée") which was a long barreled version with improved firing arrangement and breech-loaded charge which fired a 179 lb (81 kg) bomb 2,265 yards (2,071 m), using a propellant charge of 2 lb 13 oz (1.3 kg).[2] This appears to be the bomb configuration adopted by USA.
French estimates were 80 bombs needed to destroy a strong shelter with a roof of concrete or rails and concrete.

58 mm Mortars

The Mortier de 58 mm type 2, also known as the Crapouillot or "little toad" from its appearance, was the standard French medium trench mortar of World War 1.
3 types of bomb were available :[1]

• Light L.S. bomb - 18 kg, containing 5.35 kg explosive. 6 wings, hollow plugged tail.
• Heavy D.L.S. bomb - 35 kg, containing 10.0 kg explosive. 6 wings, hollow plugged tail.
• Medium A.L.S bomb - 20 kg, containing 6.4 kg explosive. 3 wings, unlike the other 2 it has a hollow tail which contained the propellant charge, placing thrust closer to center of gravity and increasing range and accuracy. Not yet in common use as at March 1918.

Examples of capabilities :-[2]

• To destroy a 3m x 3m blockhouse, 60-80 D.L.S. bombs or 100-150 L.S. bombs were required.
• To cut a passage through barbed wire defences 40mtr wide x 30mtr deep, 120 D.L.S. bombs or 200 L.S. bombs were required.