A post from @new3.2 regarding Trench Mortars lead to some research I had meaning to do for while, understand the development of Trench Mortar capability. The initial arrangements were very much ad hoc, both in terms of the ordnace and the organisation,  as the British Expeditionary Force transitioned to positional warfare. The ordnace and the organisation developed over the period of the war. 

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Mortars were first used in the 15th century and continued in use into 19th century. They were used in the Russo-Japanese  war where the Germans observed their use during the Siege of Port Arthur. This lead them to develop the  Minenwerfer mortar which was used throughout World War One. 

1914

When Britain declared war in August 1914 there were no mortars in service with the British Expeditionary (BEF) when they mobilised. [1]. As the war transition from one of mobility to position on the Aisne and in Flanders, an operational requirement for mortars was identified. [1]. On 28th September 1914 the War Office Siege Committee proposed a special trench ordnace. [3]. On 20th October, GHQ BEF submitted a request for trench ordnance. [3].

Initially ad hoc arrangements were made to bring trench mortars into service. 

In December 1914, 40 x Coehorn Mortars (ancient black powder) obtained from French. Nicknamed "Toby Mortars" after the officer who suggested acquiring them. [2]

Soldier of Ox and Bucks LI with an ancient trench mortar ('Toby' mortar)

Experimental mortars arrive in France, but proved to be inaccurate and likely to premature explosion. [2]

Trench mortar demonstration at the Royal Engineers Yard, Armentieres. 30th December 1914

Source:  IWM Q 51562

1915

Other initiatives included the the production of 12 x 4 inch mortars made from bored out 6 inch shell and 20 x 3.7 inch mortars designed by Indian Corps. [3]  

In January 1915 Vickers introduced a 1.57 inch trench howitzer. [3]

             

Source: WM ORD 74

A major development in January 1915 was the invention of a 3 inch mortar by Wilfred Stokes, a weapon system which would bear his name. [2]

Wilfred Stokes with Mortar

In March GHQ BEF accepted into service the 2 inch mortar. [3] The mortar was nicknamed the "Toffee Apple" or "Plum Pudding " as a result of the ammunition it fired. It consisted of a wooden staff which fitted into the mortar's barrel, on top of which was mounted a spherical bomb.  It could fire a 51 lb (23 kg) high explosive bomb at minimum range of 100 yards (90 m) out to a maximum range of 570 yards (520 m). [4]

2 inch Mortar - "Toffee Apple / Plum Pudding"

 

In April 1915 Trench Mortar Battery (TMB) establishments were authorised. [1] The Second Army established a Trench Mortar School at Berthen, 10 miles (16 km) south west of Ypres. [6]

By July 1915 the ordnace in France available to equip the Trench Mortar Batteries were  127 x Vickers 1.57 inch trench howitzers, 25 x 2 inch mortars , 125 x 3.7 inch mortars and 40 x 4 inch mortars. [3]  In August 1,000 Stokes Mortar were issued. [2] The 3 inch Stokes Mortars fired a 10 lb 11 oz (4.84 kg) high explosive bomb out to a range of 800 yards (731 m). [5]

3 inch Stokes Mortar

A 4 inch Stokes Mortar was introduced which fired smoke and gas shells. 

In December 1915 it was decided to classify mortars into Light / Medium / Heavy Mortars. [2] The light mortars, 3 inch Stokes mortars would be manned by infantry units. The Gunners would man the medium mortars, 2 inch "Toffee Apple" / "Plumb Pudding" and heavy trench mortars, 240 mm French mortars. [2] The allocation would be 2 x light TMB and 1 medium TMB per Brigade, 1 heavy TMB per Division. [1].

1916

In early 1916 a the appointment of Divisional Trench Mortar Officer (DMTO) was established. [1] Medium TMB's were numbered in relation to the Brigade they supported eg the145th Trench Mortar Battery supported 145 Brigade (part of 48 (South Midland) Division). [11]

145th Trench Mortar Battery

The heavy TMBs would be initially equipped with French 240 mm heavy trench mortars. [1] The Mortier de 240 mm fired a 180 lb (81.6 kg) high explosive bomb out to a range of 2,265 yards (660 m). [7]. 

Mortier de 240mm

Mortier de 240mm

In March 1916 GHQ issued Medium & Light Trench Mortar Batteries - Policy and Organization. [8] It outlined how the light and medium mortars would be used.[9]

7. Principles underlying above Organization

The tactical principles on which this organisation is based are:-

- The Medium Mortars are weapons of position, unfitted by their own weight and the wight of their ammunition to accompany Infantry in the actual assault, though they will move forward in support as soon as possible.

- The Light Mortars are especially suitable for sending forward with the Infantry in the assault, and it will usually be advisable to hold at least some of them in hand for this purpose.

The medium trench mortars would come under control of the Divisional Artillery, the light trench mortars would be controlled by Infantry Brigades. [2] The TMBs were redesignated names and numbers.  The medium TMB's were designated X, Y and Z together with the Division. [2] For example in the 48th (South Midland) Division, the 143rd / 144th and 145th TMBs became X/48, Y/48, Z/48. The heavy TMBs would be designated V eg V/48. 

eg X2 / Y2 / Z2 and heavy TMB, designated V, eg V/2. [2]

Y/2 TMB - Y Medium TMB 2nd Division  Source: @Y2 TMB

The Light Stokes batteries under each Brigade took their number from the Brigade eg 102 TMB [2]

102 Light Mortar Battery Tyneside Scottish

 The 240 mm French mortars proved to be  be unsatisfactory. [1] It was replaced by a modified version of the Mortier de 240mm which was produced under licence by the British and designated 9.45 inch heavy mortar. It differed from the French version by loading the charge through the muzzle, rather than a brass case into the breech. It was nicknamed  the "Flying Pig". [10]  The 9.45 inch heavy mortar fired an 152 pounds (69 kg) high explosive bomb to a range of 2,265 yards (2071 m). [10]

In June 1916, 30 x 9.45 inch mortars (Flying Pigs) were issued to Heavy TMBs, with a further 200 issued in December. [2]

9.45 inch Heavy Trench Mortar "Flying Pig"

9.45 inch Heavy Trench Mortar firing position

1917

The 2 inch "Toffee Apple" / "Plumb Pudding" were problematical in their operation with the danger of a premature explosion.

1917

The 2 inch "Toffee Apple" / "Plumb Pudding" would be replaced by 6 inch Newton Mortars which were first issued in June 1917. [1] The 6 inch medium mortar fired an 52 pounds (24 kg) high explosive bomb to a range of 1,950 yards (1,780 m). [12]

   

6 inch Newton Medium Trench Mortar   

 

6 inch Newton Medium Trench Mortar 

         

  NZ Gunners load 6 inch Newton mortar 1918

A further  reorganisation of trench mortar organisation was proposed by GHQ in December 1917. [1] The Heavy TMBs would be  reduced to 1 per Corps. , and the Medium TMBs to 2 per Division.  The units would be redesignated, the Heavy TMB would be know as "Mortar Battery RGA" and the Medium TMB as "Mortar Battery RHA/RHA". [1]

1918

In February 1918 the proposed changes to the TMB organisation were enacted.  In the Divisional Artillery the Z (Medium TMB) was broken up and men redeployed to X / Y (Medium) TMB.  [2]   

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TRENCH MORTAR ORGANISATION

[1] Medium TMBs come under control of Divisional Artillery

[2] Reduction in number of TMBs and redistribution of manpower

TRENCH MORTAR BATTERY ORDER OF BATTLE

Trench Mortar Orbats August 1916

[Source: Trench mortar battery - Other Great War Chat - The Great War (1914-1918) Forum (greatwarforum.org) @Ron Clifton]

Light Trench Mortar Battery - 2 sections, each 4 x 3" Stokes mortars

Captain, 3 Subalterns, 2 Serjeants, 8 Corporals or Lance-Corporals, 32 Privates, 4 Batmen.

Medium Trench Mortar Battery - 4 x 2" mortars

2 Officers, 1 Serjeant, 4 Corporals or Bombardiers, 16 Privates, 2 Batmen.

Heavy Trench Mortar Battery - 4 x 9.45" mortars

Captain, 2 Subalterns, 3 Serjeants, 1 Fitter, 4 Corporals, 4 Bombardiers, 47 Gunners (incl 3 telephonists), 2 Orderlies, 1 Clerk, 1 Cook, 3 Batmen.

TRENCH MORTARS

TRENCH MORTAR EFFECTS

September 1916 photograph showing the different sizes of depth and diameter caused by the three types of TMB courtesy of @Simon Birch

The ordnace in operation at that point in time would have been the 3 inch Stokes mortar, the 2 inch "Toffee Apple / Plum Pudding", though trials were in progress for the 6 inch Newton Mortar [13], and the 9.45 inch mortar. 

Light Trench Mortar - 3 inch Stokes mortar firing a 10 lb 11 oz (4.84 kg) bomb - crater size 5 feet diameter (1.5 m) / 2 feet deep (0.6 m)

Medium Trench Mortar - 2 inch "Toffee Apple / Plum Pudding" firing a 51 lb (23 kg) bomb - 52 pounds (24 kg)

Medium Trench Mortar - (Trialled) - 6 inch Newton Mortar firing 52 pounds (24 kg) - 52 pounds (24 kg)

Heavy Trench Mortar - 9.45 inch mortar - firing a 152 pounds (69 kg) bomb - crater size 25 feet diameter (7.6 m) 12 feet deep (3.6 m)

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TRENCH MORTARS IN ACTION

9.45 inch Heavy Trench Mortar

2 inch Medium "Toffee Apple / Plumb Pudding"

3 inch Light Stokes Mortar

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TRENCH MORTAR PROFICIENCY BADGE

In December 1915 a letter from the Commander in Chie BEF to the War Office recommended "a proposal that personnel of Trench Mortar Batteries should be provided with a distinguishing badge, and suggesting that the badge should be similar to that authorised for Grenadiers but of a distinctive colour. These badges to be granted to all men serving in Trench Mortar Batteries and will be retained only while the men are actually serving in the batteries" . [14]

The badge was a blue grenade and worn on the upper right arm. [15]

Blue flaming grenade trench mortar badge

Grenadier Guards Trench Mortar Badge

2in mortar Trench Mortar Section  [Source: Postcards @mark holden]

Trench Mortar Warrant Officer [Source: Postcards @trenchtrotter]

Trench Mortar Signaller [Source: Postcards @GWF1967]

20335 L/Cpl H Dakin (Mentioned in despatches) 5th Sherwood Foresters, att. 139 Trench Mortar Battery, 46th North Midland Division [Source: Postcards: @Dawley Jockey]

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Sources:

[1] Farndale. History of the Royal Regiment of Artillery Western Front 1914-18.  Annex G

[2] The British Trench Mortar Batteries in the First World War - The Long, Long Trail (longlongtrail.co.uk)

[3] History of Ministry of Munitions Volume 1 Pages 33-34

[4] 2-inch medium mortar - Wikipedia

[5] Stokes mortar - Wikipedia

[6] War Diary Army Troops. SECOND ARMY: Trench Mortar School Catalogue reference: WO 95/398/2

[7] 240 mm trench mortar - Wikipedia

[8] https://www.greatwarforum.org/topic/218875-trench-mortar-batteries/ GHQ O.B/166 9 March 1916 "Medium & Light Trench Mortar Batteries - Policy and Organization

[9] https://www.greatwarforum.org/topic/218875-trench-mortar-batteries/?do=findComment&comment=2365691 GHQ O.B/166 9 March 1916 "Medium & Light Trench Mortar Batteries - Policy and Organization Section 7

[10] 9.45-inch heavy mortar - Wikipedia

[11] 48th South Midland Division (somme-roll-of-honour.com)

[12] Newton 6-inch mortar - Wikipedia

[13] WO 95/398/2  War Diary Secondary Army Mortar School - Sep 24 1916.

[14] National Archives' reference WO 95/28/6 GHQ files a letter from C-in-C to War office (ref Q/4000)                                                                https://www.greatwarforum.org/topic/302602-creation-of-trench-mortar-badge-dating/

[15] https://www.greatwarforum.org/topic/302602-creation-of-trench-mortar-badge-dating/?do=findComment&comment=3184011