Field Fortifications: How thick was thick enough?

The ability to construct quick but effective fortifications in the field has historically fallen to the soldiers of the Royal Engineers. This dirty and backbreaking work may not be precise, but it is based very soundly on experience and scientific experimentation. During the 19th and 20th centuries, as weaponry and warfare evolved at a staggering rate, the data provided to frontline engineers also evolved. I have drawn on an extensive collection of military manuals spanning from 1888 through to the modern day to collate this data and publish it in a single place.

The data is for field fortifications such as trenches, and does not include permanent fortifications such as stone built forts. The majority of data reflects protection against small-arms or rifle fire.

Field Works by Major General C.B. Brackenbury (1888)

Protection in the field against artillery

The following table of data was extracted from the Manual of Elementary Field Engineering and updated for inclusion into the 1888 publication.

Gun	Concrete Butt	Sand & Clay Butt	Rammed Clay	Extreme Penetration	Advised Parapet Thickness
7-inch B.L. rifled gun¹	7ft 9in	12ft 11in	18ft 3in	21ft 11in	25 to 30 feet
70-pr B.L. rifled gun²	6ft 4in		14ft 3in	17ft 0in	25 to 30 feet
40-pr B.L. rifled gun³	6ft 1in		11ft 8in	16ft 4in	18 to 25 feet
20-pr B.L. rifled gun⁴		7ft 0in	10ft 3in	13ft 3in	15 to 18 feet
12-pr B.L. rifled gun⁵		3ft 2in	4ft 0in		6 to 9 feet
10-inch S.B. gun⁶	3ft 10in		11ft 0in	12ft 0in	15 to 18 feet
8-inch S.B. gun⁷	3ft 6in		11ft 5in	12ft 9in	15 to 18 feet
68-pr S.B. gun⁸	4ft 0in	7ft 6in	14ft 10in	21ft 6in	25 to 30 feet
32-pr S.B. gun⁹	2ft 8in	5ft 8in	9ft 5in	14ft 0in	18 to 25 feet
9-pr M.L. gun¹⁰			4ft 0in		9ft (Sand) 12ft (Medium Soil) 15ft (Clay)
16-pr M.L. gun¹¹			6ft 0in		9ft (Sand) 12ft (Medium Soil) 15ft (Clay)
13-pr M.L. gun¹²			7ft 0in		9ft (Sand) 12ft (Medium Soil) 15ft (Clay)
12-pr B.L. gun¹³			17ft 0in		9ft (Sand) 12ft (Medium Soil) 15ft (Clay)
22-pr B.L. gun¹⁴					9ft (Sand) 12ft (Medium Soil) 15ft (Clay)

Mean penetration data for projectiles fired at 1,060 Yards.

Protection against rifle bullets

The following table gives the thickness of material which may be considered proof against rifle bullets of existing service arms (in 1888!) at any range:

Protective Material	Thickness Required
Earth	2 feet
Iron Plate	⅜ inch
Steel Plate	¼ inch
Fir Logs	12 inches
Fir planks, 3-inch thick	6 planks (18 inches)
Pack Logs	6 inches
Oak planks, 2-inch thick	3 planks (6 inches)
Gabions filled with earth	1 gabion
Filled sandbags, crossways	2 sandbags
Filled sandbags, lengthways	1 sandbag
Rope mantlet	6 inches
Loose cotton	4 feet
Compressed cotton in a bale	2 bales

The image below from the Imperial War Museum shows a destroyed field fortification from the mid-C19. Experiences such as this would have helped inform measures for ballistic protection. This fort has been constructed primarily using wicker gabions that would have been earth filled. They have also utilised hessian sand bags for additional protection in areas.

THE CRIMEAN WAR 1854-56 (Q 71081) View of the ruined interior of the Great Redan, Sevastopol. Copyright: © IWM. Original Source

Manual of Military Engineering (1901)

The parapet of an infantry trench should be thick enough to keep out a rifle bullet – 2 feet will suffice. Overhead cover was not considered necessary at this time as the enemy’s artillery fire would be frontal, however to protect against oblique fire a thickness of 6 to 9 inches of earth supported by brushwood was advised. The oblique cover was intended to keep out shrapnel, bullets or shell splinters.

Military Engineering, Field Defences (1908)

The following table gives the thickness, in various materials, proof against a bullet from the short Lee-Enfield Service Rifle at 30 yards range. The bullets of some continental armies have, however, greater penetration.

Material	Thickness Proof	Remarks
Clay	5 feet	Varies greatly. This is maximum for greasy clay
Earth, free from stones (un-rammed)	3 feet	Ramming earth reduces its resisting power
Sand	2ft 6in	Rather more than enough. Very high velocity bullets have less penetration in sand at short than at medium ranges
Sand between boards	18 inches
Brickwork	9 inches	If well built.
Soft wood (fir)	48 inches	24 inch proof at 500 yards
Hard wood (oak)	27 inches	15 inch proof at 500 yards
Wrought iron, or mild steel	½ inch
Hardened steel plate	¼ inch	1/16 inch proof at 600 yards
Special hard steel	1/5 inch
Shingle	6 inches
Coal (steam)	2ft 6in
Chalk	1 foot	When freshly excavated

The 1.457-in Vickers-Maxim gun (pom-pom), which may be taken as an example of light quick-firing gun, fires a 1 lb common shell, with a bursting charge of 340 grains of black powder. Its penetration into wrought iron is 2.25 inches at the muzzle. The bursting charge of the shell is so small that its effect against earthworks is insignificant.

While no data is given for the action of artillery shells, a useful plate was included in this manual showing the effect of anti-infantry artillery shells.

The caption read: Plate I gives an idea of the action of various kinds of projectiles. It will be observed that practically the only one which has any backward effect after burst is the howitzer common shell, fired at high angle of elevation.

Manual of Field Engineering (1911)

Rifle fire

The following table gives the maximum penetration of the pointed bullet in various materials. In order to obtain proof cover, a percentage must be added to these numbers (this number is unspecified in the manual).

Material	Maximum Penetration	Remarks
Steel plate, best hard	7/16 inch	At 30 yards normal to plate; 3/16 inches is proof at not less than 600 yards, unless the plate is set at a slope of 3/2 when 3/16 inches is proof at 250 yards.
Steel plate, ordinary mild or wrought iron	¾ inch	As above
Shingle	6 inches	Not larger than 1in ring gauge
Coal, hard	9 inches
Brickwork, cement mortar	9 inches	150 rounds concentrated on one spot will breach a 9 inch brick wall at 200 yards
Brickwork, lime mortar	14 inches	As above
Chalk	15 inches
Sand, confined between boards or in sandbags	18 inches	Very high velocity bullets have less penetration in sand at short than at medium ranges
Sand, loose	30 inches	As above
Hard wood (oak, with grain)	38 inches
Earth, free from stones (unrammed)	40 inches	Ramming earth reduces its resisting power
Soft wood (fir)	58 inches	Penetration of brickwork and timber is less at short than at medium ranges
Clay	60 inches	Varies greatly. This is maximum for greasy clay
Dry turf or peat	80 inches

Logs are used to protect a building in Mercatel. This photograph was taken in 1915 © IWM Q 51085

Tests such as the one below helped to educate and guide the policy on levels of protection required for troops in the field and for equipment being developed for the battlefield. For these, it was essential to have examples of the enemy ammunition and weapons to conduct realistic tests.

MINISTRY OF INFORMATION FIRST WORLD WAR OFFICIAL COLLECTION (Q 14636) Experiment with 3/4inch mild steel armour plate for armoured cars, outside view. One German ‘S’ bullet penetrated point first, another failed point first, and a third ‘reversed’. Wormwood Scrubbs, 1915. Copyright: © IWM. Original Source

Artillery fire

Field guns

Both shrapnel shell and high explosive shell are fired by the field artillery of most foreign nations. Shrapnel with time fuzes can be used up to a range of about 6,000 yards. With percussion fuzes shrapnel can be used effectively against troops behind 14 inch brick or 2 feet thick mud walls as they penetrate before bursting.

Manual of Field Works (All Arms) (1925)

Material	Penetration	Minimum thickness to be provided	Remarks
Steel plate	¾ inches	1 inch
Shingle or broken stone	6 inches	9 inches	If pieces do not measure more than 1 inch placed between boards
Coal (hard)	9 inches	13 inches	Between boards
Coal (kitchen)	15 inches	23 inches	Between boards
Brickwork in lime mortar	14 inches	21 inches
Chalk	15 inches	22 inches
Sand, confined between boards or in sandbags	18 inches	27 inches
Sand, loose	30 inches	45 inches
Earth, free from stones, unrammed	40 inches	60 inches	Ramming earth reduces its resisting power
Sawn timber, hard wood (oak)	38 inches	57 inches
Soft wood (fir)	56 inches	84 inches
Freshly cut timber logs, 12-in diameter and over	24 inches	36 inches
Poles 4.5 to 8 inches in diameter	38 inches	57 inches
Clay	60 inches	90 inches	Varies greatly. This is a maximum for greasy clay
Dry turf or peat	80 inches	120 inches
Snow, rammed		60 inches	Varies greatly. Soft snow has little power of resistance

Tunnelled dug-outs

Provided that the ground is suitable and that the site is not too close to the enemy, tunnelled dug-outs are the most satisfactory type of shell-proof accommodation on any scale.

Complete protection against shells of large calibre (8-inch and over) can seldom be gained without descending to impracticable depths. The thickness of cover required under these circumstances will be:

Material	Thickness
Made earth	35 feet
Clay	30 feet
Gravel	25 feet
Chalk	25 to 20 feet
Hard rock	15 feet

Royal Engineers Pocket Book (1936)

Protection against the rifle bullet

The data in this publication is the same as the data published in the Manual of Field Works (All Arms) 1925, given above.

Artillery shells

Shrapnel

The bullets come down at a steep angle and have very little power of penetration. A brick wall 9 inches thick, a bank of earth 18 inches thick or the roof or floor of a good building will be sufficient to stop them.

High Explosive (H.E.) shells with instantaneous fuze

The shells burst directly they touch the ground. The effect is mainly lateral, and is stopped by a 9 inch wall or a bank of earth 2 feet thick.

H.E. shells with non-instantaneous fuzes

These burst after penetrating for some little distance, and are of more value against material than against personnel. The force of the explosive tends to shatter surrounding material. Good protection against the effect of splinters from shells can be afforded by slit trenches.

Little can be done in hasty defences to protect against direct hits of these shells, as the amount of material needed for safety is too great; in deliberate defences, dug-outs can be made deep enough to afford protection.

Field Service Pocket Book, Field Engineering (1944)

The following table shows the thickness of various materials which should be provided to give protection against bursts of 20 rounds armour piercing bullets from LMGs up to 7.92mm or against splinters from 100-lb bombs bursting not less than 30 feet away.

Material	Safe Thickness
Earth of loam as in parapets	60 inches
Chalk as in parapets	60 inches (variable)
Clay as in parapets	72 inches (variable)
Sand, loose or between boards	30 inches
Brick rubble confined between boards	18 inches
Coal between boards	24 inches
Road metal 1.5 – 2 inches between boards	14 inches
Sandbags, filled with rubble	30 inches
Sandbags, filled with earth	30 inches
Sandbags, filled with road metal	20 inches
Sandbags, filled with shingle	20 inches
Sandbags, filled with sand	30 inches
Brickwork in lime mortar	18 inches
Concrete, unreinforced	12 inches
Mild steel plate	1 ¾ inches
Timber	60 inches (variable)

Field Engineering and Mine Defences, Field Defences, All Arms (1957)

The thickness of any material which will stop a bullet, shell fragment, etc, depends to some extent on the velocity and range of the missile and on the quality of the material. With ordianry earth for instance it varies according to how well the earth is compacted.

Material	Bullet-proof	Splinter-proof	Half-Value Thickness¹⁵
Ordinary soil	60 inches	18 inches	5-9 inches
Compacted sand, eg, in sandbags	30 inches	18 inches	9 inches
Brick	18 inches	18 inches	5 inches
Timber	60 inches	9 inches	18 inches
Concrete	12 inches	4 inches	4 ½ inches
Mild steel	1 ¾ inches	½ inches	1 ½ inches
Loose snow	140-160 inches	n/a	n/a
Packed snow	70-80 inches	n/a	n/a
Packed and frozen snow	48 inches	n/a	n/a

Royal Engineers Pocket Book (1979)

Material	Bullets	Splinters, shell fragments	Gamma radiation (to at least 75% reduction)
Ordinary soil	1.5m	45cm	45cm
Compacted sand (sandbags)	75cm	45cm	45cm
Brick	45cm	45cm	25cm
Timber	1.5m	20cm (variable)	90cm (variable)
Concrete	30cm	12cm	30cm
Mild steel	4cm	1cm	8cm
Loose snow	3.5-4cm
Packed snow	1.8-2cm
Packed and frozen snow	1.2cm

This information was also replicated in the Commanders Pocket Book, Tactical Aide Memoire 1988.

Comparison Table

A good way to represent the advancement of weapons and ammunition is to plot the increase in protective measures required.

Material	1888	1908	1911	1925	1944	1979
Earth	24	36	40	40	60	60
Gravel / shingle		6	6	6
Chalk		12	15	15	60
Clay		60	60	60	72
Sand loose		30	30	30
Sandbags		18	18	18	30	30
Soft wood	18	48	58	56	60	60
Hard wood	6	27	38	38
Iron	0.4	0.5	0.75	0.75
Mild steel		0.5	0.75		2	2
Hardened steel		0.25
Concrete					12	12
Brickwork, lime mortar		9	14	14	18
Brickwork, cement			9			18
Dry turf, peat			80	80

All measurements in inches using the data tables in this article. The 1979 measurements in metric have been converted and rounded-up to imperial.

1
1863 Newhaven experiments
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1863 Newhaven experiments
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1863 Newhaven experiments
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1863 Newhaven experiments
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1863 Newhaven experiments
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1863 Newhaven experiments
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1863 Newhaven experiments
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1863 Newhaven experiments
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1863 Newhaven experiments
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1882 Shoeburyness experiments
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1882 Shoeburyness experiments
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1882 Shoeburyness experiments
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1885 Lydd experiments
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1885 Lydd experiments
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HVT is the thickness of material required to reduce the intensity of gamma radiation by half. A double HVT will reduce radiation by 75%