MANUAL OF NATO SAFETY PRINCIPLES FOR THE STORAGE OF MILITARY AMMUNITION AND EXPLOSIVES (May 2010)

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Section IV - Explosives Hazards Mitigation Methods.

3.2.4.1. Facility Layout

A single-chamber facility with a straight access tunnel leading from the

chamber to the portal is a “shotgun” magazine because blast and debris

behave as if fired from a gun. More complex facility layouts will provide

reductions in exit pressures.

The side on pressure, the side on pressure impulse, dynamic pressure and

the dynamic pressure impulse decrease as the volume increases.

Distributing munitions over several storage chambers may control the size of

an initial explosion. Proper separation or hazard mitigating constructions can

limit subsequent damage.

3.2.4.2. Exits

The exits from underground storage sites should not emerge where they

direct blast, flame, and debris hazards to Exposed Sites, ES, such as other

entrances, buildings, or traffic routes.

Connected chambers and cave storage sites should have at least two exits.

Exits should be separated by at least the chamber interval.

3.2.4.3. Branch Passageways

When a main passageway has one exit, branch passageways should be

inclined at an angle where they join the main passageway to direct the flow

field towards the exit. This inclination should provide for vehicle access.

Angles between 40 degrees and 70 degrees are normally appropriate.

The rock thickness between the chamber and the main passageway should

be at least equal to or greater than the chamber interval. Otherwise, an

explosion in a chamber might destroy the main passageway and prevent

access to stocks of ammunition and explosives in the other chambers.

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3.2.4.4. Blast Closures

High-pressure closures are large blocks constructed of concrete or other

materials that can obstruct or greatly reduce the flow of blast effects and debris

from an explosion from or into a storage chamber. For chamber loading

densities of about 10 kg/m³ or above, closure blocks will contain 40 percent or

more of the explosion debris within the detonation chamber, provided the block

is designed to remain intact. If a closure block fails under the blast load, it will

produce a volume of debris in addition to that from the chamber itself.

However, since the block’s mass and inertia are sufficient to greatly reduce the

velocity of the primary debris, the effectiveness of other debris-mitigating

features, such as debris traps, expansion chambers and barricades is

increased. Debris traps and expansion chambers intended to entrap debris

must be designed to contain the full potential volume of debris, based on the

maximum capacity of the largest storage chamber.

These debris mitigation features were investigated in the tests described in

Reference [7]. These tests showed that such measures can be very effective,

however, no quantitative figures for the reduction of the adit debris throw were

derived. Furthermore, it was shown that a proper design of the mitigation

measures is very important. Sample drawings of the features that proved to be

effective for the tested configurations are in Reference [5].

An alternative, full-scale tested; design for a high-pressure closure device, the

Swiss-Klotz [4], is shown in Figure 2-III. This device is highly effective up to

chamber loading densities of 28 kg/m³. A special advantage of this Klotz is that

it is movable and can be closed during times when access to the storage

chamber is unnecessary.

In case of an explosion inside the storage chamber and a Klotz in closed

position, practically all of the hazardous debris as well as the explosion gases

will be trapped inside the storage chamber, thereby reducing these hazardous

effects to virtually insignificant levels. In case the Klotz is in open position, it will

be pushed into the closed position by the explosion gases within approximately

100 ms, letting pass only a small fraction of the total amount of debris and

gases.

In any case, using a properly designed high-pressure closure device in

conjunction with a portal barricade will lower the debris hazard to a level where

specific debris QD considerations will not be required. Other combinations of

mitigation features will also reduce adit debris throw to a great extent. The

remaining adit debris hazard has to be assessed based on the actual facility

layout and quantified by means of suitable tests.

f) Blast doors that are protected from primary fragments have proven effective for

loading densities up to 10 kg/m³.

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3.2.4.5. Expansion Chambers

Expansion chambers are so-named because of the volume they provide for

the expansion of the detonation gasses behind the shock front as it enters the

chamber from a connecting tunnel. Some additional degradation of the peak

pressure at the shock front occurs as the front expands into the chamber and

reflects from the walls.

Expansion chambers have other practical purposes. They serve as

loading/unloading chambers, as weather protected areas for the transfer of

munitions from trucks to storage chambers, and as turn-around areas for

transport vehicles. Figures 2-IV and 2-V illustrate underground facilities with

and without expansion chambers.

3.2.4.6. Constrictions

Constrictions, which may be used for mitigating explosives hazards, are short

lengths of tunnel with reduced cross sectional area.

A constriction at a chamber entrance reduces the magnitude of airblast and

thermal effects entering chambers near one in which an explosion might

occur. A constricted chamber entrance also reduces the area, and hence the

size of a blast door installed to protect the chamber contents.

A constriction intended to reduce airblast issuing from an exit of an

underground storage facility should be located within five tunnel diameters of

the exit.

Although constrictions located more than five tunnel diameters from exits will

reduce pressures by delaying the release of energy [8, 9], their effects on

pressure versus distance must be considered on a site-specific basis.

3.2.4.7. Debris Traps within the Underground Facility

Debris traps are excavations in the rock at or beyond the end of sections of

tunnel, designed to catch debris from a storage chamber detonation. Debris

traps should be at least 20 percent wider and 10 percent taller than the

branch passageway from the chamber whose debris it is intended to trap, with

a depth (measured along the shortest wall) of at least one tunnel diameter.

An expansion chamber may be effective for trapping debris. Tunnels entering

or exiting the chambers must either be offset in axial alignment by at least two

tunnel widths or its axis must be offset at least 45 degrees from the centerline

of the tunnel associated with the chamber [5].

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3.2.4.8. Blast Traps

Blast traps may be used to reduce the intensity of blast leaving or entering a

passageway. They may be used to attenuate the blast issuing from the adit of

an underground site, thus reducing hazard to people and property in the

vicinity. They may also be used to reduce the blast entering an adjacent

underground site, and to diminish the hazard to other ammunition. The effect

of various blast traps will be a function of the geometrical design of the blast

traps, and the peak side on pressure, the side on pressure impulse, the

dynamic pressure and the dynamic pressure impulse of the incident blast

wave. Fixed reduction figures can therefore not be given. The design of

effective blast traps is a specialized subject.

b) Various types of blast traps are shown in Figure 2-VI. The relative decrease of

pressure and impulse, and thereby the effect of these blast traps, is in most

cases dependent upon their locations. Some of the limitations are also

indicated in the figure. It is noteworthy that not all designs of blast traps are

reversible.

c) For maximum blast reduction, the length of blast traps built as dead end

tunnels should be at least half the length of the blast wave. This may result in

a considerable extension of these traps in the case of large quantities of

explosives.

3.2.4.9. Portal Barricade

Airblast

Airblast exiting the portal of an underground facility involve directional, very

intense gas flow fields along the extended centerline of the tunnel exit.

Therefore, the shock wave on the extended centerline does not attenuate as

rapidly as that of a surface burst. However, a barricade in front of the portal

intercepts this intense flow field and directs it away from the extended

centerline axis. This redirection of the flow field allows shock waves traveling

beyond the portal barricade to attenuate as an above ground distributed

source so that isobar contours become more circular. Figure 2-VII provides an

example of a portal barricade.

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Figure 2-VI:

Blast Traps

Turns, crossovers, obstacles and changes of cross section can be used to reduce

the peak overpressure and positive impulse of blast in passageways. The diagrams

in this figure illustrate some of the many possible designs. The Blast k assumed to

travel from the point indicated by a cross to that shown by a dot. Critical dimensions

are indicated as multiples of passage diameter “b”.

Some designs have comparatively little effect reducing the blast by only 10 %

compared with the straight-through passageway in Ref. No 1, whereas others reduce

the blast by as much as 80 %. It is therefore necessary to determine the actual effect

of a chosen design by measurements in a model using properly scaled and located

explosive charges.

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Figure 2-VII:

Portal Barricade Location, Height and Length

Tunnel

a. Plan View

S = Stand off distance from portal (1 to 3 tunnel widths)

R = Turning rasdius of munition transport vehicles

V = Width of transport vehicles

L = Length of barricade

W = Tunnel width at portal

φ = Single angle [10 degrees minimum]

Portal

Tunnel

a. Elevation View

C = Crest Width [See DEF 421-80-04]

H = Height of barricade

h = Height of tunnel

θ = Elevation angle [10 degrees minimum]

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CHAPTER 3 - QUANTITY-DISTANCES

Section I - General

3.3.1.1. Types and Effects

Types

This section details how to predict QD based on criteria given in para

3.1.1.2. for the underground storage of military ammunition and

explosives. Underground storage typically includes natural caverns and

excavated chambers. Recommendations in this section shall only be

used when the minimum distance from the perimeter of a storage area

to an external surface exceeds

600 mm and 0.1·Q^1/3

(m, kg).

Otherwise, use aboveground siting criteria. This section addresses

explosives safety criteria both with and without rupture of the cover.

Ground shock, debris, and air blast from an accidental explosion in an

underground storage facility depend on several variables, including the

local geology and site-specific parameters. These parameters vary

significantly from facility to facility. Consequently, distances other than

those listed below may be used provided approved experimental or

analytical data indicate that the desired protection can be achieved.

See below for default methods to determine QD.

The QD for tolerable ground shock is the same in all directions for

homogeneous, geological media, whereas QDs for other hazards

(blast, thermal, impulse, etc.) vary markedly in different directions.

Variations in QDs in different directions arise from configuration-

specific features such as the locations of adits and ventilation shafts,

hazards mitigating designs, and terrain. The acceptable QD in a given

direction is generally taken as the maximum QD determined for the

various hazards.

QD siting requirements of this section may be determined from the

applicable equations or by interpolating between figure entries.

Effects

The following effects, peculiar to underground storage sites, must be taken

into consideration for quantity-distance purposes:

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Inside the Underground Installation:

The volume available to an expanding shock front is less in an

underground configuration than it is in an aboveground configuration.

Because of this limited space, an explosion in an underground facility

typically results in long-duration, high pressures and temperatures that

spread throughout the entire volume available to the shock front.

Unless robust engineered designs (doors and/or other closing devices)

are used to separate various parts of the facility, these long-duration

blast effects spread throughout the entire underground complex. Doors

or other closing devices must be properly designed and, in the case of

doors, closed to provide the desired separation.

An initial event in Hazard Division 1.2 and 1.4 materials usually starts a

fire, which is sustained by burning packages and components of the

ammunition. This process causes additional explosions, likely at

increasing frequency, until combustible materials in the site have been

consumed. The results of these repeated explosions in the confined

space underground will depend on the type and quantity of the

substances in each unit of ammunition and the type of explosion

produced.

Outside the Underground Installation:

Blast waves from adits exhibit highly directional flow-fields along the

extended centerline of the passageway. Consequently, the blast wave

effects (overpressure and impulse) do not attenuate as rapidly along

the centerline axis as they do off the centerline axis.

The following effects should be considered for an external ES:

Blast from tunnel adits

Blast from craters, if the rock cover is insufficient.

Debris from tunnel adits

Debris from cratering

Ground Shock

Flame and hot gases

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3.3.1.2. Quantity-Distances

Inside the UG Installation

QD should be determined for the following:

Chamber Intervals

Loading/Unloading Dock

Explosives Workshop Distance (EWD)

Inspection

Outside the UG Installation

QD should be determined for the following:

Inhabited Building Distance (IBD)

Public Traffic Route Distance (PTRD)

Explosives Workshop Distance (EWD)

Earth-covered Magazine Distance (ECMD)

Aboveground Magazine Distance (AGMD)

3.3.1.3. Net Explosives Quantity (NEQ)

For siting purposes, the NEQ is the total quantity of explosives material that

must be included in defining a potential event. Part I, paragraph 1.4.2.5.

provides guidance for finding the appropriate NEQ for sites containing

materials with different Hazard Classes.

3.3.1.4. Measuring Quantity-Distances

Inside the Underground Installation.

The Chamber Interval is the shortest distance between the walls of two

adjacent chambers. The subdivision of a cavern requires construction of

massive barricades to close the gaps in the natural rock and to isolate one

site or chamber from any other. The thickness of these barricades should be

equal to the chamber intervals.

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Outside the Underground Installation.

Distances to ESs outside the underground facility are normally measured as

radial distances (see below) unless conditions make such a procedure clearly

unreasonable:

Distances determined for airblast, debris, and thermal effects issuing

from tunnel openings shall be the minimum distance measured from

the openings to the nearest wall or point of the location to be protected.

Extended centerlines of the openings should be used as reference

lines for directional effects.

A distance determined by ground shock should be measured from the

nearest wall of a chamber or a cavern containing ammunition or

explosives to the nearest wall or point of the location to be protected.

A distance determined for air blast and debris from a breached cover

shall be the minimum distance from the centre of the breach (CCB), at

ground surface level, to the location to be protected (See Figures 3-

XXIV and 3-XXV).

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Section II - Hazard Division Material Dependence

3.3.2.1. Hazard Division 1.1, 1.3, 1.5 and 1.6 materials

Distances shall be determined from the total quantity of explosives,

propellants, pyrotechnics, and incendiary materials in the individual chambers,

unless the total quantity is subdivided to prevent rapid communication of an

incident between subdivisions. All Hazard Divisions 1.1, 1.3, 1.5, and 1.6

material subject to involvement in a single incident shall be assumed to

contribute to the explosion yield.

A connected chamber or cavern storage site containing Hazard Division 1.1 or

1.3, 1.5 and 1.6 materials shall be treated as a single chamber site, unless

explosion communication is prevented by adequate subdivision or chamber

separation.

HD 1.3 material should be treated as HD 1.1 material when it is stored

underground.

3.3.2.2. Hazard Division 1.2 materials

The hazard to exterior ESs from primary fragments where a line-of-sight path

exists from the detonation point to the ES is the only explosives safety hazard

of concern for HD 1.2 materials.

When line-of-sight conditions exist, use distances common to aboveground

situations.

QD requirements do not apply if the exterior ES is located outside the line-of-

sight or if barricades (constructed or natural) intercept fragments issuing from

an opening.

3.3.2.3. Hazard Division 1.4 materials

Exterior: Exterior explosives safety hazards are not normally significant for

Hazard Division 1.4 materials. Accordingly, QD requirements do not apply for

Hazard Division 1.4 materials.

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Section III - Chamber Interval

References [7-10] deal with chamber intervals.

3.3.3.1. Hazard Divisions 1.1, 1.3, 1.5, and 1.6

Three modes by which an explosion or fire can be communicated are rock

spall, propagation through cracks or fissures, and airblast or thermal effects

traveling through connecting passages. Minimum storage chamber separation

distances are required to prevent or control the communication of explosions

or fires between donor and acceptor chambers.

The minimum chamber separation (D_cd) is 5 m for HD 1.1, 1.3, 1.5, and 1.6

materials.

Prevention of major damage by rock spall.

The chamber separation distance is the shortest distance

(rock/concrete

thickness) between two chambers. When an explosion occurs in a donor

chamber, a shock wave propagates through the surrounding rock. The

intensity of the shock decreases with distance. For small, chamber separation

distances, the shock may be strong enough to spall the rock/concrete walls of

acceptor chambers.

For hard rock with no specific protective construction, the minimum, chamber

separation distance, D_cd, required to prevent major damage by spall depends

on the chamber loading density (γ) as:

1/3

. ⋅Q

(γ≤

50 kg

)

Eq. 3.3.3-1

and

1/3

. ⋅Q

(γ>

50 kg

)

Eq. 3.3.3-2

Example (γ≤

50 kg

Q = 200,000 kg

D_cd = 1.0 · 58.48 = 58.5 m

For soft rock (See para 3.3.4.3.a), at all loading densities, the separation

distance is:

1/3

. ⋅Q

Eq. 3.3.3-3

Example:

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Q = 200,000 kg

D_cd = 1.4 · 58.48 = 82 m

Prevention of propagation by rock spall

If damage to stored munitions in the adjacent chambers is acceptable, the

chamber separation distance can be reduced to the distance required to

prevent propagation of the detonation by the impact of rock spall against the

munitions. For smaller distances, propagation is possible. Propagation by rock

spall is practically instantaneous because time separations between donor

and acceptor explosions may not be sufficient to prevent coalescence of blast

waves. Unless analyses or experiments indicate otherwise, explosives

quantities subject to this mode must be added to other donor explosives to

determine NEQ. For loading densities up to 270 kg/m³, when no protective

construction is used, the separation distance, Dcd, to prevent explosion

communication by spalled rock is:

1/3

. ⋅Q

Eq. 3.3.3-4

Example:

Q = 200,000 kg

D_cd = 0.6 · 58.48 = 35 m

When the acceptor chamber has protective construction to prevent spall and

collapse

(into the acceptor chamber) the separation distance must be

determined on a site-specific basis but may be as low as:

1/3

. ⋅Q

Eq. 3.3.3-5

Example:

Q = 200,000 kg

D_cd = 0.3 · 58.48 = 17.5 m

Prevention of propagation through passageways

Blast, flame and hot gas may cause delayed propagation. Time separations

between the original donor event and the potential explosions of this mode will

likely be sufficient to prevent coalescence of blast waves. Consequently, for

purposes of Q-D siting, only the maximum credible explosives quantity need

be used to determine NEQ.

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In order to protect assets, blast and fire resistant doors must be installed

within multi-chambered facilities. Evaluations of design loads on doors must

be made on a site-specific basis.

Propagation by Flame and Hot Gas through Cracks and Fissures

Consideration must be given to the long-duration action of the explosion gas.

These quasi-static forces might form cracks in the rock that extend from the

donor to an adjacent (acceptor) chamber, thus making it possible for hot

gases to flow into this chamber and initiate an event. Significant factors for

this mode of propagation include the strength of rock, the existence of cracks

formed before the explosion incident, the type of barriers in cavern storage

sites, the cover and the loading density in the chamber. This mode of

propagation must be considered when final decisions about chamber

separation distances are made.

Thus, because of these cracks and fissures, propagation may occur beyond

1/3

. ⋅Q

Eq. 3.3.3-6

Example:

Q = 200,000 kg

D_cd = 0.3 · 58.48 = 17.5 m

but not likely beyond;

1/3

. ⋅Q

Eq. 3.3.3-7

Example:

Q = 200,000 kg

D_cd = 2 · 58.48 = 117 m

Site-specific analyses, using a sound geological survey, should be made to

determine proper intervals between chambers.

3.3.3.2. Hazard Division 1.2

Intervals between a chamber containing ammunition of Hazard Division 1.2

and adjacent chambers should be at least 5 m of competent rock unless structural

considerations apply. This applies also to barriers used to isolate chambers in a

cavern storage site.

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Section IV - Inhabited Building Distance (IBD)

IBD must be the largest of the distances for protection against airblast,

debris, and ground shock [1, 7].

3.3.4.1. Airblast [8-15]

The side-on overpressure of 5 kPa defines IBD.

An explosion in an underground storage chamber may produce external

airblast from two sources; the exit of blast from existing openings (tunnel

entrances, ventilation shafts, etc.) and the rupture or breach of the chamber

cover by the detonation. Required IBDs are independently determined for

each of these airblast sources, with the maximum IBD used for siting.

A breaching chamber cover will produce external airblast. Use the

following table to site for IBD due to airblast produced by breaching of

the chamber cover. Values of IBD for airblast through the ruptured

cover are:

CoverThickness

IBD

Equation

1/3

Cover≤

1⋅Q

IBD forSurfaceBurst

Eq.334−1(a)

1/3

1⋅Q

<Cover≤0

2⋅Q

12IBD forSurfaceBurst

Eq.334−1(b)

1/3

2⋅Q

<Cover≤0

3⋅Q

14IBD forSurfaceBurst

Eq.334−1(c)

1/3

Cover>

3⋅Q

NegligibleAirblastHazard Eq.334−1(d)

This paragraph defines airblast IBDs from openings in an underground

storage facility. The IBD for airblast must be considered for any opening.

The method for calculation of air blast in underground storage can be

divided in the following 3 steps:

Calculation of air blast at the chamber exit

Calculation of air blast at the tunnel adit

Calculation of air blast outside the tunnel adit

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To a first approximation, the overpressure in the storage chamber

could be estimated with an algorithm of the form:

2/3

⎡

⎤

= 1200

⋅

Eq. 3.3.4-1

⎢

⎥

⎣VC

⎦

where:

p_c:

overpressure at the chamber exit, kPa

Mass of explosives material, kg

V_c:

Volume inside the chamber that is engulfed by

blast waves at the time the blast arrives at the

location of interest (m³).

Air blast at the tunnel adit

The reduction of peak overpressure and change of duration from the

exit of a detonating chamber to the tunnel adit are calculated by using

different types of tunnel elements (see figure 3-I - 3-VI) to resemble

the actual configuration of the tunnel.

The following parameters and figure 3 and 4 are used to calculate the

reduction of peak overpressure in a tunnel element (friction element)

with a constant cross section and without junctions and turns.

L_R = L_S - 5 · d₀

[8]

χ = α · L_R

[9]

τ = α · t₁

[10]

⎛

⎞

2/3

1/3

20⋅L

⋅ d

⋅⎜

⎟

[11]

⎜

⎟

⎝

⎠

L_s= Length of tunnel or tunnel element (m)

L_k=Length of chamber (m)

d_k=Average equivalent diameter of the chamber(m)

L_R=Effective length of tunnel (m)

d₀=Average equivalent diameter of the tunnel (m)

α =Friction coefficient for concrete (α = 1), shotcrete (α = 4) and

rock (α = 6)

χ =Coefficient for distance (m)

p₁, p₂=Peak overpressure at the beginning respectively end of

the tunnel element (atm).

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Figure 3-III: Description of tunnel elements

1−

⋅p

180°

⋅

180°

⋅t

⋅p

= 0.25 ⋅ p

⋅t

⋅p

⎛

⎞

0.8⋅⎜1−

⎟⋅p

⎝

180°⎠

⋅t

⎛

⎞

⎛

⎞

⎜

⎟

0.9

−

0.6

⋅

⎜

⎟

⋅ p

⎜

180

⎟

⎝

⎠

⎝

⎠

⎛

⎞

⎜

0.2

0.6

⋅

⎟

⋅ p

⎝

180

⎠

⋅t

≥2⋅L

⋅p

2⋅L

use tunnel elements(junctions)above

⋅p

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Figure 3-VI: Change of pressure and duration for sudden and continuously

constriction tunnel element

F₁ and F₂ are tunnel

crossections

Air blast outside tunnel adit

The resulting pressure in the tunnel adit could be expressed:

⎡

⎤

⎡

⎤⎡

⎤

⋅

Eq. 3.3.4-3

⎢

⎥

⎢

⎥⎢

⎥

⎣

⎦

⎣

⎦⎣

n−1

⎦

The required distances for inhabitant building distance, public traffic

route distance and explosive workshop distance could then be

calculated. The distances calculated are valid for the axis of the tunnel.

0.74

⎡

⎤

IBD

=1.64⋅d

⋅

Eq. 3.3.4-4

⎢

⎥

⎣

⎦

0.72

⎡

⎤

IBD

=1.64⋅d

⋅

Eq. 3.3.4-5

⎢

⎥

⎣

⎦

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0.66

⎡

⎤

IBD

=1.64⋅d

⋅

Eq. 3.3.4-6

⎢

⎥

⎣2

⎦

Where d_teis the equivalent diameter in the tunnel exit (d_te =

)

For a simple horizontal geometry (no barricade, a rapidly rising rock

face, an extended centerline normal to the rock face) the following

equation for off centerline axis can be used.

IBD(θ)=IBD(θ = 0)x [1+(θ/56)²] ^-0.74

Eq. 3.3.4-7

where:

θ:

horizontal angle off centerline in degrees

Large variations in directivity have been observed

(Figure

3-VII).

Therefore, it is recommended that carefully constructed models and

realistic exit pressures should be used to investigate directivity for an

actual site.

High-Pressure Closure Block Designed to Remain Intact

References [4,

5] contain illustrative examples of a closure block

designs (Figure 3-VIII).

Figure 3-VIII:

Example of a Closing Block

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Figure 3-VII:

Directivity Versus Azimuth with the Centre Line as Reference

1.0

7 mbar

0.9

0.8

0.7

140 mbar

0.6

10-15 mbar

0.5

Model test

5 to 50mbar

50 mbar

0.4

0.3

0.2

50 mbar

0.1

30⁰

60⁰

90⁰

120

150⁰

180⁰

LEGEND:

“Free field overpressures resulting from shock waves emerging from open-ended shock tubes.”

Ballistic Research Lab. Mem. Report 1965.

“An investigation of the pressure wave propagated from the open end of a 30 x 18 in. Shock tube.”

Atomic Weapons Research Establ. AWRE Report No. 0 - 60/65.

“Underground Explosion Trials at Raufoss 1968. Measurement of air blast outside the tunnel.”

Intern Report X - 124. FFI 1969.

4. U.S Navy Gun Blast Committee: “Survey of Research of Blast”. First Interim Report, 1946.

“Model tests to investigate external safety distances.” Fortifikatorisk notat 36/67, FBT

1967.

“One-dimensional blast wave propagation.” Fortifikatorisk Notat 49/69, FBT 196

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For chamber loading densities greater than or equal to 10 kg/m³, IBD

may be reduced by 50% when a high-pressure closure block, designed

to remain intact in case of an explosion, is used.

For chamber loading densities lower 10 kg/m3 (but greater than 1.0

kg/m3), determine the reduction by the formula:

y(%) = 50 ⋅ log

(

)

Eq. 3.3.4-5

where, y is the percentage reduction in IBD, and γ is loading density in

kg/m3. For loading densities lower than 1.0 kg/m3, use y(%) = 0.

Portal Barricade

When a properly designed and located portal barricade [5, 7] is in front

of the opening, IBD for airblast along the extended tunnel axis may be

reduced up to 50 percent. Although the total airblast hazarded area

remains almost unchanged, its shape, for explosives safety

applications, becomes more circular.

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3.3.4.2 Debris

Debris from an explosion in an underground facility may issue from

adits or other openings; failure of nearby structures

(portal,

barricades, etc.); and breaching of the geological cover over the

PES (crater debris).

Adit Debris

Introduction

Debris throw from the adit is one of the most relevant hazardous

effects to be expected in case of an explosion in an underground

installation in rock. Adit debris consists of parts of ammunition and

its packaging, technical installations such as ventilation equipment,

doors and fire fighting installations, chamber and adit lining and

other reinforced concrete construction elements as well as of rock

rubble produced by the explosion effects. All these pieces of debris

are accelerated by the explosion gases escaping from the

installation and are thrown into the surroundings in front of the adit

portal.

Adit debris throw mainly depends on the explosives weight Q

(NEQ, [kg]) stored in the installation and the geometry (ratio of

length to diameter - l_a/d_a-ratio) of the adit section just behind the

portal.

Other factors, such as the loading density (explosives weight /

chamber or system volume), the centre of the explosion in the

chamber, the construction of the portal area and the geometry of

the whole adit may also influence adit debris throw. However, the

available data from tests and accidents was insufficient to derive

reliable relations.

Form of the IBD Contour Line and Influences

a) The general shape - resembling a cloverleaf - of the IBD contour

line for adit debris throw is shown in Figure I. The IBD contour line

is defined by points where the fragment density is one hazardous

fragment (energy greater than 79 Joules) per 56 m² (≅ 0.0179

#/m²).

b) No closed formula exists for the shape of the IBD contour line.

Therefore, the line has to be constructed gradually, point per point.

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c) The shape and size of the contour line is influenced by the

following three parameters:

- Net explosives quantity of the stored ammunition

NEQ [kg]

- Relevant length of the adit section behind the portal

l_a

[m]

- Average equivalent diameter of the adit (l_a)

d_a

[m]

d) The ratio l_a/d_a defines the portal parameter f_p and the standard

deviation σ.

The portal parameter f_p takes into account that a long small adit

leads to a more focused debris throw than a short adit with a large

cross section area. Large l_a/d_a values, therefore, lead to far

reaching but narrow IBD contour lines. The f_p parameter influences

the maximum range of the IBD contour line.

The standard deviation σ defines the width of the IBD contour line.

Procedure to Calculate an IBD Contour Line

Calculate the maximum range R_{o max} of the IBD contour line (Figure

3-IX).

− 4.025

−

R_{o max} = f_p x

The parameter f_p is a function of the l_a/d_a ratio. It is to be calculated

according to Figure 3-X. Typical examples on how to define the

l_a/d_a ratio are given in Figure 3-XI.

The A and B values are both a function of the NEQ.

A = -5.25 + 1.0 x ln(NEQ)

0.25

B = -0.0085 -

NEQ

A, B and f_p values for typical amounts of NEQ and l_a/d_a ratios are

also given in the tables in Figure 3-XII.

For a suitable number of R_o values (8 to 12, freely chosen, but R_o <

R_{o max}) calculate the reference value D_o (Figure 3-IX).

D_o =

exp(A + B x R_o/f_p) where: exp(x) = e^xand e = 2.718

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MANUAL OF NATO SAFETY PRINCIPLES FOR THE STORAGE OF MILITARY AMMUNITION AND EXPLOSIVES (May 2010) - page 16