WR Blasting Method
What is WR? WR stand for the words Water Resistance and is a granular anfo explosive with a specialty surface coating.
This coating increases the water resistance and allows it to be used in wet conditions. Some background information on the
beginning of WR may be helpful to understanding WR.
In the late 1950’s there was an abundance of surplus TNT on the market from the US Department of Defense. Many explosive
companies purchased this government surplus explosive and used it in commercial explosive compositions. Oriard Powder
Company in Marshal, Washington was the largest Atlas Powder distributor at that time and also purchased surplus TNT. At that
time Oriard Powder Co. added a high percentage (about 40%) of granulated TNT to anfo and sold it as a special water-
resistant product, which could be poured into wet boreholes. Gradually, the surplus TNT ran out, but explosive users were still
interested in a granular pourable explosive that could be used in wet conditions. In the early 1960’s conversations between
Hugh Oriard and Harold Sheeran led to a new anfo product without TNT, which could be used in wet conditions with the aid of
dewatering. These formulations were the early beginnings of WR. Through time, experimentation, and various formulas, a
useful and successful product began to emerge. In addition, explosive users in the Northwest US began to develop an efficient
and reliable blasting method to use this new anfo product with increased water resistance, which was manufactured and sold
exclusively by Oriard Powder. These efforts have now developed into the product known as WR Anfo. The complementary
blasting technique to use WR is known as the WR Blasting Method.
Though WR is water resistant, some people mistakenly may assume WR can be poured through water.
This is not the case, so here are a few basic facts:
- An ANFO based product with increased water resistance
- Designed for light and medium water conditions
- Used in conjunction with dewatering
- Predominately used in 3-5" diameter borehole, but can be used in bulk with any diameter borehole
WR also produces advantages in comparison to other wet hole explosives:
- Full borehole loading in wet small diameter boreholes
- Highly economical and AWS similar to ANFO
- Excellent fragmentation and heave
- Can be used in more difficult water conditions with a "stacking out" process
Water Resistance. The water-resistance of WR is not derived by individually ‘water-proofing’ each individual ammonium
nitrate prill. If this was possible, there is a high probability this type of anfo composition would not detonate properly.
Excessive amounts of water would fill the spaces between the prills and quench the reaction by absorbing heat from the
reaction and not allowing the detonation to be sustained. In WR the additive on the surface of the prills thickens quickly and
forms a bridge between prills, which minimizes further water penetration. Wet prills turn into a slurry-like water-restraining
barrier, which detonates fully in the explosive reaction.
This barrier material is very close in nature to a slurry composition.
When water comes in contact with WR, both air and fuel are
captured and held in place, as a result of the quick swelling action.
This material can be considered ”insensitive” slurry, which
detonates fully with the aid of the dry prills in close proximity. Water
in the barrier material is converted to steam in the detonation
process and results in higher borehole pressures. Higher borehole
pressures translates into more heave and better fragmentation. In
fact, WR in wet conditions produces better results than in dry
conditions, because of its effective usage of absorbed ground water.
In contrast to WR, when regular anfo gets wet, the prills dissolve, allowing the air and fuel to escape to the top, leaving a
heavy dense mix at the bottom of wet prills in a saturated ammonium nitrate solution. This wet anfo material has little
chance of detonating and adding energy to the shot.
Energy. Thermodynamically, WR Anfo is very similar to anfo. There is no water or any inert materials in WR. This
produces a high absolute weight strength, which shows the amount of energy per gram of material.
AWS 859 cal/g
ABS 739 cal/g
Typically, emulsions or slurries include 10-15% water in the composition. Water can help increase borehole pressures,
but has no energy value. Therefore, WR-Anfo produces more energy per pound of explosive than emulsions or slurries!
Manufacture. The manufacture of WR Anfo is quite a simple process. First, raw low-density ammonium nitrate is loaded
into the storage hopper. The prills are then oiled to the correct percentage and put into the holding tank. Next, oiled prills are
augered under the additive tank and the appropriate percentage of WR premix is added. Prills are then mixed in the vertical
and horizontal mixing augers until they are discharged into a bulk truck for loading. In a plant situation, mixed WR would be
augered into a bagging hopper and packaged in bags appropriate for handling. WR can be used immediately, but benefits
somewhat from aging.
Density. .82-1.0 g/cc. The density of WR Anfo depends on the density of the anfo used for its manufacture. Typical anfo has
a density around .82 g/cc. The density of WR is only slightly higher, about .84 g/cc, as a result of the additive. However,
ammonium nitrate prills with both lighter and heavier densities can be used to produce WR. In particular, the “mini-prill” has
been used with very good results. This prill has a density of about 1.0 g/cc and an average diameter of about .6-.9 mm.
Typical anfo is about 1.2-1.5mm. The smallness of the prill allows for a higher amount of water resistance by reducing the
size of the spaces between prills, making it more difficult for water to penetrate. In addition, the higher density of this prill
allows for more explosive per foot of borehole, which may be desirable in some situations.
Not all prills are appropriate for WR. Prills with a diameter much larger than 1.5 mm are generally not recommended.
Larger prills result in larger gaps between the prills, which can reduce the amount of effective water resistance. Also, large
high-density prills are less sensitive than smaller prills and may not be appropriate for WR.
VOD. The velocity of detonation of WR tends to be about 5-7% faster than the anfo it is made from based in the
corresponding borehole diameter. Geological conditions may also play a role in VOD as it does for all explosives. VOD
testing of WR at the Demenigoni Project in Hemet, CA showed velocities to average about 14,700 ft/s in 6 ¾ inch boreholes.
Sleep Time. Since the water resistance of WR is accomplished by the quick swelling of the surface coating and barrier
formation, it is helpful to understand the nature of this barrier. Initially, when the barrier is first formed from water
penetration, it is about ¼ to ½ inch in thickness. It is soft in its texture and will hold back water from penetrating into the
explosive column. However, the barrier continues to grow very slowly in thickness with time, in a process similar to
osmosis. The initial rate of growth is about ½ inch per day, becoming slightly less in following days. Therefore, WR is
designed to be loaded and shot on the same day. However, many shots have slept overnight, and even over the weekend,
with excellent results. But the time factor should be kept in mind.
Good Heave and Fragmentation. WR is known to produce good heave and fragmentation. This is a result of full borehole
loading in wet conditions, higher velocity than anfo and the effective use of water during the detonation process. With
correct usage of the WR Blasting Method, WR will produce a deep power trough, which is indicative of good gas expansion.
WR Blasting Method
As mentioned earlier, the WR blasting method was developed through trial and error by explosive users in actual field
conditions. The current method is both highly reliable and produces excellent fragmentation and heave. This method
consists of dewatering, priming, stacking out, loading and double priming.
The first step in the WR blasting method is dewatering and is required if the borehole is making water or has standing water.
WR does not pour through water well, and tends to bridge and seal off. In addition, pouring into standing water will increase
the ratio of wet-to-dry prill and will not produce maximum results. Therefore, good dewatering procedures should be
observed. Wet and dripping boreholes with little standing water may not require dewatering.
Dewatering pumps most commonly used with WR are BDS, Dryrite and
Legra. BDS is an economical pump, which utilizes pressurized air and
an inflatable bladder to seal a borehole and force water out. This method
works well and is the most economical pump. However, in fractured
geology compressed air is able to escape around the seal and the
effectiveness of this pump is reduced or made inoperable. The Dryrite
pump is a double diaphragm pump, which dewaters boreholes through
suction and a special dewatering nozzle. This pump works in all types of
geology, including fractured rock. Legra is a hydraulic pump for 6” and
larger boreholes. It is normally mounted on a pickup truck or bobcat. Its
advantages include faster dewatering, especially at lower depths.
Disadvantages include higher initial cost, higher wear and maintenance
factors and high cost of losing a hydraulic pump head down the borehole.
In addition to the above mentioned dewatering pumps, it is possible to simply shove an airline down the borehole, turn on
the compressor air and allow the water to “rain down” around the hole. Unfortunately, this is quite messy and the reason
most blasters invest in dewatering equipment. Although dewatering requires some extra time, often the time spent on
dewatering can be made up through faster borehole loading times, and avoiding problems of loading through water.
Borehole collars are recommended during dewatering as it will reduce the potential of rock falling into the borehole during
dewatering and losing a pump head.
The next step in the WR Blasting Method is adequate and proper
priming. WR Anfo is slightly less sensitive than anfo and should
have a 1lb cast good priming will cause better initiation and
generally give better results. This also applies to WR.
After priming the process of stacking out is generally
used. Blasters tend to add two or three sticks of an
emulsion or slurry product immediately after priming
and before loading WR. The WR pours around
thebottom sticks and fill the empty spaces, producing
full borehole loading. This in effect produces a
heavier ‘toe’ load and puts the power where it is
needed most. In addition, it may be necessary to add
additional sticks in the stacking out process,
depending on the amount of water the boreholes are
WR requires good dry contact with priming and should be loaded onto the top of a dry stick or primer. If the stick is
submerged in water, then additional sticks should be added until the top is above water. This technique is
somewhat of a self-adjusting method. Boreholes that make water faster may require a few more sticks, while
boreholes producing water more slowly will require none, other than the toe load. Typically, after the primer and the
first two sticks are loaded, the blaster will listen for sounds down the borehole. If the last stick produced a “splash”,
then another stick is added, until a “thud” or the sound of a dry stick hitting a dry stick is heard.
The loading of WR is the next step. WR Anfo is loaded into boreholes in the same manner as regular anfo. Simply slit the
bag and pour. WR will produce full borehole loading and protect the explosive column from water incursion.
Besides loading from bags, WR can also be loaded from a
common anfo bulk truck, when appropriate. Although WR can
substantially reduce the usage or more costly packaged stick
explosives, WR Anfo will not totally replace all emulsions or
slurry usage. Besides the helpful usage of these products in
the toe load, there may be holes that are considered
“undewaterable”, where the water returns as fast as it can be
taken out. In these cases, a water-resistant emulsion or
equivalent is used to stack out of the whole borehole in the
traditional fashion. This may be necessary occasionally,
depending on your water conditions. However, often in wet
shots 80-90% of boreholes can be loaded with WR Anfo. It is
good to keep in mind; “Use WR where you can. Use emulsion
where you have to!” This will lead to best results and greatest
The double prime is the final step to consider and is an important aspect of the WR Blasting method. In wet holes it is
always possible that active water flow can wash out an area of the explosive column.
WR is somewhat more susceptible to washout from active water
flow than emulsions or slurries because of the barrier texture. A
separation in the explosive column from flowing water has the
potential to cause a cutoff. Double priming has proven to be
effective insurance and good protection from cutoffs. In addition, if
for some reason the explosive had fallen into low order detonation,
the second prime acts to boost the explosive column back into
high order and provide additional high energy. Double priming is
always considered a good investment in effective blasting.
WR can also be used with bulk loading equipment. If an onsite mixing system is used, then WR can be manufactured and
augered directly into a typical anfo truck for immediate loading into boreholes. Manufacturing from a fixed setup is generally
recommended. This produces a high-quality WR product and reduces equipment cost.
It is desirable for the basic WR Blasting Method to
be used in bulk situations, as it will produce the
best results. Boreholes with light water conditions
will require just dewatering, priming and loading
WR in bulk. However, boreholes producing faster
water may require the stacking out process,
according to the traditional methods, and the use of
“chubs” or some packaged product. In other
extremely wet and undewaterable boreholes, a
highly water-resistant emulsion may be required
through the total hole. Basic WR loading
procedures always apply, however, the technique
should be adjusted to produce the most efficient
loading and effective blasting results, taking into
account the individual site conditions.
Construction. WR has found many popular application in the construction industry. Trenching is one of the most common
applications. Often shallow wet and dripping trenching boreholes cause anfo misfires. WR gives great protection in these
situations and often may not require dewatering. WR also gives protection against rain runoff and other weather-related
loading problems encountered in construction projects. Road construction is another popular use of WR. In the Northwest
US and Western Canada WR is the preferred explosive for construction of logging roads. With the high natural rainfall in this
geographic area, most every shot must contend with water conditions. WR has become the leader in this highly competitive
and cost-conscience market.
Quarries. WR is commonly used in many quarry situations. The usage of WR may be used to help reduce the usage of
more expensive slurries and emulsions stick products. In addition, the full borehole loading of WR produces excellent
fragmentation and heave.
Permafrost. WR has good application in cold northern climates with permafrost. Anfo in permafrost conditions tends to melt
the frozen ground, which leads to the formation of a dense ammonium nitrate solution in the bottom of the boreholes. This
can lead to misfires and cutoff. WR is used to eliminate these problems by reducing melting. In addition, WR does not have
to be maintained at warm temperatures for usage during exceptional cold weather.
Difficult Geology. Certain difficult geological situations may also be good applications for WR. In the recent construction of
the Domenigoni Resevoir Project in Southern California, difficult geological conditions were encountered. Alternating layers
of hard and soft metamorphic rock formations (quartzite and gneiss) were combined with fracturing and water conditions.
Soft rock formations tended to absorb shock energy of emulsions, while the more water-resistant emulsions do not produce
enough gas heave, for adequate displacement and excavation. Onsite testing showed that WR Anfo produced better
results that 30/70 emulsion with the same drilling pattern and borehole size in these conditions. WR was chosen as the
primary explosive because it produced the best combination of water resistance, shock energy and heave.
WR has a long history of over 30 years usage in a variety of wet conditions and geological formations. Its development in the
Northwest US was based on the need and preference of explosive users. The continued success of WR shows both its
effectiveness and reliability for construction, quarry and mining projects. By using WR in the applicable situations, explosive
users can maximize their blasting results and reduce their costs.