Artificial Rigging Anchors for the Present and Future
John H. Ganter and William
K. Storage
Information and disclaimers
IF CAVES ARE TO BE
MAINTAINED in natural condition then visitors must try to minimize their
impact. We tolerate minor exceptions to this rule that will return relatively
large rewards in terms of more documented passage, or a reduction of hazard.
The exceptions are most obvious in vertical caving, where either exploration or
visitation may require artificial anchors to be placed for rigging. There are
analogies to the physical infrastructure (roads, bridges, etc.) that is built
and maintained for the common good. Our intent here is to provide some reliable
advice on where and how these investments are appropriate and how they should
be maintained. An important theme is that of false economy: cheap materials
and/or laziness will waste effort, will result in more damage to the caves, and
can result in hazard.
Permanent rigging
article discusses self-drives and stainless hardware (Nov. 1999)
|
We began
with three simple observations: 1. At some percentage of
vertical drops there is no way to secure ropes to existing cave features to
minimize abrasion, and thus artificial anchors must be installed; 2. Artificial anchors
will deteriorate over time, particularly if they are not maintained properly; 3. Cavers can come to
rely excessively on artificial anchors, placing them even where natural
anchors exist; From here we tried to
assemble information that would help the caver to make responsible decisions.
This is an account of both research and exploration. What resulted was a
complete re-assessment of both the published English-language literature and
our own beliefs. The result is that we may have to reconsider how we build
our infrastructure in the future, and regard past investments with increasing
caution. TRENDS
IN ARTIFICIAL ANCHOR TECHNOLOGY Why is the population of
anchors increasing, and what will the effects be as this population ages?
Which anchors will be most reliable for which applications? To consider these
questions, we must first examine two trends. The Increased
Availability of Anchors and Hardware Over the past 10 to 15
years there has been a gradual increase in the number of artificial anchors
used in caving. The reasons are numerous. In part, we are pushing more
difficult caves further. In part, we are more aware of expedition caving and
aid climbing where artificial anchors have played a large role. And there is
definitely a difference in availability: anchors, a variety of hangers,
hammers, etc. are all available from caving equipment dealers. So there is
much more chance that "Joe Caver" will have a "bolt kit" and use it. In the early 1970s
increased availability resulted from interest in rock climbing. Petzl and
Troll marketed products aimed specifically at cavers. To understand these
effects, one must consider the vertical caving techniques of European and
British cavers. The earliest experiments with SRT occurred in France during
the 1930s and 1940s, but ladders (and winches for long drops) were favored
into the 1960s (Worthington, 1989). Alpine conditions, cold water and thinner
ropes have since given SRT and artificial anchors a major role. This approach
has allowed small teams to push cold, remote caves to depths of over 1000
meters. When the same approach has been attempted by less competent cavers in
heavily-visited caves, there have been problems with poorly placed,
deteriorating, or simply unnecessary anchors. |
OUTLINE TRENDS
IN ARTIFICIAL ANCHOR TECHNOLOGY OBJECTIVES
FOR ARTIFICIAL ANCHORS THE
MAJOR OPTIONS IN ANCHORS 2 Studs CHOOSING
ANCHORS: BALANCING STRENGTH AND RELIABILITY |
Rechargeable Hammer Drills:
Painless Drilling
The second important change
is the introduction of battery-powered hammer drills. According to Peter Ludwig
(1988) the original AC-powered hammer drill was developed by HILTI Company of
Liechtenstein. This was superior to the traditional "impact drill" incorporating a rotating serrated
disk that alternately pushes the drill bit forward as it turns. It was
necessary for the user to push the impact drill hard to make it work. However,
the hammer drill has a solenoid that operates a pneumatic cylinder to hammer at
about 4000 impacts per minute (Hilti, 1989). It puts most of its energy into
impacts (about 1 Joule each), and it does not have to be pushed hard by the
user (Gebauer, 1986). After HILTI's patents expired, it was Bosch of West
Germany who produced the first DC (battery-powered) hammer drills. Others,
including HILTI, quickly followed.
Using a rechargeable hammer
drill, a caver can set an anchor in less than a minute and a power pack will last
for 10 to 20 holes (depending on various conditions like rock hardness, ambient
temperature, etc.) Clearly this 4 kg (9 lb) tool has the potential to change
the way in which we cave, because it makes placing artificial anchors so easy.
Effects of the Trends
The result of these two
trends is that we have to re-examine what we know about anchors and how we use
them. Due to the marketing and convenience, we have tended to use self-drill
anchors over the past 10 years (self-drill refers to anchors which have
drilling teeth on them; they are both a disposable drill and an anchor). For
drilling holes by hand, self-drills are the choice of many experienced cavers.
Other "sleeve-type" anchors with internal threads are also in common use.
Figure 1: The two
basic categories of artificial anchors, and related terminology
But these are all turning
out to be more prone to deterioration than might have been expected.
Interestingly enough, they have long been out of style for surface climbing.
Now the hammer drill provides the opportunity to drill holes easily, even with
awkward orientations. Should we set other anchors that will last longer? In
what orientation should anchors be set? What hangers should be used?
OBJECTIVES
FOR ARTIFICIAL ANCHORS
What is an Artificial
Anchor?
To begin, anchors fall into
two broad classes. Each is a metal fitting that goes in a hole drilled in rock
(Figure 1). The self-drill has teeth that allow it to first be used as a drill.
An expander cone is then placed in the open end, and the anchor driven home. A
set screw, usually called a "bolt," attaches a hanger to the anchor and the rock surface.
Hangers connect the caving
rope to the anchor. This is usually through a carabiner or Rapid-link, although
some hangers support the rope directly. Hangers are of two basic types: those
that are radially loaded and those that are omnidirectional.
The stud is driven into a
hole drilled with a bit. Some type of protrusion then acts as a barb to keep it
from being withdrawn. The end of the stud is threaded, and a nut is used to
hold the hanger against the rock. There are variations and hybrids on these
themes, but this is sufficient for general discussion. Later, we will give a
more complete classification of anchors.
What is a Safe Anchor?
To be safe, an anchor must
provide not just a place to hang a rope, but also for the avoidance of hazards.
A good anchor allows the caver to be on rope while staying away from features
of the cave which are judged to be hazardous: sharp and/or abrupt lips, loose
rocks, water, etc. It should be strong enough to take the dynamic loads that
would result from failures of other equipment or errors on the part of the
cavers. And it should be reliable for users who do not know its history, and
who may be less competent than those who installed it.
To be reliable, the
placement must minimize susceptibility to deterioration if the anchor is left
in place. Both the anchor design and the anchor placement must be damage
tolerant. The strength of a newly placed anchor is almost irrelevant. More
important is the strength of the aging anchor and the detectability of its
deterioration.
How strong should an anchor
be?
Anchors, artificial or
natural, should be at least strong enough to hold the maximum loads that a
caver could survive. Eavis (1981) suggests 1200 kgf (2640 lbs) as a maximum
force survivable in a harness. (This force would be exerted by a 77 kg (170 lb)
person taking a 15.5 g fall, i.e. decelerating at 15.5 times gravity). For very
short durations, accelerations of 35 g's have been survived, but 15 g's is an
accepted limit where the back bends forward to limit motion (Damon and Stoudt,
1966).
A Brief Mechanics Tutorial
1 Loading
Judging the quality of an
existing anchor requires some knowledge of the mechanics of the system. When
the anchor (except for the adhesive types discussed below) is secured in its
hole, a large compressive force is developed along the anchor-rock interface.
This force provides the friction that resists pullout (axial direction, tensile
force). The importance of a tight fit for pullout loading is thus obvious.
Figure 2: Idealized
self-drill and stud anchors in tight holes. Axial preload enables the
rock/hanger interface to oppose the load applied by the rope (W), with friction
force (F). Note that the self-drill anchor is optimally placed just below the
rock surface.
The rock stress from this
compressive force exists with no applied load. If an axial load is applied, the
rock stress increases until the rock breaks along a conical plane of maximum
stress. Shear loads will cause a slightly different failure shape.
Most small anchors are
stronger in the pullout direction than in the perpendicular or radial loading
(shear force) which is more common in cave use. This applies for both anchor
failures and rock failures. Still, there are several reasons radial loading is
preferred. While undesirable, it is possible to use a radially loaded anchor
which is loose (Brook, 1965), with the hanger bearing directly on the anchor or
bolt. Many combinations of anchors and hangers result in the hanger being
coupled fairly tightly to the wall. Thus minimal bearing occurs and in normal
loading the "shear" loading actually results in little applied shear stress to the anchor.
Tables of anchor shear strength are thus often mis-applied. A common
misconception (e.g. Seddon, 1986; Meredith and Martinez, 1986) is that the
stress due to applied shear loading and torquing are directly additive.
In most anchor systems, a
nut or bolt is torqued down, squeezing a hanger against the flat rock surface
(Figure 2). This squeezing is called tensile preload, since it is a tension or
pull induced in
the anchor before it is
loaded by the caving rope. This preload results in a frictional interface
between the hanger and rock wall, which supports part of the load. As long as
this coupling is maintained, the only significant force (and resulting stress)
in the anchor is the tensile preload.
Unfortunately, maintaining
this coupling requires that the preload, resulting from torquing, be somewhat
higher than the applied load. A fall, the failure of another anchor, or
possibly high loads during ascending may result in decoupling the hanger from
the wall. This results in the hanger bearing down on the bolt or stud directly.
A much different stress state then exists.
Figure 3: A self-drill
anchor with a loose hanger resulting from a lack of preload. The hanger bears
on the bolt directly. If the hanger is thin, the bearing stress is very high.
Shear and bending stress also occur, and result in extension and compression
within the screw. There is no pure axial tension.
The new stress state is
complex, a combination of shear, bending and compression (bearing). Shear
stress, from the radial loading, attempts to deform the anchor as shown in
Figure 3. Bending stretches the top half of the anchor, adding axial tension.
Unfortunately 8mm
(1/4-inch) bolts are often not strong enough to withstand the preload that
would be required to prevent decoupling under the loads established above. It
is difficult to imagine that optimum preload could be applied or maintained in
the cave environment. We conclude, as have most others, that 8mm (1/4-inch)
bolts are risky and should not be used as rappel anchors.
In the case of self-drills,
a similar stress state will exist if the bolt is torqued, with the hanger
coupled to the anchor instead of the wall when the anchor is slightly underdrilled
(Figure 4). The test results of Brindle and Smith (1983) (Figure 5) show the
results of increased bending stress from a 2mm protrusion.
Figure 4: A self-drill
that is underdrilled, but has a torqued bolt. Since a preload exists, it
experiences only axial tension, bending, and shear.
Studs have some advantages
where stress is concerned. First, the preload is distributed over the entire
hole diameter, not just the central bolt in a self-drive or sleeve-anchor.
Second, the need for high preload is reduced because of this greater bearing area.
For a more detailed discussion of the relationship between bolt preload, stress
and shear load capacity, we suggest an engineering design textbook such as
Juvinall (1983).
2 Axial and Other Loading
Angles
In cases where various load
angles are basically directed at the head of the bolt (Petzl Clown and Petzl
Ring, for example) the anchor strength will vary predictably between that
achieved in radial and axial loading. Some older hanger designs cause leverage
tending to increase anchor loads as mentioned for axial loading. The newer
Petzl designs greatly reduce this tendency. On the basis of our stress
analysis, and testing by Brindle and Smith (1983), small variations from
straight radial loading do not significantly affect anchor strength.
Figure 5: How strength
decreases in an improperly placed anchor
Because radial loads are
always applied at some small distance from the wall, there is a tendency for
the hanger to pivot about its bottom end. This results in leverage and some
axial (pullout) component to any applied radial (shear) load. Lawson (1982) and
Brindle and Smith (1983) have noted that the minimum net axial force will
result from load application at some angle between radial and axial directions.
This varies from straight axial by 15 to 40 degrees depending on hanger
geometry. We agree with their observations on minimum net axial force but
disagree with the conclusion that they represent "optimum loading angle." Loading at these angles will result
in minimum stress only if no shear or bending is present in the anchor/bolt.
Achieving such an "optimum loading angle" in caves would often mean placing anchors in overhanging walls where
drilling is difficult and the consequences of poor placement are severe. We
support Lawson's contention that increasing the load angle beyond "optimum" rapidly increases stress to
dangerous levels, and feel that this is a further argument for anchor placement
that results in loading which is close to straight radial.
3 Adhesive Anchors: A
Special Case
Adhesive anchors, discussed
in detail below, consist of a stud glued into a hole. Manufacturers of adhesive
anchors claim that no expansion stress is placed on the rock and that true
bonding of the anchor to the rock occurs. Their strength testing in weak
concrete supports this. Until a load is applied to adhesive anchors, induced
rock stress around the hole is essentially zero. This obviously leaves a larger
percentage of the rock's strength to withstand applied loads.
Understanding Corrosion
Some popular corrosion
fallacies exist in caving circles; one of these is stress corrosion. Stress
corrosion cracking is a well known phenomenon where some metals, in a state of
high mechanical stress, undergo accelerated electrochemical decay. The mechanism
is complex and interesting, but largely irrelevant to caving. Stress corrosion
is not observed in the combinations of alloys, heat treatments, stress levels,
and environments encountered when commercial anchors are used in caves (ASTM
Committee on Wrought Stainless Steels, 1978; Scharfstein, 1977).
Figure 6: An aluminum
hanger and carabiner attached to a carbon steel stud. The diagram shows the
probable sequence of events leading to degradation each component. A stainless
stud would help the situation. Note that the effects of "galvanic corrosion" are secondary; electrically
insulating the parts would not reduce corrosion rates. Items 1, 2 and 3 can
occur independently-- the sequence is not necessarily a cause-effect
relationship. The progression from 3 to 6 is definitely causal.
A color versionn of Figure 6 is here
Another corrosion myth
results from the table of electrode potentials found in chemistry and physics
books. It is commonly held that the corrosion susceptibility of an anchor is a
consequence of the difference in electrode potentials of the various materials
(bolt or nut, hanger, anchor, etc.) (e.g. Riley, 1984a,b). While combinations
of greatly different materials are undesirable, this belief is inaccurate. On
the basis of electrode potentials, steel pivot pins in aluminum carabiners
should not corrode, but they do. The conditions for which the table is valid
(film-free metals in solutions with their normal activity of ions) rarely exist
in cave environments. While beyond the scope of this article, the reasons for
this are well documented (Evans, 1960).
For fasteners in caves,
electrical cells may be set up even if all materials are the same. An
electrical current may result when the portion of the anchor buried in rock
limits the flow of water and oxygen to the metal surface. Oxygen exhaustion
creates a small anode and the large exposed anchor surface becomes a cathode;
corrosion follows (Evans, 1960). In such situations, the presence of grease is
beneficial because it reduces ionic activity.
Corrosion mechanisms are
intricate. Corrosion rates are greatly affected by the presence of trace
quantities of salts and metals in solution. One part per 50 million of copper
in water will cause pitting of aluminum, when calcium bicarbonate, oxygen and a
chloride are present (Porter and Hadden, 1953). Carbonates and bicarbonates
sometimes inhibit and sometimes facilitate steel corrosion (Wallen and Olssen,
1977). It has been found that 100 parts per million (ppm) of calcium carbonate
in groundwater can reduce corrosion of mild steel (Coburn, 1978). It is almost
impossible to predict what will occur outside of carefully controlled
laboratory conditions.
A more productive approach
for cavers is to employ the history of industrial applications for guidelines.
The majority of anchors in caves today are pre-expanded studs, self-drills and
other similar sleeve anchors. Conservation considerations aside, for short-term
exploration these may be adequate; for longevity they definitely are not. These
fasteners are zinc plated or galvanized carbon steel, typically 1020 or 1030
alloys. Industrial experience tells us, beyond any doubt, that these will
eventually corrode. The mechanism is not complex. They just rust away,
progressively losing strength. Our testing of old pre-expanded studs from a
rock climbing area indicates a loss of strength directly predictable from the
loss of section thickness (Storage, 1980). It is inevitable that a significant
percentage of anchors will be unsafe after 10 to 20 years of service. How old
are they now?
The corrosion of aluminum
hangers is much less predictable than that of steel bolts and anchors used in
caving. We have some samples with uniform, multicolored corrosion products and
others with a few deep pits. Several alloys used for hangers (2000 and 7000
series) corrode severely in cave environments. Stress corrosion at low stress
levels is observed in these alloys even under surface conditions. The corrosion
may be intergranular in nature, with extensive subsurface damage. The presence
of steel anchor corrosion products accelerates the aluminum corrosion. A
significant loss of strength can accompany a negligible loss of mass.
As is often the case, the
strongest alloys are among the worst in terms of corrosion susceptability. It
is ironic that our single-minded quest for high strength has sometimes left us
with inferior products. Conscientious manufacturers have selected weaker alloys
with better corrosion resistance, despite competitive pressures to increase
strength. We conclude that, while their light weight is useful for some
applications like aid climbing, even the best aluminum hangers have no place in
permanent rigging.
Since carabiners are left
with fixed rigging in some parts of the world, the same concern applies. They
are designed for strength, not corrosion resistance. Thin anodizing is merely
ornamental, and probably accelerates aluminum corrosion rates where it is
scratched. We have samples of deeply pitted carabiners which have sat in caves
for a few months (Figure 6). Steel rapid-links corrode more evenly and
predictably, and thus we consider them to be a safer choice. Stainless steel
rapid-links are even better.
From a corrosion position
alone, stainless steel seems to be the obvious choice. However, strengths of
materials must be considered. A discussion on balancing strength and
reliability appears below.
THE
MAJOR OPTIONS IN ANCHORS
Terminology
The first order of business
is to agree on a vocabulary. There are many types of anchors available for a
range of uses in construction and industry. Brand names only add to the
confusion, because they tend to be inconsistent. Here we will use generic names
that refer to the way that the anchor works (Figure 7).
1 Self-Drill Anchors
Overview Rock is hard. It can only be
drilled by tools made of even harder steel, which even then become dull fairly
rapidly. A popular solution has been the "self-drilling" anchor, which carries its own
disposable drill. Once set, the anchor accepts a bolt and a hanger to which a
carabiner or rope is rigged. Although marketed for securing machinery and other
fixtures to masonry, this has been seen as a reasonable system for artificial
anchors in caves. The "overhead" is a hammer and a driver to hold the anchor so that it can be hammered.
The supply of sharp anchors is whatever the cavers want to carry. Instructions
on setting self-drill anchors appear in a variety of publications.
Underdrilling and
Overdrilling
Occasionally one will see the results from a caver who apparently got tired in
the middle of drilling a hole and set the anchor anyway. The assumption seems
to be: Half in means half as strong and that's plenty. This is completely
wrong. Unfortunately the placement will probably hold for the fool that set it,
and then lie in wait for the naive caver who comes along later. This
underdrilling leaves the anchor and hanger sticking out from the wall,
resulting in a tremendous increase in bending stress. As can be seen in Figure
5, underdrilling by just 2mm can cut the strength of the whole system roughly
in half (Brindle and Smith, 1983). Strength is also reduced if the lip of the
hole is irregular and cone-shaped.
The other extreme is
overdrilling. Fortunately, an anchor that is placed too deep produces less
serious consequences. Assuming that the expander cone is still well in place,
the loss of strength is due to loss of contact between anchor and screw. In
these situations, high thread stress contributes to thread damage, an
increasing concern in Britain (George, 1990).
Figure 7: Anchor types
and terminology. All illustrations are stylized to demonstrate how the anchors
work. The products of two widely-available manufacturers are given as examples.
How much torque? Lawson (1982) warns that one should
not "overtighten the bolt since doing so can drastically reduce the load it
can support." This concern is valid, although it seems unlikely that bolt yielding
and loss of strength would occur without being obvious (i.e. the bolt head
twists off). Jim Smith (pers. comm.) reports that several 1/4-inch bolts have
been broken in Sistema Huautla by overtorqing. Our testing supports Smith's
observations. However, we were unable to break 8mm or larger bolts with the
wrenches that we use underground.
Small diameter bolts are
not strong enough to take the preload (and torque) necessary to maintain
hanger/wall coupling and prevent shear and bending stress. Large diameter bolts
can withstand the shear and bending, so the preload is unnecessary for stress
considerations. Considering the difficulty of knowing what torque is actually
applied under cave conditions, this is another argument against small
self-drill anchors.
Reports of Failure While some anchors are unreliable
to begin with and some are visibly deteriorating, reports of failures have been
scarce until recently. The majority are almost certainly unreported. Most that
are reported seem to be non-catastrophic, i.e. a caver noticed the problem
before loading the anchor and removed and/or replaced it.
In a rare reported instance
of short-term failure, cavers had chosen to do a pitch despite "all the signs of [it] having been
rigged by a half asleep caver in the middle of the night" (Warild, 1988). After exploration
to -945 metres, the cavers were ascending quickly through water when an anchor
pulled out, dropping one a short distance onto a ledge. The caver above repaired
things, then he in turn fell 2 metres and "just above him swung the belay [rig
point], a football-sized rock still attached by the tie-off sling." Clearly errors due to caver fatigue
and time constraints played a major role in this situation.
Reports of failure in older
anchors that are used heavily are beginning to appear. Apparently, threads are
suffering in high-traffic caves where each party installs their own bolt and
anchor. Nigel Robertson (1990) had passed beneath a rebelay in Rowten Pot when the
bolt pulled out of the anchor, dropping him 2 metres and resulting in a broken
back. In subsequent testing, the bolt "jumped the threads" when tightened into the anchor.
Robertson concludes that "this damage seems to be caused solely by abuse (i.e. gross negligence
and irresponsibility). Overtightening of bolts, using bolts with dirty or
damaged threads will all damage the threads in anchors." While warning against these same
practices, Dave George (1990) also states that "Many of the existing anchors have
been in place for over 10 years and are simply wearing out."
The problem undoubtedly
stems from both causes. Anchors rust over time. Rusty anchors are more
susceptible to damage. A single negligent or inexperienced group can seriously
damage the entire set of anchors in a cave, destroying the investment of time
and money they represent.
We believe that no
self-drive anchor can withstand the repeated insertion and removal of bolts in
these high-traffic caves. Even stainless studs will suffer from the abuse to
their threads. Perhaps the best solution would be to install anchors such as
the Petzl P38/P39 (discussed below) which have a captive hanger. While more
expensive at the outset, these anchors will neither require nor permit any
fiddling or abuse by subsequent visitors. While the technology is available, a
change in mindset will be required on the part of cavers if this investment in
new infrastructure is to succeed. If there is an anchor in the cave, it might
as well be of stainless steel with a highly reliable hanger affixed permanently
to it. This tiny difference in visibility and aesthetic appearance must be
balanced against the inevitable alternative: multiple, rusting anchors,
abandoned holes, failures and accidents. Surely in high-traffic caves there can
be little debate over which is the lesser of these two evils.
Maintenance Like most things, anchors will last
longer and be more reliable if they are maintained. This is particularly
important in the case of self-drills since lubricant will reduce deterioration
of low alloy and carbon steel dramatically. To service an existing anchor, the
bolt should be carefully removed. The threads in the anchor can then be blasted
out with a jet of spray lubricant. This will remove rock dust and rust,
displace water and penetrate into the inner parts of the anchor. The anchor
should then be squirted full of grease (Elliot, 1985). This can be petroleum
jelly in a squeeze tube. Heavy bearing grease is even better. This can be loaded
by using a spatula to fill a hypodermic syringe (with an enlarged nozzle) and
then squirting this into the squeeze tube for transport into the cave.
Of course it is even better
to grease the anchor when it is first installed. Sealing with silicone when the
anchor is inserted may also be effective in protecting the metal/rock interface
from deterioration. The best solution by far is to use stainless steel anchors,
thus avoiding the need for grease altogether.
2 Studs
The stud anchor is an
opposite approach to the self-drill; it provides a protruding threaded shaft
for the hanger, which is held on by a nut (see Figure 1). There are several
advantages. The stud is monolithic; a single piece of steel extends from the
back of the hole to the hanger. The result, generally, is that a 6mm stud
equals the strength of a 12mm OD self-drill anchor with an 8mm bolt. In
addition, the stud is never abused as a drill (Gebauer, 1986). The stud has no
internal opening to allow water to reach the inside of the anchor, nor will it
fill with mud or other sediment. Finally, studs are available in 302, 303, 304
(all roughly equivalent) or 316 (a more expensive form for marine applications)
stainless steel from a variety of sources. This alone is an important advantage
over self-drill anchors.
The disadvantages are that
a drill bit must be carried for drilling the holes. Since diameter control is
often critical to the strength of the anchor, drilling with an impact hammer
will produce better results. Drilling must be done very carefully with the
manufacturer's recommended bits.
Collar Studs There are several types of studs.
Those used commonly in caving are what we term Collar studs. Expansion
comes from a collar which encircles the stud. The collar is spread by the
cone-shaped portion of the stud just above the base. Depth control is not
critical, and in fact the hole can be intentionally overdrilled so that the
stud can be hammered in to close off the hole after use (often desirable in aid
climbing).
Wedge Studs Wedge studs are expanded like
self-drills. Unlike collar studs, depth control is critical. For a given
diameter, these anchors have nearly the same strength as collar studs. We are
not aware of any available in stainless steel.
3 Adhesive-Mounted Studs
Another option is to make
custom studs from stainless steel bolts or rod which do not expand in the hole.
Alan Brook (1989) has a set of these anchors, made from 1/2-inch rod, which are
in good condition after 10 years at the entrance to Jingling Pot. Alan uses
industrial-grade Araldite Epoxy Resin (Ciba-Geigy Plastics) to secure the
studs. The epoxy is not affected by water and most chemicals; Alan remarks that
it is used to secure roof bolts in mines. Given a source of fairly cheap
stainless steel rod, this appears to be an attractive option for high-traffic
caves. Petzl offers a "Ring" (P40) that apparently is set with an epoxy (Petzl, 1988?).
Most manufacturers of
expansion anchors now market adhesive anchor systems to be used with 3/8 to
1/2-inch rod or bolts. In very soft rock (under 900 kg/cm2 [1000 psi]) these
anchors offer markedly increased strength, due to the more even load
distribution along the buried portion (Raleigh, 1989). A 3/8-inch by 2-inch
adhesive mounted stud, properly placed in 3400 kg/cm2 (4000 psi) rock can
withstand shear loads of over 2700 kg (6000 lbs).
To use these anchors, a
hole slightly larger in diameter than the stud is drilled and then a glass
capsule is inserted. The capsule contains the correct proportions of epoxy (vinylester
or polyester resin), sand and hardener in separate chambers. The stud, with a
properly beveled end, is then used to fracture the capsule and mix the
contents. This must be done very rapidly by using a hammer drill (with the
impact turned off) to spin the stud.
Some suppliers also market
the adhesives separately. These can be used to seal and reinforce normal
expansion-type studs. Although discouraged by the manufacturer because of the
tendency for the adhesive to splatter everywhere as the stud is driven into the
hole, this combination will greatly reduce water seepage, corrosion and rock
deterioration.
4 Petzl Long-Life Anchor
System (P38/P39)
Petzl has recently
introduced a combination anchor/hanger made of stainless steel. The P38
requires a 12mm hole, the P39 a 1/2-inch hole. The P37 is a double-expansion
version for soft rock which requires a 14mm hole. Strengths are high (2100 kg
[4800 lbs]). While somewhat expensive and requiring large holes, these anchors
are very well engineered, with obvious forethought into minimizing bearing and
bending stresses. A wrench is not required for installation and no parts are
removable after placement. For high-traffic caves where artificial anchors are
proliferating, these appear to be excellent choices to provide long-term
reliability.
5 Non-Calking or "Sleeve" Anchors
When an anchor expands into
its hole, it is said to "calk" (e.g. Rawl, 1981). This refers specifically to the placement of soft
metal (typically lead) anchors which greatly deform in the hole and are not
safe for life support. Here we use the term "non-calking" to refer to anchors that are
removable after they have been placed. This is an attempt to clarify
terminology: in Britain these are often called "Rawlbolts." They have also been called "sleeve anchors" by Padgett and Smith (1987).
Montgomery (1976) describes two models, Centurion and Austin McLean.
The idea behind the
non-calking anchor is that it may be removed periodically, inspected and
greased (Brook, 1985). In some British caves, cavers provide their own anchors
for existing holes. The disadvantage to this is that the large holes (1/2-inch)
had to be drilled by hand. Today the holes could be drilled with hammer drills,
and the anchors have performed well, but the monolithic stainless steel studs
are certainly more attractive alternatives. In cases where rapid rock
deterioration is a concern, non-calking anchors can be removed for periodic
hole inspection. However, this approach does nothing to prevent hole weathering
and frequent anchor removal in soft rock will undoubtedly cause wear and
increase the rate of deterioration.
6 Pre-Expanded Nails or
Studs
These were some of the
earliest and most popular anchors used in caving and aid climbing. They are
simple, a one-piece stud split in the middle and then hardened so that two
opposing flanges are bent and compressed as it is driven into the hole. The
stud is threaded; the nail has a head and is not removable. Again, these terms
are misused and interchanged often in the literature. These anchors have
declined in popularity because they tend to pull out, sometimes under very
little force (Davison, 1977). The problem seems to arise in several ways. Some
limestones may not be hard enough to fully depress the flanges. While tests in
granite gave very good results (Montgomery, 1976), data from Molly (Emhart,
1989) indicates extremely low pullout loads in soft concrete. In other cases
weathering and solution, sometimes after the anchor is placed, may make the
rock too soft to hold the anchor. Dozens of climbing accidents have occurred
from use of these anchors (Leeper, 1977). Thus pre-expanded studs are not
recommended for long-term placements.
CHOOSING
ANCHORS: BALANCING STRENGTH AND RELIABILITY
Having established a
reasonable working load for anchors of roughly 1200 kg (2500 lbs) earlier (see What
is a Safe Anchor? above), we must now think about how to actually achieve
this goal with confidence underground. Margins of safety are used in design for
two main reasons. The first involves the level of confidence that the strength
of an individual item is the same as the samples that were tested and analyzed.
Obviously, we have limited confidence in the rock strengths. The second reason
involves deterioration and reduced strength as the item ages.
Fastener manufacturers,
such as ITW Ramset/Redhead (1989), recommend 25% of measured ultimate
(breaking) load as a safe working load, to account for strength scatter and
imperfect placement. The International Congress of Building Officials (1988)
recommends an additional 50% reduction where inspection is impossible.
These recommendation result
in a desired safety margin of 8 (or a theoretical 9000 kg [20000 lbs]
capability). This would require unacceptably large anchors; a 1-inch self drive
drilled deep into very strong rock, for example.
Redundancy is a better
approach. If parallel redundancy, or shared loading as described in a variety
of publications is used, the applied load to each anchor is halved. The
probability of simultaneous failures is low, and the likelihood of either
failing is reduced because of the divided load.
We feel that two anchors,
each intended to be capable of taking the 1200 kg (2500 lbs) load, is a safe
system, provided that they do not suffer significant loss of strength over
time.
In general this means a
10mm stud of a suitable stainless steel, placed properly. The strength of
smaller SAE grade 8 (a stronger material than stainless) bolts may be adequate,
but these corrode quickly. Since self-drives cannot be made from stainless (it
generally cannot be heat treated), studs have a clear advantage for long-term
placements.
The preference for
stainless steel eliminates many studs from consideration. The wedge stud design
is acceptable, but they do not seem to be available in stainless. Stainless
sleeve studs are available, but for a given hole diameter they will always be
weaker than collar or wedge studs. Sleeves offer somewhat better load
distribution than collars in soft rock, but nowhere near that of
adhesive-mounted studs. Stainless collar studs and adhesive-mounted studs
emerge as obvious winners for permanent placements.
THE
ETHICS OF ARTIFICIAL ANCHORS
Anchors beget more anchors.
Cavers sometimes place them poorly and even the best deteriorate. The next
cavers come along, don't like the placements or deterioration or sizes, and set
still more anchors. Where does it end?
Perhaps the most serious
problem is somewhat unobvious; the decline of good caving skills. Cavers who
learn that caving is pounding in anchors, or get in the habit of seeing them at
every drop tend to lose the ability to recognize and use natural anchors. Dave
Elliot (1983), a caving instructor who is a major proponent of artificial
anchors, has said, "In contrast to the fertile imaginings of the purists
among us, there are in fact very few natural belays in caves suitable for SRT,
artificial anchors are necessary on almost every pitch." Clearly a judgement has been
passed; don't bother looking because you won't find anything. Once these
beliefs about how drops are to be rigged spread, they can go to ridiculous
lengths. Paul Lydon (1986) has reported finding two easy 4-foot climbs rigged
from an anchor. Dave Brook (1987/88) remarks in reviewing Elliot's rigging
guide for the Yorkshire Dales that "the authors love messing about on rope, but
don't like certain aspects of caves such as climbs, crawls and especially
water."
The other side of the story
comes from the Oxford cavers (Rose, 1983) who have descended numerous deep
systems in Spain using artificial anchors on less than half the pitches. Kevin
Downey (1987) reports on trips to several deep European systems that have been
rigged using rebelay techniques, but with no artificial anchors. Explorers of
Mexico's Sistema Huautla have noted that about 70% of its roughly 600 pitches
have been rigged with natural anchors.
Naturally a lengthy and
heated debate has ensued, but to us some points seem worth noting. Anchors can
be thought of by less-experienced cavers as "hard core" and applied indiscriminately.
Anchors can be hammered in (often badly) by anyone who can buy a kit. Terry
Raines (1986) has noted that Sotano de la Golondrinas was descended regularly
for 16 years before the first anchor was placed. Now there are over a dozen. An
anchor is a permanent defacement of the cave, so poor technique affects
everyone.
Some drops unarguably
require anchors to be descended safely. In other cases, it is a judgement call
and the skilled caver can manage with careful use of natural anchors, rope
pads, etc. Steve Foster (1986) gives a good introduction to natural rigging,
and more articles on this topic are needed in the caving literature. Like
mountaineers and rock climbers, we may begin to see separate ethics for
artificial aid near home and far away (Mitchell, 1983). On Everest, just about
anything goes; on the local climbing face, a single anchor might be considered
very poor form. Too much technology can destroy the experience of caving. As
Mike Boon (1980) has observed, "How many bolts are needed before the exercise becomes pointless is a
matter for individual judgement." Ultimately it is a question of using
technology to enhance, but not overwhelm, the aesthetic experience of working
within the challenges of nature.
SOME
SUGGESTIONS FOR CONSIDERATION
Learn to find and use natural anchors safely
Set anchors responsibly, as an investment for the caving community
Use stainless steel studs
Use stainless steel hangers and bolts for existing self-drives, and
sleeve-anchors where anchors are removed.
Use grease on all self-drill anchors
Anchors, bolts and hangers should be placed well and left in place.
Subsequent visitors should not remove the bolts and hangers.
Don't use 1/4-inch anchors or studs
Don't leave aluminum hangers in caves
ACKNOWLEDGEMENTS
Alan Brook generously
shared his knowledge from many years of placing artificial anchors. Ed Leeper
and Steve Worthington contributed information and comments, but do not
necessarily agree with all our opinions. Bill Torode (National Speleological
Society Librarian) and Ray Paulson (BCRA Librarian) were very helpful in
tracking down articles. Tom Davinroy brought recent rock climbing literature to
our attention.
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Handbook, Vol. 3, 9th Ed., pp. 56-77. Metals Park, Ohio: American Soc. for
Metals.
Boon, Mike. 1980.
Editorial. Caving International Nos. 6 & 7, Jan & Apr 1980, p. 3.
Brindle, D. and R.A. Smith.
1983. Strength of Rock Anchors. Caves & Caving 20, May 1983, p. 22-26.
Brook, Alan. 1985. Placing
1/2" (12mm) Rawl Bolts: A Viable, Permanent Belay with a life of 10 or 20
years +. Caves & Caving 29, Aug. 1985, p. 13.
Brook, Alan. 1989. Letter
to J. Ganter, 6 March. West Yorkshire, England. 5 pp.
Brook, Dave. 1988. Book
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p. 24, also in Caves and Caving 38, Winter 1987, p. 35.
Coburn, Seymour K. 1978.
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Stoudt, et al. 1966. The Human Body in Equipment Design. Cambridge,
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rebelay fray [letter]. NSS News March, p. 47, 66.
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Author's email: jganter@sandia.gov, storage@nerve-net.com
Publication History
|
1990 |
NSS News
[National Speleological Society USA] 48:5, May 1990, pp. 120-128 |
|
|
1990 |
Der
Abseiler, 11, December 1990, pp. 25-36. |
Translated
by Daniel Gebauer |
|
1991 |
Cave
Science, Transactions of the British Cave Research Association 18:2, August
1991, pp. 111-117. |
Adapted
and updated version |
|
1998 |
online
version (HTML and Adobe Acrobat) at http://www.nerve-net.com/jg/c/pubs (v08, 19 July 1998, HTML on 4
September 1998, 14 December 1998). Quote marks fixed 1 November 1999. |
Slightly
revised from Cave Science |
Ó 1990-1998 by John H. Ganter and
William K. Storage.
All Rights Reserved. No part of this document may be reused in any way without
the written consent of the authors. Non-commercial distribution of the complete
document is permitted, provided that this notice and all other portions are
retained.
Disclaimers: This document is an account of
independent research, and is written for the consideration of experienced and
properly-trained cave explorers only. The authors do not make any warranty,
express or implied, or assume any legal liability or responsibility for the
accuracy, completeness, or usefulness of any information presented herein.
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used for descriptive purposes only.
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