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Examination of a rock-climbing fatality caused by equipment failure
Engineering Failure Analysis 22 (2012) 21–27
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Examination of a rock-climbing fatality caused by equipment failure Michael Law ⇑, Stephen Hawkshaw Climbing Anchors Australia, 3/1 Hi Tech Drive, Toormina, NSW 2452, Australia
a r t i c l e
i n f o
Article history: Received 19 April 2011 Received in revised form 2 December 2011 Accepted 20 December 2011 Available online 8 January 2012
a b s t r a c t Climbers rely on anchors for safety; the type of anchor used depends on the rock type. This work describes a fatality caused by rope failure, which in turn was due to the failure of a anchor. Testing of similar anchors revealed insufficient strength in the rock type where the accident occurred. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Rock climbing Testing Load analysis
1. Introduction, description of climbing The normal method of climbing with a rope for protection is that one person climbs (the climber in Fig. 1) while the other (the belayer) secures the rope by threading the rope through a device which provides extra friction for holding the rope, this action is called belaying. The climber clips their rope through anchor points which may be fixed (permanent mechanical anchors in pre-drilled holes), or more specialised equipment which uses existing rock features such as cracks or spikes which is removed with each ascent. In this paper the term anchor implies a fixed anchor. The climber clips their rope through anchor points as they climb. If the leader falls from a certain distance above the last anchor point, they fall twice this distance (Fig. 1) until the rope comes tight on the belayer. The energy of the falling climber is absorbed by a combination of rope slip through the belay device and stretch in the rope. When the climber reaches a ledge where there are anchors (generally two or more), they secure themselves to these anchor points and belay the other climber up the section of the climb. As the rope is above the second climber, any falls they take are small. 2. Description of climbing accident In 2009 two climbers were attempting Bunny Bucket Buttress, a popular 275 m climb in the Blue Mountains, NSW, Australia. The anchors on this route are all fixed. They were not carrying a route description and climbed off route after the 5th belay; they moved right 20 m to a line of anchors on an unknown climb. At some point on this unknown climb the more experienced climber said that they were no longer on Bunny Bucket Buttress, and that the anchors on this unknown climb were not standard for the Blue Mountains, and were probably not safe. They continued climbing and higher on this unknown climb they belayed on a ledge using a single anchor. Above this belay the rock was blank and could not be climbed. To bypass this section the leader hung his bodyweight on one of the anchors. With both hands now free he attempted to clip a loop of climbing rope to the next anchor with a ⇑ Corresponding author. Tel.: +61 2 9717 9102; fax: +61 2 9543 7179. E-mail address: [email protected] (M. Law). 1350-6307/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2011.12.003
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Fig. 1. Climber and belayer before and after a fall (drawing courtesy of Simon Mentz).
Fig. 2. Diagram of rope position before the accident while trying to clip the next anchor with a stick.
M. Law, S. Hawkshaw / Engineering Failure Analysis 22 (2012) 21–27
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karabiner attached to a stick (Fig. 2). There is a significant amount of outwards force during this manoeuvre, while attempting to clip the next anchor the anchor he was hanging on failed. The leader fell past the belay ledge and his single rope was severed by the band of sharp ironstone rock. The belayer was attached to an anchor 3 m higher. The position of the anchor allowed it to be loaded downwards, in shear, where it had higher strength than when exposed to an outwards load. This anchor was later removed easily by hand by pulling outwards with less than 0.2 kN load. The climbers responsible for placing these anchors were identified a group of foreign climbers unfamiliar with the weak rock in the Blue Mountains who were trying to do a new route. This group had been warned that the bolts they intended to use were unsuitable for the weak Blue Mountains sandstone [1]. An early website report of the first ascent of this climb (in Croatian) mentioned that the bolts could not be satisfactorily tightened; the nuts only offered a low resistance to tightening and the shafts kept pulling out of the rock; it appears that this report is no longer available. The bolts were identified as being 8 mm diameter 80 mm length Raumer brand wedge bolts; recommended for caving in strong rock; but not suitable as anchors for rock-climbing or in weak rock. After the accident all bolts were removed from the climb, most of them by hand at loads of less than 0.3 kN. There is video of the bolt removal [2].
3. Description of anchor Mechanical expansion bolts expand as the bolt is tightened; there are two common types of expansion bolt, sleeve and wedge anchors. Sleeve anchors have a full length sleeve (Figs. 3 and 4) which is compressed between the nut and the cone when the nut is tightened; the cone then expands the sleeve. Wedge anchors have a short expansion sleeve (Figs. 3 and 4). When a wedge anchor is placed in strong rock, the sleeve is larger than the hole and must be compressed. The compressed sleeve presses outwards against the sides of the hole and the frictional force generated is overcome by hammering the anchor in. When the nut is tightened the cone pulls back inside the sleeve and expands the sleeve, giving a very solid anchor with typical axial load bearing capacity of >20 kN. The bolts in this accident had a twin cone and sleeve design (Fig. 3).
4. Anchor strength and rock strength Anchor performance depends on rock strength. Rock strength is measured by the unconfined compressive strength (UCS) to ASTM standard D 2938 ‘‘Method for Determining the Unconfined Compressive Strength of Intact Rock Core Specimens’’. While most climbing worldwide is performed on rock with good strength having a an of 50 MPa or greater, Blue Mountains rock is much weaker, having reported strengths in the range of 24–56 MPa dry and 8–32 MPa wet [3,4]. Much of the rock in the Blue Mountains is damp internally, so the lower strength values for wet rock are better indicators of strength. For this article, strong rock is defined as rock with UCS >50 MPa. Weaker rock affects both the possible anchor strength, and also the types of anchors that can be used. Compressive strength testing of rock similar to that of the accident site gave UCS values of between 5 and 10 MPa [5]. ‘‘Simple Means’’ testing [6] estimates the rock strength by hammer blows, crumbling by hand, etc. and has good correspondence with the results of unconfined strength testing. ‘‘Simple Means’’ testing of the rock in the accident area gave similar values. The UIAA strength criteria [7] is 20 kN axial load bearing capacity (outwards) and 25 kN shear load bearing capacity when tested in a concrete block of 50 MPa UCS. For an anchor placed on a vertical wall (Fig. 5) the axial and shear loadings correspond to the outwards and downwards directions.
Fig. 3. Top, the type of bolt in the accident, an 8 mm double wedge bolt. Bottom, sleeve bolt.
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Fig. 4. (a) Sleeve anchor components, (b) sleeve anchor tensioned with cone expanding sleeve, (c) wedge anchor components (d) wedge anchor tensioned with cone expanding sleeve. Note that the sleeve in (d) has moved to a wider section of the cone.
Loads in climbing are principally downwards (shear on the anchor), with a small outwards component (axial on the anchor). Loads generated by falling climbers are rarely above 10 kN, loads of less than 5 kN are more common [8]. Outwards loads are lower, but are significant as they can lead to anchor failure.
5. Analysis of loads leading to anchor failure To estimate the loads that caused the anchor to fail the likely position of the climber was copied (Fig. 2). A heavy spring balance was used to connect the climber to the anchor, replacing the blue sling in Fig. 2. Many positions were possible which enabled lifting the stick and rope above the climber. Approximately half of these positions generate loads of 1 kN or more in the outwards direction. At the same time the direction of force could vary from almost straight downwards, through the outwards direction, to almost vertically upwards. When the anchor failed the climber fell (Fig. 2) from 3 m above the belay till the rope came tight. The total amount of rope that was in play was the 3 m the climber was above the belay, and a doubled loop of 1.5 m which was the length of rope
M. Law, S. Hawkshaw / Engineering Failure Analysis 22 (2012) 21–27
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Fig. 5. Axial and shear loading on anchor.
involved in trying the clip into the next anchor. The climber fell 3 m till he was level with the belay then another 3 + 1.5 + 1.5 = 6 m till the rope came tight on the belayer. The total fall was approximately 9 m. The fall was 3 m longer than would be normal because of the extra 3 m of slack rope involved in the loop of rope the leader was using to clip the next anchor. It is difficultly to estimate the load in this fall; the belay device is designed to allow the rope to slip at 3 kN to limit peak loads, this is an upper limit to the load. 6. Test program Two sets of tests were performed. The effect of rock strength on finished hole size was investigated by drilling a number of holes in strong and weak rock. The failure loads of a number of Raumer wedge bolts were tested in weak rock, in both shear and axial directions. 6.1. Rock strength Hole drilling and strength testing was performed on blocks of Sydney construction sandstone rock which was stronger, the UCS was estimated by the ‘‘Simple Means’’ test to be 25 MPa, significantly stronger than the rock where the accident occurred. 6.2. Finished hole size It was observed that the finished drill hole size tends to be larger when drilling into weak rock than holes in strong rock. There are two likely reasons for this: on hard rock any drill misalignment does not enlarge the hole, while on weak rock the sides of the rock can be seen to remove material. The debris from drilling may also act as an abrasive and remove further material. To test the effect of rock strength on finished hole size, holes were drilled with a masonry drill bit of 8 mm nominal size (measured 7.8 mm). Five holes were drilled in strong volcanic rock, the holes ranged from 7.9 to 8.1 mm. Six holes were drilled into blocks of Sydney building sandstone (stronger than the rock at the accident site); the holes ranged from 8.3 to 9.1 mm. The sleeves on the Raumer bolts measured from 8.4 to 8.7 mm (average of 8.6 mm) indicating that in the larger holes found on the weaker rock there would be no interference between the sleeves and the inside of the hole. After the holes were drilled, bolts were placed in the holes. When tightening the anchors, it was noted that the nuts on the anchors turned with little resistance (a torque of 3 Nm was measured on a torque wrench) while the shafts pulled out of the rock. 6.3. Failure loads Testing was carried out with a hydraulic pull ram, a 50 kN Enerpac pull cylinder BRC 46, with a 150 mm stroke, the test speed was limited by the hand pressure pump. Typical test speeds were 1.5 mm per second. The system pressure at failure was captured on a pressure gauge with a hold needle which records the highest pressure for any test, the accuracy was estimated at ±0.1 kN. The failure loads achieved (Fig. 6) were shear (9.3 and 10.3 kN) and axial (1.3, 0.6, 0.8, and 1.6 kN). The axial failure loads were higher than those seen during the anchor removal operation (in weaker rock) where most anchors were removed by hand at loads of 0.3 kN or less. For comparison, in the simulation of the climber’s position attempted in Section 6 loads outwards of over 1 kN were easy to generate, many of these anchors would have failed at these loads. 7. Reasons for reduced strength When wedge anchors are placed in a drill hole in weak rock the sleeve may ream the hole out to a greater diameter. This has two effects;
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M. Law, S. Hawkshaw / Engineering Failure Analysis 22 (2012) 21–27
Fig. 6. Left: wedge bolt during pull test, note that the sleeve is in the unexpanded position and the gap between the sleeve and the cone is packed with debris. Right: un-used and tested wedge bolts, note packed debris and unmoved sleeves on tested bolt.
As the sleeve reams the hole the diameter increases, which reduces the available frictional force. The sleeve may then slide out of the hole as the nut is tightened, rather than being held in place by the interference fit. The debris from reaming may fill the area between the sleeve and the cone with rock dust. This may stop the sleeve from interacting with the cone, and interfere with the process of expanding the sleeve. The rock dust caught between the cone and the sleeve pushes the sleeve straight back out of the hole during tightening, rather than forcing it to expand. The sleeves on the Raumer bolts measured from 8.4 to 8.7 mm (average of 8.6 mm) indicating that in the larger holes found on the weaker rock (which ranged from 8.3 to 9.1 mm) there would be only minimal interference between the sleeves and the inside of the hole. On strong rock the turning resistance of the nuts increases, which indicates that the expansion is wedging the anchor in place. On weak rock when the nuts are turned the bolt continues to pull out of the rock with no increase in torque. Anchors at the accident site showed this behaviour. The nuts also could not be tightened when testing was carried out in stronger Sydney building sandstone. By all these mechanisms the anchors, which have good performance in strong rock, are dangerous in weak rock.
8. Rope cutting The possibility of a rope cutting is always a possibility during climbing. This possibility may be dealt with by choice of a route with less sharp rock, or by retreat from the climb if the danger is regarded as unacceptable, use of longer extensions on anchors to keep the rope free of sharp edges, or use of double ropes [9]. The use of double ropes is a common strategy as there has been no known case of two ropes cutting in an accident. There is no current standard for assessing the resistance of a rope to cutting, the one standard that existed [10] was withdrawn due to variable test results. The rope in this incident was a near-new 10.5 mm rope, a standard thickness for climbing. The rope was reported to be near new and in good condition (as assessed by the survivor and climbers involved in the rescue and body recovery).
9. Conclusions The primary cause of the accident was the rope cutting in a fall. The fall was a direct result of the failure of an anchor. The anchor was not suitable for the rock type; local knowledge and best practice in placing climbing anchors was ignored. Contributing factors were a lack of knowledge of the climbing route, trusting weak anchors, the increased length of fall due to having a loop of rope in the system, and subjecting weak anchors to a dangerous loading (leaning out on a weak anchor). Rope failure led directly to the fatality, however, if the rope had not failed then the belay anchor may have failed and a second climber would have also fallen. The anchors had low shear failure loads (9–11 kN) and axial failure loads (0.6–1.6 kN); for comparison typical anchors in this weak rock are chemically bonded machine bolts or ‘U’ bolts which have strengths >30 kN in both directions.
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The accident has led to a major test program of anchors in weak sandstone [5] as there were only two studies on this subject [11,12]. As a result of the testing, many climbs have had poor quality anchors replaced. Due to the rope failure there is increased awareness and use of double ropes. References [1] http://www.onsight.com.au/news-blog/articles/36/nicks-accident-what-happened. [2] http://www.youtube.com/watch?gl=AU&hl=en-GB&v=xQt4AbQmJjE. [3] O’Connor J, Ray A, Franklin B, Stuart B. Changes in the physical and chemical properties of weathered Maroubra sandstone in Sydney. AICCM Bull 2001:20–5. [4] Pells PJN. Substance and mass properties for the design of engineering structures in the Hawkesbury sandstone. Aust Geomech 2004;39(3):1–21. [5] http://routes.sydneyrockies.org.au/display/thelab/Home. [6] Hack R, Huisman M. Estimating the intact rock strength of a rock mass by simple means. In: van Rooy JL, Jermy CA, editors, Proceedings of 9th congress of the international association for engineering geology and the environment. Durban, South Africa; September 16–20, 2002. p. 1971–7. [7] EN 959:2007. Mountaineering equipment. Rock anchors. Safety requirements and test methods. [8] Vogwell J, Minguez JM. The safety of rock climbing protection devices under falling loads. Eng Fail Anal 2007;14:1114–23. [9] Ernst B, Vogel W. Determination of the redistribution shock load in climbing double rope systems. Eng Fail Anal 2009;16:751–65. [10] UIAA 108. Sharp edge resistant dynamic ropes. Union International des Associations d’Alpinisme. [11] Allen B. The development of the bolt belay. Thrutch (Sydney Rockclimbing Club newsletter); April–May 2003. [12] Hawkshaw S. Strength and reliability of chemically bonded rock climbing anchors in sandstone. Honours Thesis Sydney University; 2003.
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Examination of a rock-climbing fatality caused by equipment failure Michael Law ⇑, Stephen Hawkshaw Climbing Anchors Australia, 3/1 Hi Tech Drive, Toormina, NSW 2452, Australia
a r t i c l e
i n f o
Article history: Received 19 April 2011 Received in revised form 2 December 2011 Accepted 20 December 2011 Available online 8 January 2012
a b s t r a c t Climbers rely on anchors for safety; the type of anchor used depends on the rock type. This work describes a fatality caused by rope failure, which in turn was due to the failure of a anchor. Testing of similar anchors revealed insufficient strength in the rock type where the accident occurred. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Rock climbing Testing Load analysis
1. Introduction, description of climbing The normal method of climbing with a rope for protection is that one person climbs (the climber in Fig. 1) while the other (the belayer) secures the rope by threading the rope through a device which provides extra friction for holding the rope, this action is called belaying. The climber clips their rope through anchor points which may be fixed (permanent mechanical anchors in pre-drilled holes), or more specialised equipment which uses existing rock features such as cracks or spikes which is removed with each ascent. In this paper the term anchor implies a fixed anchor. The climber clips their rope through anchor points as they climb. If the leader falls from a certain distance above the last anchor point, they fall twice this distance (Fig. 1) until the rope comes tight on the belayer. The energy of the falling climber is absorbed by a combination of rope slip through the belay device and stretch in the rope. When the climber reaches a ledge where there are anchors (generally two or more), they secure themselves to these anchor points and belay the other climber up the section of the climb. As the rope is above the second climber, any falls they take are small. 2. Description of climbing accident In 2009 two climbers were attempting Bunny Bucket Buttress, a popular 275 m climb in the Blue Mountains, NSW, Australia. The anchors on this route are all fixed. They were not carrying a route description and climbed off route after the 5th belay; they moved right 20 m to a line of anchors on an unknown climb. At some point on this unknown climb the more experienced climber said that they were no longer on Bunny Bucket Buttress, and that the anchors on this unknown climb were not standard for the Blue Mountains, and were probably not safe. They continued climbing and higher on this unknown climb they belayed on a ledge using a single anchor. Above this belay the rock was blank and could not be climbed. To bypass this section the leader hung his bodyweight on one of the anchors. With both hands now free he attempted to clip a loop of climbing rope to the next anchor with a ⇑ Corresponding author. Tel.: +61 2 9717 9102; fax: +61 2 9543 7179. E-mail address: [email protected] (M. Law). 1350-6307/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2011.12.003
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M. Law, S. Hawkshaw / Engineering Failure Analysis 22 (2012) 21–27
Fig. 1. Climber and belayer before and after a fall (drawing courtesy of Simon Mentz).
Fig. 2. Diagram of rope position before the accident while trying to clip the next anchor with a stick.
M. Law, S. Hawkshaw / Engineering Failure Analysis 22 (2012) 21–27
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karabiner attached to a stick (Fig. 2). There is a significant amount of outwards force during this manoeuvre, while attempting to clip the next anchor the anchor he was hanging on failed. The leader fell past the belay ledge and his single rope was severed by the band of sharp ironstone rock. The belayer was attached to an anchor 3 m higher. The position of the anchor allowed it to be loaded downwards, in shear, where it had higher strength than when exposed to an outwards load. This anchor was later removed easily by hand by pulling outwards with less than 0.2 kN load. The climbers responsible for placing these anchors were identified a group of foreign climbers unfamiliar with the weak rock in the Blue Mountains who were trying to do a new route. This group had been warned that the bolts they intended to use were unsuitable for the weak Blue Mountains sandstone [1]. An early website report of the first ascent of this climb (in Croatian) mentioned that the bolts could not be satisfactorily tightened; the nuts only offered a low resistance to tightening and the shafts kept pulling out of the rock; it appears that this report is no longer available. The bolts were identified as being 8 mm diameter 80 mm length Raumer brand wedge bolts; recommended for caving in strong rock; but not suitable as anchors for rock-climbing or in weak rock. After the accident all bolts were removed from the climb, most of them by hand at loads of less than 0.3 kN. There is video of the bolt removal [2].
3. Description of anchor Mechanical expansion bolts expand as the bolt is tightened; there are two common types of expansion bolt, sleeve and wedge anchors. Sleeve anchors have a full length sleeve (Figs. 3 and 4) which is compressed between the nut and the cone when the nut is tightened; the cone then expands the sleeve. Wedge anchors have a short expansion sleeve (Figs. 3 and 4). When a wedge anchor is placed in strong rock, the sleeve is larger than the hole and must be compressed. The compressed sleeve presses outwards against the sides of the hole and the frictional force generated is overcome by hammering the anchor in. When the nut is tightened the cone pulls back inside the sleeve and expands the sleeve, giving a very solid anchor with typical axial load bearing capacity of >20 kN. The bolts in this accident had a twin cone and sleeve design (Fig. 3).
4. Anchor strength and rock strength Anchor performance depends on rock strength. Rock strength is measured by the unconfined compressive strength (UCS) to ASTM standard D 2938 ‘‘Method for Determining the Unconfined Compressive Strength of Intact Rock Core Specimens’’. While most climbing worldwide is performed on rock with good strength having a an of 50 MPa or greater, Blue Mountains rock is much weaker, having reported strengths in the range of 24–56 MPa dry and 8–32 MPa wet [3,4]. Much of the rock in the Blue Mountains is damp internally, so the lower strength values for wet rock are better indicators of strength. For this article, strong rock is defined as rock with UCS >50 MPa. Weaker rock affects both the possible anchor strength, and also the types of anchors that can be used. Compressive strength testing of rock similar to that of the accident site gave UCS values of between 5 and 10 MPa [5]. ‘‘Simple Means’’ testing [6] estimates the rock strength by hammer blows, crumbling by hand, etc. and has good correspondence with the results of unconfined strength testing. ‘‘Simple Means’’ testing of the rock in the accident area gave similar values. The UIAA strength criteria [7] is 20 kN axial load bearing capacity (outwards) and 25 kN shear load bearing capacity when tested in a concrete block of 50 MPa UCS. For an anchor placed on a vertical wall (Fig. 5) the axial and shear loadings correspond to the outwards and downwards directions.
Fig. 3. Top, the type of bolt in the accident, an 8 mm double wedge bolt. Bottom, sleeve bolt.
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Fig. 4. (a) Sleeve anchor components, (b) sleeve anchor tensioned with cone expanding sleeve, (c) wedge anchor components (d) wedge anchor tensioned with cone expanding sleeve. Note that the sleeve in (d) has moved to a wider section of the cone.
Loads in climbing are principally downwards (shear on the anchor), with a small outwards component (axial on the anchor). Loads generated by falling climbers are rarely above 10 kN, loads of less than 5 kN are more common [8]. Outwards loads are lower, but are significant as they can lead to anchor failure.
5. Analysis of loads leading to anchor failure To estimate the loads that caused the anchor to fail the likely position of the climber was copied (Fig. 2). A heavy spring balance was used to connect the climber to the anchor, replacing the blue sling in Fig. 2. Many positions were possible which enabled lifting the stick and rope above the climber. Approximately half of these positions generate loads of 1 kN or more in the outwards direction. At the same time the direction of force could vary from almost straight downwards, through the outwards direction, to almost vertically upwards. When the anchor failed the climber fell (Fig. 2) from 3 m above the belay till the rope came tight. The total amount of rope that was in play was the 3 m the climber was above the belay, and a doubled loop of 1.5 m which was the length of rope
M. Law, S. Hawkshaw / Engineering Failure Analysis 22 (2012) 21–27
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Fig. 5. Axial and shear loading on anchor.
involved in trying the clip into the next anchor. The climber fell 3 m till he was level with the belay then another 3 + 1.5 + 1.5 = 6 m till the rope came tight on the belayer. The total fall was approximately 9 m. The fall was 3 m longer than would be normal because of the extra 3 m of slack rope involved in the loop of rope the leader was using to clip the next anchor. It is difficultly to estimate the load in this fall; the belay device is designed to allow the rope to slip at 3 kN to limit peak loads, this is an upper limit to the load. 6. Test program Two sets of tests were performed. The effect of rock strength on finished hole size was investigated by drilling a number of holes in strong and weak rock. The failure loads of a number of Raumer wedge bolts were tested in weak rock, in both shear and axial directions. 6.1. Rock strength Hole drilling and strength testing was performed on blocks of Sydney construction sandstone rock which was stronger, the UCS was estimated by the ‘‘Simple Means’’ test to be 25 MPa, significantly stronger than the rock where the accident occurred. 6.2. Finished hole size It was observed that the finished drill hole size tends to be larger when drilling into weak rock than holes in strong rock. There are two likely reasons for this: on hard rock any drill misalignment does not enlarge the hole, while on weak rock the sides of the rock can be seen to remove material. The debris from drilling may also act as an abrasive and remove further material. To test the effect of rock strength on finished hole size, holes were drilled with a masonry drill bit of 8 mm nominal size (measured 7.8 mm). Five holes were drilled in strong volcanic rock, the holes ranged from 7.9 to 8.1 mm. Six holes were drilled into blocks of Sydney building sandstone (stronger than the rock at the accident site); the holes ranged from 8.3 to 9.1 mm. The sleeves on the Raumer bolts measured from 8.4 to 8.7 mm (average of 8.6 mm) indicating that in the larger holes found on the weaker rock there would be no interference between the sleeves and the inside of the hole. After the holes were drilled, bolts were placed in the holes. When tightening the anchors, it was noted that the nuts on the anchors turned with little resistance (a torque of 3 Nm was measured on a torque wrench) while the shafts pulled out of the rock. 6.3. Failure loads Testing was carried out with a hydraulic pull ram, a 50 kN Enerpac pull cylinder BRC 46, with a 150 mm stroke, the test speed was limited by the hand pressure pump. Typical test speeds were 1.5 mm per second. The system pressure at failure was captured on a pressure gauge with a hold needle which records the highest pressure for any test, the accuracy was estimated at ±0.1 kN. The failure loads achieved (Fig. 6) were shear (9.3 and 10.3 kN) and axial (1.3, 0.6, 0.8, and 1.6 kN). The axial failure loads were higher than those seen during the anchor removal operation (in weaker rock) where most anchors were removed by hand at loads of 0.3 kN or less. For comparison, in the simulation of the climber’s position attempted in Section 6 loads outwards of over 1 kN were easy to generate, many of these anchors would have failed at these loads. 7. Reasons for reduced strength When wedge anchors are placed in a drill hole in weak rock the sleeve may ream the hole out to a greater diameter. This has two effects;
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M. Law, S. Hawkshaw / Engineering Failure Analysis 22 (2012) 21–27
Fig. 6. Left: wedge bolt during pull test, note that the sleeve is in the unexpanded position and the gap between the sleeve and the cone is packed with debris. Right: un-used and tested wedge bolts, note packed debris and unmoved sleeves on tested bolt.
As the sleeve reams the hole the diameter increases, which reduces the available frictional force. The sleeve may then slide out of the hole as the nut is tightened, rather than being held in place by the interference fit. The debris from reaming may fill the area between the sleeve and the cone with rock dust. This may stop the sleeve from interacting with the cone, and interfere with the process of expanding the sleeve. The rock dust caught between the cone and the sleeve pushes the sleeve straight back out of the hole during tightening, rather than forcing it to expand. The sleeves on the Raumer bolts measured from 8.4 to 8.7 mm (average of 8.6 mm) indicating that in the larger holes found on the weaker rock (which ranged from 8.3 to 9.1 mm) there would be only minimal interference between the sleeves and the inside of the hole. On strong rock the turning resistance of the nuts increases, which indicates that the expansion is wedging the anchor in place. On weak rock when the nuts are turned the bolt continues to pull out of the rock with no increase in torque. Anchors at the accident site showed this behaviour. The nuts also could not be tightened when testing was carried out in stronger Sydney building sandstone. By all these mechanisms the anchors, which have good performance in strong rock, are dangerous in weak rock.
8. Rope cutting The possibility of a rope cutting is always a possibility during climbing. This possibility may be dealt with by choice of a route with less sharp rock, or by retreat from the climb if the danger is regarded as unacceptable, use of longer extensions on anchors to keep the rope free of sharp edges, or use of double ropes [9]. The use of double ropes is a common strategy as there has been no known case of two ropes cutting in an accident. There is no current standard for assessing the resistance of a rope to cutting, the one standard that existed [10] was withdrawn due to variable test results. The rope in this incident was a near-new 10.5 mm rope, a standard thickness for climbing. The rope was reported to be near new and in good condition (as assessed by the survivor and climbers involved in the rescue and body recovery).
9. Conclusions The primary cause of the accident was the rope cutting in a fall. The fall was a direct result of the failure of an anchor. The anchor was not suitable for the rock type; local knowledge and best practice in placing climbing anchors was ignored. Contributing factors were a lack of knowledge of the climbing route, trusting weak anchors, the increased length of fall due to having a loop of rope in the system, and subjecting weak anchors to a dangerous loading (leaning out on a weak anchor). Rope failure led directly to the fatality, however, if the rope had not failed then the belay anchor may have failed and a second climber would have also fallen. The anchors had low shear failure loads (9–11 kN) and axial failure loads (0.6–1.6 kN); for comparison typical anchors in this weak rock are chemically bonded machine bolts or ‘U’ bolts which have strengths >30 kN in both directions.
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The accident has led to a major test program of anchors in weak sandstone [5] as there were only two studies on this subject [11,12]. As a result of the testing, many climbs have had poor quality anchors replaced. Due to the rope failure there is increased awareness and use of double ropes. References [1] http://www.onsight.com.au/news-blog/articles/36/nicks-accident-what-happened. [2] http://www.youtube.com/watch?gl=AU&hl=en-GB&v=xQt4AbQmJjE. [3] O’Connor J, Ray A, Franklin B, Stuart B. Changes in the physical and chemical properties of weathered Maroubra sandstone in Sydney. AICCM Bull 2001:20–5. [4] Pells PJN. Substance and mass properties for the design of engineering structures in the Hawkesbury sandstone. Aust Geomech 2004;39(3):1–21. [5] http://routes.sydneyrockies.org.au/display/thelab/Home. [6] Hack R, Huisman M. Estimating the intact rock strength of a rock mass by simple means. In: van Rooy JL, Jermy CA, editors, Proceedings of 9th congress of the international association for engineering geology and the environment. Durban, South Africa; September 16–20, 2002. p. 1971–7. [7] EN 959:2007. Mountaineering equipment. Rock anchors. Safety requirements and test methods. [8] Vogwell J, Minguez JM. The safety of rock climbing protection devices under falling loads. Eng Fail Anal 2007;14:1114–23. [9] Ernst B, Vogel W. Determination of the redistribution shock load in climbing double rope systems. Eng Fail Anal 2009;16:751–65. [10] UIAA 108. Sharp edge resistant dynamic ropes. Union International des Associations d’Alpinisme. [11] Allen B. The development of the bolt belay. Thrutch (Sydney Rockclimbing Club newsletter); April–May 2003. [12] Hawkshaw S. Strength and reliability of chemically bonded rock climbing anchors in sandstone. Honours Thesis Sydney University; 2003.