Pull-through, a full history

Vertical caving terminology and methods > SRT basic terms

Pull-through ("retrievable rigging" or "retrievable anchor" in the USA)

A potholing through-trip with entrances at the top and bottom of the cave. Pitches in between can be rigged using a rope that is pulled down the pitch once every person has descended, so that the same rope can be used for the next pitches. If there are no issues, then abseiling is the only thing needed, not prusiking. However, a spare rope is needed for subsequent pitches, just in case one fails to pull-through correctly. It is also normal for at least one caver to bring prusiking equipment, so that they might be able to return up a pitch and untangle the rope if needed. Uses a unique rigging style relying on a doubled rope where one end is safe for abseiling, and the other end is used to pull the rope down the pitch. Abseiling on the wrong side results in a freefall, so each person must know how to safely use it. Normally, there is no traverse line at the pitch head, since it cannot be retrieved later.

For safety, the two sides may be done with different ropes, perhaps different colours, and maybe even a very thin cord for the derigging side (since it does not need to hold the weight of a caver). This allows each caver to immediately see and feel which side is safe. Pull-throughs are only possible with anchors which are permantly left in the cave, which are smooth enough for the rope to attach directly to them and run smoothly through them, such as P-hangers, or naturals that can perform the same function.

The most common rigging uses a knot with a loop half way along the rope, with a carabiner that is clipped to it. One end of the rope is fed through the anchors, then through the carabiner, and lowered down the pitch, with a safety knot tied in it. This end is used for abseiling and prusiking. The other end, when pulled, causes the carabiner to slide down the rope, and derigs the pitch, as long as the safety knot has been untied first, and there are no tangles. This approach results in a special form of Y-hang, which is quite different from a usual Y-hang. Alternatively, a large knot can be tied in the middle of the rope, which is too large to fit through the anchors, such as a flat double overhand bend. The carabiner is not used in this case, and nothing is clipped around the rope used for abseiling and prusiking. When the rope is fed through the anchors, the knot jams against them, and supports the weight. This method is prone to the knot getting stuck in the anchor, and puts the load sideways through a single anchor, instead of in the usual direction through a pair of anchors, so it is not recommended. In the worst case, if the knot somehow managed to pull through the anchor, it would result in a freefall, which would not be possible with the Y-hang approach. It is, however, the only method that allows deviations (but not rebelays) to be reliably used, though deviation slings and carabiners cannot be recovered. This method is most often used with a single anchor, just in case it was not already controversial enough. With either approach, a failure of one of the anchors causes a significant shock load, which is worse than a normal Y-hang, so pull-thoughs are generally only used with very trustworthy anchors.

Unlike a standard Y-hang, the angle between the arms of the pull-through's Y-hang cannot be controlled, and will naturally settle to about 120°. However, because the rope passes through the anchors and between them as well, each anchor sees a force of about 193% of the load on the rope below, pulling in a direction about 15° below horizontal. This means that anchors need to hold 2.7 times the load that they would for a standard 90° Y-hang. It is not possible to control where the rope hangs between two walls of a pitch. With the knot jamming approach, the anchor that the knot jams against sees a force of 100% of the load on the rope below, pulling horizontally, while the other sees a force of 141% of the load on the rope below, pulling at an angle of 45° below horizontal. This makes it significantly less stressful on the anchors than the Y-hang approach. Again, the position of the rope cannot be controlled, and it hangs vertically below one of the anchors. Since anchors are usually placed in walls, this makes it more likely that the knot jamming approach will suffer from rub points.

History

The most basic pull-through approach had probably existed for a long time in sailing and mountaineering; tie a loop knot in the end of a rope, loop it around an object such as a rock, descend the rope, then try to flick the loop off the object by shaking the rope. This often failed, and mountaineers would then be forced to abandon that rope, and use another rope as needed to continue their descent. This advanced to using a doubled rope, looped around an object, such as a rock. One end could be tied to the mountaineer, or tied to a stick which they could sit on as a boatswain's chair (an ancestor of the sit harness). They could then hold the other end of the rope, and slowly lower themselves with it, the most simple version of doubled rope technique. This would allow the rope to be pulled down afterwards. This method was mentioned both for mountaineering and as a fire escape in Oekonomische Encyklopädie volume 13 (page 81) by Johann Georg Krünitz, in 1778 in Berlin, Prussia, Holy Roman Empire (now Germany), and had probably existed for some time before that, possibly used by some sailors or well diggers during previous centuries. This then progressed to the pull-through; loop a bight of rope around the object, descend using one or both strands, and then pull the rope down afterwards.

Between 1860 and 1862, British mountaineer Edward Whymper invented the approach of using a separate pull-down cord tied to a metal loop which the main rope passed through after looping around an object, exactly as cavers might now use a carabiner (Scrambles Amongst the Alps, Edward Whymper, 1872, page 51). French mountaineer Jean-Estéril Charlet-Straton is perhaps the first to have documented using the more basic doubled rope technique, descending both strands at once so that one end could be pulled afterwards to retrieve the rope, which he used during an 1876 ascent of Petit Dru in the French Alps. His innovation was very minimal, and in fact was a step backwards compared with that of Edward Whymper, but at least it did not require a specially prepared rope. It is significantly more risky than Edward Whymper's approach, as it relies on both strands being pulled almost exactly the same amount, with the friction at the anchor being the maximum difference allowed between the two strands. A standard pull-through, which matches Edward Whymper's approach, only needs the load on the main loaded strand to be more than or equal to that on the strand used to pull down the rope, which will always be the case during normal use, since only a single strand is used, and the other one will not be caught by mistake with more force than the loaded strand is experiencing. However, Jean-Estéril Charlet-Straton's approach is safer from the perspective of the user not needing to know which strand is which, since both strands are used at the same time.

Cavers used variations of either approach with ladders, body abseil or handlines before SRT developed. The earliest known record of its use with ladders was by French caver Édouard-Alfred Martel in 1892 in Aven de Vigne Close, and described with ladders in 1898 in the British publication Encyclopaedia of Sport. In the second case, ladders were retrieved for use on lower hangs of the same pitch, while the rope was used as a pull-up cord on the way out. German mountaineers were depicted using both Edward Whymper's approach and a ring jamming approach in Anwendung des Seiles, published by the Bavarian section of the German Alpine Club in München in 1907. The 1930 edition then showed a variation of the knot jamming approach, in addition to Edward Whymper's approach. French cavers Pierre Chevalier, Hélène Guillemin and François Guillemin started using ropes to retrieve ladders, leaving a carabiner or metal ring (presumably a piton with a ring) at the top of the pitch holding a rope in the Dent de Crolles system near Grenoble in France in 1935. Pierre Chevalier, Fernand Petzl, and others would also use that approach to retrieve both ladders and ropes from pitches from 1940 onwards, in the same cave system. In 1944, American sailor Clifford Warren Ashley suggested several methods for retrieving a rope in the Ashley Book Of Knots (#391-399), but all of them are extremely dangerous, as they rely on slipped knots, and could accidentally untie with even a slight pull on the tail of the knot. In 1945, French mountaineer Pierre Allain created the Décrocheur, a rather terrifying device that dropped the rope down the pitch as soon as the weight is taken off the rope (later versions needed a cord to be pulled, making it marginally safer).

The first known SRT pull-through took place in 1969, by French cavers Paul Courbon, G. Dou, M. Lopez and A. Mattéoli, in the Gouffre de la Pierre-Saint-Martin, in France/Spain, using the Décrocheur. In Britain, it was described in Descent 3, in the same year. The pull-through technique with a carabiner was recreated by French caver Daniel Martinez in 1972, as an adaptation of pull-up cords, for use with cord technique. Pull-throughs with Y-hangs set up for it first appeared in France at the end of the 1970s. This was initially done using equipment such as maillons that were left in the cave, or using naturals, or cord technique. Cavers continued to use pull-throughs with ladders even after SRT developed.

This history section only covers pull-throughs. This article also has a detailed history of many of the other devices and techniques that are used for vertical caving.

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