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What is the technique involved in crevasse rescues?
Presuming a group are all moving together on a glacier and the leader of the party has fallen into a crevasse (collapsed snow bridge, etc.). The fall was held so all other members of the party are secure. How do the other members of the team go about retrieving their colleague?
What are the main techniques etc. that need to be learned to perform this activity?
This is a very complicated topic, and you can take an entire course where you learn and practice the techniques. Reading an answer on SE is not going to be enough. You need to practice. The following is just an outline. There is an entire chapter in Freedom of the Hills on glacier travel and crevasse rescue.
The first step is always going to be to construct an anchor so that the team can get out of arrest position. If the leader is the one who's fallen in, then the person at the opposite end of the rope would normally be the one who gets up and builds an anchor, while other team member(s) stay in arrest position. You build the anchor between the lip of the crevasse and the nearest person, so that everyone has tension released from the rope and can stand up. While doing this, you want to make sure not to create more victims, so e.g., the person building the anchor needs to be on a Prusik.
Now talk to the climber and discuss the situation. You want to know if they're hurt.
If possible, you can try to prepare the lip of the crevasse so that the rope doesn't become entrenched. For example you can see if you can get an ice ax or a pack under it. This will be easier if the victim is able to get to a stance and unweight the rope. In most cases, it is not possible to keep the climbing rope from becoming completely entrenched. If you have enough extra line, you may want to prepare a separate portion of the lip and throw down a separate rescue line.
After this, you do not immediately start hauling the victim out. That's a last resort, and often it doesn't work. The following are various options to consider, roughly in order.
See if the victim can climb out under their own power, while belayed from above.
There may be an easy way to walk out of the crevasse at the bottom. If so, then you could lower the victim.
If the victim isn't hurt, he should be able to Prusik up the rope. Getting over the lip may be physically difficult, especially if the rope is entrenched. It may be helpful if there are two ice tools that the victim can use.
As a last resort, such as in situations where the victim is unconscious or so badly injured that he can't Prusik up, consider trying to set up a pulley system and hauling him out. This may or may not work. If the rope is entrenched, you can't just haul the victim over the lip -- people have been killed that way.
This is a varied and complex subject. As a good thorough example of a crevasse rescue done well check out This video.
Even with this thorough example, there are things that could be improved or argued that x should be done differently. As with many mountaineering techniques, practice and experience are the key.
As a summary, typically there are 3 steps to a crevasse rescue attempt.
Typically in this scenario, all members of the party will be lying down holding the falling person using self-arrest technique (ice axe pushed into the snow, etc.). To start the self rescue one person needs to be removed from this brace position. This needs to be done carefully, ensuring that the other member of the party are not pulled into the crevasse themselves.
One of the party needs to slowly release themselves from the tension in the rope and stand up. This needs to be done carefully, ensuring that the other members of the party remain secure.
Once a person is released they must build a good snow anchor. You need to weigh up time vs security here. A dead man is very secure but can take time to build, whereas an ice axe plunged deep into solid snow is less secure but is faster. The technique will depend on the circumstances so a degree of experience in building such anchors is essential.
Once the anchor is built, the person in the crevasse is secured to this anchor. This is done be attaching a prusik to the rope and securing onto the anchor.
Now that the person is secure all other members of the party should be able to join the rescue. One person must remain to ensure the anchor/potentially take the weight should the anchor unexpectedly fail.
Now the person is reasonably secure, we need to add redundancy into the system and build a system to hoist the casualty out of the crevasse. This is where a Z pulley comes in.
The first stage is to build a second, "bomber" (very secure), anchor further back from the temporary one built above. You should be able to put a bit of time into this building a good solid dead man or snow bollard. Whatever you choose it needs be very secure, since this will take the majority of the strain when hoisting the person out.
The rope is then tied to this anchor. You should now have at least two anchors (the initial, temporary one) one and the bomber one above. If anyone fails you now have a back up.
Hoisting a dead weight of a person and all their equipment vertically is very heavy. 2 people will struggle to lift a person on a dynamic rope. This is where the Z pulley comes in.
A Z pulley adds a ratio into the rope reducing the weight of the person being pulled out. To build a Z pulley you take the dead end of the rope attached to your snow anchor and re-attach it to the end of the rope connected to the person (again using a prusik).
Now that the person in the crevasse is secure and you've built your Z pulley, you can begin hoisting them out. You'll want to keep the prusik on the line as a backup and to allow you to release the rope, but basically you now pull the end of the Z pulley (possibly using multiple people) until the victim is free.
There are several variations on this technique and this is designed as a rough guide but hopefully I've highlighted the main features.
