Sand Capping is one of those golf construction terms that gets way to generalized IMO.
Just about every golf construction project has some manner of plating over sub soils with a rootzone material - almost always on greens and often on tees and approaches.
In this discussion it is more about plating with sand imported onto the site. In that case the type of sand and depth of plating is simply a science and cost benefit exercise that should ALWAYS be science based.
To clarify a couple of statements made in above posts - I've worked on three Fazio renovations, none were sand capped. No project where I've had input has the depth or selection of sand been made without guidance by an accredited lab. Of course cost is important and part of the discussion, but the choices are made based on science with input from the consultants and professionals hired to guide that part of the project. Agronomists' recommendations are not ignored because it costs too much. There have been cases where the cost of sand capping was simply too high, and adjustments in surface drainage and storm drainage were made in place of capping.
As Tom Doak mentioned at Memorial Park in Houston the fairways and greens surrounds were capped, about 38 acres of capping. That required 68,000 tons of sand to cap at 9" depth. We tested 5 sands and the results were we had to cap from 7" to 14" based on the each sand's ability to release water.
https://en.wikipedia.org/wiki/Water_retention_curveThe 7" material was too coarse and had to much gravel, the 14" material was finer, less expensive by a little, but the lab and agronomist feared it would lock up sooner as organics built up over time, and it would have taken 50% more sand. We selected a sand that gave us good infiltration in a range of 7" to 10" and used a 9" depth as our baseline.
The capping process is basically the same as building a green in that you get the subgrade right then move the material onto the subgrade and work it until you have the proper depth. In our case that was thousands of off road truck loads moving sand and then a process of checking depths and working the sand until it's dialed in. On every fairway we dug 100's on holes checking depths because its very difficult to get it perfect during the hauling process. The biggest pain with capping is all the traffic and how it impacts other tasks like cart path, irrigation, drainage...it's a logistical challenge receiving, moving, installing all that sand, especially on a project that closes in Jan and opens in Nov. So you've got to get the material In place to open up for everything else going on. And you've got to make room for more sand to be received.
87,000 linear feet of perforated drain lines were installed under the sand cap at Memorial. On a green you install 10'-12' of perf drainage per 100' sq ft, under this cap we installed about 5' -6' per 100' sq ft.
Every project is different. As Tom noted they had abundant sand at Pac Dunes and they were able to plate thicker, probably in place of installing a bunch pf drainage, but they had an expert who was making sure the water had somewhere to go away from the surface. If you don't have abundant sand, then you go thinner and add more drains. I like helping to solve those puzzles, and sometimes no capping at all and other drainage methods is the more practical solution.
At Memorial the tournament sponsor and Tour knew it rains a lot in Houston and wanted to play the ball down as often as possible, and build a course that could handle heavy play without getting destroyed when wet from all the traffic. At this year's tournament it rained 1" in 30 mins on Thursday morning. By Sat they were playing the ball down. Marc Lieishman was the leader after Thursday's round and in his presser after the round he said this after being asked how the course handled the rain, "it was unbelievable. I don't know what they did underneath this place, but in the fairways, greens, and tees it was dry. Whoever designed the drainage, they did a great job" What we did was let science drive the decisions and we had a client who trusted that process.