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Typically, we are concerned with vertical elements within the working width of MGS or any barrier system, but we sometimes install underground pull boxes behind MGS for ITS facilities, and I was asked what the appropriate offset is from the guardrail. To allow for post rotation, I assumed 7” is the minimum (measured from back of post) based off TTI testing of MGS in a concrete mow strip with leave outs (test report no. 608551-01-1-5), and since this testing was with grout leave outs I thought that rounding up to 1-foot would be prudent in case we have less stiff soils out there. If we cannot place the box so it is between two posts, do you think 1-foot is adequate for the offset? If the post rotates around the bottom (I assume it probably does not), then that would allow 16.5 degrees of rotation. Is there an established point along the embedment depth of standard MGS that it typically rotates around (for a TL-3 impact)?

Picture of an existing installation (at an AGT and not sure why there are 2 blockouts):

You are correct that posts require space behind them to effectively rotate in the soil, and constraining the post from rotation has adverse effects on guardrail performance. As you noted, there have been multiple studies over the years at MwRSF and TTI looking at posts in rock and mow strip leaveouts. As such, you 1’ spacing seems reasonable. I believe our original post in rock guidance was a little less than 15” of clear space behind the post. TTI’s mow strip had a shorter clear distance of 9”.

https://www.roadsidepooledfund.org/wp-content/uploads/2011/03/TM-GuardrailPostInstallationinRock-rev2.pdf

In terms of the rotation point of the post, a post is soil tends to rotate about a point 2/3 of its embedment. This can vary some, but it is our general rule of thumb.