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Since we do not have any standards in the above area outside the needed 2' of soil support behind the guardrail posts, I am trying to establish the best practices for Metro District for guardrail posts adjacent to various slopes. 

Sometimes due to the site limitations, we cannot provide the 2' of soil support behind the posts therefore, we need to establish a consistent approach to this issue for the District staff.  Andy Halversen came up with the below draft guideline for use in standard soil:

For slopes 1:6 or flatter use 6' posts with 6'-3" post spacing

For slopes steeper than 1:6 to 1:3 use 7' posts with 6'-3" post spacing

For slopes steeper than 1:3 - with posts 1' from shoulder PI use 7' posts with 6'-3" post spacing

For slopes steeper than 1:3 - with posts at the shoulder PI use 7' posts with 3'-1.5" post spacing

Do you agree with the above? 

How should we change the above if we have weak soil? 

Your input is greatly appreciated.

We would recommend the following guidelines.

For standard W-beam guardrail:

1. Standard W-beam guardrail placed adjacent to any slope with 2' of level soil behind the posts is acceptable.
2. For w-beam guardrail placed 1'-2' adjacent to a 6:1 or flatter slope, standard 6' W6x9 posts at standard spacing are recommended.
3. For w-beam guardrail placed 1'-2' adjacent to a 3:1 to 6:1 slope, 7' W6x9 posts at standard spacing are recommended.
4. For w-beam guardrail placed less than 1' adjacent to a 3:1 or steeper slope, 7' W6x9 posts at half spacing are recommended.

For MGS guardrail:

1. Standard MGS guardrail placed adjacent to any slope with 2' of level soil behind the posts is acceptable.
2. For MGS guardrail placed 1'-2' adjacent to a 6:1 or flatter slope, standard 6' W6x9 posts at standard spacing are recommended.
3. For MGS guardrail placed 1'-2' adjacent to a 3:1 to 6:1 slope, 7' W6x9 posts at standard spacing are recommended.
4. For MGS guardrail placed less than 1' adjacent to a 3:1 or steeper slope, 9' W6x9 posts at standard spacing are recommended.

However, recent research has indicated that more aggressive installation guidelines for MGS exist. First, there are options for 6-ft, 7-ft, and 9-ft long posts used in conjunction with MGS at the slope break point (SBP) of a 2:1 ditch. These resources may be useful to update your post length requirements.

https://mwrsf.unl.edu/reportResult.php?reportId=57

With regards to adjusting the installations for weak soil, we do not recommend adjusting guardrail installations for weak soil types.