analytical support for h/3 setback requirements
analytical support for h/3 setback requirements
(OP)
My apologies for jumping over from the EE forum with what may be a dumb question. We are designing a deep pile foundation in layered sandstone and my geotech engineer, who is very experienced in this area, says we could use piles as shallow as 10 feet into the competent rock. The city wants around 26 feet based on the h/3 setback rule and 65% grade. My guy says the rule was picked out of the air and is primarily intended for soils, not rock. If we are to challenge them, where would we go for analytical support??
Thanks
Howie
Thanks
Howie
RE: analytical support for h/3 setback requirements
RE: analytical support for h/3 setback requirements
The slope is 1.5:1 (apx 65% grade) average. 106 ft horizontal to about 65 ft vertical, sloping down from the street. The setback rule says the bottom most outward point of the foundation structure needs to be one-third of the height of the slope, up to 40 ft maximum, horizontal distance from daylight or from the interface of the rock and the soil if the soil is not competent. In this case the entire slope down to flat land is 300 ft or so, so the 40 ft applies, putting us about 26-27 feet deep for the ends of the piles. The rock is layered sandstone with layers slanting back into the slope so they cannot slip parallel to the slope. Drilling is not really too bad--the rock is firm but not really hard like granite.
Howie
RE: analytical support for h/3 setback requirements
This set-back rule seems to be based on something other than engineering principles (see Meyerhoff or NAVFAC DM7.2 (I think or 7.1) for foundations on slopes. With this rule, I would never be able to build short retaining walls on hairpin curves in mountainous terrain.
RE: analytical support for h/3 setback requirements
The H/3 slope setback is applied to new, private development (residential, commercial, etc.). I commonly see plans for roadway construction in these same cities and counties that do not provide the same H/3 slope setback criteria that my Clients must adhere to.
RE: analytical support for h/3 setback requirements
Following such things blindly - or municipalities forcing it blindly (probably with little technical understanding on their part - although, admittedly some will have this capability) is bordering on ridiculous. If not, then, just plain sad.
RE: analytical support for h/3 setback requirements
When I am confronted with this problem, a Slope Stability Study is the answer. It is time for the Geotechnical to be an ENGINEER. Sometimes this appears to be a novel concept for the Building Official or, heavon save us, the Planning Department.
RE: analytical support for h/3 setback requirements
We have a favorable slope stability analysis that deals with the entire slope down to the foot of the hill. It does not address any stability issues related to imposed structural loads. Is there some accepted analysys that would suggest appropriate depth related to the structure being placed partway down the slope?
Our jurisdiction is LA city.
I spoke with the head of grading (he used to work for my geotech engr) who told me that if we had a geologist and a soils engr sign off that they would consider releiving us of the requirement. We did that, but they did not automatically accept the recommendtion. We are preparing to discuss it with them--hence the reason for my posting here. My engineer is old school--been doing this forever and is well regarded. He is not up on all the latest computer-aided design stuff available, so I thought I would do a little checking to see if newer analyses might have been developed recently.
We would not drive piles, they would be drilled and a poured in place. This practice is pretty common in this area, it just that the foundation contractors tell me that it gets more difficult for them over 20 ft down. We would be 30ft total counting the going through the soil, so there is some cost impact in conforming to the rule.
Howie
RE: analytical support for h/3 setback requirements
As far as "blindly following Codes", in many instances I and the Client simply have no choice but to adhere to the criteria of the Codes. As I mentioned before, the H/3 foundations setback criteria is a particularly hard one to get around in these parts. It could turn into a battle with the reviewers that would simply add many months to the review process. Many clients don't want to get embroiled in this debate, especially with the housing market turning around. Unfortunately, it simply boils down to "picking your battles". This goes well beyond good solid geologic input and geotechnical engineering principles.
RE: analytical support for h/3 setback requirements
I also agree to a point - about picking your battles. But it is so frustrating, I would think why/how generalistic guidelines overrule sound geotechncial (or structural) principals. It also begs a question of why/how a non-PE can overrule a PE in review of a design. But that is, perhaps, the question for one of the other forums.
RE: analytical support for h/3 setback requirements
While I understand that picking your battles is a fact of life; there is no reason to do so silently. Everyone should be made a where of the fact no code can answer all questions correctly and that is why ENGINEERING JUDGEMENT exists. As engineers we either use it or we loose it!