analytical support for h/3 setback requirements

[howiestover]

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

 

[BigH]

Sorry - can you explain the 'set-back rule' and 65% grade. Pertaining to what? I can't see how or why anyone would want you to go, for a typical foundation, 16 ft (5m) into rock for a pile foundation support - you'll end up having to core or star-drill it out.

 

[howiestover]

Thanks for your response
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

 

[BigH]

Have you (or your geotechnical consultant) actually talked to the city and explained the geological information? Given the situation, he should provide you engineering reasons for adhering to a general guideline - not just quote the guideline as gospel in all situations. If the chap is not a professional engineer, ask to speak with the city's professional engineer (either on staff - or, as some cities do, their consultant).

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.

 

[epongra2]

Where I work (So. California), H/3 is a standard that had been adopted by most cities. In fact, it is part of the CBC (California Building Code). Most property owners, developers, etc. accept this fact and don't question it. Occasionally, we are asked by a Client to reduce the standard H/3 slope setback recommendation. If the geology is favorable, the slope stability analyses OK, then we tell the Client we will attempt it. The Client can then file a "Request for Modification" with the City, County, etc. However, the jurisdictions I work in don't typically accept approve this, and I let the Client know going in that his/her chances aren't very good.

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.

 

[BigH]

epongra2 - but how do you put in piles into rock 16 ft or so? You can't drive piles - they would have to be churn drilled or cored caissons.

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.

 

[emmgjld]

As is so much in the code, Engineering should determine if the written requirements are too harsh or not sufficient. However, I have been confronted with many who take the written and add a substantial amount of arrogance. In the IBC I usually work under, Chapter 1805.3 deals with the issue and 1805.3.5, Alternate Setback and Clearance, provides the means for engineering to occur.

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.

 

[howiestover]

Thanks for all of your input!
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

 

[epongra2]

BigH - drilling sixteen feet into Tertiary sandstone in this area is no big deal. What is commonly used in this area is a bucket-auger drill rig that can drill borings for 18" to 5' in diameter. Unless the sandstone is well-cemented, this drilling won't take long at all. The steel is then lowered into place and the concrete placed.By the way, this is the same way we here investigate deep landslides.

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.

 

[BigH]

Thanks on the info of drilling of the sandstone in your area. This is one reason why local practice is so important. As JHeidt said, we are "all prisoners of our experience" (Thanks Jim - ) When I was first starting out, I once sat for hours and hours (including overtime hours - and we didn't get paid for them) of trying to churn drill out a 5ft thickness of Ontario limestone (using a star-bit). And, of course, rocks in northern Canada (being on the shield) would be impossible to drill out using your method.

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.

to all.

 

[GeoPaveTraffic]

Another example of why engineers are not as respected as other "professionals". Blindly following codes reduces all that we do to looking something up in a "cook book"; and if that is all there is to your job, then you are not a professional.

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!