Dynamic Equations and Cohesive Soils

[foreng]

I'm working on a project in a remote location, where the use of dynamic equations such as Engineering News (ENG) is commonly used for capacity measurements. The structures tend to be small in scale (< 200 KN (45 kip)/pile). Most sites have deep cohesive soils that do not correlate well to Dynamic Equations. The closest PDA machine is about 10 hours away and not seen as an option at this time.

Any suggestions on other equations being used? I noticed FHWA is using a modified Gates.

We tend to construct very conservative foundations, lots of deep piles. As an employer rep I realize we are paying for some overly conservative foundations, looking for some ammunition to take back to my supervisors in order to justify the extra expense of soils testing.

 

[aeoliantexan]

Just some ideas for things you could do out there in the wilderness:

1. Study the geology to understand the types of soil conditions expected. Do the cohesive soils tend to get stiffer or softer with depth?

2. Based on the geology, assume, say, stiff clay with an ultimate skin friction of 1 ksf. Drive a pile to the calculated depth needed based on a static formula, let it set up over night, and re-tap it. If you don't like the results, let it set for 3 days and try again.

3. Size test pile as above and run a quick load test on a 3-day old pile using the crane as the reaction. Or run a tension test using timber pads for reaction.

4. Do your own sampling using an open-end pipe with an air fitting near the top. Drive it a few feet, pull it out, and blow the plug out with compressed air. Rinse, repeat. Test the clay with a pocket penetrometer, torvane, or your thumb. Use a static formula to design a skin friction pile
(See NAVFAC DM-7.2 page 196).

 

[foreng]

Thanks for the advice.

The piles tend to get slightly stiffer with depth. The problem arises when the dynamic formula has specified 5 blows/in and all you can get is 2 at about 150 ft. I have got them to retap after an overnight setup with little to no difference. Item number 4 is interesting, never thought about blowing out a plug and testing it.

Most of the geotechs in the area say to expect a setup of about 2 times the initial driving after about 4 to 5 days, but we never seem to let them set long enough, and without any setup I worry about plunging type failure.

 

[aeoliantexan]

I saw a Raymond pile crew "predrill" for a pile using the open pipe trick and blow the plug out with steam. I thought it might be worth a try for you.

Have you considered buying or renting an electric cone penetrometer and pushing it with the weight of the pile hammer? You should be able to get classification data and undrained strength with a friction cone.

Can you tell us where these sites are located? Is this a bay deposit, delta, lakebed deposit, or what? 150 feet of soft clay is not too common. Is it all geologically recent? Are the clays organic, calcareous, highly sensitive, or normal clays?

Have you considered a balanced mat foundation? You could excavate enough soil to equal the total foundation load, fill it with geofoam, and pour a footing on top. The dead load will need to be enough to keep it from floating.

 

[oldestguy]

A word of advice:

Most of the members here look at several different "rooms" and posting the same item on these different "rooms" is a duplication that is not needed.

On your job where soil sensitivity is not present (seems unusual), that should have been in the original post. My remarks in the other post, therefore were not useful, I see.

This likely is an area problem and have you searched other jobs to see what eventually happened? It therefore would seem that local engineers and contractors have already addressed the situation and may have some advice for how they handled the problem.

Finally, did you know the ENR pile driving formula has a safety factor of 6 theoretically, but practically maybe is 3?.

 

[foreng]

I apologize for posting in two locations, I had two versions of the post prepared and somehow I managed to copy a version in each posting.

To answer a few questions:

The area in question is the northern part of Canada in the provinces of Alberta and British Columbia (Alaska Highway Country). Most of the work is in the oil rich areas comprised of thick lacustrine deposits formed during the last ice age. The deposits are glacial meltwater lake deposits with average depths of around 30-40m. The 150 ft pile is a bit of an anomaly but not uncommon. The clays are weakly calcareous and saline. These soils are also associated with other deposits moderately calcareous and silty in texture.

I have spoken to other contractors and engineers and they have the same advice, get some sampling done or drive till you get refusal. Most geotechs in the area are quite young, and really have never worked in a situation where you don’t have soil data. They have a hard time keeping people in such remote locations. Most of the seasoned guys are retired.

I realize ENR has a factor of safety of 6, which in some situations just makes this situation more painful. I see the FHWA has adopted the modified Gates formula, when I quickly run some numbers with a FS of 3.5 the blow count is reduced by about 30-40%.

Again, I'm not overly concerned about structural failure as this is very rare, probably because installations are overly conservative; I'm merely trying to walk the fine line between safety and economy.

I have not looked into cone penetration with the hammer, when I spoke to some of the soil testing firms, they suggested a cone test from a truck, but unfortunately the access was marginal. Most of the time roads are constructed during frozen conditions a week or so prior bridge construction. Doesn't leave much time for testing and analysis.

Thanks for the advice, I will research some testing alternatives. We really need to consider getting some information in order to complete an initial static calculation prior to construction, or have things set up to analyize on site.