Pile load test with peat layer 1

[eric1037]

I have a project that will have auger-cast piles extending through existing fill and buried peat. There will be additional fill at the site. I anticipate there will be downdrag on the piles due to the skin friction of the fill when the peat compresses. For arguments sake, let's say the downdrag will be 5 kips.

The piles were designed for a structural load of 70 kips. Therefore, the ASTM load test requires the pile to be loaded to 2 times its capacity or 140 kips.

However, there will be load on the pile from the downdrag that the load test will not take into account.

Should the load test actually be 2 times the structural load plus the load from downdrag, i.e. 2*(70+5) = 150 kips?

Thanks in advance!

 

[jgailla]

eric1037,
I recently ran into the same problem. You are correct. Downdrag forces will not manifest themselves during the load test, as they are applied later when fill is added. If you have a 70 ton pile (with FS applied) and 5 tons of downdrag (no FS applied) you should end up with a test to 150 tons.

Many people seem to be confused about downdrag. In my undergrad foundations class, it appeared only about 10% of the class had an understanding of downdrag. Many civil and structural engineers in my experience do not have a clear picture of downdrag. In a pile seminar I recently attended, there was some question in dynamic testing of a pile whether downdrag forces were added in determining a factor of safety.

Some could argue that the correct load test would be 145 tons(i.e. 2*70+5). We usually specify a test load in the report. In this case, we would specify the test be performed to 150 tons instead of getting too specific. I recently found that because of confusion on the part of the structural engineer on a fairly large project, piles that could have been 80 tons (with downdrag included) were designed for 65 tons (downdrag included twice). Sorry for the ramble.

 

[GeoPaveTraffic]

In you example the test load should be 150 tons. If I were very certain of the downdrag calculations, and I have never been this certain, then I would use a test load of 145 tons, i.e. no FS on the downdrag.

 

[eric1037]

Thanks for the input!

I am not entirely certain with the downdrag calcs either. Especially since the thickness and depth of fill will vary across the site. I would rather be on the safe side.

 

[cbosy]

150 tons is correct and that provides a FS=1 since only 145 tons will make it thru the fill/peat and then another 5 tons must be deducted due to the long-term downdrag load of 5 tons.

 

[BigH]

I think this is a 6 of one, half dozen of another. I know of some who add the downdrag onto the 2x design load and others who do it (DL+DD)x2. You will see both in the literature. In your example, I would do 150 kips - it is nice number whereas 145 kips isn't really that "nice". Keeping things to 25 or 50 kip units just seems better - at least to me.

I would suggest that you do a search of B. Fellenius who has written extensively on downdrag of piles. He has some interesting observations and clearly has studied, in detail, the topic of downdrag. I apoligise I didn't keep the URL's of many of my downloads - but one reference is:

Fellenius, B. 1984. Negative Skin Friction and Settlement of Piles. 2nd Int'l Seminar on Pile Foundations. Nanyang Tehnological Institute, Singapore, Nov. 1984.

 

[DRC1]

Downdrag is present in all piles and is a function of pile length more than soil type. The downdrag is present during load testing as only small movements are nessesary to mobilize downdrag.
Big H is correct that Dr. Fellenius has written extensively on the subject. Go to

http://www.unisoftltd.com

or

http://www.fellenius.net

for a collection of papers.

 

[dmoler]

Be safe, include the downdrag in your report, and I would recommend that you include it in your capacity recommendations in bold. The reason is that the structurals want an easy number to design to (no knock against you structural people) and just give them a number that includes that. You dont want to be sued for this.

 

[jgailla]

DRC1,

Downdrag as I understand it is not present in all piles. The small movement of the pile is necessary to mobilize skin friction. Skin friction normally works to hold up the pile. Downdrag occurs when the skin friction is pulling down on the pile. This only happens when the soil is compressing under a load exterior to the pile, such as when fill is added to a site and compresses soft materials below the surface. All skin friction above and in the soft, compressing layer then acts to pull the pile down.

 

[DRC1]

Downdrag is a function of excess capacity in a pile. For a given pile load there is defined length of pile plus any tip capacity that is required to support the pile. The dividing line between the length of pile in the lower portion of the pile and the balance of the pile is the neutral plane. Below the neutral plane the pile is moving relative to the soil (penetration)developing supporting friction. Above the neutral axis, the soil is moving relative to the pile (settlement), thus developing downdrag. Downdrag does not effect the geotechnical capacity of the pile but it does effect the structural capacity of the pile.

 

[eric1037]

DRC1:

That is probably the most succinct description of downdrag I have heard. I assume the key is determining where that neutral axis lies along the length of the pile. In some situations, such as the one I described at the beginning of this thread, it is relatively simple to determine. However, for a typical site without easily compressible soils, it could become quite complex.

I suppose in those cases the downdrag has much less impact on the design of the pile than in my case. However, if grades are raised, there is always downdrag.

Interesting discussion.

 

[dmoler]

I agree with DRC1 to an extent. With a peat layer, the neutral axis is somewhere below the peat layer, regardless of end bearing/side friction ratios. At a minimum, load transferred above the peat layer should be negated. The load transferred from the pile to the soil above the peat layer has the vertical shear component that acts on the compressible peat layer. The load transferred to shear above the peat layer/neutral axis will slowly be reduced from mobilized, then residual, then zero, then downdrag, as the peat layer compresses and the soil above moves down relative to the fixed pile.

For a load test, you will see (if using strain gauges) load transferred along the shaft including above the peat layer. However, because compression of the peat is long term, loss of mobilized shear will not be evident in a 24 hour load test.