Pile load test with peat layer
Pile load test with peat layer
(OP)
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!
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!





RE: Pile load test with peat layer
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.
RE: Pile load test with peat layer
RE: Pile load test with peat layer
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.
RE: Pile load test with peat layer
RE: Pile load test with peat layer
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.
RE: Pile load test with peat layer
Big H is correct that Dr. Fellenius has written extensively on the subject. Go to www.unisoftltd.com or www.fellenius.net for a collection of papers.
RE: Pile load test with peat layer
RE: Pile load test with peat layer
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.
RE: Pile load test with peat layer
RE: Pile load test with peat layer
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.
RE: Pile load test with peat layer
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.