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H-Pile Toe Relaxation in Sedimentary Bedrock

(99.03.27) 

Query -- We are currently working on a project in which some evidence of relaxation has developed for piles driven to refusal in a local bedrock formation comprised of  shale/sandstone/mudstone that is heavily folded/weathered.  The piles penetrate several feet(2-3 m) to tens of ft (perhaps 10 m) into the  bedrock before reaching refusal. They  seem to exhibit little change in  behavior within short-term restrikes  (a few days to a week.)
    When revisited several weeks later the capacity appears to have degraded for some piles, even when the piles have  been well isolated from the rest of the action.
     I recall some discussions years ago regarding similar behavior observed in the high strain testing community.  Are you aware of any published articles addressing this phenomenon?  Any advice is greatly appreciated!  Mike Holloway

Following are the replies received to date concerning the above email query.  These have been quite useful in documenting the observations of these phenomena, and in postulating the geotechnical mechanism(s) involved.  If you have any further meaningful information to contribute, please submit it.  Thank you to all....

Replies --

1.    I have similar problems in my testing jobs before. When I tried to explain my hypothesis to the local enginners, many don't believe what I said.  This problem occured in shale/sandstone material with high silt content, and usually the piles were short say not more than 12.0m deep. The harder you drive, the lower the capacity in the pile after a period of time.   My explanation to your problem is as follows:
    When driving a pile through shale with a small set, the energy delivered to the pile have to be dissicipated in the pile. There isn't any vertical penetration to absorp the energy. Therefore the energy delivered to the pile would go to the side way vibration in the pile.  This would creat a gap between the pile and the surrounding soil.  At the same time, the high silt content shale material would be subjected to fissure crack around the pile tip. At such situation after a period of time, ground water would start to seep into the toe. When fissure shale subjected to water would soften the soil and reduce the end bearing capacity of pile.  The problem would persist even redrive the pile to set again. Any dynamic load test should pick up such phenomena.  My proposal to my client was to downgrade the pile.
    In such situation, bored pile would be the solution. Mun Kwai Peng

2.     I have met with this phenomenon in a number of cases.  It is a frequent problem in all of Ontario, for example. The mechanism is similar to relaxation of piles driven in dense silty sand: The soil dilates and the thus created negative pore pressures cannot be relieved during the blow.  Consequently, the effective stress increases and grabs the pile creating a temporary increase of shear resistance.  We see the similar effect when piles are driven into shale capped with impervious till: the driving into the rock creates pieces of rock that have to be displaced as the pile to advances.  The pieces roll over one another and the soil-rock volume dilates causing a sucking response in the water. The till is  impervious enough to prevent the water from flowing into the voids between the rock pieces which causes the effective stress to increase creating the temporary resistance evidenced by the relaxation of capacity.
    So, the phenomenon is easily explained.  However, I have not found any sure means of predicting that a particular site will havethe problem other than experience that it might occur.  For example, I have found it only over a part of a site without any obvious reason as to why the trouble --- and it was, for sure ---would not be everywhere over that site.
    I do not think the phenomenon has been written up properly anywhere. I think I presented it somewhat loosely to a User’s Day in Boulder back in the early 80’s.  Bengt Fellenius

3.     I co-authored a paper with Niel Schwanz at the Corps about some h-pile we tested statically/dynamically near Minot, ND.  Static capacity of h-pile driven to shale was less than dynamic predictions, assumed due to relaxation.  Paper is in Stress Wave 96 proceedings.  William K. Cody

4.     See Thompson&Thompson, Real and Apparent Relaxation of Driven Piles. ASCE, Philadelphia, May 16-19, 1983, "Dynamic Measurement of Piles and Piers."
    GRL also did some work in the Ohio valley shales, mostly for Penn DOT in Pittsburgh with lots of relaxation.  Drive/redrive/redrive etc.  I doubt that was published.  Frank probably had the most central seat in the Pittsburgh series of tests.
    In my experience and my perceptions of others experience, the finer grained material, ie clay shales, are the most prone to relaxation.   It has definitely been seen before.  Bert Miner

5.    We have seen relaxation in piles driven into shale material.  In fact, it has usually been very soon after initial driving, not weeks as you've discribed. I've been of the belief that the cause of the relaxation has to do with water flowing along the sides of the piles and into the previously dry shales. this leads to a degradation of the rock and hence the relaxation.  I will look to see if we have any published articles to support my feelings. Bill Beloff

6.     I recall several similar cases with cast in situ piles (Franki piles).  We attributed this effect to the softening of the shale when coming into contact with wet concrete. Having a problem with precast piles (steel or concrete) is new to me...
    How deep was the groundwater level?  Are you planning to repeat the driving test? Maybe you could inspect the stress wave shapes and learn from changes there?  Best of luck, would be interesting to hear about the resolution of the problem. Rainer Massarsch

7.    During the Case project we first found this problem and published a report in 1977 which addressed this topic.  Reference would be “Piles and Pile Driving Performance for H-piles Driven to Bedrock”, Nov 1977,  199 pages, Department of Civil Engineering, Case Western Reserve University, authors G.G. Goble, G.E. Likins, Jr., and W. Teferra.  Data case was from piles here in Cleveland . Mohamad and I “published” a case for H piles in Pittsburgh for a Users Days many years ago (approx 1983?).
    Fellenius also wrote about relaxation in the first stresswave conference 1980, but they were 2 cases of concrete piles.   The PDA manual mentions the problem of H piles in soft or weathered rocks….  It is a reasonably well known problem.  Garland Likins

8.     We did a project funded by PennDOT several years ago. We were a subcontractor and had little control over the project.  Fairly extensive testing was done in the Pittsburg area where relaxation in the shale had been frequently observed.
    They wanted to quantify the magnitude of the relaxation.  As I recall the results were not very conclusive.  I believe that a report was prepared.  Those characteristics were often observed in eastern Ohio and western Pennsylvania.  You can probably get a copy of the report from the Cleveland office.  George Goble

9.     Although not common, it is not unusual for "Relaxation" to occur in Australia. It occurs predominantly in fine grained soils and sedimentary rocks.   I haven't had time to look through my technical literature in any detail, but a paper that immediately springs to mind was one from the 4th SWC: "The effects of pile relaxation on toe capacity and stiffness" by Seidel & Anderson, & Morrison", pp 619-626, 4th SWC, 1992.  Slav Tchepak

10.    If the shale/sandstone is typical Calif. material, it is quite sensitive to moisture intrusion. Most of the weathered Calif shales/sandstones are rather weakly cemented (or precompressed). They readily absorb water, and weaken considerably. Do the piles provide a path for migration of water?  If the piles are in a dry environment, neither moisture migration nor dilation could be involved. In this case, there must be some mechanism for relaxation of the lateral pressures created during driving. Since the piles are isolated from "the action", lateral vibrations from their neighbors may not be the cause, but stress waves travel through the shales and sandstones for considerable distances at depth.

Good luck. Maybe you can lobby for a few load tests to gather more evidence.  DewaineBogard
 

Created by Jason Ernst Holloway - Last modified: 03.07.31 (lkh)