Slope Retained wall failure
Slope Retained wall failure
(OP)
My firm (acting as a subcontractor) has installed a soldier pile lagging wall that has failed by excessive deflection (2 ft in some cases). The wall is 15 ft high, with precast lagging panels and HP14x89 beams in 24" concrete filled holes. The purpose of the wall is to retain a landslide in a rain forest that continues to cover a mountain roadway. The wall was designed by the FHWA, who are claiming that we did not drill the piles far enough into rock (they required 7' of rock}. At the start of the job we did ask how we would know when to stop the piles and were told that it would be obvious as we were drilling through rock, and the inspector would confirm the pile depth. We are sure we drilled 7' into rock, but they are now using the term "bedrock" in an area where the bedrock is covered with weathered rock. Nevertheless the inspector verified we had enough tip depth.
The question here is the original design - the calcs are for a standard flexible retaining wall with a level backfill and homogeneous soil conditions, with no accountability for rock or a unstable slope, or considering water behind the wall (it is a rain forest near Cuba). We have pictures of a 7' diameter boulder that is now behind the wall, which was not there when we drilled the piles, and the GC is constantly regrading the backfill slope as it is unstable. I think the original design is not adequate for the actual conditions, but they keep pressing us that we did not drill deep enough. Do you design walls to retain unstable slopes like this? Any opinions?
The question here is the original design - the calcs are for a standard flexible retaining wall with a level backfill and homogeneous soil conditions, with no accountability for rock or a unstable slope, or considering water behind the wall (it is a rain forest near Cuba). We have pictures of a 7' diameter boulder that is now behind the wall, which was not there when we drilled the piles, and the GC is constantly regrading the backfill slope as it is unstable. I think the original design is not adequate for the actual conditions, but they keep pressing us that we did not drill deep enough. Do you design walls to retain unstable slopes like this? Any opinions?
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RE: Slope Retained wall failure
My best estimate is that the wall may not have been designed for the proper loading conditions. Soldier pile walls can and are designed to resist the forces of an unstable slope, however, the loads are different than for a standard retaining wall design.
My suggestion is that your firm needs to hire an experienced geotechnical engineer to review the original design and the existing conditions. Based on this review you should be able to determine the best path forward.
RE: Slope Retained wall failure
First, a 15' cantilevered, soldier beam wall is pushing the limits for even a non-landslide condition. With a landslide condition, the earth loads can be significantly higher. Therefore, the odds are that the HP14x89 soldier beams could be too small and or the embedment into rock (especially if weathered or fractured) could be insufficient. You need a slope stability analysis, not a simple retaining wall design. A slope stability analysis is only as good as the accuracy of the soil and rock properties you use. Hire a geotech who can get the proper soil and rock values and run the analysis.
Second, I never recommend using precast lagging panels for a wall being built by top-down construction - especially in a landslide condition. What holds up the already unstable hillside while you open cut to install the soldier beams and precast lagging? Precast lagging panels usually need to be installed from the bottom up, after the full height cut has been made. This can lead to an unsafe, unstable, illegal working condition.
RE: Slope Retained wall failure
RE: Slope Retained wall failure
Pslem: We have measured the top deflection and the movement at groundline (roadway elev). The pile has moved at the roadway elev. and you can see that the HP beam has broken through the concrete pile. It appears the concrete pile is trying to stay in place and the HP is moving outward.
RE: Slope Retained wall failure
RE: Slope Retained wall failure
RE: Slope Retained wall failure
RE: Slope Retained wall failure
The bending of the H pile should at least give the actual bending moment by back calculating.
Can you insert any sort of pressure measuring device between the earth and the wall, such as one would install on a wall before backfilling it?
Can you measure the deflecting of the lagging unints and relate to pressure? They might be taken into a lab and loaded to get their "spring constant".
Can some installation of strain gages be done to accomplish this?
Could you cut a vertical notch in a lagging unit after installing a moment measuring "device" outboard to take the bending tensile part lost by the cut. That might be a steel strap with strain gauge as an idea for that.
RE: Slope Retained wall failure
I go back to my original post, get an experienced local (if possible) geotechnical engineer involved. That is the only chance you have of convincing the original designer that his design was inadaquate. Otherwise, I expect that you will be paying for the new wall.
RE: Slope Retained wall failure
RE: Slope Retained wall failure
The H piles (as it seems they should) for the seen part are not stressed in the apparent length as to cause bending deflection. The slope pressure is then more than anything arcing towards the lower surface. Without knowing what happens under ground level it is difficult to ascertain if there is bending or rigid rotation underground but the photo seems to indicate rigid body rotation and lack of embedment.
The design seems flawed by just not being designed properly as a retention system as the slope stability case demands. The designer needs to determine a critical wedge for a long (active) slope and this differs from the common retaining wall design. However pressures can be determined from a properly determined critical wedge.
In any case you may have drilled enough in the weak rock, the piles and slabs be strong enough ... and the weak rock fail due to embedment be not enough.
RE: Slope Retained wall failure
Judging from the picture the soil appears to have a fair amount of plastic material with a significant amount of moisture in it as well.
A quick check of the cut using the steel section decribed above and asuming 3ft spacing between piles and that the total pile length is about 25ft, we get the following:
1. If the 7ft embedment in rock is verified, the pile would not be expected to deflect more than 2-4in. (Joints not acounted for)
2. If the rock embedment is not there or limited, the pile tends to rotate (and this is evident in the picture) and yes, the 2ft deflection or more becomes plausible. A 3ft embedment gets you close to 10-12in of deflection (assuming the same quality soil continues to the surface of the rock)
Bear in mind that these are sketchy (but indicative) calculations based on the limited information available.
Moral of the story, weigh your move carefully before you react, and "get the help of a professional geotech" and drill a couple of holes to verify the soil quality in the embedment zone including the quality of the "rock".
RE: Slope Retained wall failure
The spacing between the piles is 5'. With typical pile depth of 20' bgs (36' total, 7' rock embedment), 16.5 deg slope, full water pressure behind wall, no contribution to stiffness from concrete (it is cracked), the deflection at the top using a p-y analysis is 21". The original design assumed a backfill unit weight of 105 pcf and a phi of 23 deg. I used gamma of 125 pcf for my analysis (the slope is a saturated clay w/ boulders).
A geotech report was done but is of limited value as only 2 borings were done at the road level, one each side of the slope covered roadway.
RE: Slope Retained wall failure
Interesting photo. My first thoughts when I opened the photo was, where are the tiebacks (nails/deadman/etc). This system is at the brink of its cantilever ability - we generally start thinking of some form of tieback system when we need to support an excavated wall (stable) of approximately 4.5 metres or greater. The need for a suitable tieback system would be even greater when the slope is unstable.
You mentioned boulders and it made me think about the paleo-channels (buried gullies) we get on our mountain slopes - the water that flows down these gullies is often very powerful, especially after intense rainfalls. Is it possible that you may have such a buried channel or gully at this point of the soldier pile wall where the water becomes trapped behind the wall and the water pressure builds. It appears quite muddy in front of the failed part of the soldier wall.
Next question - were the steel soldiers joined? and how were they joined? I noticed that other soldier piles appear shorter in the photo and I would assume the joint is located deeper in the ground, where as the failed pile length appears much longer?? Possibly indicating variable rock penetration, hence variability in the rock? Two borings may not have been enough information on the rock quality.
Just some thoughts from viewing the photo.
RE: Slope Retained wall failure
What was the timing of the failure (occurred suddenly while a D8 was tracking fill along it, overnight when nobody was there to see it, gradually yielded during fill placement)?
How was the fill being compacted against the wall? Against a stiff wall, the pressure can get much greater than active or even Ko. This one was probably pretty soft, however.
Was the designer counting on much help from passive resistance?
_______________________
Somebody check me on the following. (Took my last structural engrg class in 1980.)
HP 14x89: Section modulus = 131 in^3. If it's 60 ksi steel, the yield moment is 7860 k-in or 655 k-ft or 655,000 lb-ft. The yield moment is when the outermost fibers of the beam reach yield. The plastic moment (when the full section is yielding and full resistance is mobilized) would be higher than that, but I'm too lazy to figure it out.
Assume Ka = 0.5, gamma = 125 pcf, height of fill = 15', spacing between soldiers = 5'.
Bending moment = force*moment arm = [area supported by each soldier]*[avg. pressure]*[moment arm]
= [5'*15']*[1/2*0.5*15'*125 pcf]*[15'/3] = 176,000 lb-ft
Changing ht. to 20':
Bending moment = [5'*20']*[1/2*0.5*20'*125 pcf]*[20'/3] = 416,000 lb-ft.
If it's only 50 ksi steel, the yield moment would be 546,000.
___________________
Regards,
DRG
RE: Slope Retained wall failure
RE: Slope Retained wall failure
The OP said that the wall supported a landslide in a rain forest. I believe that best thing to do is check the original design to see if heavy enough design loads were used (I doubt it) and to check the borings and drill logs to check the competency of the "rock" socket.
RE: Slope Retained wall failure
RE: Slope Retained wall failure
RE: Slope Retained wall failure
From the photo provided (and the apparent size of the landslide), the slide plane would seem to be much deeper than 13' below grade. Am I correct in assuming that the photo was taken near the middle of the slope failure? Yikes!
"A geotech report was done but is of limited value as only 2 borings were done at the road level, one each side of the slope covered roadway"
Sounds like the borings were done on the sides or lateral margins of the landslide. You would expect bedrock to be shallower on the margins of the landslide, rather than the center of it.
RE: Slope Retained wall failure
RE: Slope Retained wall failure
RE: Slope Retained wall failure
RE: Slope Retained wall failure