Slope Retained wall failure 1

[kleo]

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?

 

[epongra2]

"With typical pile depth of 20' bgs (36' total, 7' rock embedment)"

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.

 

[kleo]

The flange of 229 was damaged - I saw it. The GC twisted the section before it was placed in the drilled hole. This view is from behind the wall-you can see that 229 is twisted.

 

[dgillette]

Kleo - could you post a sketch of the cross section? I want to be sure I understand where the base of the slide is, what the piles are set into, etc.

 

[PEinc]

The 229 beam looks like it is not in the same orientation as the adjacent soldier beams but I still do not see the flange damage you refer to. If you saw damage, I'm sure it is there. However, your problem is more than one damaged soldier beam. One damaged soldier beam will not cause a wall to fail as shown in your first picture. HP14x89, GR50, soldier beams are fairly strong and they still appear to be fairly straight, although no longer plumb. I suspect that the lateral load was much greater than the design load and that the passive resistance, supposedly in rock, was less that that counted on in the design. 15' is above the normal cantilevering height even for competent soils. A landslide condition makes it even less appropriate to cantilever 15 feet. The only thing favoring the original design is the close, 5' beam spacing (which is usually too close and too expensive). The rest of the design is, to me, suspect.

 

[PEinc]

When I look at the original picture, the height of the precast lagging panels appears to be not much less than the 14.695" flange width of the HP14x89 soldier beams. There are as many as 19 lagging panels high showing in the picture. If the panels are only 12" high, that's a 19' high wall - not a 15' wall. At 14" high, the wall would be as high as 22 feet! Please double check the 15' wall height. A 15' wall with 19 panels would mean the panels are less than 10" high. 10" lagging panels seem too small for what I see in the picture and for what I have designed in the past.