Soldier Pier Retaining Wall (Globally Unstable Soils) 1

[THERR]

Hi All,

I am currently working on a project dealing with very unstable soils wanting to slide out and down into a park path (It has already destroyed part of a road from movement). There is an existing retaining wall (very old, worn, falling apart, i.e. historic - 8ft. in height) that faces the path and the client does not want to remove it, fix it, etc. Therefore, we need to stabilize the soils from behind the wall. Sheet pile was originaly suggested, problem is there is some extremely hard bedrock just 10' below grade so it will make sheet piling difficult. Another option is soldier piers (drilling 18" diam piers about 3' apart) we have done some work regarding this in very cohesive soils and have had no problems. This site has some pretty coarse gravel so I am not sure it will work. Can anyone tell me if we could get soldier piers to work or if there is any other option (no soil nails or helical piers - contractors option). Thanks in advance.

 

[BigH]

As Focht3 would say - Welcome to the Threads!!

A couple of questions beg asking, viz.,

1. What is the type of unstable soils? Sands, sandy silts, soft clays, etc. that form the slope up against which the retaining wall is placed. It is so very important to identify the soil stratigraphy.
2. What is the groundwater level regime? Has there been excessive rain?
3. Do you have a geotechnical engineer on the project.

If you don't have 3 - get one and get one fast. He can address the importance of points 1 and 2 - explain them to you and show you how they effect the nature of the design for stabilizing the slope.

Now, at first glance, given the "historic" nature of the 8' retaining wall, the slope has been there awhile. Is there any signs of "old" movements? . . . or have the movements started recently? My guess is that the movement is recent. Unless there has been a change in the geometry of the site (local steepening of the slope, loading atop the slope, etc), I would opine that you have had your fair share of rain this year - in this case, then, it is possible that the slope has a higher than normal groundwater regime. In cohesionless slopes, the raised water level will reduce the resistance to sliding (by reducing the effective normal stresses acting on the frictional sliding plane). In cohesive slopes, there might be water in minor tension cracks. In any event, it seems that water is the culprit, at this early stage of our assessment.

Given that water may be the culprit, groundwater control might be a wise consideration besides just trying "structural" means of stabilizing the slope. An experienced local geotechnical engineer, as stated before, will be able to address these issues and you may save a lot of time and money in not going back and forth on design options that really are not appropriate.

 

[THERR]

Thanks for the quick response BigH. Thanks for the welcome also.
The soil is a sandy clay at two boring locations and "mixed rubble" (assuming rubble is coarse gravel) at the other. The borings were taken about 6 months ago. During that time the borings indicate that the water table was roughly 3 ft. above bedrock (nothing significant - bedrock is 10' below grade). The movements seem pretty recent and significant though there is evidence that this wall has seen some minor movement in the past. One of the options thrown on the table was mechanically stabilizing the soil through mixing the soil with slurry. It was thrown off the table due to the fact the client does not want to do much excavation (a road runs next to the existing retaining wall). Assuming this is a cohesive soil your thoughts on water infiltrating minor tension cracks in the soil intrigues me. The funny thing is the geotechincal report mentions nothing of the slope stability problem and this problem seems to have been apparent during the time of the borings.

 

[BigH]

You've indicated that you had a soil boring done. Was it done by a drilling contractor who just handed you the logs, or was it done under the guidance and supervision of an experienced geotechnical engineer (i.e., firm/organization) with proper internal reviews. In my view, it seems quite obvious that there is a slope stability problem from the information at hand and I am surprised if, in the geotechnical report (if an interpretative report rather than a strictly factual report) didn't address this issue. What did the report say was the cause of the distress? What did the report recommend for remedial actions? Most thorough geotechnical reports will raise a number of options that might be available to the designer of the remedial works. If yours didn't, perhaps you need a fresh look at the information. Many times, the client - a non-technical man - simply knows he should do a boring and hires a drilling company. They give him the logs. Then, he gives this to the designer - but in a proper arrangement, the designer should engage the geotechnical engineer so that, as the programme is set up, it can be tailored to the job.

If there is a road at the crest of the slope (or slightly back), the condition of the slope stability (whether in the sense of a soil slope stabilty or excessive pressure on the retaining wall - or perhaps deterioration of the retaining wall) is a major concern. The client's liability might be huge depending on the legalities and interaction of him and the authorities.

Anyway - some more thoughts. By the way - where is the project (roughly)?

 

[THERR]

The soils report does not mention any slip critical plains or slope stability issues on the site. The borings were provided by a drilling contractor. Who in turn handed them to a geotech. Who in turn handed them to an engineer. Who in turn used them for a structure located in the general vicinity of the project (about 50 ft. away). So the borings were originally taken to provide information about a structure and not the wall in question, but since they are relatively right next to each other the client now would like to use them for the wall. The problem is the soils report just describes foundation type to be used (drilled piers of course), bearing capacity and skin friction. The report also provides the soil borings showing the areas of rubble and cohesive soils. There is no mention of soil instability. When I called the geotech at his firm yesterday I found out ...drum roll please..."he is not working for us anymore." So I asked if anyone else is familiar with the project and I was told they will get back to me. Btw. the project is located near Highlands Ranch in Colorado.

 

[Focht3]

Hmmm,

Sounds like there's more to this story than you have been told -

You need more information than you apparently have right now. Additional borings, inclinometers, topographic surveys, etc. That is, if the owner wants to save the wall.

And, oh, one more thing: you either need a new geotech on the team, or the owner and existing firm need to kiss and make up...



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[DRC1]

If you have a couple of borings, you may have enough information to do a slope stability analysis. The fact that you mention that there has been evidence of minor movements over time may sugguest the slope was marginally unstable and had been creeping. Soldier piles and lagging will not solve a global stability problem as the failure plane will pass around the piles. You will probably need to drive socketed soldier piles and then drive sheets so that the soldier piles are in the down hill bellies. Trying to make this wall work is probably going to be more expensive than replacing it. My best sugestion is to have the owner go out, have his picture taken next to the wall, then tear it down and build a new engineered wall that adressess the global stability issue.

 

[Focht3]

Hmmm,

Not so fast. Evaluating unstable slopes isn't cookbook engineering. [blue]THERR[/blue] needs to know what the loads are in order to design a fix. In order to do that s/he needs more information - and someone to evaluate both the cause(s) of the instability and the depth of the unstable zone. Without that crucial information, [blue]THERR[/blue] may create a design that fails, one that works but is unnecessarily expensive, or gets lucky and gets it right. I, for one, don't like 1 in 3 odds...

And if the slope has been "marginally unstable and been creeping," then a geotech's involvement is crucial unless [blue]THERR[/blue] has done a lot of these before and has a thorough understanding of the site and area geology. From the tone of [blue]THERR[/blue]'s posts, I find this unlikely. And it is important to remember that the existing soil borings were taken six months ago for a building about 50 feet away from the wall, so they may or may not represent the current conditions directly in front and behind the wall.

[blue]THERR[/blue]: Why put your firm - and possibly your professional license - at risk unnecessarily? You didn't create the problem; don't put yourself in a position to have to pay for the proper fix - and maybe find yourself explaining your actions to your state board. Remember that damage to a public park, or access to it, could trigger the involvement of public officials - and the press.

I'd withdraw from the assignment if the owner refuses to properly address the issue -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[THERR]

Great posts by all. I really appreciate all of your help. After much deliberation on this project (there was plenty), and the uphill battle it seems I will be fighting with all the existing conditions on the site (not to mention the schism between clients and original consultants that seems to have been existing way before I became entangled in this mess) I have decided not to take the project. These are tough times and every project does help. But you all are right. This project is just not worth the risk of my professional career.
I am new to this web board and just the short time I have been on here it has really impressed me. Your comments are all well thought out, intelligent and professional. I will be sure in the future to check back here often. Maybe one day I will be able to help you all as much as you have helped me. Thank You.

 

[BigH]

Good comments/warnings Focht3.

Ditto for Therr on turning it down.

I once had a similar situation. A highly political report to write - Conservation agency doesn't want hillside developement but says if a geotech says it is ok, they will consider, but they won't really. Luckily my old mentor, over a Whopper helped me sort out the correct/best tack to take.

Soils do change. I had another site where 5 borings showed good till from the surface - one boring at the toe (about 20m away from one of the other borings) of the embankment showed artesian sands - flows so big I had massive troubles trying to plug.

 

[Robert168]

From this story, I have found out again that there is a big defferience between a soil test report and a geotechnical investigation report. The results of a soil test report, no matter how perfect they are, can not mean anything unless they correctly interpreted by a qualified geotech taking all the influencing factors into consideration.

No matter who will take this type of project, he or she needs a geotechnical investigation report, rather than a soil test report.