earth retention_tiebacks vs struting 5

[krainaoz1]

Hi,

I'm new to this great forum and posted for the first time.

I'm a geotech engineer working in a team that is involved in a 4-level basement design for a proposed building in an urban setting.
The ground profile consists of 2-3 metres of residual soil and then 6-9 metres of weak weathered rock (gradually improving with depth) and then into competent bedrock (excavation depth is around 13m).

We have an issue where I cannot use tiebacks due to a lack of permission from the neighbours (train authority land) unless I can show that the anchors are a superior earth retention solution to struts/rakes.
From our client's point of view the use of soldier piles+tiebacks is far more convenient that struting. The soldier piles+tiebacks shoring solution satisfies all the ground movement/stability criteria.
Are there any geotechnical/structural arguments or a process that we could go through, which would give us a chance to convince the authority about the beneficial effects of using tiebacks as opposed to struts?

Thanks for any ideas/thoughts and I appreciate your help!

 

[Mad Mike]

Krainaoz1,

I think the neighbour's issue is not so much to do with the suitability of your tiebacks, but the future implications for his site. Obviously once the tiebacks are installed they cannot be tampered with in any deep excavation / piling operation that might take place. That places a restriction on the adjacent land owner, which is the main reason their permission is required.

All the best,
Mike

 

[hokie66]

Perhaps if you make the case well that your tiebacks are temporary ground anchors, and can be cut in the future if necessary, the authority would be receptive.

Payment for an easement also might entice them to look more favourably on your proposal.

 

[GeoPaveTraffic]

There is no geotechnical advantage to tiebacks over internal bracing.

Mike Lambert

 

[PEinc]

GeoPaveTraffic is correct - assuming that the braces are installed properly and are pre-loaded. The advantage for tiebacks is in the cost savings they produce for the excavation and concrete work. Installing a brace may cost about as much as installing a tieback anchor but the braces need to be removed later on from the basement (often in small pieces) and the foundation walls & waterproofing need to be patched up where the braces penetrated the walls. A 4-level basement with multi-tier cross braces or raker braces may not allow enough room for construction equipment in the hole. The benefits for using tiebacks instead of braces are generally for the contractor and project owner, not the adjacent property owners. If braces were better than tieback anchors, braces would be used more often. Perhaps someone should emphasize to the adjacent owner that each tieback anchor is tested to prove its ability to support its design load without any creep reduction. The same can't be said for braces.

http://www.PeirceEngineering.com

 

[SlideRuleEra]

...each tieback anchor is tested to prove its ability to support its design load without any creep reduction. The same can't be said for braces.

A wise statement. Here is a "before" photo of a drainage canal that we (bridge contractor) fixed on a design-build basis. The EOR had to pay for repairs, even though the bracing failure happened because of an added surcharge & dynamic loading never expected.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[krainaoz1]

I am very grateful for your responses and help so far and look forward to other opinions/arguments.

We are considering the use of fibreglass anchors, so there would be no excavation restrictions on the neighbouring land in the future. The anchors would also be distressed and cut after permanent basement floor slabs/walls are constructed (they become the permanent excavation support system), so if the fibreglass anchors are damaged/cut it would have no effect on the excavation stability.

PEinc - your suggestion about rigourous anchor testing to prove its ability to support its design load without any creep reduction is a great argument!!

 

[PEinc]

Fiberglass anchors will have much lower capacity and more elongation than steel anchors and will require many more anchors. Skip the fiberglass. Field tests by US contractors have shown that abandoned, temporary tieback anchors do not present any problem for future excavation. Hydraulic backhoes can easily dig right through abandoned, steel, tieback anchors safely and without any problems.

http://www.PeirceEngineering.com

 

[rapt]

" do not present any problem for future excavation"

As long as they are detensioned!

 

[PEinc]

Not true, rapt. When an abandoned but tensioned tieback is encountered and broken by a backhoe during excavation, it is surrounded by soil when it gets broken. The surrounding soil confines the tieback and prevents a problem. If overburden above the tieback anchor gets slowly removed, eventually the decreasing overburden will allow the tieback anchor to creep until it looses its tension. Again, no problem.

http://www.PeirceEngineering.com

 

[rapt]

Where I come from it has always been a requirement that they be detensioned.

Not sure if I would want to be on the backhoe when at a strand tieback anchor with several metres of fully tensioned free strand length before the grouted length when it 'releases" from the soil.

Other problem is failure of the strand due to corrosion causing a length of strand and attached anchorage to fly into the basement car park!

 

[hokie66]

I agree they should be detensioned. PEinc is probably correct most of the time, but why take the risk? They are "temporary", so should not be left tensioned.

 

[Ingenuity]

Permissible to use a fully removable REMO-anchor?

Link

 

[PEinc]

In the areas that I work, temporary tieback anchors get detensioned when the new foundation wall is being backfilled. When the backfill reaches a tieback level, the tiebacks are cut and the wales are removed before continuing backfill to the next higher tieback level or finished grade. If the new foundation wall is poured directly against the temporary (but left-in-place) sheeting wall, the tiebacks cannot be detensioned without spending significant time and cost.

Removable tieback anchor are not completely removable. Removable tieback anchors have limitations and are not a great as the suppliers claim them to be.

http://www.PeirceEngineering.com

 

[PEinc]

"Not sure if I would want to be on the backhoe when at a strand tieback anchor with several metres of fully tensioned free strand length before the grouted length when it 'releases" from the soil."
A backhoe would break a strand well before the operator would ever know about it. The soil fully surrounding the tieback tendon would prevent any problems.

"Other problem is failure of the strand due to corrosion causing a length of strand and attached anchorage to fly into the basement car park!"
In almost all situations, tendon corrosion would be a very gradual process, not a sudden full-load failure of the tendon. As the tendon slowly corrodes, it will stretch elastically until the load is relieved. Elastic Stretch = PL/AE which means that as the cross-sectional area is slowly decreased, the load slowly dissipates as the elongation increases. Also, there are little if any documented cases of tieback anchors failing from corrosion down in the ground. There has been documented corrosion at or very close to unprotected (no trumpets, no caps) anchor heads but that is an area where there may be sufficient water and available oxygen to cause corrosion. Steel bearing piles don't corrode significantly down in the ground. Neither do tieback anchors unless the subsurface soil is extremely aggressive - in which case, you should not be using non-encapsulated tiebacks.

http://www.PeirceEngineering.com

 

[rapt]

PEInc

"Not sure if I would want to be on the backhoe when at a strand tieback anchor with several metres of fully tensioned free strand length before the grouted length when it 'releases" from the soil."

I will make sure I am not a backhoe operator in your neck of the woods. In Australia, you would be banned from operating by Workplace Health and Safety if you put someone in that position.


"Other problem is failure of the strand due to corrosion causing a length of strand and attached anchorage to fly into the basement car park!"
Having seen corroded tendons that have exploded out of a wall into open space when they finally failed, I can tell you categorically that you are wrong on this one! Fortunately no-one was in the firing line, but only by a few metres.

 

[PEinc]

Obviously, I am not talking about excavating equipment hitting a tieback anchorhead at the face of an in-service wall. That would be a problem. I am talking about excavating behind a backfilled, out-of-service, tiedback wall for which the tieback anchors have not been detensioned. Digging down and hitting the tendon will safely break it while it is still confined by the soil.

I am also not advocating hitting and breaking a tensioned tieback anchor while the retaining wall is still in service. Of course, the anchorhead could fly off the face of the wall. It sounds like you are talking about totally different situations than I have described.

http://www.PeirceEngineering.com

 

[DanMcGrew]

krainaoz1. I have worked on projects where we have been that deep and used internal bracing. Usually we work with the contractor to design a bracing system that works with their equipment. Bracing levels are important so that braces are not in the way of floor finishing as you work back up out of the hole.
As for detensioned tiebacks not being a problem for future excavation, that is generally true except tunnel boring machines must have them removed before boring can begin. These detensioned tiebacks have caused major problems for tunnel boring in Seattle.

 

[rapt]

I give up!

 

[JSD1986]

Rapt- im with you, we call it naturally de-stressing of anchors once excavation above anchors occur. essentially zero risk leaving stressed anchors in the ground. Once the earth above them shifts a bit the tension is peacefully released.