earth retention_tiebacks vs struting
earth retention_tiebacks vs struting
(OP)
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!
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!





RE: earth retention_tiebacks vs struting
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
RE: earth retention_tiebacks vs struting
Payment for an easement also might entice them to look more favourably on your proposal.
RE: earth retention_tiebacks vs struting
Mike Lambert
RE: earth retention_tiebacks vs struting
www.PeirceEngineering.com
RE: earth retention_tiebacks vs struting
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.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: earth retention_tiebacks vs struting
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!!
RE: earth retention_tiebacks vs struting
www.PeirceEngineering.com
RE: earth retention_tiebacks vs struting
As long as they are detensioned!
RE: earth retention_tiebacks vs struting
www.PeirceEngineering.com
RE: earth retention_tiebacks vs struting
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!
RE: earth retention_tiebacks vs struting
RE: earth retention_tiebacks vs struting
Link
RE: earth retention_tiebacks vs struting
Removable tieback anchor are not completely removable. Removable tieback anchors have limitations and are not a great as the suppliers claim them to be.
www.PeirceEngineering.com
RE: earth retention_tiebacks vs struting
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.
www.PeirceEngineering.com
RE: earth retention_tiebacks vs struting
"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.
RE: earth retention_tiebacks vs struting
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.
www.PeirceEngineering.com
RE: earth retention_tiebacks vs struting
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.
RE: earth retention_tiebacks vs struting