Hello, I need some suggestions on a 30 ft high retaining wall. I never designed a retaining wall more than 9ft high. The wall is also part of the building, and we do not want to transfer the load to the building. I attached a preliminary geometry. Please take a look to see it makes sense and let me know your suggestions and anything I need to consider. Thanks very much!
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30 ft high retaining wall 3
- Thread starter Engin1
- Start date
HTURKAK,
The service life is going to be dependent on a lot of things. 30 year service life on the anchors is a reality if you are going to be installing black bar in 6" dia. grouted holes. 8" holes with either epoxy coated or galvanized all thread rod pregrouted in a corrugated HDPE sleeve with an added corrosion inhibitor in both the pregrout and tremie grout will certainly get you 75 year service life in normal soil conditions.
Providing proper drainage on the backside of the wall and possibly waterproofing the concrete will be important.
The service life is going to be dependent on a lot of things. 30 year service life on the anchors is a reality if you are going to be installing black bar in 6" dia. grouted holes. 8" holes with either epoxy coated or galvanized all thread rod pregrouted in a corrugated HDPE sleeve with an added corrosion inhibitor in both the pregrout and tremie grout will certainly get you 75 year service life in normal soil conditions.
Providing proper drainage on the backside of the wall and possibly waterproofing the concrete will be important.
In past, we used soil nailing system for temporary retaining of a deep excavation. To my best knowledge ( please somebody correct me if I am wrong ) , the soil nailing system typically installed through drilling , placing the reinforcement and grouting. The nails are not prestressed after installation for this reason sometimes called passive anchorage . The nails will get active once the soil mass starts to move.
Instead of reference , I will tell my experience: I have experienced that the buried concrete loosing strength so that one can excavate with hand tools ( if the concrete is not protected properly). Just for curious, how the grout can be protected against corrosive soils for a design life 50+ years ?
P.S. Railway departments still insisting for gravity retaining walls for abutments and for walls retaining railway track.
Instead of reference , I will tell my experience: I have experienced that the buried concrete loosing strength so that one can excavate with hand tools ( if the concrete is not protected properly). Just for curious, how the grout can be protected against corrosive soils for a design life 50+ years ?
P.S. Railway departments still insisting for gravity retaining walls for abutments and for walls retaining railway track.
Dear STrctPono,
Thank you for your explanation. I did not see your respond since I was writing my comment. (pls notice the time 22 Sep 20 18:39 and 22 Sep 20 18:40 ) . My concerns are , deterioration of grout in long term rather than corrosion protection of reinforcement and the requirement of mobilisation of soil ( development of active thrust ) for activation of passive anchors. That is, at least some portion of active thrust will be resisted by bldg. perimeter walls moreover, at rest pressure will develop in long term and mostly will be resisted by again building perimeter walls .
Kind regards..
Thank you for your explanation. I did not see your respond since I was writing my comment. (pls notice the time 22 Sep 20 18:39 and 22 Sep 20 18:40 ) . My concerns are , deterioration of grout in long term rather than corrosion protection of reinforcement and the requirement of mobilisation of soil ( development of active thrust ) for activation of passive anchors. That is, at least some portion of active thrust will be resisted by bldg. perimeter walls moreover, at rest pressure will develop in long term and mostly will be resisted by again building perimeter walls .
Kind regards..
I have seen calculation for Shear Strength for the Heel before
where they neglect bearing pressure.
causing maximum shear to be high value
say for instance your wall
Shear @ critical point = 1.4D = 1.4 (14*32*120 + 14*3*150) = 84 kip/ft
and shear strength phi V_n = phi * 2 * (fc)^0.5 * 12 * d
your case assuming 4000 psi
.0.75*2*(4000)^0.5*12*33.6 = 38.3 k/ft < 84 k/ft
where they neglect bearing pressure.
causing maximum shear to be high value
say for instance your wall
Shear @ critical point = 1.4D = 1.4 (14*32*120 + 14*3*150) = 84 kip/ft
and shear strength phi V_n = phi * 2 * (fc)^0.5 * 12 * d
your case assuming 4000 psi
.0.75*2*(4000)^0.5*12*33.6 = 38.3 k/ft < 84 k/ft
HTURKAK, permanent, anchored, soldier beam walls have been used in the United States for over 40 years, and I'm sure for a longer time in Europe. I think you will have a very hard time finding literature about any failed anchored wall that was built correctly and with proper corrosion protection for the anchors and anchor heads. If there were a significant number of anchored wall problems, the FHWA, AASHTO, and state DOT's would not still be building these walls. FHWA's Geotechnical Publications web site has much downloadable (some free) information on anchored walls, ground anchors, and soil nails.
Let me to express my concerns again;
- The proposed system is soil nailing ( or sometimes called passive anchorage ) system .The nails will get active once the soil mass starts to move. That is, some portion of active thrust will be resisted by building perimeter walls and moreover, at rest pressure will develop in long term and will be resisted by again building perimeter walls .
- The corrosion protection measurements are applicable for protection of reinforcement. My concern is , deterioration of grout in long term rather than corrosion of reinforcement,
- In my city , there are R.C. buildings which are 100 years old or more. The service life of bldg could be the more than service life of passive anchorage system.
P.S. Sorry for disappointing you but I have no intention to search the web for finding literature about any failed anchored walls .
However, looked to the I am retired engineer and sharing my experience without conflict of interest .
HTURKAK,
What PEinc and I said earlier is that the soil nail wall should absolutely be isolated from the main portion of the building. A 30ft tall standard cantilever retaining wall (which you supported) would also very much be susceptible to movement at the top of the wall. You seem to focus a lot on it being a passive system, yet I don't see any benefits to this being an active posttensioned tieback system. And I would argue that a cantilever retaining wall is a pretty passive system as well.
You mention that you are worried about deterioration of grout in the long term. Please explain what is the source of deterioration? Are you specifically talking about sulfate attacks? I would agree that a high cement content low aggregate mix of grout is more susceptible to washout, lower durability, and more prone to fatigue cracking... I would argue, however, that the corrosion of the internal steel is a greater threat to the longevity of the grout than the deterioration of the grout by itself. But we've discussed how that can be easily mitigated. If the grout itself is your main concern, I'm curious how you justify augered cast in place piles or micropiles constructed using grout?
I respect your opinion on this site and think you are a good source of information but I think your assessment of soil nail walls may be a bit biased. Not to mention that the OP stated this is a cut condition. Building a 30ft tall cantilever retaining wall in a cut condition seems like the wrong choice when there are other options that are more suitable and probably much more cost effective. "If the only tool you have is a hammer, all problems begin to look like nails."
What PEinc and I said earlier is that the soil nail wall should absolutely be isolated from the main portion of the building. A 30ft tall standard cantilever retaining wall (which you supported) would also very much be susceptible to movement at the top of the wall. You seem to focus a lot on it being a passive system, yet I don't see any benefits to this being an active posttensioned tieback system. And I would argue that a cantilever retaining wall is a pretty passive system as well.
You mention that you are worried about deterioration of grout in the long term. Please explain what is the source of deterioration? Are you specifically talking about sulfate attacks? I would agree that a high cement content low aggregate mix of grout is more susceptible to washout, lower durability, and more prone to fatigue cracking... I would argue, however, that the corrosion of the internal steel is a greater threat to the longevity of the grout than the deterioration of the grout by itself. But we've discussed how that can be easily mitigated. If the grout itself is your main concern, I'm curious how you justify augered cast in place piles or micropiles constructed using grout?
I respect your opinion on this site and think you are a good source of information but I think your assessment of soil nail walls may be a bit biased. Not to mention that the OP stated this is a cut condition. Building a 30ft tall cantilever retaining wall in a cut condition seems like the wrong choice when there are other options that are more suitable and probably much more cost effective. "If the only tool you have is a hammer, all problems begin to look like nails."
I would be interested in seeing a floor plan of the building. We know the building is 80' long in one direction and we know that the retaining wall is 30' long. Does this mean the building footprint is 80' x 30'? If so, it would be less problematic to design the building to retain soil pressure than one which has an 80' x 80' footprint, but still no piece of cake.
If a tieback wall can be built such that the long term lateral movement is held to one or two inches, it may be a good choice, but it would need a life expectancy of more than thirty years.
BA
Engin1 (OP) said:
If a tieback wall can be built such that the long term lateral movement is held to one or two inches, it may be a good choice, but it would need a life expectancy of more than thirty years.
BA
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1
- #49
I have been following this topic for a bit because it is very similar to a structure we just designed. Our structure was a roughly 80'x80' square structure with CMU walls at the bottom 2 levels on three sides with concrete over steel deck. Above these two levels was wood framed diaphragm and roof and walls. The shear walls were only located at the perimeter, on three sides - the front was open. We too had a roughly 35' tall retaining wall and went through months of coordination, back and forth, around in circles designing many different options. We looked at H-piles, cantilever stepped walls, cantilever sloped walls, using Geofoam infill behind the wall, soil nails, buttresses and using the diaphragm and shear walls to brace the wall. Additionally the walls on the rear of the building were approximately 12' from the property line and on the sides were 3 to 9'. This project was wind controlled with a higher Kzt due to local topography and based on the soil characteristics we were in SDC A.
When trying to use the diaphragms and shear walls we needed to use the online tools from Vulcraft/Verco to design the diaphragms as the publications didn't come close to what we needed capacity wise. We ended up with around 7" concrete over 3", 16 ga deck with double rows of welds at 6" o.c. and perimeter and sidelaps with around 7 ksi concrete (if my memory recalls correctly). We ran other options as well and when going thinner ended up in the 10 ksi concrete range. Additionally thinner didn't always work for gravity as these spans were long due to architectural needs and the fact that little to nothing stacked - Not a Feasible Option
The H-Piles at this location were W14x426 spaced at 5' o.c., embedded into soil rock around 40' additionally this is in a remote location up a long windy and in some places narrow road - Not a Feasible Option
The stepped and tapered walls were similar, with around 30" thick at the base - The client didn't like this thickness and the contractor claimed they could not build this due to how close it was to the property line and being that there were other structures up the hill from this site.
Geofoam infill was liked, but again, not able to be constructed per the contractor.
Buttresses were a similar constructability issue.
A few Geotechs were bought on board and a few soil nail companies looked at it and eventually one company put together a plan to build a soil nail wall that acted as a permanent retaining wall and could be constructed from the top down. The top 12' or so was to be geofoam following the natural grade (infill) to reduce any loads at the top of the wall where it was relying on the building structure for "retaining" and then below that level the soil nail wall did all the work. Their engineers were not worried about deflection of the wall, claiming their wall would deflect almost immediately upon building and stated, in writing, their wall would not impart any loads on our structure for the duration of the building life. Additionally a gap with compressible geofoam was place between their wall and the building structure.
When trying to use the diaphragms and shear walls we needed to use the online tools from Vulcraft/Verco to design the diaphragms as the publications didn't come close to what we needed capacity wise. We ended up with around 7" concrete over 3", 16 ga deck with double rows of welds at 6" o.c. and perimeter and sidelaps with around 7 ksi concrete (if my memory recalls correctly). We ran other options as well and when going thinner ended up in the 10 ksi concrete range. Additionally thinner didn't always work for gravity as these spans were long due to architectural needs and the fact that little to nothing stacked - Not a Feasible Option
The H-Piles at this location were W14x426 spaced at 5' o.c., embedded into soil rock around 40' additionally this is in a remote location up a long windy and in some places narrow road - Not a Feasible Option
The stepped and tapered walls were similar, with around 30" thick at the base - The client didn't like this thickness and the contractor claimed they could not build this due to how close it was to the property line and being that there were other structures up the hill from this site.
Geofoam infill was liked, but again, not able to be constructed per the contractor.
Buttresses were a similar constructability issue.
A few Geotechs were bought on board and a few soil nail companies looked at it and eventually one company put together a plan to build a soil nail wall that acted as a permanent retaining wall and could be constructed from the top down. The top 12' or so was to be geofoam following the natural grade (infill) to reduce any loads at the top of the wall where it was relying on the building structure for "retaining" and then below that level the soil nail wall did all the work. Their engineers were not worried about deflection of the wall, claiming their wall would deflect almost immediately upon building and stated, in writing, their wall would not impart any loads on our structure for the duration of the building life. Additionally a gap with compressible geofoam was place between their wall and the building structure.
Interesting, Aesur. If the footprint in this case is 80' x 30', and if the Main Floor slab was, say an 8" or thicker flat slab, diaphragm action would likely be easier to achieve. But design problems like this don't occur every day; when they do, it pays to study all of the options available.
BA
BA
You are right ..My concern is not the corrosion of the internal steel which can be mitigated .
The concern is the deterioration of concrete..
The soource of deterioration could be ( sulphate , chloride attack, ground water, alkali cement reaction, groundwater,aggregate itself, humidity of soil..) In past , i observed that the buried concrete deteriorates and loose strength so that one can brake with hand tools easily if the concrete is not protected properly (e.g. bitumen coating, epoxy bitumen painting, membrane protection).
Regarding the CIP piles , in general 3 to 4 in clear cover provided and the basic loading is compression ..
I suggested to change the perimeter columns to walls similar to buttresses ) so the perimeter wall (the main structure itself ) will retain the soil loading or , provide setback and construct semi gravity or cantilever retaining wall so the building structure .
The OP is free to make his/ her opinion and choose any option .
r13 said:
Would you want an expansion/contraction joint between the wall and the rest of the building? If the wall is a tieback wall, hopefully it won't move much, but it could deflect an inch or so. If the wall relies on cantilever action and passive pressure, it will deflect a good deal more.
BA
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