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30 ft high retaining wall
3

30 ft high retaining wall

30 ft high retaining wall

(OP)
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!

RE: 30 ft high retaining wall


Apparently the retaining wall dimensions are O.K. IMO, the most critical problem, long term tip deflection which may be harmful for the surrounding pavement and structures. You could taper the stem for this height ..

RE: 30 ft high retaining wall

I too have never designed such a high retaining wall. Have you considered a tieback system?

BA

RE: 30 ft high retaining wall

The wall is 3 foot thick over its entire height which is unnecessary. Consider at least 3 steps in thickness.

At that height though I’d be considering a tie back system.

RE: 30 ft high retaining wall

Make sure free drainage behind the wall. You might need several layers of weep holes too. Provide taper on the front face; tie back is a good idea to save space and materials.

RE: 30 ft high retaining wall

Quote (Engin1)

The wall is also part of the building

That's really the key point. You need to look at how it interacts with the building.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: 30 ft high retaining wall

I'd taper the wall from 3' at base to 1' at top with the slope on the outer face. When the wall tilts, the outer face will become more vertical.

Dik

RE: 30 ft high retaining wall

Also consider to use fly-form and low slump concrete mix for continuous concreting without over stress the bottom form work.

RE: 30 ft high retaining wall

Is there a way to NOT connect it to the building?

1) To be able to use the active earth pressure values, the wall needs to move a little; other wise you have to use the in-situ (k_naught) earth pressures. At 30 ft of height, you might see 2 inches of movement at the top.

2) If you are connected to a building, then you do not really have a cantilevered section. You have a flat plate supported on 2 sides. This means there will be some horizontal moments and shears in the section.

At smaller scales, these issues don't matter as much, but at 30 ft height they do.

Also, as other people have mentioned, I would definitely batter one side of the wall.

RE: 30 ft high retaining wall

Lots of good advice here already. My addition is that while tiebacks are good to consider, 30ft cantilever walls are feasible. I just built one in a high-seismic zone with similar proportions.

----
just call me Lo.

RE: 30 ft high retaining wall

Good advice has been given. But if built as a cantilever as shown, you must keep it isolated from the building. You could build the wall footing around building piers, or else cantilever the building over the wall.

RE: 30 ft high retaining wall

Hokie's right... you don't want it to be connected to the building.

Dik

RE: 30 ft high retaining wall

(OP)
Thanks everyone for good suggestions. To be part of the building is the client's request. I will try tie back method. thanks

RE: 30 ft high retaining wall

Will you have perpendicular walls at the ends of the retaining / basement walls? Because I don't care how thick the wall is, it will not be as stiff as the side walls. Also, if you have a concrete slab at the top of the wall (or somewhere along its height) that will also be stiffer than the wall itself.

Not sure I understand why we can't allow the retaining wall force to go into the building at all.

RE: 30 ft high retaining wall

If the land is not an issue, I will design the retaining wall on steps - 2 setbacks 5' each. 30' wall will require much more than 3' thick at the bottom.

RE: 30 ft high retaining wall

(OP)
JoshPlumSE,

The Wall will be 30ft long, it has perpendicular walls at the ends. It will be metal deck roof a the top. It does not have locations can fit x bracing to resist the whole retaining forces. We could allow partial forces go the building and use moment connection, but how to determine how much load will go to the building?

RE: 30 ft high retaining wall

(OP)
retired13,

Thanks! It's the client's request to use it as part of the building wall. We suggested to setback 5'.

RE: 30 ft high retaining wall

Engin,

It makes design difficult, but not impossible. Is the wall will enclose the basement? If so, how to install tiebacks? What is the seismic zone? Is this how it looks?

RE: 30 ft high retaining wall

(OP)
retired13,
It has 10ft basement. It cannot taper inside the building. I am thinking to put the ties above the basement and the top to reduce the deflection of the wall. The 30ft is from the basement floor to top of the exterior grade. There is no openings on this wall. Seismic B.

RE: 30 ft high retaining wall

I am a little confused on your situation. Can you sketch the plan and cross section? If it is as shown below, then the building itself is a retaining structure.

RE: 30 ft high retaining wall

(OP)
retired13,

See the section.

RE: 30 ft high retaining wall

I don't understand why you're considering this as a cantilever retaining wall? to me, the slabs at the basement and main floor level provide lateral support to the wall, same for the roof deck. That loading needs to go somewhere obviously but the side walls should provide the level of shear wall in that direction required provided the diaphragms at each floor level have the capacity to transfer the backfill loads to the side walls.

It is really tough to give suggestions without knowing all the contraints and dimensions. What does the plan look like? Is this an addition or is all of the construction new?

RE: 30 ft high retaining wall

You need a smaller scale section through the building showing the grade on the other side.

BA

RE: 30 ft high retaining wall

I see. You might need more than one level of tieback though. The wall needs to go above the slopping ground to catch loose/sliding materials. A bench below the top of wall, as a buffer zone, should be considered.

RE: 30 ft high retaining wall

(OP)
BAretired
See section.

RE: 30 ft high retaining wall

If this is new construction, have you ever considered a "secant pile wall"? I came across one on a project in the 1990's. I got the sense that it saved a lot of time and money due to the deep excavation that would have otherwise been required for the basement walls.

With a 30 ft excavation on one side, and the large thickness required for this wall, it seems to me that a secant pile wall may be a more efficient way to go.

RE: 30 ft high retaining wall

Engin1,

Thank you. I would expect that the basement walls and Main Floor slab will have a significant effect on the performance of the retaining wall. The differential height of soil is 20', still a very high retaining wall, assuming no shear walls occurring above Main Floor.

BA

RE: 30 ft high retaining wall

As it's shown you really don't have a choice but to include the lateral earth pressure in your building lateral system analysis and design.

Based on your sketch it looks like grade may continue to taper off on the low side, if it hasn't been done already I'd strongly encourage the Geotech to do a global site slope stability analysis as if this doesn't check out it won't matter if the retaining wall design works or not. I'd also be very careful of global sliding in this case.

My Personal Open Source Structural Applications:
https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:
https://github.com/open-struct-engineer

RE: 30 ft high retaining wall

Scant and tangent wall are viable solutions for deep excavation like this. But I would suggest to look into "slurry wall", or diaphragm wall, which can be utilized as part of the building, after the slurry is replaced by termite concrete. Link

RE: 30 ft high retaining wall

(OP)
JoshPlumSE,
I haven't used secant pile wall. thanks for your info, I need to do some research on "secant pile wall".
BAretired, Yes the main floor does have a significant effect. 20' is still a lot. The retaining wall is 30ft long, and at the end it will have perpendicular walls which can resist some shear forces.

RE: 30 ft high retaining wall

If the new wall is a tiedback wall, it too should be isolated from the building to prevent differential movement and structure damage. There should be some type of expansion joint to allow the wall and the building to move independently.

www.PeirceEngineering.com

RE: 30 ft high retaining wall

An open cut to put in the retaining wall is the problem, and a type of wall that is installed as you dig out for the building makes a lot more sense.

Can soil nail walls be installed as you cut down? I am unsure, but I could see a soil nail wall making sense, with a permanent cast in place facing that is used for the finished building face.

RE: 30 ft high retaining wall

To me this isn't a retaining wall in the regular sense of the word. Based on that section, it is a building wall, retaining soil. There is almost zero likelihood that the building would be removed but that wall would remain standing since the building frames into the wall. Design each diaphragm for the forces it will see, give them a load path to the ground, and then all you are doing is designing a 20ft tall wall with soil pressure, and a 10ft wall with soil pressure, or a continuous uneven two-span wall with soil pressure if you would prefer. Either way, I would not be designing this as a 30ft tall cantilevered (or tied back) retaining wall that must stand alone.

Really without a full plan view indicating the length of this wall between potential support (shear) walls, we can't know for sure what are possible solutions.

RE: 30 ft high retaining wall

If the wall is only 30' long with a support at each end, it makes economic sense to connect the retaining wall to the building, using building elements to resist the soil pressure. Removal of the building may be a concern a long time in the future.

The use of tiebacks may be a consideration if the adjacent property belongs to the building owner, but tying into a neighbour's property may be asking for trouble in the future.



BA

RE: 30 ft high retaining wall

(OP)
Thank everyone for your suggestions. Our senior engineer suggested to use soil nails to reduce the wall thickness and footing sizes. First, I am going to try using the building to resist the soil pressure first and see if it works out. And I will also try the tie back method using soil nails or other kinds ties. thanks!

RE: 30 ft high retaining wall

I wanted to comment on your post earlier but didn't get a chance. Perhaps I missed it earlier but is this a cut or fill condition? That completely governs my stance on the design approach you should take.

If in a cut condition, I liked the idea of the soil nails only if the wall is not carrying any vertical loads from the building. Soil nail walls resist lateral loads very well but should not be used to carry vertical loads.

A cast-in-place final facing for the soil nail wall is probably the nicest finish and most durable but also will have some expensive forming costs. This is a tall one-sided form. I have been a part of some soil nail projects where we formed the outside CIP facing by drilling and epoxying GFRP dowels 4" into the face of the initial shotcrete layer and tying those into the formwork walers. I was sweating a bit when they were pouring the concrete as I knew a failure would have meant dumping about 50 yards of concrete right into some pristine ocean shoreline but it worked. Most Contractors are going to look at that one-sided form and automatically assume that they will have to shore and buttress it from one side which will be costly for a 30ft tall wall.

RE: 30 ft high retaining wall

Quote (Engin1
Thank everyone for your suggestions. Our senior engineer suggested to use soil nails to reduce the wall thickness and footing sizes.
....)



I will not recommend the use of soil nailing for permanent use adjacent to human occupied building. The building service life could be more than 50 years while soil nailing could be in the range of 30 years. Another issue, if the adjacent area where soil nailing will extent is a private area, they may excavate and disturb the nailing .

I will suggest to change the perimeter columns to walls similar to buttresses ( in this case,e.g. b= 16 in h= 80 in ) and the wall thick . could be 16 in. Another option, provide setback and construct semi gravity or cantilever retaining wall so the building structure will get free .

RE: 30 ft high retaining wall

(OP)
STrctPono, It's a cut condition.
HTURKAK, the adjacent property belongs to the same owner. That's a good point. The client does not want a setback retaining wall.

RE: 30 ft high retaining wall

HTURKAK, can you please give a reference for the 30 year life of a soil nail wall? That would be unwelcome news to US highway departments which insist on much more than a 30 year wall life.

www.PeirceEngineering.com

RE: 30 ft high retaining wall

Another suggestion that architect would love :)

RE: 30 ft high retaining wall

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.

RE: 30 ft high retaining wall

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.

Quote (PEinc ,
HTURKAK, can you please give a reference for the 30 year life of a soil nail wall? That would be unwelcome news to US highway departments which insist on much more than a 30 year wall life.)


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.

RE: 30 ft high retaining wall

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..

RE: 30 ft high retaining wall

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

RE: 30 ft high retaining wall

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.

www.PeirceEngineering.com

RE: 30 ft high retaining wall

Quote (PEinc
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.
www.PeirceEngineering.com)


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 www.PeirceEngineering.com. I am retired engineer and sharing my experience without conflict of interest .

RE: 30 ft high retaining wall

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."

RE: 30 ft high retaining wall

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.

Quote (Engin1 (OP))

the adjacent property belongs to the same owner. That's a good point. The client does not want a setback retaining wall.

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

RE: 30 ft high retaining wall

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.

RE: 30 ft high retaining wall

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

RE: 30 ft high retaining wall

@BAretired, I agree, on ours if we had shear walls at 30' o.c. we would have been much better off making the building work supporting the wall.

RE: 30 ft high retaining wall

Thanks for sharing Aesur! Good to hear that you guys thoroughly thought this one through.

RE: 30 ft high retaining wall

Quote (STrctPono
HTURKAK,

....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?)


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 .

RE: 30 ft high retaining wall

IMO, the retaining wall can be part of the building, but it should be a self-standing wall, without relying on other building components.

RE: 30 ft high retaining wall

Quote (r13)

IMO, the retaining wall can be part of the building, but it should be a self-standing wall, without relying on other building components.

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

RE: 30 ft high retaining wall

Prepare for remodeling, reconstruction, changes... down the road.

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