INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Guidance for the Design of a Reinforced Concrete Retaining Wall

Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Hello everyone, please find attached a drawing of the retaining wall i am designing according to EC7 together with an excel file of my calculations.
Its a cantilever L-shaped reinforced concrete wall comprised of a stem and foundation slab.

Backfill soil is well graded gravel with unitweight = 20kN/m3 and angle of friction = 25degrees (conservative value)
Wall height is 4.20m, Height of soil retained by wall is 2.70meters.
Surcharge load = 10kPa, A seismic force has also been considered.

According to my calculations my problem here is that the foundation of the wall needs to be at least 1.80meters wide in order to get a safety factor of 0.94 against sliding, which i think its too much for this design.
So for a 2.70m hight soil i get a 1.80m wide foundation slab in order to resist sliding. Dont you guys think that This is a really expensive design? am i doing something wrong? or did i get tit right?

The excel file is sectioned as follows:

0. Retaining Wall Properties
1. Gross Pressure Method
2. Eurocode Comb 1
3. Eurocode Comb 2
4. Seismic
5. Bending Reinforcement
6. Deflection (not complete)

Can you guys please have a look at my work and help figure this out? does the wall foundation need to be that wide?

http://res.cloudinary.com/engineering-com/image/upload/v1514841058/tips/Retaining_Wall_Details_-_forum_lrme3i.pdf

http://res.cloudinary.com/engineering-com/raw/upload/v1514841074/tips/Retaining_Wall_Design_y9vhvs.ods

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

The proportions seem fine to me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Interesting that the surcharge stops at the "virtual face". 25 degrees friction angle for the backfill seems way too small. 30 plus would be expected.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

CODE --> oldestguy

Interesting that the surcharge stops at the "virtual face" 

The surcharge load is not from compaction machinery, but from a building at 3.0m away from the wall, however i have pushed the surcharge load even nearer to the wall but it stops at the virtual face because in the Eurocode approach the surcharge load is never considered as favourable load in resisting sliding or overturning.

CODE --> oldestguy

25 degrees friction angle for the backfill seems way too small. 30 plus would be expected. 

I am being conservative with the angle of friction of the backfill because this project is something like a charity, if not enough money is collected then we will have to use a fine grained backfill which is cheaper than a good granular fill and ofcourse use a drainage blanket on the wall to help with drainage.

I tried my design with a 30 degree angle of friction for the backfill and I got a safety factor of 1.15 against sliding. Do you guys feel ok with such a safety factor according to Eurocode or do you think i should try increase the safety factor ?




RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

That building just beyond the footing edge certainly applies a load on that wall footing, so sliding is very unlikely. That thrust is not likely uniform with depth, but near zero at surface, a triangular more likely.. If that building is already there, how are you going to build the wall without some significant bracing to hold it during the excavation next to it? Such bracing may have to remain in place and affect the design of the wall. It may have to be underpinning of the building. Run a lab test on your proposed backfill and my bet is no test will come under 30 degrees, even ultra loose. A simple test is a loose pile and measure the slope (at the storage pile).

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

Am i doing something wrong?

Yes, you are. To start with... you are defining the loads, trying to comply with code, and designing the retaining wall in one grand combined step - a spreadsheet that is supposed to "do it all". This approach is obviously not be working... you tell us that you do not believe the spreadsheet's results.

I'll happy to share my views, one step at a time, on the details (both technical and procedural) of what to do about this... but only if you are interested. I don't want to waste my time or yours making comments that will be ignored. My first suggestion will be to forget the spreadsheet, at least for now.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (oldestguy)

If that building is already there, how are you going to build the wall without some significant bracing to hold it during the excavation next to it?

the building is not already there.

1. first the retaining wall will be build around the perimeter of the plot.
2. the plot will then be filled with soil to raise the level of the plot.
3. then the building will be build on top of the added soil

Quote (oldetguy)

Run a lab test on your proposed backfill

how can i test a backfill soil?

this is what I asked in another thread (see link below) and i was told that i should use a standard value from a text and that there are no lab tests for backfilled soils

http://www.eng-tips.com/viewthread.cfm?qid=433781



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

This approach is obviously not be working... you tell us that you do not believe the spreadsheet's results.

The reason i do not believe the results of the spreadsheet is because am not that experienced, this is my first retaining wall design.
Also i have seen some other retaining wall designs being build with 2.0m backfill and have a foundation slab, of b = 1.20m my design is 2.70m backfill with b = 1.80m

Quote (SlideRuleEra)

I'll happy to share my views, one step at a time, on the details (both technical and procedural) of what to do about this... but only if you are interested. I don't want to waste my time or yours making comments that will be ignored. My first suggestion will be to forget the spreadsheet, at least for now.

As I said, i am not that experienced and this is my first retaining wall design, I would really appreciate it if you could share your views with me, i am here to learn, I am guessing you are an experienced engineer and i respect your knowledge,
please guide me on how you want to proceed

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

kellez - Thanks for sharing your retaining wall experience level, that will make the discussion easier. Yes, I've been practicing a while, 48 years, 43 of them as a PE. Most of that time in heavy industry. Mentored six young engineers, one at a time, for time periods of a few months to 10 years. They have turned out well - two are vice-presidents, two are in middle management, and two are senior staff.

Back to your problem... put the spreadsheet and other concerns aside for a while. You are trying to make everything top priority... low cost backfill, low cost retaining wall, "conservative" design values. Instead of looking for precise (but not necessarily accurate) answers to specific questions, look at how math models a "real world" retaining wall. The results of your own analysis will tell you which way to proceed.

Start with Ka, your "active earth pressure coefficient". The goal is to (realistically) make that value as low a practical... that means a "high" value of φ (good granular soil). What does having soil with "high" value of φ do for the calculations (in principal... not a detailed answer)?

1. Reduces thrust on the wall.

2. Look at the soil table PEinc provided on your other thread. Tends to have a heavier unit weight... providing more load on the wall's heel slab to resist sliding and overturning.

3. Reduces the surcharge loading from the building that is 3 meters behind the wall.

4. Reduces the soil mass that is affected by seismic acceleration.

5. Probably increases the soil friction resistance for sliding. I have seen this coefficient of friction taken as tan (2/3 φ).

That's enough for now, think about it. Do you want to continue?

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Some other things to ponder:

You've assumed the conservative soil parameters in case you can't afford the good backfill, but then your wall seems expensive so you can't afford the good backfill. This is a self-fulfilling prophecy. Try with the decent backfill parameters then check whether you can afford the wall.

Notwithstanding that comment, is your base width actually out of proportion? You cite a 1.2m base for a 2.0m high wall as another example: B/H=0.6. Your design is 1.8m base for 2.7m height: B/H=0.67. Not too different at all. What do you know about the other wall? Did it have access to good fill, lower design surcharge, other factors in its favour?

Some textbooks (eg Bowles) give B/H from 0.5 to 0.7 as a guide. You're within that range. My own experience from looking at old retaining walls is that many show signs of movement (leaning and sliding) to the point that I'd say they were 'under-designed', so I'm not too confident in the lower end of any old rules of thumb.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

Start with Ka, your "active earth pressure coefficient". The goal is to (realistically) make that value as low a practical... that means a "high" value of φ (good granular soil). What does having soil with "high" value of φ do for the calculations (in principal... not a detailed answer)?

I do understand how the angle of friction affects the retaining wall but to be honest it helps a lot when you describe it the way you do here, especially when you put it down on paper.
It helps a lot with thought processing and makes it easier to understand the design problem in hand.


1. Reduces thrust on the wall. I completely understand the soil mechanics behind this

2. Look at the soil table PEinc provided on your other thread. Tends to have a heavier unit weight... providing more load on the wall's heel slab to resist sliding and overturning. ofcourse, it seems logical

3. Reduces the surcharge loading from the building that is 3 meters behind the wall. again I completely understand the soil mechanics behind this, similar to reducing thrust on wall, mentioned above in point 1.

4. Reduces the soil mass that is affected by seismic acceleration. I do understand this as well, however this is not true when using the Seed and Whitman Method to calculate the seismic force, their equation does not consider the active earth pressure coefficient and also doesnt consider the angle of friction of the soil.

However the Peudo Static and Mononobe Okabe method do consider the angle of friction in their calculation



5. Probably increases the soil friction resistance for sliding. I have seen this coefficient of friction taken as tan (2/3 φ). Could you please elaborate? Do you mean wall friction value?



That's enough for now, think about it. Do you want to continue?


Ok, I have given this some thought and my main observations is that, the design of the wall is mainly affected by the type of wall used for the retaining wall (in this case reinforced concrete cantilever wall) and the type of soil used to backfill the wall.

Good granular fill obviously will reduce the forces on the wall and create a more economical design, it will also provide good drainage and avoid any hydrostatic pressures to build against the wall.

Ok, this is very clear in my mind right now, you have helped me to take a step back and view this design with a clearer mind, and take everything step by step.


Please advise me on how you want to proceed next.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote:

5. Probably increases the soil friction resistance for sliding. I have seen this coefficient of friction taken as tan (2/3 φ). Could you please elaborate? Do you mean wall friction value?



See Page 5-48 of CalTrans Retaining Walls. I'm assuming the foundation soil has properties similar to backfill.

Concerning the next step:

1. Take oldestguy's advice to determine the angle of repose (friction angle) and the unit weight of the proposed backfill. I'll bet unit weight and is less than the assumed 20 kN/m3. Do this even if you know the results will not be precise... some information is better than a guess. Use the values obtained for the design. IMHO, the time to be conservative is by increasing the safety factor, not by designing using "wrong" material properties.

2. How did you compute the surcharge load on the wall? Revisit that calculation by applying the 10 kPA load where it is (beginning 3 m from the wall) not at an assumed closer location. My simplified calcs indicate the surcharge will have minimal effect on the wall. Ignore the surcharge load contribution for resistance to overturning and sliding.

3. On your other thread, BAretired mentioned that the proportions of the wall are not right; I agree. The proposed geometry is not efficient. The loads are too near the toe (Very little lever arm to resist overturning moment. Loads too close to the toe to get proper distribution of load on the soil supporting the retaining wall.) The resultant load from supporting soil needs to the in the middle third of slab. Reproportion wall shape/dimensions for more favorable loading characteristics. Iterate for an improved design, change the initial assumed dimensions based on intermediate calculations. Don't expect to be right the first time.



4. Buy and use CRSI Design Guide for Cantilevered Retaining Walls that BAretired recommended.
Alternatively download US Army Corps of Engineers - Retaining and Flood Walls.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)



See Page 5-48 of CalTrans Retaining Walls. I'm assuming the foundation soil has properties similar to backfill. Ok i get that, its the angle of friction between footing and foundation soil

Concerning the next step:

1. Take oldestguy's advice to determine the angle of repose (friction angle) and the unit weight of the proposed backfill. I'll bet unit weight and is less than the assumed 20 kN/m3. Do this even if you know the results will not be precise... some information is better than a guess. Use the values obtained for the design. IMHO, the time to be conservative is by increasing the safety factor, not by designing using "wrong" material properties. Ok, i will change all my values so that my design problem represents the real conditions as much as possible

2. How did you compute the surcharge load on the wall? Revisit that calculation by applying the 10 kPA load where it is (beginning 3 m from the wall) not at an assumed closer location. My simplified calcs indicate the surcharge will have minimal effect on the wall. Ignore the surcharge load contribution for resistance to overturning and sliding. I already ignored the surcharge load contribution for resistance to overturning and sliding.

Ok, I will recalculate my surcharge load, i think this is the main reason why my wall is overdesigned, due to the combination of the seismic load and surcharge load,
I still think its good to include a surcharge load near the wall which will represent compaction machinery, but i should not include this with the seismic force
Therefore my thought is to include the surcharge load of 10kPa exactly next to the wall for the compaction machinery but i should not include that in the seismic design.


3. On your other thread, BAretired mentioned that the proportions of the wall are not right; I agree. The proposed geometry is not efficient. The loads are too near the toe (Very little lever arm to resist overturning moment. Loads too close to the toe to get proper distribution of load on the soil supporting the retaining wall.) The resultant load from supporting soil needs to the in the middle third of slab. Reproportion wall shape/dimensions for more favorable loading characteristics. Iterate for an improved design, change the initial assumed dimensions based on intermediate calculations. Don't expect to be right the first time. I agree the alternative shape of wall you suggest is more efficient, however the reason i chose an inverse L-shaped wall and not an inverse T-shaped wall is because the wall is located exactly on the boundaries of the plot, therefore its not possible to extend my footing into the neighbouring plot, in addition i dont think its good idea to move the wall further inside the plot, its a waste of land. If my only coice is an inverse L-shaped wall what would be your suggestion to overcome this?

4. Buy and use CRSI Design Guide for Cantilevered Retaining Walls that BAretired recommended.
Alternatively download US Army Corps of Engineers - Retaining and Flood Walls. I will have a look in that

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

...my thought is to include the surcharge load of 10kPa exactly next to the wall for the compaction machinery...

I do not advise doing that. You will get back to very situation which prompted your original post: The wall appears overdesigned and you don't know why.

Apply the building's 10kPa load where it is... beginning 3 meters from the wall. Then you will know how much influence the permanent 10kPa load has on the wall.

There is no limit to other loads you can voluntarily superimpose. Apply a temporary construction (compaction) load adjacent to the wall... you are not "forced" to make it 10 kPA and can see what effect it has on the design. Also, the wall loading from compaction has different characteristics than a surcharge. Finally, the compaction loading is more of a point load (not a surcharge uniform distributed load) and it is temporary - a lower safety factor can be considered for a temporary load.

See "Pressure on Retaining Walls From Compaction Effort". In particular, look at "Figure 3".

Quote:

If my only choice is an inverse L-shaped wall what would be your suggestion to overcome this?

I understand your reason of the inverse L-shaped wall, looks like the way to go.
See if you can excavate some for the wall's foundation slab. Just a depth that is reasonable without causing problems... even, say, a 0.5 meter deep excavation will help. Good idea to embed the slab in soil to minimize possible erosion problems (believe you told us frost depth is not a concern). The primary reason is to increase the amount of backfill on top of the slab... more weight to resist overturning/sliding and the centroid of this weight is located to help keep the foundation resultant force in the middle 1/3 of the slab.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

I do not advise doing that. You will get back to very situation which prompted your original post: The wall appears overdesigned and you don't know why.

Apply the building's 10kPa load where it is... beginning 3 meters from the wall. Then you will know how much influence the permanent 10kPa load has on the wall.

Ok i will apply the buildings surcharge load at 3 meters from the wall, which i have changed to 20kPa (i was wrong about the 10KPa).

I am looking into text books in order to find out how i can calculate the effect the surcharge load will have on the wall but i am having some difficulties. I have never tried to calculate this with a surcharge load that is at a certain distance from the wall.

I have looked into the CalTrans Retaining Walls paper (page 39, equation (5.5.5.10.2-2)) you posted above and found this equation (see photo attached). This equation is used for a Uniformly Loaded Strip Parallel to the Wall, however my building load is not actually a strip but an RC concrete slab (raft foundation) parallel to the wall (approx, 10m x 13m) , therefore do you think I could consider this as a very wide strip or do i need to use another equation?

The value i get is 8.17kN, which seems a bit high considering that the building is 3.0 away from the wall. Also i do not know at what height (point) this acts on the wall, if you want to have a look i have attached the excel file below, its only one calculation. Am i wrong using this equation?

http://res.cloudinary.com/engineering-com/raw/upload/v1515583756/tips/Surcharge_load_at_a_destance_x_from_wall_n7kqjx


RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Equation 5.5.5.10.2-3 tells you where the resultant force acts.

The ArcelorMittal piling handbook has an alternative formula. Worth checking a few different methods since they tend to be gross approximations, then picking a number that you're comfortable with. Another alternative might be good-old Coulomb wedges: change wedge angle to find the worst case.

Is the load 3m from the wall stem, or 3m from the heel/virtual face?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (steveh49)

Equation 5.5.5.10.2-3 tells you where the resultant force acts.
thats a big equation, thanks, i will try it out

so i will need to determine this in order to check if the pressure actually acts on the wall, it could be lower than the base of the wall?

Quote (steveh49)

Is the load 3m from the wall stem, or 3m from the heel/virtual face?
Its 3.0meters from the wall stem

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

You might need to use equation 5.5.5.10.2-1 to determine the distribution over the height of the wall then integrate to find the resultant force. Equations 5.5.5.10.2-2 and -3 might assume that the wall extends below the horizontal pressure zone.

For a sanity check, consider that your load starts just 1.2m behind your heel/virtual face (or 1.5m, have you changed the sketch since first posting?) and extends over a large area, so isn't too different to the 'full surcharge' case shown on the sketch in your original post - ie a surcharge that continues up to the heel/vurtual face. The 'full surcharge' load is Q*Kh*H = 20kPa*0.33*2.7m = 17.8kN/m for 30degree friction angle backfill. Your first calculation (8.17kN/m) is half of that. Is it really an unreasonable load that needs to be reduced further? How accurate is the calculation since it doesn't have any factors that represent the shear strength of the soil? Is it impossible for there to be any other surcharge between the building and wall in future?


RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (steveh49)


The 'full surcharge' load is Q*Kh*H = 20kPa*0.33*2.7m = 17.8kN/m for 30degree friction angle backfill. Your first calculation (8.17kN/m) is half of that. Is it really an unreasonable load that needs to be reduced further? How accurate is the calculation since it doesn't have any factors that represent the shear strength of the soil? Is it impossible for there to be any other surcharge between the building and wall in future?

I dont think there will be any surcharge load in the future, however, first i would like to design my wall using the most accurate load cases that best describe the real life loads, and then i can add any additional loads i want.

No, the 8.17kN/m i calculated is not an unreasonable value, however, most importantly i need to determine at what height the surcharge load will act on the wall, which i think it will act below the base of the wall. It will probably act upon the footing/foundation of the wall however i will neglect that since its a favourable action against sliding and overturning.



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
I had a look in ArcelorMittal piling handbook and found another equation for determining the effect of a surcharge load on the wall, a lot simpler than the one found in CalTrans Retaining Walls the equation is shown below and can be found at page 137-139.

One observation is that again the equation is not using any soil material properties. The equation is shown below with an example



I used this equation in excel and these are my results

INPUT DATA:
Surcharge, q = 20.00kPa
Distance, d = 3.00m
Strip load width, b = 2.00m
Height of wall, h = 2.70m

First I calculated the Increasing Lateral Pressure, Δσv (kN) and then I sum up the values to get the resultant lateral pressure, Is this correct?

Another observation is that if a plot the force diagram of the resultant lateral pressure, it will be triangular, which i think its not realistic in such a load case.
do you guys think i should use the The Boussinesq Equation





RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

I have never tried to calculate this with a surcharge load that is at a certain distance from the wall.

Fine, now you are making progress. You want the mathematical model to "fit" the problem... not just be application of your existing skill set. But the solution does not have to always be complicated. Since this is your first retaining wall design, suggest starting with a semi-graphical solution to get an understanding of how the remote surcharge affects the wall. I like the simplified method shown and explained on pages 28 & 29 of Allan Block Retaining Wall Engineering Manual.


The Arcelor Mittal Piling Handbook that steveh49 mentioned has a similar approach - see Chapter 4.

For your wall, I get approximately 3.3 kN thrust (assuming the foundation slab is 0.5 meters thick).



Quote (kellez)

I would like to design my wall using the most accurate load cases that best describe the real life loads, and then I can add any additional loads I want.

Good, I have a couple of suggestions for additional minor loads to check. But you should have enough info now to make a first order iterative design. Let's see what you come up with.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
I am working on it, i will post my sketch in a bit

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

For external stability, it is usual to calculate the destabilising forces that act on the virtual face of the wall. In that case, your surcharge load is 1.2~1.5m away depending on what your current base width is. This would increase the load acting on the wall compared with assessing the load based on the 3m distance of the surcharge from the stem.

The Piling Handbook gives you the distribution of load vs depth below ground. You have to work out the height of the resultant by integration. For information, the diagram and equations from your previous post [now deleted] were under the heading "Concentrated and Linear Surcharge" in previous editions of the Piling Handbook.

From a few posts ago: "First I calculated the Increasing Lateral Pressure, Δσv (kN) and then I sum up the values to get the resultant lateral pressure, Is this correct?" This isn't correct. Δσv is in kPa; it's the vertical pressure at a point in the soil due to the surcharge and varies depending on the location of the point in question relative to the surcharge load. You multiply Δσv by the horizontal pressure coefficient to get the lateral pressure, then integrate the lateral pressure distribution to get the resultant total force and location of the resultant.

The current Piling Handbook appears to give a couple of different methods for assessing the lateral pressure resulting from the area surcharge, and the Allan Block method is different again. There will be other methods out there. I hope it's clear that these equations only give order-of-magnitude estimates of the load applied to the wall. If the load due to this surcharge is a significant part of the total horizontal load, tread carefully.



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (steveh49)

For external stability, it is usual to calculate the destabilising forces that act on the virtual face of the wall. In that case, your surcharge load is 1.2~1.5m away depending on what your current base width is. This would increase the load acting on the wall compared with assessing the load based on the 3m distance of the surcharge from the stem.

That is a valid point you made there, i will adjust my calculations and post my sketch. I need to take this step by step. I will show you my sketch to let me know if i am correct

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
Ok this is where i am right now

I have used the equations in The Piling Handbook (see below) in order to determine how the load is distributed



Therefore, I calculated a, c and d (see below)



And this is the shape I have come up with

CASE 1: STRIP LOADING





CASE 2: INFINITE LOADING - THIS IS MORE REALISTIC FOR MY CASE SINCE THE BUILDING IS 10m wide




My thinking now is that I should first calculate the whole surcharge load as if it was exactly next to the wall

Active thrust due to surcharge: Pq = Ka x q x h = 0.33 x 20 x 2.70 = 18.0kN

Then i should calculate the surcharge load according to triangle B

Active thrust due to surcharge from Triangle B: Pq = Ka x q x a/2 = 0.33 x 20 x (0.98/2) = 3.28 kN

now subtract B from A

Active thrust due to surcharge from Shape A: 18.0kN - 3.28 kN = 14.72 kN

Therefore the total thrust of the surcharge load is 14.72 kN

Everyone agrees with this?



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (steveh49)

For external stability, it is usual to calculate the destabilizing forces that act on the virtual face of the wall. In that case, your surcharge load is 1.2~1.5m away depending on what your current base width is. This would increase the load acting on the wall compared with assessing the load based on the 3m distance of the surcharge from the stem.

Thank you steveh49, I did not know that. It makes sense.

kellez - The 14.7kN surcharge thrust sounds good to me... for an initial trial. Make the (preliminary) calcs for all the loads, you have to start somewhere - don't "wait" until you have "better" information. The preliminary results will guide you to the next step.

Did you decide to keep the footing sitting on top of existing ground (total height 2.7m) ?

The surcharge loading will be larger (more important) than I originally estimated. Is an allowance for the building's live load included in the 20kPA value? If the building is a warehouse or for heavy equipment live load may be considerable... not so much for an office or residence.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)

Did you decide to keep the footing sitting on top of existing ground (total height 2.7m) ?

Yes I will keep it that way for now and carry out the design as it is, i will see how it goes an decide if i should excavate and site the wall at a lower level within the excavation.


Quote (SlideRuleEra)

Is an allowance for the building's live load included in the 20kPA value? If the building is a warehouse or for heavy equipment live load may be considerable... not so much for an office or residence.

No i did not account for a live load in the surcharge load, however this is only a 1-story residence. Its a reinforced concrete structure with a 45cm thick foundation slab/raft foundation, thats why the 20kPa surcharge load.


I am currently working on my calculations and checking everything, i did find some careless mistakes, that also contributed to the overdesign, however now my surcharge has increased as well, so i should be back at the same design as before. I will do my calculation and let you know

Any advise on how to apply the seismic load, I have used the Seed and Whitman Method to determine the seismic load, and then added it to the existing thrust from soil and thrust from surcharge.

However i think the Seed and Whitman Method is way to conservative than the Monobe Okabe method which is also suggested by the Eurocode 8, and because my main issue here is the


I have one very important question that greatly affects the design:


Load case combinations and safety factors greatly affect the design, therefore my question is, The surcharge load from the building is considered as a permanent load right?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

I think deepening the excavation will result in diminishing returns as you end up increasing the amount of surcharge and soil pressure the wall sees. But that's just a thought.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

Any advise on how to apply the seismic load, I have used the Seed and Whitman Method to determine the seismic load...

Seed & Whitman is as good as any. I see that the design ground acceleration = 0.25g. Take a look at Development of Improved Procedures for Seismic Design of Buried and Partially Buried Structures, published by the Pacific Earthquake Engineering Research Center. In particular, see the "Conclusions & Recommendations" on page 157:



More about that in a minute.

Quote (kellez)

Load case combinations and safety factors greatly affect the design, therefore my question is, The surcharge load from the building is considered as a permanent load right?

Yes, I would consider the building's surcharge load as permanent for horizontal thrust on the wall. As discussed earlier, no credit for the surcharge to resist sliding or overturning.

Considering load combinations - for this first order calculation I suggest applying the following loads simultaneously:

1. Thrust From Soil, with Overturning & Sliding Safety Factors of 1.5

2. Thrust From Building Surcharge, with Overturning & Sliding Safety Factors of 1.5

3. Seismic Force, with Overturning & Sliding Safety Factors of 1.0 (Recommendations given above are used as justification for no added safety factor).

Make the calcs... let's see what you get. This is a wonderful project... you'll learn a lot more by having to deal with what I believe is a first effort with "issues". There will be other concerns to address later.

jayrod12 is right, but like everything else there may be compromise advantages to an excavated foundation. First, make the above calcs.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (SlideRuleEra)



Yes, I would consider the building's surcharge load as permanent for horizontal thrust on the wall. DONE

As discussed earlier, no credit for the surcharge to resist sliding or overturning. DONE

Considering load combinations - for this first order calculation I suggest applying the following loads simultaneously:

1. Thrust From Soil, with Overturning & Sliding Safety Factors of 1.5

2. Thrust From Building Surcharge, with Overturning & Sliding Safety Factors of 1.5

3. Seismic Force, with Overturning & Sliding Safety Factors of 1.0 (Recommendations given above are used as justification for no added safety factor).

The load combination I have used is 1) Thrust from soil + 2) Thrust from building surcharge + 3) Seismic force

I am using 2 types of load combinations, according to eurocode, safety factors are shown below, Combination 1 is used to factor the loads, Combination 2 is used to factor the material properties




For the seismic force i am using a safety factor of 1.0 as you suggested


Make the calcs... let's see what you get. This is a wonderful project... you'll learn a lot more by having to deal with what I believe is a first effort with "issues". There will be other concerns to address later.

Indeed i have learned a lot up to now while trying to solve all the issues with your help and everyone elses help.

jayrod12 is right, but like everything else there may be compromise advantages to an excavated foundation. First, make the above calcs.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
I have done the calculations using the following properties for the wall and soil

THE Retaining Wall Foundation is still at 1.80m wide but i have more confidence in my calculations now
As you can see from the results below I still have an issue with when the seismic load is added in the calculations any advice on that?




These are the results i get

SLIDING: EUROCODE COMBINATION 1



SLIDING: EUROCODE COMBINATION 2



OVERTURNING: EUROCODE COMBINATION 1



OVERTURNING: EUROCODE COMBINATION 2



SEISMIC: EUROCODE COMBINATION 1

For the seismic load case, I have calculated the 1) Total Horizontal Force due to soil + 2)Total Horizontal Force due Surcharge, + 3) Seismic force according to the Seed and Whitman Method which i think is a bit more conservative than the Mononobe Okabe Method. I will also give Mononbe Method a try and see





As you can see my SF for seismic load case is still below 1, this is my biggest issue now, i need to concentrate on this one now





RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
I tried calculating the total horizontal thrust using the Mononobe Okabe method but i have one issue.
The example I am following is only about a gravity mass retaining wall therefore there is no wall footing/foundation thereofe there is no soil on top of the foundation to resist sliding and overturning,

Therefore my question now is:

What do i need to do in order to take into account the resistance to sliding and overturning that is provided by the soil which sits on top of the footing/foundation.
Another way to set the question is how much of the soil sitting on top of the footing will contribute to the seismic force and how much will contribute to the resisitnace against sliding/overturning?

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
One more question regarding the Mononobe Okabe method

Which triangle from the two below best describes the seismic force



RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

kellez - Concerning how seismic force acts on a cantilever retaining wall. Your second sketch is in accordance with US Army Corps on Engineers practice:



Seismic force comes from the "driving wedge" and acts on the "structural wedge". See Seismic Analysis of Cantilever Retaining Walls.

I'm still working on a response to your spreadsheet results. We have some more fundamental issues to look at. Will take me a while to put together an explanation... should have something later today.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

kellez - Good, you made the initial cals... now you can begin to iterate the design. I'm not familiar with the Eurocodes, so can't help you on that. But the calcs indicate that "something" is wrong. Refer to my comments on January 5:

Quote (SlideRuleEra)

The resultant load from supporting soil needs to the in the middle third of slab. Reproportion wall shape/dimensions for more favorable loading characteristics.

Believe me, I don't make this stuff up. smile



You have checked overturning and sliding, but don't appear to have checked the more important soil pressure distribution under the footing. Having soil pressure distribution Case I or Case II is essential for basic loading conditions. For "extraordinary" loading, a limited amount of Case III may be considered acceptable.

Evaluate the proposed geometry (base is 1.80 meter long). Use the basic calculated active thrust from the soil and active thrust from the surcharge... NO adjustments, NO added safety factors, just the calculated values. For the time being, ignore seismic loading too.

If my math is right (I find metric to be very "difficult"... my problem, not yours), the soil pressure distribution under the footing is Class III... too much eccentricity in the loading.



The footing geometry MUST be changed for it to work properly with the existing assumptions. The wall does not fail by being Case III, but it indicates that the design is absolutely NOT right. This is your project, think about how do you want to proceed. There are several options to consider, propose what you would do.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)
You are right i need to deal with this before i move to the seismic load. I did carry out the calculations for eccentricity and bearing capacity of the foundation, please see below.
You are right the vertical force of the wall is acting outside of the middle third of the wall base, which will cause an increase in pressure on the ground below the foundation.
this is due to the fact that the weight of the wall is not spread uniformly across the whole area of the foundation.

NOTES REGARDING MY CALCULATIONS:

As you will notice I have also added a small force for wind loading, what do you think about that (shall i remove it)?
Shall i also consider the surcharge acting on the base of the wall when calculating the Resisting Moment?






RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

Quote (kellez)

...added a small force for wind loading, what do you think about that (shall i remove it)?

The chances of having design wind loading AND a design earthquake, at the same time are very small. The traditional approach is to pick the greater of the two loads, and ignore the other - I agree with that. Chances are the seismic loading will govern for this project.

Quote (kellez)

Shall i also consider the surcharge acting on the base of the wall when calculating the Resisting Moment?

IMHO,that is a reasonable potential solution to consider. On this project, if the heel slab was directly below the surcharge load, that should work. But the slab is not below the load, so my answer to that is no.

The surcharge load is fairly close the the heel slab, so the surcharge may put some vertical loading on the heel slab. But the soil properties are in doubt. Without detail knowledge of the soil properties and control of backfill placement, I would not consider that approach either.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

(OP)

Quote (Kelles)

NOTES REGARDING MY CALCULATIONS:

As you will notice I have also added a small force for wind loading, what do you think about that (shall i remove it)?
Shall i also consider the surcharge acting on the base of the wall when calculating the Resisting Moment?

Just to be clear, both questions quoted above, are in regards to the case of determining the eccentricity of the wall.

RE: Guidance for the Design of a Reinforced Concrete Retaining Wall

The eccentricity of the wall needs to be calculated for more than one load combination. The basic combination is called "Usual Loading" by the US Army Corps of Engineers. For this project's usual loading the horizontal active thrust from the soil plus the horizontal active thrust from the surcharge are resisted the weight of the concrete stem and heel slab plus the weight of the backfill directly above the heel slab. IMHO, the horizontal active thrust from the surcharge acts on the structural wedge and the horizontal projection of the heel slab.

If the usual loading eccentricity is too large (which it is on the current design)... stop. Take the steps needed (often redesign of wall geometry) to lower the eccentricity before considering other load combinations (which may include wind loading).

This is a good time to bring up what some may consider a minor issue. The assumed weight of concrete seems high, 25 kN/m3 (159 PCF). In the US, 150 PCF (23.6 kN/m3) is considered the weight of "normal" reinforced concrete. This value is typically sightly conservative... if (per the usual case) the concrete has to be supported by falsework and forms. For uplift and buoyancy calculations, I consider 150 PCF to be overly optimistic. Note that for this retaining wall the weight of concrete is most important for resisting the overturning moment (uplift). For uplift/buoyancy I use 145 PCF (22.8 kN/m3).

Just suggesting that you use a realistic value of concrete weight for this project... whatever that value may be. What on first glance "seems" conservative, may not be.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close