Retaining Wall - Driving Moments Greater Than Resisting Moments

[CWEngineer]

I have a retaining wall (floodwall) with water on the left hand side and the soil only about 2 ft above the heel and the toe. So the retaining wall is being driven from left to right. I am making point A at the bottom of the toe (which is on the right side). Due to the loads and limited construction space I plan to put this retaining wall on piles.

When I calculate the driving moment (Water, At Rest Pressures, and Uplift) and resisting moment (Weight of water above heel, weight of concrete, weight of soil and passive pressure)about point A (Bottom of Toe) the Driving Moments are Greater than the Resisting Moment. So when I Sum the moments I obtained a moment in the clockwise direction.

When I try to figuere out the location of the Vertical resultant (pointing up), the resultant has to be to the right of Point A (not within footing), for the Sum of the Moments to be equal to zero.

I guess typically, the Resisting Moments are greater than the Driving Moments so you have a counterclose-wise moment at point A, resulting in the Vertical Resulant Point Up, and to the left of point A (within footing).

Does it make sense to have the Vertical resultant to the right of point A (not within footing) because of the Driving Moments are greater than the Resisting Moments? Or is this something that is not appropriate and need to consider another proccess?

THANKS, Really Appreciate Your Comments.

 

[Yavoran]

Hi!!

Looks more like a dam for me.
Why not try dam & earth retaining fora here.

Otherwise i would simply try to change the dimentions so i would not have such a big diference in the moments.

for a dams if i am not mistaken(please corct me team)you should never get an uplifting at the back of the foundation.
The other thing to take into account is the type of the soil. Make sure the water do not pas underneath your wall.

Y

 

[Qshake]

You will definitely need to consider flow under the wall for this condition.

I recommend that you check out the Corps of Engineers Engineering Manual (online) to get some idea of what your doing....sounds like a beginning effort.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[DaveAtkins]

The resultant CANNOT be to the right of point A. This would mean the structure is unstable.

DaveAtkins

 

[jdonville]

gman,

Shouldn't your piles be providing uplift resistance? This could be your problem. Note that uplift is not usually included in overturning calculations as an external force. Usually any buoyancy issues are addressed by modifying the submerged unit weight of the materials.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

 

[CWEngineer]

Due to my construction space of less than 20 ft I cannot construct a levee. Intially I was designing a floodwall (approximately 16 ft) but I was not able to fit my footing within about 20 ft of right of way. I was not able to meet the sliding safety factor due to the water pressures. Yes, geotech is looking at seepage under the wall. At this point I am not really sure if the piles should be providing uplift resistance.

Appreciate your advice and any suggestions on what I should might consider doing.

THANKS

 

[damstructural]

You don't say how tall the floodwall is, so its hard to give a judgement or check this, but if your net moment is overturning, as you describe, then your structure is unstable. End of story. Check loads and/or Redesign.

You should have a net restoring moment and the issue should be whether or not the resultant is in the middle third of the base, or whatever criteria you feel comfortable with.

 

[CWEngineer]

I have to design the wall about 16 feet high, which support pressures due to water. My initial intent was to make this a fooldwall, but the footing was too long for my construction area.

So now I looking into designing a pile supported retaining wall. I am using the methodoly by Foundation Engineering: Peck, Hanson and Thorburn.

I have increased my footing width to make the resisting moments (173 k-ft) greater than the overturning moments (165 k-ft). I know this is just by 8 k-ft. Also by doing this the resultant is within the footing area - but just barely.
Should my intent still be to place the resultant within the middle 1/3 even if I am using piles? Appreciate all your help and advice.

THANKS

 

[Boonie]

I would suggest using a helical pile to "tie down" the footing. This type system will provide anchorage and give you the ability to resist the moments and driving forces needed to keep your footing within the limits of the right-of-way. Helicals can be placed wherever you require a resisting force on your moment arm. Typically, you can produce 30 - 50 kips of force (depending on soil types) with each helical screw. I have used anchors on cantilevered walls on 10' spacings to resist the uplift pressures discussed in the Corps of Engineers flood wall manual to help decrease the footing lengths.

 

[DaveAtkins]

If your resultant is "just barely" within the footing, then chances are you are exceeding the allowable bearing pressure in the soil. When your resultant force is not within the kern limit, then the bearing pressure at the toe of the footing is equal to (2*P)/(3*a*B), where P is the magnitude of the resultant force, a is the distance from the resultant force to the toe of the footing, and B is the width of the footing (usually 1', since we design a 1' strip).

DaveAtkins

 

[shockstressed]

ANOTHER OPTION IS TO RETAIN THE BALANCE OF THE SLOPE AND USE A REINFORCED STRUCTURE OF SOILS THAT ALLOWS THE EXCHANGES OF LOW FLOW DURING HIGH FLOOD AND HAS THUS TO BE OF ADEQUATE PERMEABLE STATE TO TAKE THE FLOW WITHOUT STRESS IN THE STRUCTURE THROUGH FINE MATERIALS CAUSING A SURGE EFFECT AND OF COURSE ANY EROSION AT WEAK POINTS. THIS IS NOT ASSISTED BY SUBSIDENCE ZONES.
LOADINGS THEN HAVE TO BE TAKEN AS FOR A SEMI-SATURATED SOIL AS THE TERMINOLOGY GOES AND OF COURSE SOME SEEPAGE TOLERATED AT THE REAR WALL SITE
OR OF COURSE JUST RAISE AND BED THE SEDIMENTS ABOVE THIS BUT THIS ADDS TO THE SURGE TOWARD THE FLOOD.
REGRADING ALWAYS LEADS TO SOME SETTLEMENT WHICH YOU WILL HAVE BEEN TRYING TO AVOID ACCORDING TO YOUR DESCRIPTION AND I CONCUR WITH THE INSTABILITY COMMENTS MADE BY OTHERS REPLYING.
THE ONLY ANSWER WITH MATHEMATICS IS TO STAY WITH THE FORMULAE AND PRECEPTS FOR DESIGN AND IF IT DOES NOT WORK OUT THEN THINK OF SOMETHING ELSE AND TELL THE CLIENT SO.
CLIENT DEMANDS MAY TRY OPTIONS AND I HAVE PROGRAMMES THAT WILL PROVIDE HUNDREDS AS I WAS TIRED OF PEOPLE PRODUCING ONE ANSWER BECAUSE THE DATA DID NOT EXIST, THE DATA HAD TO BE ASSESSED EVERY TIME AND CALCULATIONS TOOK SO LONG. THIS LEADS TO RISKS AND STOPS STUDENTS FORMING ONE DESIGN AND GOING FOR COFFEE FOR THE REST OF LIFE
BUT THEY DO NEED A CAREFUL CE APPROACH AND KNOWLEDGE OF YOUR EXPECTED END RESULT, WHICH YOU DO HAVE CLEARLY.
THE ANSWER IS MUCH THE SAME, TRY OPTIONS OF MOVING THE WALL, CHANGING THE CONFIGURATION, OR POSSIBLE TRENCH IN THE BASE AND RE-DESIGN OF THE FOUNDATION, BUT AS WE ALL KNOW IF THE BIG RIVER SCOURS OUT THE BASAL SEDIMENTS WE ARE ALL IN IT, SOMETHING POLITICIANS FAIL TO APPRECIATE
MIKE STAGG

 

[bjb]

If you are using full passive soil pressure in your resisting forces you have to be careful. Developing the full passive pressure requires significant motion, which may not be tollerable. You can develop 1/2 of the passive force without much movement, or you can only count on the at-rest pressure in your resisting force calcs. This is what I believe the Corps. of Engineers recommends in their engineer manual on flood and retaining walls. Our in-house soils engineers say the same thing. Check out Engineer Manual 1110-2-2502 at the website for the Corps of Engineers.

 

[CWEngineer]

Hi,

Yes, I am designing this floodwall/wall with piles using the EM 1110-2-2502. I am using 50% passive pressures and including those in Overturning and Sliding ananlysis. And I am using At-rest pressures for the soil on the waterside.

I have also placed the water to the pool elevation on the water side. I am aware that I also have to design this floodwall for the water to be at the top of the wall per the EM.

One thing that I am still trying to get info. is the location of the water on the landside. Currently I have the water on the land side to the bottom of the footing, which gives me a triangular uplift pressures. I am not sure if I should raise the water on the landside to the top of the soil, which will give me a trapezoidal pressure BUT will also give me resistance due to the horizontal pressure.

At the momoent, I have increased my footing to 14 ft and now my resultant is about 4 feet from the toe. When I design for water at the pool elevation I have all my piles in compression, but when I place the water to the top of wall I have the piles near the heel in tension.

I guess here is another question I have and would really appreciate your feedback on it any other issue I sould look into:

1. What will give me a more conservative desgin, placing the water on the landside on the bottom of the footing or to the top of the soil?

Really Appreciate Your Feedback and Advice.
gman1

 

[JStephen]

Would sheet piling work here?

 

[CWEngineer]

That is something we consider earlier on but never really went with it. We were not sure if it would be water tight.

Does anyone have experience with sheet piling, and have an idea if it could be design to be water tight and hold approximately 15 ft of water?

THANKS