Cantilever retaining wall design 2

[Kereo]

Hey everyone:

Trivial question here, but I asked my colleague and didn't get a convincing answer

So I've recently started designing a conventional cantilever retaining wall while working through the preliminary calcs, I became aware of the K(active) and K(passive) coefficients that I'm using. I mean, I've always been taught to use these coefficients to calculate the earth pressures acting on the wall. But will the retaining wall reach these limiting states, because according to Rankine, the wall has to undergo some degree of deformation before the K(At rest) reaches one of these limiting states.

I'm not sure whether this is more conservative, because when K(At rest) approaches K(active), the active pressure gradually reduces, so wouldn't it be more conservative to design the wall at a higher "K(active)" pressure? Why do we design the wall at its failure point?

Any advice would be greatly appreciated thx

 

[DaveAtkins]

It does not take much movement to achieve active earth pressure. According to Terzaghi, a lateral movement equal to 0.001 times the height of the wall results in active earth pressure. For a 10' high wall, this means the top of the wall must only move 1/8". So active earth pressure is used for design of cantilever retaining walls.

DaveAtkins

 

[fattdad]

To DaveAtkins: 1" of rotation on the top of a 10-ft wall is the guide for mobilization of active earth pressure. No clue how you got 1/8"?

To the OP: Movement is required to engage friction. At rest earth pressure does not mobilize friction in the same manner. If you don't want ANY movement, you really have to design for base shear and at rest earth pressures. Of course there's also the weight to help in the moment stability.

f-d

ípapß gordo ainÆt no madre flaca!

 

[BridgeSmith]

I understood that the amount of movement to reach the lower active pressure condition depends on the retained soil type, with clayey materials requiring much larger movements.

As fattdad noted, if there are no adverse effects from the wall moving the amount that it will take to mobilize the internal friction of the retained soil and reduce the pressure from at-rest to the active condition, then it can be designed using the active pressure.

If it can move the additional amount required to mobilize the passive resistance (which is typically much larger than required to achieve active) of the soil in front of the wall, and that soil can be confidently assumed to be in place for the service life of the wall, passive resistance can be used to supplement the external stability (typically only helps for sliding).

 

[oldestguy]

Avoid all this and design for "at rest".

 

[PEinc]

Kereo, design of cantilevered retaining walls is covered extensively in many text books and design manuals. These references will cover when to use active and at-rest pressures and how much wall movement is required to develop these pressures for different soil types. I suggest that you check one or more of these references; then ask questions in ET.

http://www.PeirceEngineering.com

 

[Kereo]

Hi everyone, thanks for the helpful comments.

To summarise, we don't always assume the wall will reach limiting state and so before applying the relevant Earth pressure coefficients, it is important to know the approximate deformation that the wall will undergo through its service life?

Also with reference to HotRod10's comment - "If it can move the additional amount required to mobilize the passive resistance (which is typically much larger than required to achieve active) of the soil in front of the wall, and that soil can be confidently assumed to be in place for the service life of the wall, passive resistance can be used to supplement the external stability (typically only helps for sliding."

If the cantilever wall deforms such that the active limiting state is achieved, does that mean the passive limiting state on the other side of the wall (resisting) is not achieved? Because it'll require an additional amount of movement which will cause the active side to fail?

 

[EireChch]

Kereo - I think you should avoid calling it the "active limiting state" as it might just confuse things (well for me at least). Just call it Ka and Ko. Earth pressure on the retained side will be somewhere between.

If the cantilever wall deforms such that the active limiting state is achieved, does that mean the passive limiting state on the other side of the wall (resisting) is not achieved? Because it'll require an additional amount of movement which will cause the active side to fail? - Yes in short. If the wall can not move enough to mobilize (full) passive resistance then it means that the reaction that you relied upon in your calculations is not there. This is one reason why passive is ignored or sometimes factored by half. The additional active side will not "fail" as it depends what your definition of failure is. If the area is a landscaped area then movement required to reduce Ko pressure to Ka pressure is not a big deal as the grass etc will move with it, however if it was a basement wall supporting full height glazing (or something very sensitive to excessive movement) the the movement could be an issue and in this instance you would design for Ko pressures. Ko pressures increase structural forces on your wall by approximately 40%.

I have always worked off the rule of thumb:

Wall supporting landscaping, temporary works, no structural elements - Ka

Walls supporting structural elements - Ko

f-d

Where did you get 1 inch for a 10ft wall from? That equates to 0.83% of H. Ill guess and say someone told you it long ago, you took it on board, applied it to everything and havent had any problems?

I was told above 1% of the wall height will be active.

Bowels recommends a range between 0.001H to 0.004 for cohesionless which is 0.1 to 0.4% and 0.01 to 0.05H for cohesive , which is a 1 to 5% range

EC7 recommends greater and 0.05% of H is active. See below

I suppose mine and your 1 and 0.83% is conservative at it would result in more walls being designed to Ko pressures.

 

[BridgeSmith]

"If the cantilever wall deforms such that the active limiting state is achieved, does that mean the passive limiting state on the other side of the wall (resisting) is not achieved?"

Correct.

"Because it'll require an additional amount of movement which will cause the active side to fail?"

No. The passive resistance is unrelated to what's happening behind the wall. Without movement, the soil pressure on the retained side of the wall may be as high as Ko, depending on how it was backfilled. Usually, that is the assumption. If that level of pressure causes the wall to move, the movement allow the pressure to decrease as the retained soil particles begin to interlock and become partially self-supporting. This fully mobilized internal resistance is the difference between Ko and Ka. Additional movement will not reduce the pressure behind the wall below that corresponding to Ka, since it is the remaining pressure after the soil's internal resistance capacity has been reached.

In front of the wall, it takes significantly more movement to fully mobilize the interlock of particles within the wedge of soil resisting movement.

 

[ukbridge]

Make life easy (both for you, and the financial health of your project) and just design for k0.

Get it out the door.

 

[EireChch]

Although easier to design to Ko, like my above post if its just supporting landscaping above it would be an over design in my opinion.

OP- whats the context of the wall?

 

[Kereo]

A landscaping wall about 3m

Thx for the contribution everyone!

 

[fattdad]

Hi EireChch: 0.83 rounds up to 1.

Good post!

f-d

ípapß gordo ainÆt no madre flaca!

 

[PEinc]

Can anyone provide a reputable reference for using at-rest earth pressure for designing a cantilevered retaining wall?

I have never seen a reference or design manual recommending use of at-rest earth pressure for anything other than a restrained wall. Even anchored, non-gravity walls are usually designed using active pressure. Only rarely have I have seen people recommend using at-rest earth pressure when they wanted to be extra careful and wanted to limit wall movement.

http://www.PeirceEngineering.com

 

[BridgeSmith]

At-rest pressure is used where movement of the wall would be detrimental to the wall or another part of the structural system. If you want a specific directive, I found it in the AASHTO LRFD bridge design spec, at the beginning of Article 3.11.5, where it states that Ko should be used "for walls that do not deflect or move". That seems clear enough to me, so for the situation at hand, designing for Ko would be conservative.

 

[DaveAtkins]

fattdad,

0.001 times 10' or 120" is 1/8" (approximately).

I agree with PEinc. Designing a cantilever retaining wall for at rest pressure is unnecessarily conservative. The minute the wall moves a tiny amount, you have active pressure.

DaveAtkins

 

[fattdad]

DaveAtkins: Your math is correct. However; if you use 1% of 10 ft you'd actually get 0.83".

f-d

ípapß gordo ainÆt no madre flaca!

 

[BridgeSmith]

So, depending on the material, for a 10ft tall wall the movement to reach the active soil pressure condition is between 1/8" and 1". Even an inch of movement won't cause any issues for a typical landscape retaining wall (unless you're building it 1/2" away from the property line).

OTOH, a few of the retaining walls we've done couldn't move at all because there was a sidewalk poured right up against it. For those we used Ko.

We've even done a couple of walls restraining landslides, or subject to the loading of one, so we've used Kp (passive pressure coefficient) to calculate the retained soil pressure. I just depends on the site conditions and design requirements.

 

[PEinc]

I would never assume that a sidewalk was supporting or even could support a retaining wall. Sidewalks may be here today, gone tomorrow. In my experience, few cantilevered retaining walls need to be designed for at-rest earth pressure.

HotRod10 said above that "If you want a specific directive, I found it in the AASHTO LRFD bridge design spec, at the beginning of Article 3.11.5, where it states that Ko should be used 'for walls that do not deflect or move.'" Cantilevered walls are meant/expected to move and deflect a reasonable amount. As far as HotRod10's above mention of AASHTO as a reference, I interpret what AASHTO says as, if the wall will not be able to move or deflect, it should be designed for at-rest pressure. AASHTO did not say or recommend that cantilevered retaining walls be designed for at-rest pressure. I'm still waiting for a clear and reputable reference for requiring at-rest pressure for cantilevered walls.

http://www.PeirceEngineering.com

 

[BridgeSmith]

It's fairly straightforward that a wall that is only designed for Ka may move, or may try to move, at least some amount, when the backfill is compacted. If this is unacceptable from a serviceability standpoint, then the wall should be designed for Ko, so that it doesn't move or load whatever is restraining it.