Retaining Wall Shear Key 2

[PEStruc]

I'm Working on checking the sliding for a retaining structure that is in a high seismic zone and I have issue with the sliding. Tried to add shear key but thigs get worse because after adding the key, both passive and (active, surcharge and seismic earth pressure) increased, but the increases of the sum of (active, surcharge and seismic earth pressure) is greater than the passive pressure.

By consulting another engineer, he mentioned that If the key is placed away from the face of the wall receiving active earth pressure, the key will not be subject to any earth pressure and the earth pressure diminishes as you move away from the soil wedge that is bound by the retaining wall and the angle of internal repose of the soil.

Tried to look online and found some examples that are in line with the above statement and some are not. Any one knows reference that talks about this issue in particular and whether we should calculate the active earth pressure to the bottom of the shear key or to end it at the bottom of the foundation? Appreciate the help!

 

[r13]

I suggest place the shear key directly under the toe (similar to a turn down/flipped L), and assume the failure plane is at the level of bottom of the key. The pivot point of rotation should be at the bottom of the key also.

 

[HTURKAK]

One picture worth one thousand words.. ( Ref Caltrans bridge design specifications SECTION 5 - RETAINING WALLS )

In this case two pictures worth two thousand words.

 

[r13]

My thinking of rotation point, "O", differs from HTURKAK.

 

[Settingsun]

The Caltrans guidance has a sudden step between no active earth load on the shear key (shallow case) and lots of active earth load on the key (deep case). That's a pointer that it may not quite reflect reality. For comparison, British Standard BS 8002 simply says to take the active pressure to the base of the key, so does not have the Caltrans relaxation for shallow keys.

In the Caltrans shallow key model, is the assumption that the soil behind the key self-supporting after the wall moves slightly away from it?

 

[HTURKAK]

Shallow key model requirements ;
Dk < Tf Bk > Dk , when Bk < Dk , the pressure surface of height, h , extends to bottom of footing key..

IMO, your assumption is correct.. the shallow key model is chosen for to mobilize the base friction at a lower level . The soil bearing stress approaches to zero level around heel and the active pressure will be negligible since for mobilization of soil friction, passive resistance due to shear key will be necessary that is, wall moves slightly ..

You can reach the document from web

https://dot.ca.gov/SearchResults?q=BRIDGE+DESIGN+SPECIFICATIONS

 

[JoelTXCive]

I have had this exact problem several times before, and unfortunately; have never found a solution.

You have a wall that is sliding; so to solve that you want to add a shear key to get the benefit of some passive pressure.

But, by adding a shear key you pick up additional active pressure, which is greater in magnitude than the added passive pressure.

Adding the shear key actually makes the situation WORSE, unless you increase the depth of the shear key (which creates more headaches).

The question is:

Is there a reasonable rationale for NOT loading the shear key with additional active pressure when the shear key is positioned up at the toe of a wall?"

 

[HTURKAK]

Yes... look to the first picture.. the requirements are shear key depth ≤ depth of footing ( Dk ≤ Df) and the other requirement, Bk> Dk ..

The use of shear key is required when one can not get F.S. against sliding which is a common problem when the base of footing has waterproofing membrane or tanking of concrete with epoxy paint etc. Consider the the steps of construction, for a shallow key conforming the requirements, you should provide a recess at binding concrete . The friction surface will develop below the bottom of blinding under the key and for developing of shear surface, some passıve pressure should develop , that is the wall should move slightly.. behind the shear key , the active pressure developing will be negligible or zero, since the bearing pressure under the footing around heel will be minimum.

I hope this explanation makes sense..

 

[Settingsun]

I'd want to understand the thinking of the Caltrans model better before adopting it. Here are the ways I don't understand it:

1) What is the reason for the D_k < T_f limit? Is this an indirect way of limiting it to walls that don't need much resistance from the key, on the assumption that T_f is ~H/10? That is, walls that are probably OK without the key but need it for a minor increase on paper to achieve the factor of safety.

2) What if the key is near the toe, whilst still satisfying the two geometric criteria? It would then have large vertical pressure on the ground behind the key, so any active pressure would not be negligible as opposed to the case in Hturkak's previous post.

3) There's a sharp transition between no active pressure on the key and full active pressure over the full height of the key. I think that's a massive simplification. How critical? What if you showed up to the pre-pour inspection and the key had been over-excavated by 200mm? Do you tell them they have to fill the trench back up to the design level, or do you let it pass because you don't really believe the Caltrans model?

4) Stability of the rectangle of soil behind the key: It has a large earth pressure pushing it to the left and some friction from the footing also dragging it to the left. It has no reaction from the shear key (as it applies no force to the key) and has little capacity for friction from underneath as we've established that the vertical force is small. What keeps it stable as a rigid body? Or does it fail in shear Rankine-style, in which case there would be active pressure?

 

[HTURKAK]

Dear steveh49 ;

CALTRANS BRIDGE DESIGN SPECIFICATIONS including SECTION 5 - RETAINING WALLS is an open source . I tried to upload subject document but could not succeed .( If somebody tells me how to upload , I will appreciate )

I will try to reply your queries to my best knowledge. My responds in general will be based on past design experience rather than theoretical discussions..


The use of shallow key is , IMO, in order to mobilize base friction and to get limited passive resistance. If you can get Sliding Factor of safety, FSsl ≥ 1.5 without footing key,O.K., you will not need the key. But in case of Wall footing without footing key , the sliding resistance SHALL BE BASED ON friction factor (will be the smaller of friction between blinding and compacted subgrade or between footing and mebrane or blinding layer ). However, in case of footing key, the friction factor will be shear strength of subgrade .( If somebody uses the shear strength of subgrade with footing key option , I will agree ).. Some authorities ( CALTRANS also ) includes passive lateral earth pressure, not to exceed 50 percent of the available passive lateral earth pressure. Your assumption is correct (the key needed for a minor increase to achieve the factor of safety. If more passive thrust is needed , deep footing key will be necessary.



If the key is near the toe, moreover under the far end of the toe, the active pressure behind the key will be greater than passive
pressure developing at the front of the key.. so meaningless ..


You are right .. this is one of the design criterias. Other authorities' assumptions and criterias may deviate...If the key had been over-excavated by 200mm, yes ..I should tell them they have to fill the trench back up to the design level with blinding concrete..


If the key around heel and Bk > Dk, I will not agree with you.. since in order to mobilize base friction and passive thrust, the wall shall move slightly to the left . The active pressure behind the key is limited with the soil bearing pressure behind the key around heel moreover, the bearing pressure could be zero..due to relaxation of soil behind the key, the active pressure developing will be negligible.
The following snap is copy paste of the same document which shows the sliding design assumptions ;


Here is the link..

Thank you so much R 13..

Link

 

[r13]

HTURKAK,

Hope this helps.

 

[ATSE]

Quoting HT: "You are right .. ...If the key had been over-excavated by 200mm, yes ..I should tell them they have to fill the trench back up to the design level with blinding concrete.."
While HT is providing some good context, I do not agree with his statement above.
[aside: for this discussion, I am assuming that the shear key can be designed to not fail structurally]

Regardless of what codes we use, for cantilever retaining walls, adding a shear key below the footing, and/or deepening a key, always results in a higher factor of safety for sliding. If adding a key gives you a lower sliding FS, then your methodology is over-simplified to the point to being wrong.
And if your contractor over-excavates the key (unlikely), tell him to add vertical key reinforcing, then buy him a 12-pack.

I concede that most current wall design methodologies give simplified lateral driving and resisting, resulting in artificially lower FS, as JoelTX noted. However, can we really say that a shear key reduces sliding capacity as compared to no key? Certainly no.

I didn't see anyone mention USACE retaining wall design methodology. EM-1110-2-2502. Which, by the way, has the same problem. However, I think this is a better standard than what most text books show.

 

[ATSE]

BTW, if you're unfamiliar with flood walls [EM documents are free and easily downloadable], the shear keys are typically placed at the back of the heel vs what CRSI and some text books shows.
Tall flood walls are a bit more serious than drained walls that are 8' tall or less, which are relatively forgiving.

 

[r13]

USACE manual is very useful, and largely used in water related projects. It is a little too much for land based cases.

 

[jsmith234]

this is a good thread on the matter

IBC 2009 1807.2.1 Retaining Wall Keyway

https://www.eng-tips.com/viewthread.cfm?qid=332224

 

[Settingsun]

There's an opinion in jsmith234's other topic that is the complete opposite of Caltrans: you need a deep key to avoid active pressure. So, basically, do whatever you want. It's just like your opinion, man.

Is the Caltrans document current? I couldn't find it on their website and it pre-dates a relevant change to the California Building Code according to the other topic.

 

[Lifter123]

I have options for a precast wall using a cantilever design and cast in place foundation.