Lateral earth pressure on footings and keys of retaining walls 1

[bnickeson]

This may be a bit of an unintuitive question, but it is mostly geared around how software treats lateral earth pressure loads on footings of retaining walls and keys of retaining wall footings. In this instance, I am speaking of RetainPro but I believe other software treats it similarly.

In the instance of a restrained foundation retaining wall, you obviously have at-rest pressure applied to the height of your wall. That pressure is also applied to the vertical projected face of your footing in nearly all cases. However, in most software the lateral earth pressure is not applied to a key at the base of the footing. Is there a good explanation as to why making a footing deeper (like to engage more passive resistance) in turn increases your lateral earth pressure due to the pressure being applied to the footing depth, but if you place a key the lateral earth pressure does not apply to the key?

I'm working on sliding design of a basement foundation wall that's buried 14' below the soil level, so it's got some fairly significant lateral load at its base and it's quite the challenge to keep it from sliding. If I increase the thickness/depth of the footing to engage more passive pressure, I just end up with more lateral soil load as well so I end up chasing my tail until the passive pressure catches up with the added lateral pressure. That depth becomes significant. But if I keep the thickness of the footing fairly shallow (let's say 16") and add a shear key, there is no added lateral earth load on the key and the passive pressure can quickly add up to resist the horizontal loads. Does this intuitively make sense? Should there be lateral pressure applied to the key in the software? Or should there not be lateral pressure applied to the footing?

 

[JoelTXCive]

Hallelujah! Someone has found the same issue I have been wrestling with for the past 8 years.

Unfortunately, I have not found an answer.

I think that this is one of many glitches in what I like to call "retaining wall logic"

The problem can be further exacerbated if the geotech says "disregard passive pressures on the first XX feet of embedment." I have had retaining walls where thickening the footing or adding a shear key actually HURTS the sliding calculation because you pick up additional active (or in situ) pressures, but don't get any passive.

We use a software package called "QuickRWall" and it has a checkbox that allows you to select if active pressures are calculated on the shear key or not.

Unless Retainpro has a button for that, then I think you just have to exercise some engineering judgement in what the software is saying.

 

[canwesteng]

For sliding it should be included imo. Is there a basement slab you can count on to stop sliding?

 

[bnickeson]

For sliding it should be included imo. Is there a basement slab you can count on to stop sliding?

Yeah, I'm also counting on the basement slab. Even though it's 100' in length parallel to the direction of the lateral soil force, the slab only contributes about 800 plf to the sliding resistance. That's less than a quarter of the total sliding force. I was a bit shocked by that.

 

[jhnblgr]

For the soil pressure to really exist movement must occur. The backfill material will unlikely have much pressure on the footing slab side. What material are you using for backfill? Increase stem thickness and heel length. If the slab is a diaphragm and both ends of the wall backfilled equally the loads should equal in theory.

 

[jerseyshore]

Yeah, I'm also counting on the basement slab. Even though it's 100' in length parallel to the direction of the lateral soil force, the slab only contributes about 800 plf to the sliding resistance. That's less than a quarter of the total sliding force. I was a bit shocked by that.

Is there a wall on the other side? How is a basement wall going to move a 100 ft long slab?

 

[HTURKAK]

Should there be lateral pressure applied to the key in the software? Or should there not be lateral pressure applied to the footing?

If the key just near the soil side , yes . If centered or at interior side , no need to apply lateral pressure . The following doc. ( free access From CALTRANS ) Pages 5-49/50 explains the concept .

Regarding the bracing of basement walls , i preferred to use SOG or reinforce the ftg for lateral loading spanning btw the transversal walls .
The following sketch from Foundation of structures ( Dunham, 1962 )

 

[ANE91]

If the key just near the soil side , yes . If centered or at interior side , no need to apply lateral pressure .

This actually makes a lot of sense. Horizontal pressure is the effect of overlying soil bearing down and spreading out. Keys located relatively closer to the toe don’t experience this effect, as the heel carries the overlying soil.

 

[XR250]

Yeah, I'm also counting on the basement slab. Even though it's 100' in length parallel to the direction of the lateral soil force, the slab only contributes about 800 plf to the sliding resistance. That's less than a quarter of the total sliding force. I was a bit shocked by that.

Can you dowel the slab into the side walls?
IMHO, slab friction is underestimated.

 

[TRAK.Structural]

I would guess that the pressure on the back side of the key is ignored in the software because typically you sum moments about a point at the base of the footing, and in doing so any pressure on the back of the key counteracts the overturning moment.

With that being said, I find it hard to reason that you can get passive pressure on the front of the key while ignoring active/at rest on the back of the key. I don't think the presence of the heel of the footing over top of the key changes anything about the driving side soil pressures, the lateral component of the pressures still exist under the footing.

 

[Smoulder]

"Horizontal pressure is the effect of overlying soil bearing down and spreading out. Keys located relatively closer to the toe don’t experience this effect, as the heel carries the overlying soil."

Presure at toe is more than density*depth because of overturning.

 

[bnickeson]

Is there a wall on the other side? How is a basement wall going to move a 100 ft long slab?

There's only soil on one side of the building, so the load is unbalanced. There's also very few transverse walls due to the space layout and the shape of the building. So the slab sliding resistance is just 0.6D times a 0.3 coefficient of friction which doesn't add up to much.

 

[bnickeson]

Thanks for the explanations, all. The key being located away from the back of footing and wall makes sense to me for why we wouldn't apply lateral pressure to it.

 

[Greenalleycat]

We have had this debate in the office many times
The standard industry guidance here DOES put active pressure on the back of the shear key if you locate it at the rear of the wall
As such, we rarely use shear keys and, if we do, we locate them further forward so that the theoretical failure plane doesn't intersect the back of the key