Stability of soldier pile retaining wall 1

[t230917]

I am designing a soldier pile wall for the first time with the following:

Retained height - 11.2'
Ka=0.46, Kp=1.6 (after using reduction factor from AASHTO), pile spacing=8', concrete dia.=3', phi=30 degrees, unit weight-120 pcf from backfill to excavated height, and then reduces to 80 pcf, LL surcharge=250 psf on both faces.
Based on this, I am getting a depth of embedment of 21'.

I had 2 questions in regards to this. 1) Does the depth of embedment look right?
2) How do I calculate the stability of this wall? I cant find the procedure for this.

Thank you.

 

[r13]

The embedment length is almost twice of the retained height, it looks too long. I suggest to open a thread on "bridge engineering" forum, maybe someone there can help.

 

[jayrod12]

Around here, a 1.5:1 or 2:1 embedment:retained, is in the range of expected for that type of soil parameters.

 

[BridgeSmith]

The embedment length looks about right to me, retired13, considering the 3' width and 8' spacing, especially if the 250psf LL surcharge is the actual lateral pressure, as opposed to the vertical loading. That would be my first question - is the 250psf horizontal pressure or the vertical load?

Also, a LL surcharge would typically not be applied in front of the wall, where it would provide confinement and resistance, because live load is by definition a transient load, not to be counted on for resistance.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

Rod,

I was expecting in the order of 1.5:1, but yes, it will close to 2:1 with 30% increase. The active earth pressure coefficient Ka seems high for soil with Ø=30°, unless adjusted for sloped fill. One thing I don't understand is the unit weight (soil or concrete) varies from 120 pcf to 80 pcf, what is the verification accounted for, ground water?

 

[t230917]

Thanks for your replies. To answer your questions:

Yes, the unit weight goes down to account for ground water at that depth.

The active earth pressure coefficient has been adjusted for sloped backfill per AASHTO LRFD 3.11.5.3.

The 250 psf is the vehicular surcharge, not the actual horizontal pressure. The horizontal pressure due to this will be 250*Ka.

I am applying a LL surcharge on the front face of the wall since there is an access road at the front face of the wall which will have vehicular traffic.

How should I check the stability of the wall? I am not seeing any guidance for this on AASHTO for this type of wall.

 

[BridgeSmith]

In the AASHTO LRFD spec. the first diagram (fig. 3.11.5.6-1 in the 8th Ed.) is for discrete wall elements (soldier piles) embedded in granular soil. You have to solve for the embedment depth that produces moment equilibrium. I'm not sure if it's current with the spec., but we multiply that calculated embedment depth by 1.2 to provide a factor of safety for design.

Any live load in front of the wall would only add resistance to the embedment soil, and cannot be counted on to be present, so it is typically ignored in the design calculations.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[PEinc]

Soldier beam walls are not normally used to retain hydrostatic pressure. However, it is not uncommon to design for ground water at the base of the wall using buoyant soil below the base of the exposed wall. If you are designing for some retained, buoyant soil (your 80 pcf, which seems high to me), don't forget to add the water pressure associated with unbalanced water levels.

I assume that your wall is a cantilevered solder beam wall. Your soils seem to be heavy and fairly weak. Reconsider whether or not a cantilevered wall is appropriate for the soil conditions.

http://www.PeirceEngineering.com

 

[r13]

For a concrete pile with 3' diameter, it is the size of drilled pier. Is it terminated at the excavated ground, or extended all the way to the top? What is the type of lagging to be used to retain the soil? And, what is the stability check you have had in mind, over turning?

 

[oldestguy]

"2) How do I calculate the stability of this wall? I cant find the procedure for this. "
For design you can use many different references, but fr a starter try the USNavy design manuals. I have an old one NAVFAC DM-7 that has procedures.
If in doubt I'd ask an experienced geotech engineer, since you appear to be likely to miss something.

 

[r13]

Question for the soil guys - will ground water reduce the positive earth pressure resistance, or no effect?

 

[oldestguy]

Submerged unit weight is significantly less then not submerged, meaning the lateral resistance is reduced accordingly.

 

[t230917]

Yes, I am using Fig. 3.11.5.6-1 to get all earth pressures and to calculate the depth of embedment. I summed the moment at the bottom of the pile as zero, and calculated the depth of embedment based on this. I am not adding a factor of safety here since I have reduced the Kp per 3.11.5.4-1 and I dont see any other guidance on this.

The Structure Geotechnical Report suggested soldier piles would work best for our case since the soils are too hard (cobbles/weak bedrock) for driving sheet piles.

The concrete encasement starts 2 feet below finished grade. Above it is CLSM with the same diameter. CIP lagging is used to retain soil.

Yes, I want to check for overturning and sliding.

OK, I will check the US Navy Design manuals.

 

[PEinc]

If this wall is being built from the top down, in lifts of about 4 or 5 feet, using precast concrete lagging is usually a problem.

http://www.PeirceEngineering.com

 

[r13]

For soldier pile wall, there is no overturning and sliding concerns.

 

[oldestguy]

I'd do some checking on equipment needed for this wall. It may be that installing more smaller diameter "caissons" that are lighter, less costly. The lighter equipment may be better than one that must drill the large diameter and also get the depth of what is planned, especially if there is hard drilling, as with rock, etc..

 

[BridgeSmith]

retired13 is correct, there is no sliding or overturning for a soldier pile wall. There is only rotational stability. Either the wall rotates forward and falls down, or it doesn't. I guess you could call that 'overturning', but you'll only confuse yourself if you try to apply the overturning or sliding provisions of the AASHTO spec. to a soldier pile wall.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[PEinc]

Soldier pile walls do need to be checked for overturning. That's why you take moment about the tip of the soldier beam. As far as checking for sliding (summation of forces in the horizontal direction), the magnitude of the passive resistance is always much greater than the total driving force because the moment arm for the passive resistance when you have moment equilibrium is always much less than the moment arm for the driving pressure. Therefore the total passive resistance has to be much greater than the total driving pressures. It makes no sense to check sliding of a cantilevered, non-gravity wall because the passive resistance is always greater than the driving force and the wall cannot push backwards if properly designed. If the wall tried to move backwards, the passive resistance and the driving forces would switch sides, which would tremendously increase available passive resistance and reduce the driving force.

http://www.PeirceEngineering.com

 

[r13]

Soldier pile walls do need to be checked for overturning. That's why you take moment about the tip of the soldier beam.

In a sense PEinc is correct, but it would be more exact/correct if the word "checked" be changed to "designed", as it is absolutely essential in determine the pile embedment depth to maintain force equilibrium, thus the "check" has been done in the onset of design. Conventional thinking of the retained soil block slide with the retaining structure, pile in this case, will not occur is self-evident - as the pile cannot free move laterally, the retained soil block will collapse if it pushes the pile to fail.

Albeit the argument above, it is understood that the verbs "checked" and "designed" are interchangeable and loosely equal in engineering sense and usages.

After setting adequate pile embedment length, the over turning and sliding stabilities are maintained through structural design of the piling and lagging. Again, these efforts are design activities, rather than the conventional "stability check".

 

[PEinc]

Tomatoes, tomahtoes. A properly designed, cantilevered, soldier beam wall does not need to also be checked. However, a soldier beam of known size, length, and spacing could be checked for its retained soil height and its soil and water conditions.

http://www.PeirceEngineering.com