Ground anchors with tieback wall design

[talps]

I am working on a 21 foot(above ground surface) tall solider pile wall with tieback anchors. The wall will support 3 concrete tier walls. I considered my first anchor location at 10 feet from the top of the soldier pile. After the completion of hand calculations, I ran a global stability program to check the global stability of the wall system. I got a factor of safety less than 1.25 and the failure slip circle pass through above the anchor location. Then, I moved my anchor up, to 5 feet. Now, I got FS greater than 1.5. But, the factor of safety against passive failure of the retained soil above the anchor was decreased to less than 1.5. Can somebody give me an idea to fix this issue? I used a friction angle of 28 degrees for retained soil.

I really appreciate for our time.

Thanks

 

[FixedEarth]

Have you considered making the soldier beam spacing smaller? You haven't mentioned which of 3 upper walls are surcharging the tieback wall. Also since it is multilevel anchor, have you checked for the various construction stages? A cross section with distance and section sizes will help.

 

[talps]

FixedEarth,

Thanks for the quick reply. My design spacing is 7 feet and I tried with 6 feet too. I considered the surcharge from all three tiers. It is a single tie back anchor design. I attached a rough sketch of the cross-section.

Thx

 

[FixedEarth]

Sketch helps a lot. Looks like the lower two walls are surcharging the tieback wall. (18 ft setback < 21 ft wall height). You will need to determine the lateral stress of the lower two walls independently ( treat each wall as a strip load)and add it to your active earth pressure. You may consider making the wall a two level anchor to reduce your soldier beam size, equalize your -ve/+ve moments and control your deflection. You may also consider a tangent pile wall that is tied back, it is a more rigid system.