toe of caisson wall shoring 1

[ajk1]

Can anyone explain to me the meaning of the circled part of the last paragraph of the Canadian Foundation Engineering Manual CFEM)attached? Can you illustrate its meaning with a numerical example?

For example, if the distance from the lowest prestressed tie back to the bottom of the general excavation is 4 m, and the earth pressure against the shoring is say 120 kPa applied as a rectangular pressure distribution, how would you apply the CFEM statement to determine the toe embedment below the general excavation level?

try 4 m toe depth: then total at-rest soil force per unit length of wall, rectangular pressure distribution, is:

(4 + 4) x 132 = 1056 kN.

Passive soil resistance against the toe, as a triangular distribution, is:

Kp x density x ht/2 = 3.0 x 21.7 x 4/2 m = 130 kN.

If that is what is meant, then 4 m embedment would not be enough. But is that what they mean? Would need a tremendous depth of embedment? But what do they mean?

 

[FixedEarth]

You can get solved examples of tiedback walls from Fethi Azizi's book on geotechnical design found here:

http://www.tech.plym.ac.uk/sme/Azizi/geotechnics.htm

If you want a computer solution, you can try TBWALL software.

If you have 2 tieback levels or more, you need to check for the lower level excavation height stability prior to stressing that layer. There is a lot of art and risk, so partner with a seasoned shoring designer.

 

[ajk1]

FixedEarth - the output example online for tbwall shows a trapezoidal soil pressure against the wall. We wanted to use a rectangular pessure distribution to reduce the movement, on account of the adjacent building. Do you know if tbwall can do a rectangular soil pressure?

Also, they seem to embed the toe 0.1H below the bottom of the excavation, where "H" is the depth of the general excavation. Do you know where the 0.1H comes from?

 

[FixedEarth]

Yes it can also do rectangular distribution and may be slightly more conservative but still appropriate.

When you analyze a tieback wall, you can either take the exposed wall height with the active earth pressure + the embedment portion of the beam with the passive resistance and then solve for tieback forces, shear, moment, deflection and embedment. This would be similar to having an overhanging beam with multiple spans and loaded with rectangular or trapezoidal soil distribution. Or you can use Blum's method, which is very specific to tieback walls.

Blum's method states that there is an inflection point just below the base of the excavation. The distance to the inflection point as measured from the base of the excavation varies with the soil at the toe and it is a function of the exposed height, H. For a soil with an angle of internal friction of 28 degrees, it is 0.1H.

Remember, the embedment portion of the tieback should not be confused with analysis embedment of 0.1H. The movement can be reduced by placing the upper most tieback as close as practical to the top of the excavation, say @ 1.5 m depth. The movement may also be reduced by placing successive tieback strands or bars within close intervals, say @ 3 m on center.

"Foundation Engineering" book by A. R. Jumikis, 1986 & "Engineering Design in Geotechnics" F. Azizi, 2007 both do a great job of explaining Blum's method.

 

[ajk1]

Fixed Earth -thanks very much for thios information. I am checking a design by another firm, so I don't have the fredom to set the tie bacls where you suggest. I am jsut trying to check whether the design works. I notice that the Azizi book is in priced in pounds. Can I order that from here in Toronto Canada?

 

[msquared48]

Depending on what stratum you are extending the toe into, I would not consider the top 2 to 3 feet of the stratum for lateral. Just add one meter to your embediment result.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[FixedEarth]

The author, Azizi, sells his book directly on Amazon in UK. Amazon would convert the pounds to Canadian dollars automatically. This is a well written book and enjoyable read.

By the way, the 0.1h is a standard depth used by all shoring designers.

http://www.amazon.co.uk/Engineering...=sr_1_4?s=books&ie=UTF8&qid=1325002866&sr=1-4

 

[ajk1]

Aziz: thanks. I will order the book.

Mike: seems a bit strange that the embedment depth is independent of the distance from the bottom of the general excavation to the first tieback. In my case the exacation from grade is 20 m deep, the distance from the bottom of the excavation to the first tieback is 4 m and the toe embedment is 4 m below the bottom of the general excavation; the water table will be lowered to 1 m below the bottom of the general excavation.The soil is granular, about 100 blows/30 cm. The shoring was designed by a firm that specializes in it. Anyway I will ordert the Azizi book and see what it says.