Jacking Pit Thrust Block Design

[shorebob]

I read a very informative thread re: thrust blocks in jacking pits, but it has been closed. I'm designing a thrust block in a slide-rail-shored pit in which the lower panels will be raised, and the thrust block will bear horizontally on the soil, developing passive pressure. The transition has been accounted for, but is irrelevent here. My question is about arching factor. When I design soldier piles, I use an arching factor at the toe of 2.0-3.0, based on CalTrans'.08xphi. For a 12'w. x 6'h R.C. block, would an arching factor of 2.0 be extreme, yielding an effective passive pressure area of 24' x 12'?

 

[PEinc]

What would be causing an arching effect? Nothing as far as I can see. You will have a block bearing against the vertical earth face. I don't see how you can claim an arching effect.

http://www.PeirceEngineering.com

 

[PEinc]

thread255-184604

http://www.PeirceEngineering.com

 

[cntw1953]

Never heard of arching effect in soil classes, read a few papers after graduation but never full understood it. Anyone can point out a good reference? Thanks.

 

[shorebob]

Perhaps my terminology is imprecise. 'Arching' suggests soils' self-supporting tendency between 2 or more fixed points. However, CalTrans' Trenching & Shoring Manual describes a permissable increase in the Effective Width of a soldier beam's toe due to a "..wedge-shaped resisting soil configuration" (pg. 10-2). This increase is quantified by an 'Adjustment Factor', referred to therein as 'passive arching capability'. Caltrans allows this factor to = .08 x phi </=3.0 in cohesionless soils. 2.0 is a common conservative value used in soldier beam design. That said, I'm searching opinions as to this factor's applicability to, and value for, large, flat thrust blocks. Some colleagues here in SoCal have used 1.33 & 1.5.

 

[PEinc]

If you don't increase the effective size for a footing with a vertical load, why would you increase the size of a "footing" that pushes in a horizontal direction? Side shear for an embeded soldier beam can be significant relative to the embedment's horizontal bearing capacity. However, side shear of a much larger jacking reaction block should be less significant. I've never increased the area of a reaction block. Doing so may be OK but I haven't done so.

http://www.PeirceEngineering.com

 

[ishvaaag]

The old codes for loads in Spain explained that you SHOULD account trhice the ACTIVE push on a vertical element embedded in the ground, whilst not counting any passive resistance behind. Nothing was said about passive push in vertical elements embedded in the ground. So again when unfavourable was counted, when unconservative, not.

Current code CTE SE_C that cares of foundations has charts for cohesive and noncohesive mediums. Look pages SE-C-146 adn 147.

http://www.codigotecnico.org/index.php?id=33

download DB-SE C if you want.

 

[jdonville]

Shorebob,

Have to agree with PEInc on this one. I use an effective width on drilled and driven piles all the time for shoring applications. However, I would not do so for a thrust block.

J

 

[cntw1953]

"'Arching' suggests soils' self-supporting tendency between 2 or more fixed points."

Thank you for the explanation, however, I am still confused, where are the "2 or more fixed points" in your application?

 

[shorebob]

Sorry. The term 'arching' has skewed the discussion. 'Arching' is actually an active pressure phenom., allowing reduced soil load on, say, lagging between the fixed points of adjacent soldier beams. Here I'm searching opinions as to whether the 'effective width increase' allowed at the toe of soldier beams (See CalTrans reference) against passive pressure is applicable to wider elements, such as thrust blocks. Thanks to all for your continued input.

 

[cntw1953]

Sorry for continuously posting questions over your question.

Do you mean you have two (fixed) jacking points/blocks, so you wonder whether the Caltrans pressure reduction provisions (used for sheetpiling design) applies or not?

 

[aeoliantexan]

Tschebotarioff has a good discussion of the passive resistance of buried anchor blocks in "Foundations, Retaining and Earth Structures", McGraw-Hill, 1973. For a horizontal pull on anchor blocks in sand he says the ultimate resistance is Ep=Gamma (H^2)Kp'(b)/2, where H is the depth of the bottom of the block and b is the width. He has a chart for Kp' as a function of H/h, where h is the height of the block. Kp' is 6.7 when H/h = 1 and 11 when H/h = 6. This was based on experimental results with b=h in sand with Phi = 32.5 degrees.

He also discusses anchor blocks in clay.

Some caution would be in order when applying these results to a thrust block in a pit, especially in clay.