Minimum Earth Pressures 3

[pelelo]

Hello,

I am working on a shoring project.

So far in some areas I am getting very low horizontal earth pressure values. I was wondering if there is any reference which can provide minimum earth pressures values for designing purposes.

I have read somewhere to use a surcharge load of 72 psf to account for traffic, men working, any small equipment. Still including 72 psf i get somewhat very low earth pressure values (200 psf). I think there should be a reference somewhere that deals with this low earth pressure values.

Please let me know,

Thanks

 

[fattdad]

if you consider an equilivant fluid pressure of 1/2 the unit weight of soil, that'd be 60 pcf. If you are supporting 10 ft of soil, that'd be a horizontal pressure of 600 psf at the depth of 10 ft.

For long-term horizontal stresses, you'd consider a triangular distribution of stress. So, it'd run from zero at the ground surface to 600 psf at the depth of 10 ft. The total vector being 3,000 lbs/lf of wall.

In shoring design, Terzaghi concluded that the actual earth pressures for such design are basically 30 percent greater and distributed as a rectangle (trapazoid for clays). You can check out his textbook for more details, but I think NAVFAC DM-7 have these illustrations also.

I always use a surcharge for construction. A construction surcharge adds another rectangle to the stress v. depth graph.

f-d

¡papá gordo ain’t no madre flaca!

 

[pelelo]

fatdad,

Thanks for your reply.

In you example, that would be the minimum earth pressure recomended for your design (3000 lb/ft of wall). Is there any reference (book, paper, manual) that support your answer?.

I do have Terzagui´s textbook. All my earth pressures values were computed using Tezargui´s approach .

Thanks a lot.

 

[msquared48]

For the surcharge, just extend the pressure prism up two feet above the level of earth you are retaining.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[fattdad]

For the surcharge, just extend the pressure prism up two feet above the level of earth you are retaining.

Folks do this for engineering design of finished structures. For temporary shoring design, where the horizontal stress is converted to a rectangle, it just doesn't work. If you want to account for a 250 psf construction surcharge, that'd equate to a rectangle of 160 psf over the entire height of the wall (i.e., 30 percent greater than active earth pressure, my number is for Ka=0.5, which is likely too high, just for illustration).

Back to the OP. If you have Terzaghi's book that's the only reference you need. Ultimately, you need to know the position of the water table (please for construction work include a dewatering spec), the unit weight, the friction angle and all the other stuff as required by the Terzaghi formula.

f-d

¡papá gordo ain’t no madre flaca!

 

[FixedEarth]

Pelelo- From your writings I gather you need a refresher on earth pressures. My suggestion would be if there is a shoring designer, just give him the soils friction angle, F.S. to apply to passive resistance (1.33 to 1.50 is typical), soil's unit weight, applicable surcharges & they will take it from there. If you can work with a senior engineer then he can take a look at your calculations and advise accordingly. You just don't want to be in a position that you gave earth pressure values that were not suitable for a shoring project.

For your future reference, you can get the theory and solved examples from this recent book:

http://www.amazon.co.uk/Engineering...=sr_1_4?s=books&ie=UTF8&qid=1343084695&sr=1-4

 

[FixedEarth]

This manual should help:

https://www.dot.ny.gov/divisions/en...ces/technical-services-repository/GDP-11b.pdf

 

[dcarr82775]

My understanding was that the apparent pressure diagrams include a construction surcharge by the nature of their derivation. They are based on the loads measured in struts for cut/cover tunnels in major cities. Can't have that type of condition without a respectable surcharge.