height limits for different systems? 2

[koo]

Hi,

I'm doing a prelim. design for a undergrade roadway, 600' long. I am considering different options for the earth retaining systems. The largest grade difference is about 40' in the mid section. MSE, pile and lagging, anchored sheetpiles are all under consideraton, but I'm not sure what their economical height limits are. Look for some advice here.

BTW, is there a MSE or panel wall system that allows use of anchors without excavation like anchored sheetpiles?

Thanks.

Koo

 

[DRC1]

MSE walls and anchored walls can be quite high. Achored walls have been used for cuts exceeding 60 feet. Anchor spacing is a function of many variables, but for preliminary analysis, assume a level of anchors every 8 feet. Cantilevered walls (H-Pile or sheets) have a practcal limit of 12 to 15 feet, again the actual value depends on several varibles. I don't know of any MSE walls with anchors (does not mean there are not any). One option is to use soldier piles and lagging with permanent anchors secured directly to each pile. (assuming an anchored system) Then pour a concrete fascia wall, using the H-piles and lagging as a back form.

Good luck!

 

[Focht3]

Hmmm,

A 60 foot cut is pretty damn tall. Given the likely construction constraints, you're probably looking at a drilled shaft or cast-in-place panel wall. These allow the temporary wall to act as the permanent wall, too. It's probably too tall for soldier pile and lagging or steel sheet piling.

While you may need a level of anchors every 8 feet, that's going to be terribly expensive over a 60 foot height. (That's 6 or 7 rows of anchors!) The anchor spacing will be dependent on the strength and stiffness of the wall. Stronger, stiffer walls need fewer (but perhaps stronger) anchors. Frankly, I'd be surprised if you needed more than three rows of anchors.

And cantilever walls can be as tall as 22 feet - but not with soldier piles and lagging. These taller cantilever walls are built with drilled shafts -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[BigH]

Focht3 has good points in that if you are trying a conventional wall in a cut, you will have a major cut just to get to the point that you can construct the wall and then backfill.

There was a method used in the Stuttgardt Subway system years ago that will permit you to build the wall as you excavate. Take precast concrete "footings" - about 1m by 1m. Then starting at the top, excavate down 2m or so - can usually be done "near vertical". Then place the "footing" vertically on the slope at, say 2.5m spacing, and use a soil nail (soil anchor) and install through the centre. Tension the anchor after installation. Then go to the next level of excavation - stagger the "footings". Basically you are putting in "footings" with the tie-backs acting to provide the compressive force to the soil. We were going to use this on the Coquihalla Highway in British Columbia until the alignment was changed but a similar system was used in Revelstoke.

 

[dpa]

Hi BigH,

This is the second thread today for which I have given you a star. Keep up the good work.

DPA

 

[DRC1]

If your excavation is dry, you may want to consider soil nailing, which involves excavating a 5 ft cut, placing mesh and shotcreting, then installing soil nails. Selection of earth support is usually based on many criteria other than height, such as soil type water table and anticipated driving conditons.