Depth of Ground Improvement (Remove and Replace) For Wall Bearing Capacity

[Alexander87]

Hey Guys,

I'm looking at construction of a new MSE wall to support a proposed roadway embankment, so far I've determined that I need to remove and replace existing soft clay soils to satisfy bearing requirements for the wall, however the depth of this clay is on the order of 15 feet before we hit medium-dense sand and we do not want to propose removing the entire clay stratum if it isn't necessary. Unfortunately the depth of my experience as a junior engineer is limited to hand-waving for small spread footings, where engineering judgement allowed us to assume a general R&R on the order of 5 feet was appropriate.

I was thinking if we use pressure bulbs to assume a depth where new stress is on the order of 10% of current stresses then that would be OK, but with such a large embankment width that would essentially be the entire layer. Is that thinking too conservative?

Thanks!

 

[oldestguy]

You need to make a global stability analysis for the whole system, walls and its backfill as well as the embankment to be built in addition to any settlement chck. It is unlikely that any "bulb of pressure" study would apply. If anything for the settlement analysis part, the best you might do is look at some minor reduction of applied pressure out in front of the wall with a sloping edge to the pressures applied but as an estimate.

 

[wwen]

Hey oldestguy,

thanks for the reply - I have actually evaluated the wall for global stability, external stability and total/differential settlement. The only thing generating a CDR <1 is the bearing calc (per AASHTO LRFD), so we understand ground improvement is required to satisfy but are trying to quantify the extents that are required..

 

[fattdad]

something doesn't make sense. Bearing capacity relates to loading on a flat surface and the corresponding field of shear in the soils below. Global stability accomplishes the same thing!

So, I'm confused how you can have successful design in global stability and unsuccessful design in bearing capacity?

Bearing capacity calculations include the size of the loaded area. Are you considering the areal extent of the entire reinforced zone of the MSE?

Not sure I'm helping, but I'm interested in your problem.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Mad Mike]

Get an experienced geotechnical engineer involved- for 15 feet of clay that's causing trouble, it would be preferable to stabilize the system using crushed rock, whether vertical pads or a blanket; probably with a high strength geo-textile thrown in somewhere.

Without having much info, my gut feel is that 5 feet would usually suffice; but then as fattdad notes, it's a concern that your stability checks are okay while your bearing capacity is problematic. Re-check those stability analyses...don't be relying on soil cohesion, which will be lost in the long term. Throw in some pore pressures in the clay just to be sure...

All the best,
Mike

 

[Alexander87]

Hey Guys,

Thanks for the input, here are my replies:

fatdad - We have been checking bearing using Terzaghi's as per AASHTO (10.6.3.1.2), with B as the effective width of wall, what you guys are saying about concerns that global is working but bearing isn't makes alot of sense to me. We are using AASHTO Strength 1 state for highest factored bearing pressure and the determination of V1/V2/V3 values (self weight, aep, LL etc) appears to be correct to me. Terzaghi calc and resistance factor for qn to qr seems to look good to me as well!

Mad Mike - crushed rock/gravel is actually the replacement material we are going to be implementing. I think we are using fairly conservative values for the clay for both short and long term parameters that are based both on insitu testing (SPT's) and lab testing (UCS tests). The drained friction is based on PI correlations but we are using a conservative value that is probably 2-3 degrees lower than correlations. Also just FYI I double checked the parameters we are using between our bearing and our global stability analyses and they are the same between the two.

 

[Alexander87]

Just further to this, I am using a paper "Bearing Capacity of Footings over Two-Layer Foundation Soils" by Radoslaw, M. dated 1995 that presents an alternative method that should allow us to determine critical depth of ground improvement if I am reading it correctly.

 

[oldestguy]

It would help a lot if you provide a cross section view of the situation. On that show the wall and its reinforcement, and where you have new fill back of the wall system. Designate where stability checks show a safety factor of ? amount. Show your soil profile and the strength values.

 

[Alexander87]

OK Thanks oldestguy, I'll try to get that out today for everyone to see. One important update to this is that I realized during the iterations that replacement of insitu clays IS actually necessary to satisfy short term global stability!

 

[fattdad]

(surcharging can also address undrained strength and related stability. That is if you have the space and time!)

f-d

ípapß gordo ainÆt no madre flaca!

 

[EireChch]

As OG said, give us a cross section showing the MSE wall, its height, geotextile length, ground conditions with parameters assumed.

That will get you more valuable answers.

Stone columns may also be a possible option. There have been papers that have shown SC to be an economical option when you need to excavate more than 2.5m.