Peat Ground Improements - Site Class E to C?

[ocgeo]

Hi, we have a site with 30 feet of peat over 70 feet of 100 blow count till.

We are at a Site Class E due to the peat with a thickness of 10 feet and M%>40; PI>20, Su<500psf per the code.

However when we install grouted rock piles through the peat on a 10' x 10' grid, and no longer have a continuous layer of M%>40; PI>20, Su<500psf soil, we can increase the Site Class to a C correct? As the average blow count will be about 70.

Anyone have experience or can offer any guidance on this?

Thank you

 

[BridgeSmith]

As it was explained to me, the site class is related to the 'stiffness' of the soil mass for the vicinity of the foundation; an estimation of the displacement response to an earthquake, i.e. will the ground move a little or a lot during a seismic event of a particular magnitude? The addition of the site classification to the design provisions was primarily a response to the devastating Mexico City earthquake, where a moderate quake did major damage in the city because it sits on mainly silt. The same energy that moved the massive rock near the coast only a tiny amount, moved the silt under Mexico City alot.

Based on that understanding, I would say whatever foundation you put in does not change the site classification.

 

[ocgeo]

That is interesting, I think also the weaker soil has a longer period which if it is similar to the building will cause a lot of damage. I thought also based on the stiffness comment that the ground improvements stiffen the soil, however I am not sure how or if you can quantify that. Thanks for the comments

 

[BridgeSmith]

The ground improvements stiffen the soil locally, within the foundation limits, but the site classification accounts for the response of material in the broad area around the outside of the foundation. If the foundation was a solid mass of concrete, instead of a group of discrete shafts, you could not use the properties of the concrete in your site classification. As long as there is peat around the foundation, it has to be considered in the site classification, because that is what will determine the magnitude of the displacement and forces on the foundation.

If material is to be permanently removed, you can consider the average of the material in the top 100' from the new finished grade.

 

[EireChch]

Ground improvement is commonly used to improve the site class. Geopiers, stone columns, soil mixing etc are various methods used. Pre and post testing is done to confirm the level of improvement. CPTs, SPTs or shearwave velocity testing is undertaken.

You have essentially bypassed the peat with the grouted rock pile (what are those by the way?) and offered no improvement unfortunately.

HotRod - if they did replace the 30ft of peat with concrete they could use it to reclassify the site. They have improved the ground immensely. I would not recommend concrete but 30ft of compacted gravel is a great improvement

On a side note I would check your piles lateral capacity. You have zero lateral support in the peat.

 

[JAE]

The ground improvements stiffen the soil locally, within the foundation limits, but the site classification accounts for the response of material in the broad area around the outside of the foundation.

Ground improvement is commonly used to improve the site class.

The site soil shall be classified in accordance with Table 20.3-1 and Section 20.3 based on the upper 100 ft. of the site profile.

So we have ASCE using the word "site" (without any definition in Chapter 1) which seems to imply the local site itself, and not the surrounding "region". I've always perceived the idea of Site Class to be the region, per HotRod10's response as it makes sense that a column of crushed rock under the building, say in downtown Mexico City, isn't going to mitigate that bowl of Jello that the surrounding region is made up of.

But per the strict language of ASCE 7 it appears to refer to only the "site" (vs. the region). There's also no commentary for Chapter 20 which might shed light on it.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[ocgeo]

Thanks for the responses, ideas, and clarifications. EireChch, The piles planned are a mixture of aggregate and grout. A hollow steel casing is inserted to depth, displacing the peat, with aggregate and grout placed and compacted in lifts to the surface, creating 24 inch columns.

We plan to obtain lateral from a 12 inch aggregate base layer over all of the piers, with some interior (seismic) footings, and perimeter footings.

For testing, I think SPT or CPT strength tests after in the peat between the 10 foot spacing of piles will offer little to no improvement. The issue is getting out of the E category as some peat will remain, however, we will have installed this array of piers, so in essence the peat is not longer continuous, and not an E category, does this seem reasonable? Then with the 70 feet of 100 blow + till, I would think a shear wave velocity would be very high and in the C category, agree?

JAE/Hotrod, The soil that impacts the seismic response of the building, that is difficult, but even if its defined as 'site' that is outside the building area, which would require the entire site to be improved. And for this building, two of the property lines only have 5 foot setbacks.

 

[BridgeSmith]

JAE, my understanding of the term 'site' is that it includes the local area around the structure, not just the footprint of the foundation. The AASHTO spec makes this statement in Section 2.4.2:

"Current topography of the bridge site shall be established via contour maps and photographs. Such studies shall include the history of the site in terms of movement of earth masses, soil and rock erosion, and meandering of waterways."

And there's this from the commentary, Section C3.10.3:

"The behavior of a bridge during an earthquake is strongly related to the soil conditions at the site. Soils can amplify ground motions in the underlying rock, sometimes by factors of two or more. The extent of this amplification is dependent on the profile of soil types at the site and the intensity of shaking in the rock below. Sites are classified by type and profile for the purpose of defining the overall seismic hazard, which is quantified as the product of the soil amplification and the intensity of shaking in the underlying rock."

If the ground improvement extends beyond the foundation, such that it would resist the forces from that "bowl of Jello" on its own, so that those forces do not reach the foundation, then possibly you could change the site classification. I would be hesitant to do it even then, without a site-specific response analysis.

 

[BridgeSmith]

ocgeo, some of the devastated buildings in Mexico City had closely-spaced drilled shafts to a depth of more than 100'. It didn't help them. The effect of an earthquake on the foundation is due as much or more to the soil around the foundation as what's under it.