liquefaction susceptibility of a soil

[aayjaber]

I am developing a property where part is sloping up by 40% with rock outcroppings showing; the rest is flat and is a small part of the property, on the surface it is a mixture of dark soil and stones of all sizes. On the published California maps of liquefaction hazard the flat area is labeled as liquefaction risk.

I dug a couple of trenches in the area and in both I encountered very dense soil mixed with stones, the soil is dark kind. As I reached the 2 ft depth it was very hard to dug much even by the sharp ax pick I am using. As I went deeper the stones started to get bigger and bigger. The stones I am digging out of the trench as of many sizes. Overnight I thought may be I should fill the trench with water to help me go deeper, when I went to work in the morning I found almost all the water I poured in the trench sitting there nice and clear.

What can be read from the observation? Is the liquefaction hazard just speculative and the real data will confirm or reject the hazard assumption?

 

[fattdad]

Liquifaction occurs (typically) in the upper 40 ft of the soil profile under the conditions of:

Granular soils below the water table
Granular soils above their critical void ratio
Sufficient earthquake magnitude.

You obviously have soils below the water table.
You have granular soil.
You don't provide information on the density of the granular soil.
Not sure about the design earthquake.

A resounding maybe!

f-d

¡papá gordo ain’t no madre flaca!

 

[CarlB]

To me the risk sounds very minimal.

You do mention density, noting it is hard to excavate, "very dense".

The trench did not necessarily extend into the water table, the water you mention is water you poured in yourself. It could be a very "tight" soil.

Regarding your question, yes the mapped hazard is speculative (generalized), need real data to confirm or reject. The data you present indicates (to me) the risk is low, but you probably need borings to 30' to further assess.

 

[msucog]

have a geotech specifically look at the issue. as fd notes, it's the upper 40 feet or so that really matters. with the high accelerations in california, the concern is much more "real" than say here where i'm at where seismicity is pretty much non-existent in my state. it can be hard at the top and crappy below. there again, it may be absolutely okay but that's impossible to tell without evaluating it.

here's an example. there are others available

http://earthquake.usgs.gov/regional/nca/alameda/

 

[cvg]

you absolutely need to be within the water table, regardless of the quality of your soils. From your post, it doesn't sound like you are, especially since you mention exposed bedrock. Check with CDWR or with your geotech on depths to groundwater in your area.

 

[dgillette]

From your brief description of the site, there is a good chance you don't have the right geologic environment for liquefiable material to exist. However, it looks to me like you need to bite the bullet and get a drill rig out there. The building code enforcement folks will probably insist on hard data (SPT, depth to water table), not just geologic interpretation.

They most certainly won't accept "These guys I talked to over the internet but have never met didn't think it would be a problem."

 

[aayjaber]

Drilling will happen tomorrow.

Want to ask a question about drilling. What if we encounter rock at 7ft to 12 ft where the drill bit cannot go any further. Can we stop and say we hit a solid bedrock then liquefaction is not a risk or do we have to bring a different kind of drill to go deep to the 30 ft to 40 ft to shed a clear light on liquefaction?

Thanks

 

[moe333]

If you have some way to confirm that the rock you are drilling into is actual bedrock and not a cobble/boulder within a soil deposit, then you shouldn't have to drill deeper for the liquefaction evaluation.

 

[dgillette]

If you've really hit bedrock, then you've arrived. It is possible, at least in theory, for the rig to refuse on a cobble or boulder overlying liquefiable material. The geologist or drillers may have a feel for the local conditions enough to say whether that's plausible in your location. They may need to core a few feet into the rock to be sure it's bedrock and not a floating boulder. I would defer to a local geologist.

 

[msucog]

you could always do some geophysical work to compliment the drilling. with the geophysical work, you could evaluate the rock levels/layering and also liquefaction with the same data. you would have to be selective of the geophysical method though since some cannot identify "velocity shifts" in the profile.

 

[msucog]

or the additional work may not be necessary if the geotech has enough data to evaluate the scenario...i suppose it all boils down to "engineering judgement".

 

[dgillette]

msucog - I'd be hesitant to use any surface geophysics there, even though Ken Stokoe (U Texas) says he can pick up low-SWV material below high-SWV material using Spectral Analysis of Surface Waves. It's harder to apply on a steeply sloping site like aajayber has, and besides, it's probably a lot quicker and cheaper just to drill, for anything but a very large site (and even there you would need confirmation drilling). A simple surface refraction survey cannot detect low-velocity material under faster material.

 

[msucog]

you are correct...refraction is limited in its capabilities. however refraction microtremor has it's applications and can help identify velocity shifts. i've had very good results on things like land fills when compared to boring data using remi. it does take some "experience" and understanding of the methodology to get realistic results (at least that's my opinion). others may have different opinions. i've correctly identified velocity shifts on several occassions where we performed drilling too within say about 5-10% of the profile depth (with 5' at a 100' depth or within 2' at 25'). i could also see where remi might not provide highly confident results...which sort of goes back to my experience and methodology statement.

 

[dgillette]

I don't know anything about remi. Where can I get a Reader's Digest description?

 

[cvg]

again, before drilling make sure the normal water table is less than 40 feet bgs, otherwise you would be wasting your time. I would think that a local geologist could very easily confirm approximate depth to water table before taking the rig out.

 

[msucog]

dg, sure thing...i'll put another plug in for the guys over at optim. i've mentioned their name a few times on here. also search "john louie" over at the UNR.

http://www.optimsoftware.com/white_papers/

 

[BigH]

Coring a few feet "into" rock may not be enough. Our company's policy to drill 3 m (10 ft) to "prove" bedrock. I know of one company that drilled 5 ft, said it was bedrock - and it was a ledge limestone with soil beneath. If coring, ensure you have sufficient core depth.

 

[aayjaber]

Drilling went well. Here are the results.
Top 5 ft was as expected tough mixture of stone and top soil.
Then we hit very soft sand for a layer which I believe is almost 7 ft thick.
Then we hit resistance and when looked at the sample it was a pebble like material, like the round shaped little stones that have been running in a river for millions of years. This layer was 3 ft thick
Then came the rock at depth 15 ft. Looks like the serpentine rock is highly weathered for at least two ft when it was exposed in its geological days. we went drilled for 5 ft in the rock, then we added another 5 ft to make sure we were not hitting a local boulder.

In the area people report drilling up to 50 ft to hit rock, I guess I was lucky, this is understandable since my property is far from what geologically could have been a river in past times and at the edge of a hill side.

The above was for the flat part of the property, the next we attempted to drill in few locations on the slop where the house will be sitting in part. The maximum we were able to drill without massive resistance was 7 ft. I also made trench like excavation ranged between 2 to 4 ft to expose the rock; the geologist appreciated it because it was revealing about the rock formation we are dealing with.

 

[fattdad]

For further insight from this group, we now need to know the "SPT N-value" of the "soft sand" and the depth to ground water. Hopefully, you left the boring open long enough to record a stabilized water table elevation?

f-d

¡papá gordo ain’t no madre flaca!