very low SPT 2

[geo321]

Hello,

Currently working on a site where they are constructing new houses composed of 2 floors and small spans.
A SPT test was submitted for our review. The N value is so small (N=4) which send us in a very loose silty sand.
The assumed bearing capacity  is 8T/m2 according to the general notes.
Based on the spt, i made some calculations and got a bearing pressure of 12 T/m2.
Since the allowable is less than the assumed bearing, should i worry about something ?
Thank you

 

[jgailla]

Serviceability (i.e. settlement) usually controls allowable bearing pressure, not the soil capacity.
The soil capacity at failure is much higher than the allowable bearing pressure based on settlement.
A very loose silty sand will be hard to construct anything on. You should be looking at some kind of soil remediation.
How deep did you do the SPT? N=4 at 10 feet is probably fine for a mid-cost residence.

 

[geomane]

What are the loading conditions and the tolerable total and differential settlements? I assume they may be light due to being a residential structure. Also, what depth is the groundwater table?

If they are light loads, you may be able to improve the soils to a depth 1 to 3 feet beneath the bottom of the proposed footings to 95% of Modified Proctor Max Dry Density by utilizing compaction equipment.

 

[geo321]

N=4 at 4 feet.
I have compacted the soil at 95% of the modified proctor dry density to minimize the settlement.
Regarding the tolerable settlement, the drawings doesnt mention it. It just mentions that we should make sure that the soil has a bearing capactiy higher than 8 t/m2.

 

[geomane]

Sounds like the allowable bearing pressure you were given is based on the building code.

Run a settlement analysis and determine if the soils can support the houses with the improvement you have performed.

 

[oldestguy]

If the unit T is a metric ton equivalent to about 2,200 English pounds, this is a light loading. Is your sand a uniform gradation? If so, a low blow count is common, yet the soil is only moderately compressible, meaning little settlement expected. If you try compaction, you are not likely to change anything. Consider a bucket full of marbles, shake it and try to densify it, no luck.

 

[geo321]

Thank you oldestguy and yes T refer to tons

 

[geojosh84]

I'd recommend schmertmann's method outlined in NAVFAC 7.1 to determine settlement based on your allowable bearing pressure. Typically 1/2 to 1 inch are acceptable settlements. Also, I'd recommend a SM with a PI less than 15.

 

[connect2]

Watch for grade raises around the homes and the site in general.

 

[BigH]

Are you in an earthquake prone area? If so you may need to consider liquefaction. Secondly, only one SPT value? Seems strange. We would take SPT tests as part of split spoon sampling at 0, 2.5ft, 5ft, 7.5ft, 10ft, 12.5ft, 15ft, 20 ft etc for normal routine jobs. Why? Many times you do not necessarily know at what depth the footing would be placed (frost depth, basement). So you should had a lot more information than a single "N" value.

 

[bdbd]

geo321,

I understand your question and I will give you most clear answer. These calculations are for 25x25 foundation with groundwater level at 1 and depth of foundation 0. These are all for 25mm allowable tolerated settlement!!! If you don't know the settlement criteria, for a 2 story building, 50mm will be okay. So, when you calculate the bearing capacity with these formulas, multiply it with 2 which is 50mm/25mm. (If your tolerated settlement is 75 mm than you would have to multiply it with 3 which is 75/25mm)

 

[oldestguy]

Sincere: We meet again. When offering advice to those asking, it would be helpful to define each term in any formula provided. For instance, define SPT and N in the first formula. I'd say N there is usually referred to by geotechs as the standard penetration test, or STP. Subtract N from STP and what is the result?

Most residential structures have significant loading on continuous footings. Is your B that width? or is B the width of a square footing?

 

[bdbd]

Oldest guy,

You are definetely right. I am sorry.

All N values defined above are SPT-N1_60. Watch out the 1 in the N. That is for overburden correction we do on SANDS. You have to make it. 60 is for %60 energy correction, depending on your country it may already be 60%. So check out the ground investigation report to see any sign, or ask it to the related person.

B is the smallest dimension of your footing. 30x60 raft's B is 30. In case circular foundation, simply use B=square root (pi*R^2)

Dw is depth of water, Df is depth of foundation.

 

[oldestguy]

More comment by OG. The original value of N or Standard Penetration Test considered the way the value was obtained. A spinning cat-head (capstan) with one wrap of a rope hauled up a 140 pound weight and dropped it 30 inches, with number of blows in 12 inches of penetration counted. More modern methods are more efficient and so the blow count is lower than by the original method. Most of the formulas from some years back were based on the original methods. Depending on how the method was done, you may or may not need the correction.

A quick search for bearing capacity methods and formulas assuming a given degree of settlement of a footing was made on Google and some 21 different papers are discussed by this very complete summary of the subject. Some of them are shown in SY's ist above.

http://ece.umass.edu/sites/default/...-of-Shallow-Foundations-on-Granular-Soils.pdf

I think one has to evaluate whether or not his footings are rectangular, near square or long when using any of these. I'd think most are for rectangular, in which case settlement is likely less than for the same width long footings. You don't just use that that width (B) in any such determination.

Finally, with several places where width is used in the formula, or other awkward use, likely it is necessary for you to develop a chart relating loads (column, or load per unit length) versus width of footing, with lines of equal settlement then plotted. The use of that chart by the footing design engineer then avoids them making any calculations using any of the many methods available. Where ever you can find that the method was used was field checked to see how well it works, then that might be more useful than blindly adopting any method. In the case of the Hough original charts, a study by California DOT with some of their jobs showed it was quite accurate, unless the water table was shallow, in which case it under-predictged settlement. From what I know, I don[t think one can automatically apply any of these to a mat or raft foundation. In that case get samples and get actual compression related data and run the calculations.

Don't forget to have grain size tests done for sand, because uniform graded sand N values are very misleadingly low.

 

[bdbd]

geo321,

OG will never know his audience, either a professional geotechnical engineer or a structural engineer, or anyone who does not any interest on it. I think it is an important skill to know which audience you are talking to, so you can adjust your speech's level of complexity.

Use the formulas, it will be enough for you.

 

[GeoPaveTraffic]

Sincerely,

I completely disagree with your approach. It is NEVER enough to just "use the formulas". If you use formulas without understanding how they were developed and the limitations of the formulas you are destined to have problems. If you are lucky, you will just cost your company a lot of money, if you are unlucky you will end up killing someone.



Mike Lambert

 

[bdbd]

GeoPaveTraffic,

I know the theory behind these formulas. The person who asked the question is not a geotechnical engineer. He is the constructor or something like that, and he reviews the geotechnical report. I DIDN'T tell to use these formulas in every structure. We are answering the questions based on the cases.

If my house will be built with a 2 storey on a ground like that, I am completely OK with the results. If my skyscrapper will be built on a ground like this, NO I AM NOT OKAY. Because, it will need further investigation, detailed analysis with hand and FEM etc.

What I am telling is OG and now you do not judge the audience to listen the story. That's all.

 

[oldestguy]

Since: Thanks for your comment. Audiences here are assumed to be learners, not experts. For instance, it would help us if you answered all my questions of you a while back on your pile supported raft question. Your question was a basic one that all experienced geotechs should know well. In order to be of most assistance it would be helpful to have you explain your status, learner, intermediate , or old timer with experience.

 

[oldestguy]

Sincere: Here is some of your earlier post with settlement calculations.

"These calculations are for 25x25 foundation with groundwater level at "

Since the OP will be building 2 story houses with small spans, likely light loads, why would an experienced engineer provide calculations for individual foundations 25 meters wide for a house? Your admonishment to me was to think about who you are advising. Where in the housing industry are there house foundations 25 meters wide. In my area of the USA we are darn unfortunate to have those foundations even close to one meter wide. It's time to count to 10 before advising here please.

Oh, yes, you can go back and delete it.

 

[bdbd]

Oldestguy,

You are a very interesting person. I have made EXAMPLE calculations, and GAVE the formulas to him. What should I do more?

My question about piled raft is not a basic one, but you start telling it from the most basic part, and you tease afterwards like this. That question is about compensated piled rafts and there are 2-3 papers about it, by Poulos and Sales.

Anyway, I am really tired of dealing with you here. I hope people can get their answer here. But I am done with this forum. You can go ahead and dominate it with SPT corrections.