Preliminary design values for soil strength

[renko]

Hello,

Can anyone direct me to some 'standard' or preliminary values for characterstic soil strengths - e.g. undrained shear strength - for different soil classes - CLAY, SILT, SAND etc.

 

[GeoPaveTraffic]

There is no such thing as soil strenths for "soil classes". You can have a hard CLAY or a very soft CLAY. You can have a very dense SAND or a very loose SAND.

An engineer with experience in a particular geographic area MAY be able to provide an estimate of the soil properties in that geographic area based on other projects in the area, knowledge of the geology of the area, etc.

If we knew about the soil before drilling and laboratory testing, we wouldn't need to do that work.

 

[renko]

I expected I might annoy Geotechnical engineers with this vague question. I realise that undisturbed soils can exist in various states - stiff, soft, loose, dense etc.

Many Geotechs and Pile designers I work with base their preliminary design on some assumptions, prior to lab tests. I was hoping to find out what these assumptions are based on. But I suppose it is down to experience and knowledge of the local ground as you suggested.

Personally I'm all for drilling and testing.

 

[GeoPaveTraffic]

I wasn't really annoyed.

But as I said and as you commented, it really does come down to what you know about the area.

 

[fattdad]

Prior to laboratory testing (and for the case that you have drilling data) you can assign strength values using SPT N-values and pocket penetrometer data in conjunction with visual soil descriptions.

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[tails142]

Here are some details for you;

Sieve size mm
0.002-0.06 SILT
0.06-2.00 SAND
2.0-60 GRAVEL
60-200 COBBLES
200+ BOULDERS


Slightly sandy GRAVEL - up to 5% sand
Sandy GRAVEL - 5-20% sand
Very sandy GRAVEL - 20-50% sand
GRAVEL/SAND - equal proportions
Very gravelly SAND - 20-50% gravel
Slightly gravelly SAND - up to 5% gravel
Slightly silty SAND or GRAVEL - up to 5% silt
Silty SAND - 5-15% silt
Very silty SAND or GRAVEL - 15-35% silt
Slightly clayey SAND or GRAVEL- up to 5% clay
Clayey SAND or GRAVEL - 5-15% clay
Very clayey SAND or GRAVEL - 15-35% clay
Sandy SILT or CLAY - 35-65% sand
Very coarse - over 50% boulders and cobbles


Undrained shear strenghts kN/m3

Very stiff and hard clays - >150
Stiff clays 100-150
Firm to stiff clays 75-100
Firm clays 50-75
Soft to firm clays 40-50
Soft clays and silts 20-40
Very soft clays and silts 20


Safe bearing capacities kN/m2

Igneous rock - 5000
Limestones - 3000
Slates - 2000
Shales - 1000

Very stiff and hard clays - 300-600
Stiff clays - 150-300
Firm clays - 75-150
Soft clays and silts - less than 75
Very soft clays and silts - nil


Compact gravel and sandy gravel 600
Medium dense gravel and sandy gravel 200-600
Loose gravel and sandy gravel <200
Compact sand 300+

Medium dense sand 100-300
Loose sand <100
Firm organic 20-40

Unidentifiable made ground 25-50

Springy organic materials/peats - nil

Plastic organic material/peats - nil


Should be used with care; provided for academic reasons. Obviously best to get your soil tested to be sure after the prelim design stage.

 

[renko]

Thanks tails142,

These are the sort of preliminary values I've seen used. I know some can be taken from BS8004, particularly bearing capacity. Is there a good Soil Mechanics book I should refer to? (I have Craigs).

 

[BigH]

M.J. Tomlinson's Foundation Design and Construction (since you seem to be from the UK).

 

[renko]

Thanks BigH,
Ireland actually, but we run on the same lines as the UK.
I actually ordered M.J. Tomlinson last week as I have one of his books for pile design.