DCP to Allowable Bearing Pressure 1

[80smetalfm]

Im new to this forum so excuse my question if someone has asked the same.
I understand DCP results undertaken for a simple building structure for example a 3 storey RC building can be deduced to obtain Allowable Bearing Capacity (kPa). Can someone tell me exactly how this can be obtained. What formulars exactly would i Use? eg. How do i calculate the modulus or compressibility of the material tested at subgrade?

 

[garrettk]

I'm not sure I speak for all, but I never use DCP results as a means of "determining" a bearing capacity. I will use DCP results to "confirm" soil conditions which are similar to those encountered during my initial explorations with SPT data.

Depending on the technician and the soil conditions, you may only get as much as 3 to 5 feet below grade with a DCP. Your "simple" 3 storing buildings will likely tranfer loads much deeper than that below the bottom of the footing. In my opnion, DCP's should only be used to confirm that conditions are similar to your initial exploration.

I believe there is an ASTM circular (not an ASTM Standard) that correlates DCP blow counts to SPT blow counts, but this should be used with caution. I've found that different regions (geologic conditions) require different charts to provide comparisons.

 

[BigH]

also - as has been said many times on various other threads - you need to define DCP. It means different things to different folks - and in different areas of the country, North America and the world. For me, it is driving a 2 inch 60deg cone on the end of A-rods using a 140pound hammer dropping 30 inches - but this isn't the definition if you are a Sowers-man, I understand.

 

[Ron]

BigH...good point. The Sowers DCP is a hand-held device with a 15-lb weight. It's a real finger smasher!...but reliable when site-specific or soil-specific correlations are done.

 

[80smetalfm]

thanks everyone for your help.
The DCP i was talking about weighs 9kg (Australian std) and does require abit of muscle and patience to operate! To thank you all, i would like to share with you how a "careful" and "indirect" approach to converting DCP to Allowable Bearing pressure(ABC) can be attained. This is what ive used over the last 4 years.

Steps as follows:

1) Get DCP readings from site to below founding level
2) Present them in 300mm increments
3) Correlate DCP readings to SPT values by using acceptable standards. The Australian code has this correlation. NZ also does have it.
4) Guess a pad or strip footing width to be used.(i always use 1m and work backwards during design stage)
5) Use "Peck & Hanson & Thornburn, 1974" chart and
6) Presto you have an allowable bearing pressure!!

Conditions of use:
- DCP experience and feel for material required
- Know your codes well
- Apply to shallow foundations only
- Always remember ABC is different for different widths!!
- ABC is not the same as Ult bearing pressure
- have the muscles to do the DCP will help!!
- Note, Strip footings can produce greater settelment for the same width and applied pressure hence ABC should be reduced by 20% for strip foundations.

A spreadsheet over the years of DCP vs ABC for different materials is always helpful!

 

[Mccoy]

From the sound of it, it reminds to me the "light" dynamic penetrometer some colleagues use here in Italy.
There are recurrent disputes over the use of such tools, but I tend to agree with garretk, it's only to be used as a qualitative tool, its ureliability is strong, and the dataset is very much subject to outliers from rods-to-hole friction.
To be used only when necessary, in otherwise unaccessible areas, and so on, or to give a qualitative estimate of soil strenght only at shallow depths...

 

[oldestguy]

I have the same question, since all I can find on the subject refers to checking the density of roadway base courses and then relating it to CBR. Then penetration is much less than a meter.

With a 20 mm diameter probe (which is standard in some areas), I suspect that any coarse gravel will affect the result.

This probably falls into the same subject as using a pocket penetrometer to measure unconfined compressive strength. Much can affect it. However as a rough tool it works, when you consider what affects it.

 

[BigH]

oldestguy - you should check out the TRRL of UK. They have a manual on the use of the miniature dynamic cone penetrometer that is used for roadways. But, don't get caught up with "precision" when using it. I know of a firm in India that got the knuckles rapped for using this "only". You are correct that gravelly soils, like with any dynamic test (SPT, pentest) etc. would effect the blow counts. The miniature one is best for clayey type subgrades, not for engineered structural pavement layers. Also, you need to know if the correlation is to soaked or unsoaked CBRs. Important in monsoon type climates.

 

[soiledup]

The latest Quaterly Journal of Engineering Geology (QJEG) has an article regarding 'The use of a Mackintsoh Probe in site investigations in soft soils'. The Mac Probe sounds pretty similar to the instrument that you are describing.
Follow this link to find the document abstract:

http://www.ingentaconnect.com/content/geol/qjeg/2006/00000039/00000002/art00005

Again, as stated above, I've never based calculations around this alone. Always just infilling data spots between the more empirical insitu testing methods.

Regards

Soiledup

 

[eric1037]

oldestguy:

It sounds like you are referring ot the US Army Corps of Engineers DCP. The FHWA and many state DOTs have numerous studies on this device. It is used almost exclusively for pavement design applications.