Low Volume road design

[nagatalluri]

Guys


I am designing a low volume road for haul roads in a wind project. For the design, is there any established correlation that can be used for estimating the CBR value from aggregate base material gradation?

What is the governing CBR for the low volume(haul road design), is it the CBR of the subgrade or CBR of aggregate material?

Please let me know.

Thanks
NT

 

[BigH]

The CBR of the subgrade is the crucial value as it will tell you the required thickness of overlying layers - A good crushed aggregate subbase and base could usually be taken as greater than 50 and typically greater than 80 - so the real deal is getting a good subgrade. And, a soaked CBR. There are correlations and these have been provided in the past on these sites. You can check out SlideRuleEra's web site and they are posted there. There are a number of good manuals on the web (for free) on unbound trafficked roads (or gravel roads as I would call them). You can check these out.

 

[nagatalluri]

Dear Sir

Thanks for your input.

NT

 

[BigH]

http://www.slideruleera.net/CBR_Correlations.pdf

 

[nagatalluri]

Dear Sir

Thanks for the reference. Can you please provide me with the reference for the soaked CBR to be used in the design. For low volume road design, I am using the CBR based equation for calculating the EBS (elastic modulus of base material).

The equation is EBS= 17.6 X (CBR)^0.64 EBS: in MPa

I am wondering if a soaked CBR or CBR at OMC needs to be used here.

Is there any reference for this.

Thanks for your time

NT

 

[BigH]

There is always, it seems, a laxity in identifying that the CBR value is soaked (or unsoaked). Soaked is more in line with the "worse" condition - i.e., thawing ground, monsoon rains, etc. ASTM D1883 implies: "Unless specified otherwise by the requesting agency, or unless it has been shown to have no effect on test results for the material being tested, all specimens shall be soaked prior to penetration." - so I would presume unless there was evidence to the otherwise that the CBR is soaked (unless it is field CBR - which would only apply to the moisture content at the time (season/month) of the test. How do you determine the EBS value? Is it a tangential or secant modulus. This is why one really needs to understand from where such equations come and the conditions/assumptions on which the equations were developed. Go back to the original source of the equation and see what restrictions/assumptions were used.

 

[nagatalluri]

Sir

Thanks for the response.

NT

 

[DougHole]

have you considered doing any Scala penetration tests? You can correlate a CBR from these quite easily

Doug Hole
Junior Geotechnical Engineer

 

[nagatalluri]

Hi Doug

Can you please provide reference for the "Scala penetration tests", I haven't heard of them before.

Thanks
NT

 

[DougHole]

Hi NT. Here is a link.

http://groundtest.co.nz/scala/scalapage1.php

These are used a lot in Australia/NZ. Might be called a DCPT (Dynamic Cone Penetration Test) if you're in the USA




Doug Hole
Junior Geotechnical Engineer

 

[BigH]

Similar, eh, to the TTRL's Miniature Penetration Test. Just as a matter of caution again, when using terminology like DCPT - be careful - there are many variations of the so called Dynamic Cone penetration test - some use light weights and some, like the pentest in Canada use a 65 kg weight dropping 760 mm - there is Sower's version as well - and, in Malaysia, the Mackintosh Probe as well as the JKR probe. There are numerous threads discussing the pros and cons of these types of tests and the variations in equipment.