subbase material spec 3

[Aldin]

can six inch boulders be used as a subbase material

 

[GeoPaveTraffic]

Can cows fly?

Seriously, depending on the what the subase is for, what the subgrade is, and how they are placed, yes 6-inch rock can be used for a subase. Just as cows can be made to fly.

To get good/useful answers you need to provide more information. Also, 6-inch rock is not called a boulder. Boulders start at 8 or 12 inches depending on the classification system.

 

[Aldin]

many thanks geopavertraffic for the quick response
The project that i am in charge of is the reconstruction of one our major roads .I have decided to use the spec of AAshto for my subbase and base material,however a lot of the old times think that i should use larger stone six inches and more in my subbase .i do not believe this is needed due to the fact that i am following a know stanard spec

 

[Oson]

If the base is of 6" or less and the sub-base is 10" or less, I would use maximum 4" rocks on the sub base. Of course your guide will be the specifications grading requirements have to be provided for the sub-base. Also remember that probably you will be required to make density tests and 6" rocks will be on the way.

 

[Ron]

Aldin....using 6-inch cobbles for a subbase is not a good idea for several reasons. First, large aggregate pieces tend to point load the subgrade under pavements. With time, this causes punching problems into the subgrade. Secondly, ravelling of underlying and overlying smaller materials can more easily make their way into the larger interstitial voids created by such large material, thus allowing displacement.

 

[Aldin]

many thanks ron i think your response makes good first pricinple engineer sense.many thank also to oson for that insight into the specification and the need to do density
testing in the fact that the six inch might get into the way

 

[emmgjld]

Aldin
I agree with Ron's points and some of the other comments above.
I have witnessed many projects using cobble sized material, a commen aggregate source from the many rivers on the Western Slope of Colorado. The cobbles and gravels tend to be well rounded, tending to 'roll' under wheel loading.

I have been exposed to a number of 'theories' regarding testing and quality control. Testing can be difficult. I have found the real test is the 'probing' when trying to drive the stake for the Nuclear. Poorly graded material (non compactable) or poorly placed materials tend to easily roll aside from the driven stake.

About 15 years ago I began using the following criteria for subgrade improvement beneath many pavements and buildings. I apologize in advance for the log post. What I am presenting is what has worked for me, for the materials I commonly encounter and the conditions I work with. Go ahead and pick it apart.

The imported structural Fill (Hveem-Carmany R>50 , swell not critical) is to be Granular, Medium to Coarse Grained, Very low plastic (PI<4), Non Freedraining, Compactable and within the following Gradation:
Maximum size, by screening 8"
Passing the #4 screen 20% - 85%
Passing the #40 screen 10% - 60%
Passing the #200 screen 3% - 15%

The maximum aggregates size may be exceeded if the contractor can provide evidence of proper compaction of the matrix material while avoiding excessive particle size segregation of the fill material or avoiding excessive overworking of the subgrade soils.

Imported Structural Fill and Aggregate Base Course (ABC) to be compacted to 90% of its maximum Modified Proctor dry density (ASTM-D-1557) at a moisture content within + 2% of optimum moisture. The use of light weight tracked equipment will minimize subgrade degradation, vibratory compaction equipment is not recommended if ground water levels are close to the base of the fill.

During the placement of any structural fill, it is recommended that a sufficient amount of field tests and observation be performed under the direction of the Geotechnical Engineer. The Geotechnical Engineer should determine the amount of observation time and field density tests required to determine substantial conformance with these recommendations.

 

[BigH]

For subbase - I wouldn't use 6 inch cobbles. I have always found that when using larger sizes, you tend to get bad segregation during placement. It is nice that in specifications, they say to rework it, replace it - or whatever to rectifiy it - but what they are really saying is that you, as a contractor, need to rectify their mistake in specifying such large sizes. I would keep the maximum size down in the range of 75mm (3 inch). Even that can segregate if not well graded. Subbase courses should be constructed in maximum 8mm lifts, in my view, and the 3 inch max - and given some elongation and not perfectly rounded gravel or cubical crush - you should be okay.