Decomposed granite as road subbase

I've never seen or heard of DG being used as a roadway subbase or base course here in California.

My background is in geology. The term decomposed granite actually implies a degree of weathering. The rock has been broken down, either chemically or physically, or both. It is not the same as crushed granite. Moreover, granite is a generic term for a wide variety of fine to coarse-grained intrusive rocks. The degree of weathing is dependant upon many factors.

Your suspicions with respect to the mica is probably in the right direction. However, "granites" also contain other minerals (i.e. feldspars and trace minerals) that break down or weather into secondary clays.

Were abrasion tests run? It is my understanding that some dg's can physically break down under relatively light loads. What is the subgrade material along the alignment?

I note that the road is failing on the inner side of corners/curves?. What type of failures are occurring. I presume that this is now a paved road. I am not sure that your problem lies with the material in totality as I have used this material or similar material and have not noticed your problem. The addition of too much water to such materials can result in the material becoming somewhat soapy to the feel. Not sure if this may be resulting in pavement slippage which could manifest around inside of curves.

The above are guesses and better answers may be possible with further information as requested.

Interesting.

Thanks for the responses - this problem has all of our geotechnical teams guessing. I should have noted that the granite was a crushed material from a quarry we have used successfully on many previous projects - we visited the quarry to check the quality of the rock this material came from and there were no problems i.e. no signs of decomposed veins or variations in granite strength/structure.

The in-situ subgrade material comprises of compentent.

The road pavement is failing along the low points of the road, not just at the corners. The material passed all testing that we undertook, including abrasion tests. The prepared pavement structure further passed all in-situ testing and checks.

The material is sensitive to moisture and that was carefully addressed during construction - the slippage idea is interesting. There is a considerable amount of mica in this granite, put this concept with oversaturated conditions due to surface water ingress, could slippages occur at such a microscopic level that would as a whole cause the damage to the pavement structure under loaded conditions? It may be worth while undertaking direct shear testing under optimum and saturated conditions using the fine components of the failed material to see what the loss of shear strength is under different conditions.

hello folks, new in this forum but i must admit its quality-very enlightening! UK eng in oz at the moment, decomposed granite 'deco´is used a bit down under-seems to behave well if used at the correct stage of weathering. Its basically a low fines sa to a granular material that responds well to compactive techniques (in my albeit limited experience).

Clearly something i´snt working with your road though-you´ve probably gone through this chain of thought before but it may be better to approach the problem from a ´bewildered´angle. Maybe core the road, test the sub base and grade using appropriate methods, clearly define the issue. At least then you may be able to say with confidence ha)that deco is the problem. Be a shocker if advised deco was the problem, ripped it up and found 30mm of asphalt!

good luck

In our neck of the woods, the material called decomposed granite is one of the stages between hard rock and a highly plastic clay.

Have you taken up samples to see if it has somehow continued on the process between hard rock and is going toward the clay status?

I would encourage you to read a paper "Sudden Failure of a Heavily Loaded Container Pavement" by Brian Burman Vol 132, No 11 ASCE Geotech and Geoenvrionmental Journal. The Aussies look at degree of saturation of their bases since greater than 60 to 70 % have lead to failures in pavement structure. There may be some ideas to explore. This concept is rarely used in North America and I think there is some merit in its use since I have noted premature cracking of pavements on the onset of winter when bases and paving have been completed in late fall. We also tend to saturate bases for compaction. This could have implications in late fall work in NA or if poor drainability exists.

Would appreciate your findings on this project.

We have taken a failed sample of the granite subbase to a specialist geologist who is currently processing thin sections for us and then taking a look at it through an electron microsope.

We acquired another granite subbase material, one that we know has stood the test of time and we have had a closer look at that. There were significant differences that may have contributed to localised failure.

The findings so far are as follows:

1 The failed granite material is unweathered, very hard and the abundant mica particles are unweathered as well. There is very little kaolinite in the sample.
2 The failed granite underwent secondary crystalisation millions of years ago that resulted in the particles being orientated in the same direction, rather than the typical random orientation in the typical competent granite.

We are now leaning towards this scenario: the interparticle forces associated with a competent randomly distributed granite are equal in a loading situation while our failed granite in the same loading situation behaves differently as the interparticle forces are different due to re-orientation of the particles (mica, biotite, sand, etc.).

Consider the above scenario together with the ingress of surface runoff that oversaturates the subbase and one may get localised failures that we are seeing.

Any ideas are welcome? An interesting granite.

This secondary crystallization and reorientation of minerals in a preferred direction is very common, and is often a factor in both shallow and deep-seated failures within grantic rocks. Think metamorphism and rocks referred to as gneiss. However...

It would seem to me that the process of crushing the material, processing it, and placing it with mechanical compaction would render this a moot point. However, if one were to place lifts in the fill based on mineralogy or grain size (i.e. one lift composed primarily of micas, next lift composed of sand-sized particles, etc.), one would simulate what is often found in nature. Thus, you could artificially create the layered effect seen in rocks and this could lead to problems over the relatively short time span you are considering.

Do you have any pictures of the placed fill in cross-sectional view?

A test commonly conducted on aggregate base course is the LA Abrasion (placing the material in a tub and rolling it around with steel balls and seeing how much it degrades). In my local area, if the test results meet certain specs, along with several other specs (PI, sieve, etc.), then it can be labeled as aggregate base course. Have you ran a LA Abrasion test or something similar? Could moisture/pumping be the problem? The inner curbs is probably subjected to increased loading conditions due to the road inclination that shifts vehicle loads. How much has the life of the pavement been shortened?

What do the failures look like? Shallow sluffing (or sloughing if that's the preferred spelling in SA), or sliding on a sheared zone below the ground surface?

What moisture conditions are involved? Slides only in wet weather, or all the time? (Do you even have much wet weather in that part of SA?) Side slopes? No vegetative cover, right?

It seems to me that there must be a simpler option.
I believe you stated that your failure is occurring at the corners of your road. Is this true, or is it the same through the whole site? Failures in localized areas given the homogenaety of your granite, which sounds like it is produced at a quarry, would lead me to believe the problem is not in your fill, but possibly below or above that decomposed rock fill. Could it be that the corners of your road were not as compacted due to the turn radius of the roller? Or, if it is a wide spread problem, could also the problem lie in other materials, such as the asphalt mix or the native cut, given the tried-and-true nature of your granite? Have you exhausted these possibilities?