Decomposed granite as road subbase
Decomposed granite as road subbase
(OP)
We have just experienced a problem with a subbase layer for a road pavement design. The material is decomposed granite and has been extensively used in roads in the past (well at least in South Africa). The material sails through all the test specifications (CBR, swell, grading, etc) and does not contain weathered components. The constructed road is failing on the inner side of corners and we suspect that there is a transferance of fine particles occurring or there may be some chemical reaction between the mica particles and the fine particles with the introduction of water. Has anyone else experienced something similiar with decomposed granite?? Your assistance and thoughts on the matter would be appreciated.





RE: Decomposed granite as road subbase
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?
RE: Decomposed granite as road subbase
The above are guesses and better answers may be possible with further information as requested.
Interesting.
RE: Decomposed granite as road subbase
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.
RE: Decomposed granite as road subbase
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
RE: Decomposed granite as road subbase
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?
RE: Decomposed granite as road subbase
Would appreciate your findings on this project.
RE: Decomposed granite as road subbase
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.
RE: Decomposed granite as road subbase
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?
RE: Decomposed granite as road subbase
RE: Decomposed granite as road subbase
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?
RE: Decomposed granite as road subbase
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?