rough grading and compacting sand

[brnt]

I am a site inspector on a project where the contractor is presently rough grading the site. The native material is silty clay and sand.Most of the material is sand.The contractor is having a hard time passing compaction tests. He is useing a large sheeps foot packer to compact the lifts and to my eye the sheeps foot is just churning up the sand and is not compacting it.My experience is an experienced contractor just has to experiment with differant equipment and techniques to find what works for the native soil but this contractor just keeps doing the same thing over and over again with the same results. I am just trying to give him some ideas, and was wondering if anyone had experience with compacting sand on a large scale.The specifications call for 6 inch lifts--maybe with this soil he needs to place larger lifts? Maybe he needs to try differant watering techniques--I don't think he is using enough water. Maybe he needs to use differant compaction equipment? Any suggestions would be appreciated.

 

[Ron]

A sheepsfoot roller is not appropriate for compaction of sand for the reason you noted. Compaction should be done with a smooth drum vibratory roller. Make sure the moisture content of the sand is close to optimum and that the Proctor is correct for the material encountered.

 

[dirtsqueezer]

Hey, brnt

It sounds like you're pretty much flying without a cape over there. Sand won't compact without moisture, and without a troxler gauge or an accurate proctor you won't be able to tell if you're going to get settlement later on. I'm not sure what the weather's like out there, but if you're able to talk the contractor into adding some water hopefully via loaded water truck to your mostly-sand, you can check it by forming what you want to be a 'fragile hand cast' when you squeeze a handful of this sand. Sounds like a pretty big project, so have fun out there.

 

[Hoaokapohaku]

I'm not sure what you mean by, "The native material is silty clay and sand. Most of the material is sand." Do you mean that a given sample is (1)both silty clay and sand, with most of the sample being sand? Or do you mean (2) that some areas are silty clay, but most areas are clay.

If the case is (2), then the sheepfoot compactor would likely be the correct type of equipment for the silty clay areas, but the wrong type for the sand areas. Two different compactors would be required, as well as two (at least) different proctors.

If the case is (1) its a little harder to determine. While you describe the material as "mostly sand," it could have a large amount of clay and it's compaction characteristics could be like sand or like clay. In the field, you can determine if it should be treated like clay or by sand by observing it's behavior as it is being compacted. If it is being compacted in an appropriate manner for clay, and the proctor, moisture, lift thickness, etc. is all correct and it cannot be compacted with a sheepfoot roller, it's probably not clay. However, a vibratory sheeps foot roller applies the shear needed to compact clay, it usually leaves the top material to the the depth of the pad feet loose. After 4-6 passes operated in vibratory mode, the vibrator must be shut off and additional passes made until the surface material is consolidated and the roller "walks out" of the depressions made by the pad feet. If the material is being compacted like sand with a smooth drum, and again all the other compaction parameters are correct, but the material is failing to reach density and there is traverse checking created by the roller drum, the material is behaving like clay. A sheepsfoot roller, or other compaction equipment that applies shear, is required.

Over the years I've responded to countless complaints that begin with either something like, "I can't meet compaction. This material is crap," or, "I can't meet compaction. The proctor is wrong." But in well more than half those cases, the actual problem is not enough water. Second leading cause is segregated material because of improper handling. All others causes trail far behind. When the problem is not enough water (most cases), sometimes it is because they are not watering enough, but most of the time it is because they are trying to water too large of an area at once. I tell them to water in short sections and start rolling immediately behind the water truck. This solves the problem most of the time.

 

[brnt]

Your right--not enough water. We got a few days of rain and low and behold he started getting compaction with the wetter material