Wednesday, July 11, 2012

why is my road falling apart?

The story:  Underground constructed in 2007, roads 2007, no houses yet (2012), asphalt is ACB, numerous cracks, corner pushes.

ACB is a base coarse asphalt, not intended to be used as a surface course. It is open, and will allow water to penetrate. In addition, there is considerable segregation and exposed rock areas typical of cold placement of truck box corners and hopper wings. The asphalt had variable asphalt content, often to the low end. Densities were adequate.  

Separate the mechanical damage, such as the location where the crane or picker truck lifted something heavy, without adequate pads. Two rectangular spots where the rectangular pads were in contact with the AC, and is punched the structure.

Through out the project, there were issue with lift thickness, and the contractor was not cooperating with control of lift thickness. This was all reported to your man on site, but nothing changed.  

Separate trench settlements, where underground settled. Density is not enough to control; lift thickness must also be controlled. We struggled with the contractor over lift thickness when we were on site, but we were not full time. This was reported numerous time to Mr. xxx, your man. Nothing happened, the contractor continued with thick lifts.   A typical compactor has difficulty to compact the bottom portion of a 300 mm lifts, the maximum allowed. We cannot test more than 8 inches deep. A nuclear density machines density reading has surface bias, 87.5 percent of the value is in the top 50% of the depth, nearly all of the moisture reading is in the top inch. Trench settlement typically creates two longitudinal cracks, one at each side of the trench, somewhere from the straight wall to the edge. Often it shows up as three cracks where two trenches are involved. Services, and other lines that cross, can be identified by settlement in there location.

Trench settlement can also be identified by "manhold growth", which is settlement around the manholes. In several locations, it is likely that vertical ends in the backfill were created near manholes. This results in a loose column of backfill which can settle.

The lots were not graded to drain water away. There are ponds where the houses will be in portions, and water drains the rear or the curbs in locations. When subdivisions are left, it is recommended that the rear of curbs be backfilled to encourage water to drain across or away, not stand and soak in.

Traffic is more than minor. During the inspection 5 loaded trucks were noted, one loaded semi-trailer use the the 97A/89 route twice,  and on both trips, did backing practice on 97A/89 corner ( the only purpose I could think of, but I could be for other reasons)  The other truck was a tandem, hauling topsoil on Grand.

On numerous corners, the asphaltic concrete has failed by diagonal vertical shear. This is a common issue with poor compaction and pushed hand placing producing a shear plain in the asphalt during placing, which never gets knitted together without considerable rubber rolling. Thermal expansion of the mono sidewalk curb and curb produces a lateral forces on the asphalt, and the asphalt faults along the pre-existing weak plain / shear plain. 

There are several areas that have typical failures of missed cement stabilization or deep gravel infill into some trench, producing a water source for frost heave. These can be investigated by a trench, should it be desired. This involves cutting a trench about 2 feet wide, three feet deep, and 10 feet long through the problem for evaluation. Testholes often miss the issue, as these may be small. In several of the locations, these may be gas or electrical crossings, off a bit from where the plan indicates.

There are a number of areas that appear to have structural pavement failures. This can result from a few truck overloads,  traffic volume, thin structure, or some issue with the structure. It is recommended that testholes be cored, and hand augers be conducted for evaluation. Asphalt thickness, gravel quality and thickness, cement stabilized holidays, and other issues can be identified. In some cases, these are occurring at trench settlements crack locations, which will permit the ingress of water and soften the subgrade and below.   


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