Lateral Shifting of Sandy Soil (Question from an Electrical Engineer)

[ReedB]

Here's the situation. My project installed underground PVC conduit via directional boring almost 10 years ago. We used ground penetrating radar to document it's path and it was documented to be more or less installed straight (+/- 4-5") between to hand holes approximately 200-300' from one another under an aircraft taxi way, this was done about 20 times in a straight line. In my most novice electrical engineering opinion, the soil through which this bore was installed is sandy (low country of South Carolina near the ocean if that helps). So here's my dilemma, a new installation is digging a trench approximately 5' from and parallel to the conduit installation mentioned before, utilizing a backhoe and digging approximately 8' deep. The backhoe has now snagged the PVC conduit in two locations while digging its trench. SO for all the soil guys out there, is it conceivable for undisturbed soil to shift laterally underground by 5-8' in 10 years, taking what is suspended in it with it? If so, is this common? If not, could anyone offer an explanation for what I'm seeing here?

 

[GeoPaveTraffic]

While it is not impossible for the ground to shift, it is unlikely and would would have caused cracks in the ground surface of a corresponding amount.

The most likely explanation is that the conduit was not installed as straight in those locations as you thought it was.

Mike Lambert

 

[cvg]

a couple of possibilities:

- The GPR was not that accurate in locating the conduit in the first place
- your markout for the recent work was not accurate

combine both of these and you could be farther off than you thought

 

[oldestguy]

You might do a double check on location now by using or installing another wire inside and applying a signal to it and using the devices used for finding buried electric cables. There also might have been a mistake made in recording the locati0on of the GPR reading to begin with. What surface bench marks were used for recording those locations? Done by a surveyor? Probably not. Also did any one ever test that GPR to begin with to see how precise it was? My guess is that that method was pretty poor for finding a light material like a hole in the ground surrounded by light material. If this is now a legal case, some darn careful checking of the GPR is needed.

 

[CarlB]

Is it possible the pipe shifted into the trench as the trench was being excavated, which could happen due to loss of lateral support, and that the ground around the pipe was loosened during installation.

 

[oldestguy]

Carl: If that happened the obvious surface changes would be so clear that the question would not have been posed here.

 

[Okiryu]

Appears that the GPR did not work well for locating the electrical line. My experience with GPR is that it can only locate underground utilities approximately, so I do not rely on it too much. 200-300 ft is a large distance and the conduit may not be installed straight. Do you have records of the installation? Do you remember that actually 200-300 ft of conduit was installed in that location? If there are more than 200-300 ft of installed conduit, it means that the conduit was not installed straight...