metal detectors and asphalt?

[oddgrrl99]

We have recently purchased a new old house (mid 1880's) that has not been lived in for at least 15 years. It is located near downtown Lexington Ky on land that historically was a staging area for Civil War troops. The entire lot around the house has been under pavement since the 1950's and we plan on removing most of the pavement over the next couple of months. My question is, I want to make sure no relics or (dare I hope?) caches are not removed along with the pavement-will a metal detector be useful to detect anything that may be under the asphalt? I love treasure hunting and have always wanted a detector-this is a great excuse to get one-any ideas? Thank you!...Lisa

 

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[fmrUSMC_0844]

Around here at least, (Utah) I have noticed when they tear up the asphalt, they do a pretty good job at just taking the asphalt up and leaving the dirt.

I dont see why you couldnt use a metal detector through the asphalt. You could always mark any hit you get with a GPS unit. The only problem I would see is how thick that asphalt may be. That will cause you to lose a lot of ground depth. It never hurts to try though. Otherwise you never know.

If that was me I would hit the hel out of it afterr that asphalt is gone. You never know what is in the ground. Good luck!!

Im sure someone else with a lot more knowledge than me will chime in here and help you out.

 

[Tom_in_CA]

reply

There's a few bugs in going through asphalt with a detector:

a) First, the gravel rocks used in asphalt composition are mineralized more than normal soil. So it's going to depend on what type detector you're using, what your settings are, etc... You might need to lower your sensitivity.

b) Asphalt is not laid right on top of virgin dirt. There is a compaction bed made first of DG (decomposed granite). That's a type fill dirt that can be many inches thick. It is rolled and compacted, and THEN the asphalt is laid on top of THAT. So when you see them grind out old asphalt (to prepare for a fresh "lift"), they usually only grind out down to that DG layer, NOT the native soil. So you can see, that if the asphalt is 1 or 2" thick, and the compaction layer is several more inches thick beyond that, you can see that native soil is going to be fairly deep. This can be a foot of depth for public streets/roads, as they are engineered to handle more weight of continual use. But for a private driveway or lot, less thick d/t it's only handling slow traffic and parking only.

c) My experience in hunting any medium, where it changes mid-way down, is that the machine tends to track/tune/balance to the topmost "soil" minerals type. So if, in your case, the "soil" type changes 3 times (asphalt, DG, then native soil), it seems to confuse the machine in what the ground balance is, tending to sort of make a "brick wall" when your signal is encountering the changing types. Contrast to if the "soil" type is uniform all the way down, then it's not differing mineral types.

doesn't hurt to scan it though. Might have to turn down the sens, as I say. Or if you have a beach pulse type machine, bi and tri-level minerals should pose no problem, and it'll go deep, etc... However, every frickin staple, tack, nail, etc... that's down there, will give hits, d/t no disc.

We have recently purchased a new old house (mid 1880's) that has not been lived in for at least 15 years. It is located near downtown Lexington Ky on land that historically was a staging area for Civil War troops. The entire lot around the house has been under pavement since the 1950's and we plan on removing most of the pavement over the next couple of months. My question is, I want to make sure no relics or (dare I hope?) caches are not removed along with the pavement-will a metal detector be useful to detect anything that may be under the asphalt? I love treasure hunting and have always wanted a detector-this is a great excuse to get one-any ideas? Thank you!...Lisa

 

[oddgrrl99]

Tom in CA, just saw your posting about detecting & asphalt. You were right on the money about what would be underneath. We dug up about 4 inches of asphalt only to find about 8 more inches of dirt/gravel under that AND an old brick driveway under all of it. I did try the detector and dug out a bunch of old square nails but got seriously discouraged with how hard everything was underneath it. It was wishful thinking but I have gotten some experience and am ready to try other places. Thank you!

 

[Tom_in_CA]

reply

Tom in CA, just saw your posting about detecting & asphalt. You were right on the money about what would be underneath. We dug up about 4 inches of asphalt only to find about 8 more inches of dirt/gravel under that AND an old brick driveway under all of it. I did try the detector and dug out a bunch of old square nails but got seriously discouraged with how hard everything was underneath it. It was wishful thinking but I have gotten some experience and am ready to try other places. Thank you!

wow, what an old post. You're very welcome. It was worth a try though, considering the age and history of your area.

I have rarely done good under asphalt, no matter how much history and usage of an area. Now something like tennis court asphalt (which is only meant to hold up the weight of people) might have less DG layer/base. But for roads, they're normally always sterile underneath, d/t too much fill before the asphalt ever went down.

Also can depend on how old the asphalt is. Becuase extremely old asphalt would/might have gone right down on top of the yester-year oil-&-screenings (the "poor mans" chip seal jobs before asphalt was invented). And it's possible that as time went on, they just added layers, and there was not the currently requisite DG compaction layer ever there.

But alas, once there's multiple layers of asphalt (as grinders/milling machines weren't introduced to the USA till the 1970s or so), go added, then ..... rarely ever does A/C ripouts go truly down to virgin dirt.

That's why sidewalk tearouts are always better