Testing Device for Foundation Depth???

[mkoon]

Hello. I'm new to the forum and hoping to gain some insight from the experts! I have been researching trying to find a tool that can be used in the field to test the depth of a foundation. Typical foundation size is 18-24" in diameter and 3-6' deep. We are working on a project that requires us to verify foundation depth before mounting a new sign on it and doing it the old fashioned way(digging a hole next to it) just doesn't make sense and I figured there must be some equipment out there a technician could use to verify depth in lieu of digging a hole. For reference, we've tested the Olson Instruments Concrete Thickness Gauge and that didn't work for our application. I have recently found the Pile Integrity Tester from Pile Dynamics and am looking into that one, just curious if anyone else has done some research and used anything they liked that worked with a high degree of success(90+%). Thanks in advance!

 

[Tomfh]

We’ve used pile integrity testing a few times. It’s been performed by geotechnical subconsultants. It’s reasonably effective although as with any ultrasound type test can give you inconclusive results.

If you’re doing a lot of tests in the field it could be quite a useful bit of kit.

 

[Heaviside1925]

OP,
Just an off the wall suggestion. Since you are an electrical, have you considered viewing the foundation as a grounding conductor? You could sink a reference rod next to it and relate soil resistivity to ground resistance to solve for depth since diameter in known.

https://blog.nvent.com/calculating-...ound-electrode-design-principles-and-testing/

No idea the accuracy but might be worth a try if you already have the tools.

 

[LittleWheels]

The foundation's similar diameter and depth complicates selecting a non-destructive test method. Some test methods merely indicate the distance to the nearest surface, some to the furthest. Those methods measuring to the furthest surface tend to filter out 'premature responses' from nearer surfaces.

 

[Brad805]

These tools will help determine the depth, but provide no information about the reinforcement, anchor bolts, or the concrete. That is not a very large pile. I am not sure how much time I would invest into this unless there are a lot of site constraints.

 

[mkoon]

Yes, the piles are small in size, but our customer is trying to avoid removing and replacing with new if we can determine the depth of the piles and as such reuse them. Right now, we are deploying field techs to dig adjacent to the foundation to determine depth and I just can't believe there is not a piece of equipment/tool out there that could reliably tell us the depth to avoid digging the hole. We have to do 6000+ site surveys to determine the pile depth at each site so finding a solution would save us a lot of time and money! Ideally, I am hopeful to find a tool that can be put in the hands of these field techs(electricians) to do a simple test to confirm depth so we can avoid the time and pain of digging a hole to verify.

As for the anchor bolts and reinforcement, we are using new anchor bolts for our project to set the signs on the piles.

 

[CarlB]

You should contact a geophysical services firm, there are a lot of methods for detecting/mapping subsurface conditions.

One example:

https://geophysicalservices.com/loc...-gpr-magnetometers-buried-cables-voids-pipes/

 

[Brad805]

Ok, I understand.

The tool is not difficult to use. We have rented them before to measure the depth of the some piles supporting a silo. In that case it was to prove the foundation was built incorrectly, which it most certainly was based on all the evidence on site. Below is another example from not long ago. In that case they hired someone to do this part. They wanted to re-use a sign foundation for a car dealership. They tested the depth 6 times and reported the average depth of the 40" pile was 11.2'. The depth of all the tests varied from 11ft to 11.4'. I would not get involved as I had no information about the anchors or reinforcing beyond, "it has stood there for X years.."

 

[mkoon]

@Brad805 - Which tool did you use? Do you have name or website you can share? I assume you liked it, thought it worked well and was accurate? I can't tell from your dealership example, did you mean to say the pile was in reality 40" deep(or did you mean 40" diameter?), but the test suggested 11'? That's not good if I got that correctly.

 

[Brad805]

I recall I contacted Hoskin Scientific in Canada and talked with them. The device we rented was like the one pictured below. Google "Geotechnical Equipment Rental" and you should find someone near you. I do not think it was expensive to rent, but that was a number of years ago. For the silo investigation the results were consistent with what we were seeing on site. The owner insisted the piles were much deeper than we believed, and this indicated they were all bearing in shallow fill materials. We did not excavate to confirm the depth, but it was reasonable. In your case it would seem practical to test some with a device, and then dig to confirm. Once you are confident it should save some work and site repair if that is part of the project. I suspect your piles would have been installed with a bobcat. Your depth will not be exact, so it seems you are looking for a minimum depth.

The pile they were testing at the car dealership was 40" diameter. The depth measured by the device varied from 11.0ft to 11.4'. I am not sure if they confirmed that or not. We did not want to get involved. That pile diameter is not uncommon for us, and depths up to 30'-0" are typical. I am in a cold climate. A 3'-6" deep pile would jack out of the ground in our area.

 

[jayrod12]

I don't think the PIT testing will be accurate in this instance. When it's been used on projects of mine they need to install a detecting sensor at least 3 feet down the pile so they can get a reference for how fast the waves are travelling over a known distance so they can extrapolate that data for the time it takes the wave to get to the bottom of the pile and back. Since your foundations are at most 6 feet deep, you've already dug most of the way there just to install the sensor. Then I assume when the end of pile is close to the sensor that it will be tough to have any sort of accuracy in the results.

What about just driving an 8-9 foot piece of rebar on an intersecting angle and seeing if you hit concrete? How accurate of results are you after?

 

[mkoon]

Accurate enough to feel confident setting a small sign on it. We need a minimum foundation depth of around 4.5' to set the new sign on. I'm just trying to find a more cost effective and efficient way to validate that short of digging a hole. I do like your idea above, that could work....

 

[Heaviside1925]

OP,
Do you have any information as to what forces the concrete is to resist? I'm assuming controlling uplift and resisting overturning? or is this about frost depth? My only though on the rebar idea, other than that I really liked it at first, is you have no way of knowing any discontinuity issues in the foundation. If it's a small sign, then I would imagine the loads the concrete is resisting are relativity small. If you know the loads, a brute force, go/no-go method might work using a pry bar or moment arm of a certain length. If it doesn't move, it's good, if it moves, no good. If frost depth is the only driver, the rebar idea may be the way to go.

 

[dold]

So you have 2,000 of these things? What sort of signs are they?

 

[LittleInch]

How many piles are there per sign?

Are you measuring the wrong thing?

So pile capacity vertical down or up would be more useful surely?

Depth just give you weight.

Try lifting it out of the ground using a digger and a load force device instead?

Able to withstand xx N up or down without moving more than a feww mm is surely a better test?

A 3 ft pile on bedrock will beat a 6ft pile in silt.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[mkoon]

Good thoughts all, thank you for the feedback, it is much appreciated! I'll try to provide a little more context to help with your suggestions. We have ~6500 sites in the US and another 500 we need to visit
Canada, all requiring a "foundation verification survey". Essentially digging down to verify depth.

Our sole mission is to determine the depth of the foundation as we know the diameter and it has an existing sign sitting on it today. The sign we are putting on is a little heavier and has a slightly larger wind-load, as such, our structural engineers have said the foundation needs to be a minimum width and depth to feel good about setting the new sign on it(reusing the foundation vs going new). Soil conditions are not part of the requirements at this point given the foundation size and sign size being on the smaller side. So mainly our concern is depth for frost in those climates where that is applicable and structurally being of a size to handle this new sign. Liability would be on us if we place the sign on a foundation that is not big enough to handle it, hence the need to verify. Or, we go new, which obviously the customer would like to avoid as it's costly and messy.

-1 pile/foundation per sign. Typical diameter is 18", some are 24".
-Sign is a digital board, painted steel with an enclosure and roughly 6' tall, weight is 600-800lbs depending on which version we use.

When I set out on this idea, my hope/thought was there was a reliable tool to test depth. That in my mind is the path to avoiding digging a hole. Keeping in mind I will have 50-100 people doing these surveys across the US so it needs to be something a non-engineer can use.

Hope this all makes sense!

 

[Tomfh]

If you only need to verify 4.5' then probing in multiple locations with feeler rods sounds like a good option. You can use smooth solid rod (or rebar), driven with a cordless rotary hammer, which is easier than bashing away with a hammer. Or you can use a long bit:

https://www.southland.com.au/16-0-x-2000mm-sds-max-zentro-masonry-drill

I sometimes use this approach to find shallow rock/footings.

 

[Heaviside1925]

Obvious question I guess, but I'll ask anyway... Have you asked the structural for ideas? The statements they have made are correct, but have you asked them if there is anyway of verifying without excavation? I'm asking this for two reasons, the first is, they may have some ideas to help you do what you want without excavating. The second reason is, the structural may be assuming you are excavating, and I would be concerned by not excavating and using an alternative method may not meet their intent, especially if the foundation is not continuous for some reason. Maybe you have and if so, I apologize for stating the obvious.

 

[dold]

That's a ton of signs! Maybe a gas powered auger with a 4" diameter (do they make those? maybe 6" diam?) bit right up against the pile could work? That'd give you a little more access to jam a hooked piece of rebar down into it. Auger it out to 4.5' - 5', slide the rebar hook into the hole 4.5' til it catches on the bottom of the pile, or doesn't catch, then you know it's deeper than 4.5'. BUT, if you have to mobilize 50-100 people to do this then it would be quite the expenditure in equipment. Surely cheaper than buying/renting some sort of remote sensing type device though. Probably also still cheaper than hand digging?

Or, ask the structural engineers to sharpen their pencils. Maybe they just enveloped the wind loads for absolute worst case (180mph or something like you'd see in florida). If they can look at it site by site (or even state by state/province) that could potentially reduce depth requirements. I'm assuming wind load controls. 600lbs for a 18 or 24" pile is peanuts as far as bearing pressure goes. Ditto for frost depth. Southeast US frost depth is nowhere near 4.5'. Maybe they enveloped that too.