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Site Topography

Site Topography

Site Topography

I am working on a project that has several drilled shaft foundations for a DOT Noise Barrier project. I was given the boring logs for the project, but the nearest boring natural grade elevation is about 10' above the natural grade elevation where my drilled shafts will be. The project is in Wisconsin so the topography is relatively rolling.

Would the correct procedure be to assume that soil layers follow approximately the existing grade contours or is there a better method (i.e. more conservative) to use for determining what soil layers the drilled shaft is situated in? Is 10' too much vertical difference to use for design? I wouldn't think so, but I am new to the drilled shaft game.

RE: Site Topography

Answer is Maybe. However in Wisconsin there have been glaciers in a few stages, covering most of the state. As a result, you cannot generalize as to what is present away from the borehole. For a significant job, I'd rent or borrow a device that does a crude seismic measurement of depth to hard material, calibrating at a nearby bore hle. The device impacting a shock to the ground is a sledge hammer with switch attached. My quick search found several, but probably too complicated for this purpose. Years back we used an oscilloscope, with a trigger of the sweep by the hammer switch and the input then was a geophone placed a differing distances from the hammer. Plotting the data gave the answer as to depth to rock.

RE: Site Topography

BadgerPE - I am not sure if I understand your discussion on 10ft above natural grade elevation? The elevation of the borehole you are using is 10ft higher up the hill than your noise barrier? 10ft doesnt seem outrages to me but you could look silly if you make an assumption and it turns out to be way off when it comes to construction.

What is the noise barrier? Just a fence or is it an acoustic earth bund with a fence? I presume if you are using drilled they are supporting some sort of column or pole to which the barrier is attached to.

How long is the barrier? In an ideal world i would have boreholes drilled along the alignment of the barrier and would prepare a long section on which you could plot the different stratigraphy and soil strengths along the chainage. This would be very useful for a contractor.

RE: Site Topography

Thanks OG!


I believe the DOT provided alignment/bore locations is incorrect. We have an RFI in now for clarification. Thanks for the response.

RE: Site Topography

Just in case the borings were not on the alignment due to rig access, etc. think about hand sounding, by driving a rod to refusal and then jacking out. These can be 1/2' diameter, 3 or 4 feet long sections connected with well rod couplings on threaded ends. The removal can be by 4 foot farm jacks, as Farm and Fleet sells for about $50. Wrap a chain around the rods and away she goes. I've done a few of these and it works. Weld a hook on the lifting "ledge" to make it easier. Pipe wrenches or Vice grips are needed.

RE: Site Topography

Quote ( )

Would the correct procedure be to assume that soil layers follow approximately the existing grade contours or is there a better method (i.e. more conservative) to use for determining what soil layers the drilled shaft is situated in?

to answer the question requires knowledge of the geologic history that created the land surface. as OG touched on it - glacial action is one process that could lower or raise the existing ground surface without affecting the older layers below. It is prevalent in Wisconsin. So a glacier could either leave a pile of material creating a hill or could create a valley by cutting through the layers.

However thrusting and faulting can result in folding and tilting of the geologic layers which is consistent with your theory that it follows approximately the contours. This would be more prevalent in mountainous areas.

You need a geologist to interpret things for you. For a drilled shaft, you might want a closer boring or get ready for a changed conditions claim.

RE: Site Topography

If the area of the job is stated, I likely can add some info as to what to expect.

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