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Advancing hand auger probes through caving soil/stone

Advancing hand auger probes through caving soil/stone

(OP)
Our firm does frequent geotechnical investigations utilizing hand auger probings with DCP testing in lieu of other in-situ testing due to project/site constraints. For us, and maybe many of you, some of the constraints of trying to perform a geotechnical investigation within the footprint of an existing structure results are as follows:
  • Low overhead clearance
  • Limited access for drilling/sounding rigs by physical dimension, weight, etc
  • Noise
  • Cost
  • Other reasons
Typically, prior to advancing hand auger probes, any existing flooring must be cored or sawed to expose the subbase/subgrade. This task is performed by the use of concrete core drill or similar. Once exposed, the auger probing can be advanced through the subbase and finally, the subgrade can be reached. However, the majority of our problems stem from the type of material used as subbase and the thickness of the subbase. These problems may include:
  • Caving subbase (such as washed stone)
  • Trying to loosen up compacted ABC stone
  • Spending too much fighting the aforementioned items
I would like to address the first on the list. I've encountered, on numerous projects, the thickness of washed stone used as a concrete slab subbase to be 2-5 times the design thickness. Instead of 4 inches of stone, I've seen as much as 1.0 foot to 2.5 feet of washed stone! The problem with caving soil/stone is that you're not removing a column of material any more, but a cone of material with substantial volume. This also leads to a loss of subbase beneath a large portion of the slab. I understand why contractors prefer to use additional stone instead of soil in some instances, but those can be address at a later date. For right now, I'd like to focus on the following:


1. What are some effective ways to remove washed stone from a hand auger probing?
In the past, I've found that using a typical 3" diameter hand auger bucket provides poor to moderate results when the stone thickness is 6 inches or less, and as the stone thickness increases, the results become poor to non-existent. More recently, we've added the use of a wet/dry shop vacuum to are arsenal of hand auger field equipment. The results were more promising given the following:
  • Your vacuum has good suction power
  • Your vacuum hose diameter is large enough to accommodate the size of the aggregate
  • The thickness of the stone is less than 2.0 feet or so (the deeper you're trying to remove stone, the more power/airflow you'll need)
The questions I am raising are:
Are there any hand auger buckets specifically made for washed stone?
Are there any modifications that can be made to readily available hand auger buckets that facilitate use in washed stone?


Any other suggestions are welcome.


2. What are some effective tools used to prevent caving of loose soil/stone in hand auger boring?
Another additional method we've employed has been the use of a standard size 4.0" diameter PVC pipe as a casing for hand auger probings. The casing does what it is intended to do (keep the hole from caving) but it has its drawbacks.
  • PVC is weak and can damage if abused
  • A 6.0" diameter core is needed to fit the 4.5" outer-diameter casing
  • The casing and hand auger cannot be advanced simultaneously
  • The when retracting the hand auger from the casing, the friction between the hand auger, casing, and washed stone will pull the casing out of the hole
The questions I am raising are:
Has anyone used a different method of casing a hand auger probing with success?
Has anyone used a different type of material as a casing with success?
Is there a way to add vibration to the casing to help advance it through washed stone? If not, would using one of these in the washed stone help advance the casing?
(Apparently the v-word is picked up on the language filter.)

Any other suggestions are welcome.

Thank you for your time. =)

RE: Advancing hand auger probes through caving soil/stone

remove a 2 ft square section of the slab and use a shovel. Now you can actually collect a bulk sample, take a density test, and advance the hand auger.

To the rest of your post, yes all the things you mention can and do happen.

f-d

¡papá gordo ain’t no madre flaca!

RE: Advancing hand auger probes through caving soil/stone

(OP)
Thanks for the response. Unfortunately, cutting a large hole in the slab is probably not the best solution. The owner would probably be very upset with me, and I'm not a structural engineer so I can't just patch the hole in the slab and call it good. Additionally, I don't need a bulk sample or density test. I just need to stop the washed stone caving into the auger probe so I can perform testing on the subsurface soils.

RE: Advancing hand auger probes through caving soil/stone

yes, I understood your OP. It is frustrating for sure. I think some undermining is bound to happen. I think owner's need to know that our invasion will not be 100 percent back-to-normal also.

I guess it's judging collateral damages and such.

f-d

¡papá gordo ain’t no madre flaca!

RE: Advancing hand auger probes through caving soil/stone

You might try to advance a casing when digging the boring. I have used this technique when checking the depth of ASTM C-33 sand used for wastewater disposal fields. in that situation, I used a 6" double-wall HDPE pipe available from a home supply store. For washed stone, a heavier wall PVC pipe may be better suited to your application. Possibly, the stone might require a drive shoe on the bottom of the pipe to keep it from cracking. Fabricating some "teeth" in the bottom of the pipe may also make it easier to advance the casing.

let me know if it works out for you.

RE: Advancing hand auger probes through caving soil/stone

I've made use of stove pipe (sheet metal) under water and in very soft peat. Admittedly it is not the same case as your stone situation. There are (or used to be) riveted heavier gage sheet metal pipes for vacuum sawdust removal from wood working shops, which might be better. Be prepared for dmmage and some frustration, but it is surprising how sheet metal can take confined pressure without collapsing.

I found air heating duct pipes of aluminum to be just too weak to do the job.

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