Soil Investigations in WOH Sands/Silty Sands

[geomane]

What is the best in-situ testing method to determine the friction angle in very loose (Weight of Hammer) sands/silty sands? We obviously already have SPT borings in the project area, but I'm trying to decide if we should perform some CPT soundings, or maybe another testing method that I do not have experience with such as the DMT. I don't think we can obtain tube samples of the material.

 

[Ron]

One method to obtain a tube sample in these materials (assuming you are in the coastal plains area of Georgia) is to not use a new tube. When you buy them they are either fairly clean or even have a light coating of mineral oil to prevent rusting. Wash them off, give them a light rub with muriatic acid on the inside and let them sit for a couple of days. This will give a very light rust coating on the inside and the increased friction will usually hold the sands in for sampling.

Otherwise, use CPT or a Dilatometer

 

[oldestguy]

I have done in place shear strength testing of various materials, including paper mill waste. Some were done by taking my gear to the site via private air, which means not a heck of a lot of things to bring along. A basic portable "kit" consented of a one foot square "bracket" made from steel angle iron in square units (two angle irons). Inside that goes a loose one foot square 1-1/2 in. thick plywood (two 3/4" pieces 90 degrees to each other), one side roughened with corrugation saw cuts. A 8" diameter outer race from a big bearing (junk price), fitted with strain dial good for 0.001" readings, calibrated in a lab testing machine, to serve as shear force measuring method. A fairly good spring scale for measuring weight in a bucket that sits on the one foot square plywood. A portable hydraulic cylinder with hand pump (comes from automobile car frame straitening kit). On site you arrange for some earth bank or other dead weight to put force against when shearing. Dial gauges to measure shearing strain, with appropriate supports made in the shop. The above for remote sites. In town with a back-hoe, bucket resting on a strut, you can jack against it for load, using a calibrated hydraulic jack, etc. So with a little bit of shop work you easily can do in-place shear testing of that loose stuff.

Edit: For light horizontal push that ring dial gauge probably had better read to 0.0001"

Another edit: For a precise load measurement, I got my steel ring, the outer race for a big bearing from a junk box outside a construction equipment repair place, no charge. As I recall the thickness of the ring about 5/8", width about 1-1/2" and overall diameter about 6 0r 7 inches, maybe more. Small steel strips, with 2 bolts between for mounting the dial gauge and also provides a flat surface for mounting, if needed.. Being retired that stuff went to the last lab I worked with.

 

[Ron]

@OG....your post made me smile, remembering all the things we've had to do to get data or samples! I once built a hydraulic sample extruder from a truck lift gate I bought at a salvage yard for $100. The lab used it for over 25 years. Lab tech and I also built our triaxial board and backpressure saturation set up.

 

[oldestguy]

When I went on my own, my former boss, B.K. Hough gave me all his home lab equipment including the prototype universal testing machine, the model for those that went into the Cornell soil lab. Tri-ax and the works, consolidation machines suitable for large diam samples, Ro-tap sieve shaker, etc. Most still in use at a local lab. It pays to have a friend like that.

 

[GeoEnvGuy]

In very loose sands and silty sands you may want to consider determining the peak undrained shear strength ratio, rather than a friction angle. This can be done with correlations such as Olsen and stark or attempting to do ground freezing to obtain samples for laboratory testing.

 

[fattdad]

I like both the CPT and the DMT. Low blow counts in sand; however, often relate to the drilling method. Did you use augers and are you below the water table?

I'd more than likely jam a cone next to the test boring, to see if the data sets look similar.

Otherwise, use mud rotary in saturated sands.

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

I should add that uniform sands many times will show very low blow count, but the actual range of density is small beteen "loose" and dense. Thus blow count is meaningless.