undisturbed soil sampling methods 2

[nick269]

I am working in Vietnam in a project that includes the design of bridges, roads, and canal embankment modifications, so both shallow and deep foundations are involved. The ground is characterized by soft organic clays and high water level elevations.

The problem I am having is with the drilling contractor who insists taking what are supposed to be undisturbed samples by driving in a thin-walled Shelby tube with a 50 kg hammer. These samples are supposed to be for direct shear box and oedometer consolidation testing. According to Vietnamese standards, in soft clays the sampler should be pushed by the hydraulic system of the rig, but it is allowed to take samples driven by percussive methods in stiff clays. Could this be true?

In my view, the lab test values derived from these samples which are banged into the soil are invalid, and are not worth the money spent. It would be more cost effective to use SPT values and collect disturbed samples, for classification, and Atterburg limits etc.

The rational, as explained by the contractor, is that the trimmed sample has a diameter much less than the sample in the tube, and so disturbance is minimal. I don't buy that.

Am I just being a hard-assed engineer, or is my view supported by most other standards. I know that ASTM requires hydraulically pushed samples.

Nick

 

[GeoPaveTraffic]

I would suggest you extract some samples from the tubes as collected and see what they look like. While pushed Shelby tube samples would be better, these samples collected by driving the ST's may be fine. If there are fractures or other disturbances in the extruded samples, tell/require the driller to push the tubes; if not, get on with the testing.

 

[BigH]

I've seen thin-walled samplers tapped in - not with such a big hammer - but smaller hammer with short drops - why? A motorized tripod was being used and unless the driller has a piston sampler (shoving out the thin-walled tube using water pressure), it's about the only chance he has to get an 'undisturbed' sample with this type of drilling set-up. If he is tapping in the sample, I would go with a 100mm dia sampler rather than a 50mm dia.