Continuous SPTs 1

[pelelo]

Hello,

I would like to know what is your experience when performing continuous SPT borings. We are going to perform some borings at a site that is composed by clayey sands to sandy clays and silty sands to sandy silts.

The borings will be 20 ft. The project require 24 inches spoon (4 samples). The drillers are proposing to perform SPTs from 0 - 2 ft, remove spoon then insert a new spoon + rods from 2 - 4 ft, then procedure until reaching 20 ft deep.

My question is, don't they need to wash the holes using the HW casings in order to insert the rods to perform the SPT in the next depth interval?. For example if they are going to perform SPT from 10 - 12 ft, it is my understanding that the spoon + rods will not go down easily or softly due to friction and any soil that had caved in. In fact, in order to reach such depth, they propose to push the rods using the weigh of the rig (CME rig), so they push down the rods + spoon to 10 ft and then perform SPT.

The way how I envisioned it was to wash the hole using HW casings down to 10 ft, then insert the rods + spoon and perform SPTs.

I understand the way they proposed is way faster (maybe 2x faster) because they won't need to deal with inserting casings plus dealing with water, however, even though the way i propose is slower, i understand is more accurate.

Please advise.

 

[MTNClimber]

We continuously sample how your driller is proposing. They drive the first sample 0’-2’, and then immediately stick down a second spoon with a 2’ A rod on the spoon before the A to N adapter and take the 2’-4’ sample. Then they wash out to 4’ and then repeat the process. You may get some slough in the second sample but that’s why we only count the second and third 6” intervals for the N value. If you’re getting more than 6” of slough in your recovery, you know you’re N is off and should consider adjusting the drilling procedures.

I remember fatdad saying they (Virginia DOT?) would never let their driller do this for the same worries that you have but it works well most of the time. The N values are consistent and you capture the interface between two soil types better versus sampling every 5’.

 

[pelelo]

Thanks a lot,

FYI, another tool the drillers have come up with is a "stretcher", which is a tool wider than the split spoon OD.

For example, if drillers are going to perform SPTs from 8 - 10 ft, they connect the "stretcher" to the rods and push all the way down to 8 ft. That way the hole is wider. Then they remove the stretcher and connect the split spoon. By the time they place the spoon 8 feet deep, there will not be any friction issues as the hole is wider than the split spoon OD.

This alternative is only effective in clayey / silty soils where there is no water table and walls will not collapse.

Few pictures of the stretcher are attached. In this example, the "stretcher" OD is 2.5 inches.

 

[MTNClimber]

The stretcher could create a lot of slough at the bottom of the hole. Make sure the inspector understands how to identify it!

 

[coneboy]

When I run an investigation with continuous SPT tests we always advance the casing before the next test is conducted.
High water table in this area of Canada with interbedded silts/sands/sandy silts etc.
The strecher would not work.
The 24 inch SPT sampler works best I have found.
Yes, we most often ignore the 1st 6" and use the middle foot of penetration data as the SPT blow count value.

From a cost perspective I do CPT testing as often as possible as SPT/drilling is usually too expensive.
Often we lose the contract for the job if SPT testing is proposed.

 

[GeoEnvGuy]

When you say that the site is expected to encounter sandy silts to silty sands if these are saturated you will likely require wash boring to prevent heave. Typical investigation for me includes 24 inch ss at 2.5 ft intervals. I would also require vanes and/or shelby tubes in the clay if they are soft or very soft.