SPT, which way is more effective? 1

[pelelo]

Hello,

I was at job site with different drilling crews. The intent was to perform SPTs to depths between 80 and 100 feet.

Crew #1 performed the SPTs, by installing casings (site with clean sands, loose), then inserting the shoe attached to the rods to clean the hole to a specific depth, then remove the shoe and replace by the split spoon sampler then perform the blowcounts.

Crew #2, used casings as well with the wireline system to an specific depth, then made sure hole is clean, then after removing the core barrel with the overshot, they bring down the split spoon sampler and performed the blow count.

I have seen both methods and they look acceptable to me. However, I noticed that Crew #2 performed the works way faster than #1 as they did not have to bring down the rods and a shoe to clean the hole, instead, the hole was already clean and was ready to bring the split spoon sampler and perform the blowcounts.

From the budget point of view which crew do you think spent more money in tools?. Both crews are drilling for the same unit price.

Please let me know your thoughts.

 

[geotechguy1]

Option #3: Don't do any SPTs, and just do 1 machine borehole with no SPTs for every 5 to 10 CPTs. I can't for the life of me figure out why the profession is still so obsessed with SPTs when better, cheaper, more reliable and more reproducible in-situ tests have been available for decades on end.

That aside:

Most drillers I've worked with in North America charge by the hour so I would generally go for the Crew 2 method. To be honest most drillers I've worked with are much sloppier than what you described so I'd consider either to be an improvement.

 

[MTNClimber]

As long as they aren't disturbing the soil you're trying to sample then whichever is faster is the best option.

I can't for the life of me figure out why the profession is still so obsessed with SPTs when better, cheaper, more reliable and more reproducible in-situ tests have been available for decades on end.

All regions are not the same. My reasoning for still using SPTs is simple... cobbles, boulders, undocumented fill, and other random obstructions that are plentiful in my region. Additionally, the closest CPT rig is 3 states away requiring a one day drill job cost 2-3 days worth of work. The few times I've used them, a drill rig had to come out first to preclear the hole through the fill so the CPT rig can push through the clay to measure shear wave velocity. Jobs in my region that would justify the cost would have to largely benefit from bettering the site class and a require a client with deep pockets that is willing to gamble with that money.

 

[geotechguy1]

For what it's worth, there are CPT rigs that can push through soft rock and combined sonic - CPT rigs that can either vibrate the cpt cone past obstructions with the sonic rig or quickly switch back and forth between drilling and CPT testing (or combine the two in some fashion, eg by running the CPT inside the casing, pushing for 3-5m, pulling it out and the continuing to core.

Hopefully you'll have more CPT rigs near you soon! The equipment is declining in cost every year.