[secret agent walks down hallway, removing sunglasses]
[secret agent takes seat in office]
[AGENT PHI] Agent Sigma.
[AGENT SIGMA] Phi.
[AGENT PHI] I've heard it's not going well.
[AGENT SIGMA] The team is having a lot of trouble getting good subsurface
data for the state bridge project
There's a lot of internal friction, Phi.
People are comfortable with standard penetration testing, but know it has limitations.
[AGENT PHI] You've been approaching this all wrong, Sigma
[AGENT SIGMA] I'm trying to be effective, but the stress is getting to me.
[AGENT PHI] Well, you need to drill smarter, not harder.
[AGENT SIGMA] I don't get it, Phi, what's the angle?
[AGENT PHI] CPT, Sigma. CPT.
[agent sigma looks questioningly back at director phi]
[NARRATOR] CPT stands for cone penetration testing.
CPT is a field exploration and testing method that provides additional benefits beyond
those given by standard penetration testing, or SPT.
CPT rigs can come in many shapes and sizes, all with the same basic premise:
The rig uses hydraulics to push a steel cone into the subsurface.
Cone penetration testing is a fast, accurate subsurface exploration method.
A CPT rig uses hydraulics to push a steel cone into the ground.
The cone is advanced at a constant rate. The instrumented cone records continuous measurements of tip resistance,
sleeve friction and dynamic pore water pressure.
The data record allows for differentiation of soil layers and interpretation of material properties.
The cone stops at refusal in dense soil or rock.
CPTs can provide added value when you take advantage of additional in-situ testing,
such as pore pressure dissipation tests to measure hydraulic conductivity, or shear
wave velocity measurements for seismic evaluation.
And electronic CPT data can be used quickly and easily to calculate liquefaction potential,
or even to estimate skin friction and end bearing to directly calculate driven pile capacity at a site.
CPT rigs can be mounted on trucks or tracked vehicles to access a range of site conditions.
[crew members set up CPT rig on job site]
[NARRATOR] As the cone is advanced, operators add sections of rod to reach the requisite exploration
depth. CPTs can be advanced as quickly as 20 millimeters per second.
That's a 50-foot-deep hole in as little as 12 minutes; a 50-foot boring with SPT testing and
split spoon sampling would take hours.
CPT data can be viewed in real time during the exploration. The automated data collection of
CPTs makes them much less susceptible to operator error than traditional SPT drilling.
This CPT log shows the variation of tip resistance, sleeve friction, friction ratio and pore water pressure with depth.
Soil types can be interpreted from these parameters. Here the high tip resistance indicates a dense sand and the low tip
resistance indicates a soft clay.
Compare the CPT log with the typical boring log to the right.
The SPT samples at 5-foot intervals are interpreted as a continuous layer of clay, but the CPT identifies the
intermittent sand seams that could have an impact on foundation design.
You typically don't collect soil samples with a CPT, but rigs can be outfitted with a direct push
sampler to accommodate that need.
CPT and SPT can really work hand in hand to develop the geotechnical picture of a site,
with CPTs efficiently gathering lots of data, and SPTs and laboratory testing used selectively to calibrate CPTs
to develop soil properties such as unit weight and shear strength.
It's important to remember that you can't use CPT in gravelly soils or very dense or hard soils; these can result in
refusal or damage to the cone. Predrilling through dense upper fills is common.
Now, let's review the benefits of the CPT.
The CPT is precise. It provides a continuous, high-resolution soil profile that fills gaps left by SPT.
It outputs high quality data that are repeatable and do not vary with the equipment used or the driller doing the work.
It is fast, three to ten times faster than traditional SPT sampling.
The CPT provides electronic data on tip resistance, skin friction and pore water pressure that can be used directly for deep
foundation design.
And finally, numerous correlations with soil parameters such as shear strength and unit weight
Have been developed and are easy to use for geotechnical analyses.
[AGENT PHI] So, you know what you need to do, Sigma?
[AGENT SIGMA] This makes all the difference, Phi. We won't be stressed anymore by being confined to
the same old methods.