CPT is always a good idea. Why?
Cone Penetration Testing (CPT) is one of the most elegant soil investigation methods used in geotechnical engineering. It has wide range of types regarding to the purpose of measurement. Seismic piezocone test method is the most sophisticated one by its dataset for geophysical and hydraulic properties of layers.
Seismic piezocone test is like a nice adaptation of borehole + P-S logging for cone penetration.
The most significant advantage of CPT from Standard Penetration Test (SPT) is obtaining continuous data. SPT measurements in vertical direction generally range between 0.50 to 1.50 m while CPT provides 20 to 100 readings per meter!
Pore water pressure measurements along the penetration is something else for CPT. You can see an impressive example of pore water distribution curves per soil type as follows. Thanks to Robertson (1986).
Continuous data profile along the penetration provides great advantage for geotechnical engineers to acquire wide range of parameters both for physical and mechanical properties. The most significant finding from CPT is to determine soil behavior type (SBT), developed by Robertson and Campanella (1986). This approach has been modified by other researchers for lots of times. Here is an example from my studies:
The list of acquired parameters from main outputs of CPT (cone resistance and sleeve resistance) can be listed as following;
Data Integrity with Other Survey Methods
Acquired data accuracy for CPT test is much more reliable than other penetration tests when the soil strata is suitable for this method. Interpretation of CPT results are also efficient for designers to check the data integrity between the design parameters acquired from geophysical and other geotechnical (in-situ & lab) tests. Here you can see a good example for Robertson et. al.(1983) approach. This approach offers a good example for comparative study between the results coming from physical laboratory tests (Sieve size analysis), SPT-N values and cone penetration resistance (qc). You can see the following charts and example results fit perfectly with the estimated soil class.
Another example for geophysical survey result verification with CPT.
领英推荐
SPT- CPT Comparison for Continuous Data
Advantage for obtaining continuous data from CPT can be seen as following. The measured and correlated SPT-N60 values are marked in the graph. We observed that measured and correlated SPT-N60 values show consistency but, between the SPT-N measurements, you can see the data gap. Continuous data measurement by CPT is a big benefit to evaluate the whole section along the depth. This data gap may not be a problem if the measured and correlated curves fit with each other in general but, you may obtain unrealistic results with limited number of SPT-N measurements.
If the CPT & SPT interpretation would be used in a drilled shaft design with 20 m depth, probably you will have adequate SPT-N measurements to compare the results along the section. However, if you are dealing with the individual steel pile capacity for solar farm, data comparison may be controversial. Keep in mind that maximum embedment of an individual driven pile for solar panels will be 3.0 m maximum, so you will have very few data coming from SPT-N measurements while CPT presents very intense and accurate dataset along pile depth. Therefore, idealization of soil profile based on the data acquired by CPT would be much more realistic.
Recommendation for CPT Data Evaluation & Geotechnical Design
I am using Rocscience Settle3 for CPT data evaluation presenting wide range of up-to-date correlations developed by famous researchers including Mayne, Hegazy, Robertson, Campanella, Kulhawy, Jamiolkowski etc.
Besides Settle3, CPT design manual was published in 2021 by Indiana Department of Transportation and Purdue University cooperation. This study includes the up-to-date research findings in three volumes as following;
Good news, these books are open for access in Joint Transportation Research Program (JTRP) website . You can search it as you wish; you will find very valuable sources, not limited to CPT.
Calculation spreadsheets for CPT evaluation & CPT-based foundation design with example problems are also given as appendices in the website. You can see some example results produced by the spreadsheets and they are delicious!
Thanks for reading!
References
Civil/ Geotechnical engineer at Consultant to Road development Authority Sri Lanka
10 个月Ideal tool to analyse geotechnical parameters for preliminary design purposes identifying ground profile for a detailed analysis and design. Soil samples are required to identify soil properties and behaviour a prerequisite for detailed design. Ideal to evaluate at preliminary stages the load carrying capacity of driven piles.
MSc.Geotechnical Engineer - Engineering Geologist
10 个月I think cpt is very practical for evaluating the unit skin frictions and unit end bearing in order to determine and predict pile capacity at suitable soil and ground conditions. Bacause while driving along the soil it provides a simulation pile driving or pile behaviour subjected to loads especially.
Managing Director at GeoDestek Ltd. ?ti.
10 个月Tahir Bey, thanks for pointing out the benefits of CPT testing. I would also add https://www.greggdrilling.com/wp-content/uploads/2022/11/CPT-Guide-7th-Final-sm.pdf as another practical reference. We, at GeoDestek Ltd. ?ti. are proud to serve the engineering community with our 200 kN truck mounted sCPTu assembly and can share a few outputs from our recently completed seismic CPT surveys if you’re interested. Thanks for your initiative in sparing time to shape this manuscript.