Introduction to Cone Penetration Testing

Introduction to Cone Penetration Testing

Presenter: Prof. Peter Robertson
Title: Introduction to Cone Penetration Testing
Launching Date & Time: 02 February 2015 12:00pm GMT


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Session Question & Answer [CLOSED]

Allan Dahl

Do you have any experiance in using CPT i Light Weight Aggregates? We consider to use it as a method for testing of compaction of the build in aggregates.

Peter Robertson

Allan CPT has been used a wide range of aggregates, including light weight aggregates. Interpretation needs careful attention to the correct unit weight of the aggregate to get correct total and effective stress, etc. Good luck

Bill Kitch

You state that the SCPT makes a direct measure of soil stiffness. This is true, but the stiffness measured is a small strain stiffness which controls propagation of seismic waves. This is an important parameter for many applications such as liquefaction assessment. However for settlement calculations we really want a large strain stiffness. Do you have recommendations for ways to estimate large strain stiffness from the stiffness measured by seismic waves?

Peter Robertson

Hi Bill You are correct in that the modulus is measured at small strains, but there are good guidelines on how to soften this small strain modulus to the appropriate strain level for most design. Typically the shear strain level for most well designed structures is around 1x10^-1%. I covered this briefly in slide 54 and in more detail in the webinar on the SCPT ( It's often more reliable to measure the small strain modulus (via Vs) then soften to the appropriate strain level, than to try and measure the appropriate modulus directly.

Eric Peloquin

My question is in line with Bill's one. We are use to associate an in situ measured Vs with Gmax. On the bottom of your slide 54, I missed something since you say that you get a G value corresponding to approx G/Gmax of 0,4 (softening) with in situ Vs. Could you correct my misunderstanding?

Peter Robertson

Hi Eric A copy of the slides (in PDF) can be downloaded at The objective of slide 54 was to show the approx. link between the small strain shear modulus (Go or Gmax) with Young's modulus (E) needed for design (at a shear strain level of about 1x10^-1%). The small strain Young's modulus (Eo) is Eo = 2.5*Go (based on a possion's ratio of about 0.25). The average degree of softening needed for most well designed structures (and most young, uncemented soils) is about 0.4 (i.e. E = 0.4*Eo). Hence, E = 0.4*Eo = 0.4*2.5*Go = Go. I hope this has been helpful to illustrate the application of Vs and Go for design of static problems.


Thanks Peter for this great Webinar. Working on tailings, is there recommendations for interpretation of CPT data to identify chemical bonding ?

Peter Robertson

Hi Sylvie Yes - I covered this in the webinar on the SCPT that can be viewed at: The combined CPT and Vs helps identify cemented soils, since the measured Vs is higher than the estimated Vs based on the CPT

Denis Cloarec

Thank you Peter for this presentation. Here are my questions: 1-Do you have any experience in seismic CPT in offshore? With water depth <30m? Do you have an idea of the equipment? 2-In seismic CPT, how do you evaluate/measure the density of the soil (ρ), slide 54? 3- I wonder if it is possible that a small content of a very plastic clay (max 10-15%) can impact on the sleeve measure and lead to a wrong analysis of the soil nature/behavior: a clay instead of a sand. Do you have encountered this issue in the past? Thank you in advance Regards Denis

Peter Robertson

Hi Denis 1 Yes - SCPT has been performed offshore. In deep water the SCPT has been performed using seabed equipment (both CPT and remote seabed drill-rigs) using underwater remote seismic sources (hammers). In shallow water, it requires a seismic source that can operate underwater and be placed a known distance from the cone. 2 Soil density is either estimated from the CPT or measured on soil samples. You can download a short paper on estimating soil unit weight from 3 Sometime a small % of highly active clay minerals can dominate soil behavior. Hence, a soil with mostly 'sand' but with small amount of active clay minerals can behave more like a clay. In general, the CPT tends to respond to the soil behavior more than the % clay, but it is also helpful to look at the CPTu pore pressure response and dissipation test response to improve interpretation. Likewise taking a small (push-in) soil sample (using the CPT pushing equipment) also helps.