Biography

Peter K. ROBERTSON

Technical Director, Gregg Drilling & Testing, Inc.

Professor Emeritus, Geotechnical Engineering

B.Sc. (Nottingham, U.K.) (1972),

M.A.Sc. (British Columbia) (1975),

Ph.D. (British Columbia) (1983), P.Eng.

Peter has 30 years experience as an educator, researcher, consultant and practitioner specializing in the areas of in-situ testing of soils, earthquake design of geotechnical structures, soil liquefaction, pile design and soil structure interaction.  Dr. Robertson is recognized as an expert both nationally and internationally in the areas of in-situ testing and soil liquefaction.   He was the Principal Investigator of the Canadian Liquefaction Experiment (CANLEX) from 1993 to 2000, a $1.8 million collaborative project between industry, universities, and consulting engineers to study the characterization of sand for liquefaction analysis.  Dr. Robertson has been a consultant to various industrial clients and insurance companies in North America, Asia and Europe for projects involving liquefaction evaluation for major structures, stability of onshore and offshore structures, landslides, stability of natural slopes and tailings dams, deep foundations and use and interpretation of in-situ tests.  Dr. Robertson has authored or co-authored 249 publications including one book, six chapters in books, three engineering design manuals, 78 refereed journal publications, and 141 other refereed contributions.

He was an early shareholder in ConeTec Investigations Ltd. (1984-2004), an in-situ testing company specializing in the Cone Penetration Test (CPT).  He has also sat on the Boards of several private and not-for-profit organizations.  He also lectures on leadership and management and maintains an active research program in geotechnical engineering.

In his role as Associate Vice President (Research/Industry) at the University of Alberta (1999 – 2005), Dr. Robertson was responsible for leadership in the transfer of technology to the community.  Currently, Peter is working with Gregg Drilling & Testing Inc., in California.

View my CV in PDF format

Visit the Gregg Drilling website at: www.greggdrilling.com

Links

During my time at Gregg Drilling, I have contributed to their newsletter, the Gregg GeoNews with my regular column, Robertson's Remarks. Please follow the links below to view each editorial regarding Myths of the CPT

Robertson's Remarks #1: No Soil Samples?

Robertson's Remarks #2: CPT Too Expensive?

There are many websites out there with information about CPT and site investigation. Gregg Drilling & Testing maintains a website with all their equipment and services as well as technical methodology about how they conduct cone penetration testing and their various techniques to collect samples. Click below to visit Gregg Drilling's website.

Another useful link I use quite often is Paul Mayne's website at Georgia Tech. There is an excellent list of Geo Links that I couldn't possibly replicate. Click below to be taken to Paul's links page:

The site GeoEngineer.org produces a useful newsletter for the Geo-Engineering community.

For access to useful CPT Software (interpretation and liquefaction), visit GeoLogismiki's website. I have worked closely with this company to assist in the development of their CPT software.

< Links

Robertson's Remarks #4: Evaluation of Cyclic Softening in Clays

Boulanger and Idriss (2007, ASCE) recently presented an excellent summary for the evaluation of the cyclic softening in silts and clays.  They correctly suggest that the first step is the evaluation of the behavioral characteristics of the soil.  They suggest that the transition from being more fundamentally like sands to more like clays occurs at around a plasticity index (PI) of 7.  Others have suggested slightly higher values of PI to define this transition (e.g. Seed et al, 2003 and Bray and Sancio, 2006).  This issue will likely become resolved in the fullness of time as more field data becomes available.  I prefer a slightly higher PI criteria to define clay-like behavior (PI > 10) since sand-like soils tend to have a lower resistance to cyclic loading than clay-like soils and the simplified methodology requires a certain level of conservatism in its application for low to moderate risk projects or in the screening stages of high risk projects.

The resistance to cyclic loading for silts and clays is essentially controlled by the undrained strength ratio of the clay-like soil.  It is clear that if an earthquake applies a cyclic stress ratio that is close to the undrained strength ratio of the soil, large deformations are likely.  The amount of deformations will likely depend on the size and duration of cyclic loading and the plasticity of the soil.  Boulanger and Idriss suggest a value of CRR(M=7.5) = (0.8 x undrained strength ratio), to define the limit when deformations are likely to become large. 

In clay-like soils the CPT is also commonly used to obtain estimates of both undrained shear strength (su) and consolidation stress history (OCR) profiles. The CPT has the advantage of providing continuous, reliable profiles of tip resistance in a highly cost effective manner.  The undrained shear strength of clay-like soils can be estimated from the CPT using the following equation:    su =  (qt – sv) / Nkt
Where:  su  is the peak undrained shear strength; qt is the corrected total cone resistance; svc is the total overburden stress, and; Nkt is the empirical cone factor, which typically varies from 10 to 20 depending on the appropriate undrained shear strength required.

Normalized CPT parameters are used to identify Soil Behavior Type (SBT), where the normalized CPT parameters in clay-like fine-grained soils are:
Normalized cone resistance, Q = (qt – sigma vc)/sigma'vc 
Normalized Friction Ratio, F = 100 fs / (qt – sigma vc) in percent (%)
It can be shown that for insensitive clay-like fine-grained soils the undrained shear strength ratio can be estimated using the following simplified equation and CPT results: su /sigma'vc = fs /sigma'vc =  (F . Q) / 100

Hence, it is possible to identify contours of undrained shear strength ratio in clay-like fine-grained soils on the Normalized Soil Behavior Type (SBTn) chart suggested by Robertson (1990).  These undrained shear strength ratio contours can be reduced by the recommended factor of 0.8 to produce contours of Cyclic Resistance Ratio (CRRM=7.5) for clay-like fine-grained soils on the Robertson (1990) Soil Behavior Type chart, as shown in Figure 1.  Robertson and Wride (1998) suggested that the approximate boundary between sand-like and clay-like behavior could be estimated using the Soil Behavior Type Index, Ic, and that the boundary was approximately Ic = 2.6, as shown on Figure 1.  Zhang et al, 2001, recently updated the normalization of cone resistance for application to liquefaction assessment to allow for a variable stress exponent in different soils. 

The updated approach for normalized cone resistance (Q) is recommended for Figure 1.  Also included in Figure 1 are the contours for CRRM=7.5 for sand-like soils derived using the method described by Robertson and Wride (1998).  Robertson and Wride (1998) and Youd et al (2001) suggested that when Ic > 2.6, samples should be obtained and Atterberg Limits determined to evaluate whether the soils were more clay-like or sand-like.   Figure 1 illustrates that when the Robertson and Wride (1998) approach for sand-like soils is extended into the clay-like region (i.e. Ic > 2.6) the approach produces generally conservative estimates of cyclic resistance ratio compared to the clay-like response suggested by Boulanger and Idriss (2007).