The comparison of fine-grained soil classifications from the Casagrande Plasticity Chart and a proposal for a revised Plasticity Chart based on the Clay Fraction.

1. Reinterpretation of Atterberg limits test results
performed on fine-grained soils from Mexicali, Baja
California
Ing. Rafael Ortiz Hernández & Dr. Eduardo Rojas González
Posgrado Maestría en Ciencias Geotecnia
Facultad Ingeniería Universidad Autónoma de Querétaro
Santiago de Querétaro, México

2. INDEX
• About the Authors
• Background
• Atterberg Limits
• United Soil Classification System (USCS)
• USCS Plasticity Chart
• Polidori’s Plasticity Chart
• Mexicali’s soils
• Methodology
• MXLI Database
• Testing
• Applying MXLI into the Plasticity Charts
• Results
• Discussion
• Conclusions
• References

3. ABOUT THE AUTHORS
Postgraduate Student @ UAQ in M. Sc. Geotechnics
Civil Engineer, specialized in Geotechnical
Engineering / Soil Mechanics by career choice
~5 years experience in
geotechnical site
exploration in Mexicali,
Baja California
Coordinator of M. Sc. Geotechnics and Research Professor
@ Faculty of Engineering in UAQ
Level I SNI
~12 years @ Engineering Institute in UNAM
Alejandrina Prize Winner
2 First Place
1 Second Place
3 Honorific Mentions

4. BACKGROUND – Atterberg Limits
Plasticity is a very important soil property. As it describes the behavior of fine
grained soils.
The plasticity of a soil is defined by its Atterberg limits, which define the
Consistency of a soil when it has a certain amount of water content.
The two main limits used for the classification of soils are the Liquid and Plastic
limits.
Liquid limit: The water content at which the remolded soil behaves like a soft
paste (toothpaste consistency).
Plastic limit: Water content at which the remolded soil behaves like a hard
paste (soft caramel).
The difference between the liquid limit and the plastic limit is the plasticity
index or IP (IP = LL – LP).
The plasticity index has been found to be related to a number of useful soil
properties.
(Briaud, 2013)
States of Consistency and Atterberg limits
(Briaud, 2013)

5. BACKGROUND – The Unified Soil Classification System
In the Engineering practice we classify fine grained soils as those whose >50% of its soil mass has a grain size finer than 0.075 mm / 75
μm (microns), based on the Unified Soil Classification System (USCS), which is a standard adopted in many countries, such as Mexico.
In the USCS we recognize 3 main fine-grained soils groups: organic soils, silts and clays.
While these fine-grained groups do have a size classification, the USCS uses the Atterberg limits to classify them by their
consistency.
The current way of classifying them in the USCS is by using the Liquid limit and the Plasticity Index in a Plasticity Chart that places them
in 7 main zones. The zoning was done on empirical research and experience by Dr. Arthur Casagrande performed for Airfields in
WWII.
(ASTM, 2017)

6. BACKGROUND – The USCS Plasticity Chart
ASTM USCS Plasticity Chart based on Casagrande’s proposal from 1948.
(Dr. Arthur Casagrande was a very influential Geotechnical Engineer)
Group Symbol Name Conditions
CH Fat clay
Liquid limit < 50 and IP located
above A-line
CL Lean clay
Liquid limit ≥ 50 and IP located
above A-line
ML Silt
Liquid limit < 50 and IP located below
A-line
MH Elastic silt
Liquid limit ≥ 50 and IP below above
A-line
ML-CL Silty sand LL between 5-25 and IP between 4-7
OL Organic soil
Liquid limit at natural conditions < 50
and Ratio between Liquid limit at
natural conditions and after drying <
0.75
OH Organic soil
Liquid limit at natural conditions > 50
and Ratio between Liquid limit at
natural conditions and after drying <
0.75

7. BACKGROUND – The Polidori Plasticity Chart
Dr. Ennio Polidori performed research on the published experimental results of pure montmorillonite, kaolinite and quartz sand mixes.
Montmorillonite is a very plastic clay mineral, it has a high liquid and plastic limit.
Kaolinite is a very low plastic clay mineral, it has a low liquid and plastic limit.
Quartz sands is a very fine sand and has no plasticity, no liquid or plastic limit.
Montmorillonite (Mitchell, 2003) Kaolinite (Mitchell, 2003)
Quartz sand (ISM)

8. BACKGROUND – The Polidori Plasticity Chart
His research in these soil mixes didn’t match Casagrande’s classification as it originally didn’t take into account the Activity of
the soil, so he proposed a new chart based on the Activity of these soils mixes.
Group Symbol Name
CH Fat clay
CL Lean clay
ML Silt
MH Elastic silt
OL Organic soil
OH Organic soil
Soil activity is defined as the IP divided the Clay Fraction of the soil.
Clay fraction is the % of clay particles (2 μm) present in the soil mass.

9. BACKGROUND – Mexicali’s Soils
The sediments were formed in the widening of the fluvial valley caused by the floods of the Colorado
River. They are characterized by the presence of facies similar to those of floodplain alluvium and
lacustrine sediments. Its grain size or lithological composition is variable: sand, silt and clay. Thin
layers with alternating sand and silt horizons are characteristic. (SGM, 2003)

10. BACKGROUND – Mexicali’s Soils
The Mexicali municipality has a geotechnical zoning of 4 main
regions (SIDUE, 2017):
I. City
II. New River
III. Valley
IV. San Felipe
Regions I and III are geotechnically characterized as deposits of
clays of high and low compressibility (CH and CL) several tens
of meters thick and with a soft to firm consistency, with silt and
sand lenses of loose compacity that in seismic events of large
magnitude, can suffer from liquefaction effects (Santoyo, 1996).
IV
III
I
II
Google Maps, 2021

11. METHODOLOGY – The MXLI Database
A local geotechnical engineering firm in Mexicali, Baja California, ROMA SOL Ingeniería S. de R.L. de C.V. provided the authors with a
database (referred to as “MXLI”) of 112 Atterberg limit test results (Liquid limit and Plastic limit) performed on fine-grained soil samples
from the Mexicali city and surrounding valley area for this classification exercise.
Data points of the “MXLI” database of performed Atterberg limit tests in Mexicali fine-
grained soils plotted in a Liquid Limit – Plasticity Index space, n = 112.
These soils samples were obtained by standard penetration testing and
open pit testing. The purpose of these Atterberg limit tests were for the
subsoil’s geotechnical characterization as part of the soil mechanic studies
performed by the firm from 2021 to 2022.
The Liquid Limit was obtained by the Casagrande Cup and the Plastic
Limit was obtained by the rolling thread method in accordance with IMT’s
M-MMP-1-07 Standard (Consistency Limits).
The water content of these test samples was performed by electric oven
drying in accordance with IMT’s M-MMP-1-04 Standard (Water Content)

12. METHODOLOGY - Testing
Soil sampling
Atterberg Limit test samples
(Liquid and Plastic Limits)
Casagrande Cup Method
(Liquid Limit)
Thread Rolling Method
(Plastic Limit)

13. METHODOLOGY – Applying MXLI into the Plasticity Charts
The methodology was simple, we applied the USCS Chart and Polidori’s Chart into the MXLI Database and classified the materials in
both charts using Microsoft’s Excel, then we compared and assessed the results.
MXLI Database
USCS Plasticity Chart
Polidori’s Plasticity Chart

14. RESULTS – The USCS Casagrande Chart
This is the standard chart used in the United Soil
Classification System as developed by Dr. Arthur
Casagrande.
Mexicali’s soils are mostly fat (CH) and lean (CL)
clays. A small sample of silts (ML) and silty clays (CL-
ML).
There is an elastic silt (MH) but this can be
attributed to human error as it is a very uncommon soil
in Mexicali.

15. RESULTS – The Polidori Chart
The fat clays group (CH) did not have a
significant change, with the exception of 5 elements
that moved into the elastic silt group (MH).
However, most of the lean clay group (CL)
turned into silt (ML) (48 data points down to just 5).
There are now 2 organic soil (OL) data points,
but they can be attributed as an appreciation error
due to the low to null plastic limits of these data
points (IP = 0.06 and 4).

16. RESULTS – The numbers
We can see a major shift in the lean clay group as the Polidori’s chart reclassifies them as silt.
MXLI Soils according to Casagrande’s Classification
Group Symbol Description Quantity Observations
CH Fat clay 53
LL: 50.9 – 77
PI: 24.98 – 42.57
CL Lean clay 48
LL: 22.0 – 50
PI: 7.81 – 26.34
MH Elastic silt 1
Uncommon, possible
testing error.
ML Silt 7
LL: 19.8 – 47.1
PI: 0.06 – 17.56
ML-CL Silty clay 3
LL: 25.2 – 29
PI: 5.3 – 6.99
MXLI Soils according to Polidori’s Classification
Group Symbol Description Quantity Observations
OL Organic soils 2
LL: 21.09 – 28
IP: 0.06 – 4
OH Organic soils N/A N/A
CL Lean clays 5
LL: 39.8 – 48.6
IP: 14.64 – 23.08
CH Fat clays 48
LL: 50.9 – 77
IP: 24.98 – 42.57
ML Silt 43
LL: 19.8 – 50
IP: 0.66 – 26.34
MH Elastic silt 6
LL: 51.4 – 64.6
IP: 28.72 – 39.36

17. DISCUSSION – So, what does that mean?
While we expected that the fat clay (CH) materials to keep its classification, we were surprised by the change in the lean clay (CL) group.
The change of the lean clay (CL) into silt (ML) might be explained by the presence of coarser material such as sands.
Sands do not have plasticity as their grain size is not affected by the electrical phenomena that finer material experience (e.g. van der
Waals forces and Double Diffuse Layer), therefore the addition of this material to the fine soil will affect the test results (lowering its
plasticity).
Unfortunately the database does not have a record of the quantity of non-fine material associated with the test results so there is a
need to update it with this information for further analysis.
There is still debate regarding the standard procedure allowing No. 40 to No. 200 material in the test, as it was originally performed by
Atterberg and then followed by Casagrande. However Casagrande noted that this did in fact have an effect in his results
(Casagrande, 1948).
We should perform additional testing such as mineralogy and fine grain size analysis to verify these results.

18. DISCUSSION – Additional Testing Suggested
X-Ray diffraction
(Mineralogy)
Electronic Microscope
(Mineralogy)
(Mitchell, 2005)
(USGS, 2001)
(Encyclopedia Britannica)

19. DISCUSSION – Additional Testing Suggested
Hydrometer
(Clay Fraction)
Hydrometer Grain Curve
(Clay Fraction)
(Briaud, 2013)

20. DISCUSSION – What if we used a different method?
There is another method to determine the plasticity, known in Europe and Asia as the Fall Cone Test.
There is a cone with specified weight and dimensions and its penetration in a soil mass contained in a cup is correlated with its water
content.
This method can be used to determine the Atterberg limits of Liquid Limit and Plastic Limit.
The acceptance of this method in Mexico and North America is still under research and no standard has
been published for its use.
The values obtained by this method are not 1:1 with those of the Casagrande Cup and the Rolled Thread
test, so a calibration is needed to adjust the obtained Fall Cone values to those previously performed by the
standard methods.
(Briaud, 2013)

21. CONCLUSIONS – Here’s what we know now:
• Plasticity is strongly associated with the clay fraction. So this property must be taken into account as is done in the Activity
property of soils (Plasticity Index over Clay Fraction).
• The new chart maintains the distinction of High and Low compressibility in soil groups. However there must be a reappraisal of
the suspected behavior of the reclassified material.
• This chart can be used in previously performed laboratory results. No new testing methods have to be implemented for this chart,
however there is still ongoing research on better methods to determine the plasticity of materials.
• Mexicali has a silty clay that can be reclassified as clayey silt by this new chart. Additional testing and research is needed to
verify this change in behavior, such as mineralogy and fine grain analysis.
• There are still many avenues of research regarding the classification of soil plasticity. Such as localized plasticity charts fit for
specific geological regions, expanding the database of performed tests for verification and the application new testing methods.
ACKNOWLEDGEMENTS
We would like to thank M. Eng. Marco Vinicio Romano and ROMA SOL Ingeniería S. de R.L. de C.V. for their
generosity in providing the plasticity test results database for this experiment.

22. REFERENCES
Essential Reading:
• Polidori, E. (2003). Proposal for a new plasticity chart. Géotechnique, 53(4), 397-406.
• Li, K. S., Prakash, K., & Sridharan, A. (2004). Proposal for a new plasticity chart-Discussion. Géotechnique, 54(8), 555-560.
• Seed, H. B., Woodward, R. J. & Lundgren, R. (1964). Fundamental aspects of the Atterberg limits. J. Soil Mech. Found. Div., ASCE 90, No. SM6, 75–105.
• Casagrande, A. (1948). Classification and identification of soils. Transactions of the American Society of Civil Engineers, 113(1), 901-930.
• Juárez E. (2011). Mecánica de suelos I (Vol. 1). ed. Limusa. 644 p.
• Mesri, G. & Cepeda-Diaz, A. F. (1986). Residual shear strength of clays and shales. Géotechnique 36, No. 2, 269–274.
• Narsilio, G. A., & Santamarina, C. (2016). Clasificación de suelos: fundamento físico, prácticas actuales y recomendaciones. Georgia Institute of Technology,
Atlanta, GA, USA.
• Crevelin, L. G., & Bicalho, K. V. (2019). Comparison of the Casagrande and fall cone methods for liquid limit determinations in different clay soils. Revista
Brasileira de Ciência do Solo, 43.
• Wood, D. M., & Wroth, C. P. (1978). The use of the cone penetrometer to determine the plastic limit of soils. Ground Engineering, 11(3).
• Hind, K. J., Alexander, G. J., & Chin, C. Y. (2017). The Casagrande plasticity chart–does it help or hinder the NZGS soil classification process?. In
Proceedings of the 20th New Zealand Geotechnical Society Geotechnical Symposium, Napier, New Zealand (pp. 1-8).
• Wesley, L. D. (2010). Geotechnical engineering in residual soils. John Wiley & Sons

23. REFERENCES
About Mexicali:
• Campos G. J.M. (1974). Capítulo V: Mexicali, B. C., VII Reunión Nacional de Mecánica de Suelos, Tomo I. Guadalajara, Sociedad Mexicana de Mecánica de
Suelos SMMS, México
• Rangel-Núñez et. al (2010). “Efectos geotécnicos y estructurales observados en el valle y ciudad de Mexicali, provocados por el sismo El Mayor Cucapah del 4
de abril de 2010”, XXV Reunión Nacional de Mecánica de Suelos e Ingeniería Geotécnica, Acapulco México
• SIDUE (2017) “Normas Técnicas Complementarias de la Ley de Edificaciones del Estado de Baja California, de Seguridad Estructural en materia de Criterios y
Acciones de Diseño y Construcción de Cimentaciones”, Gobierno del Estado de Baja California.
• Santoyo E. y Montañez L. (1976). Mexicali, B. C., VIII Reunión Nacional de Mecánica de Suelos, Tomo II. Guanajuato, Sociedad Mexicana de Mecánica de
Suelos SMMS, México.
Testing Standards:
• ASTM D2435 (2020). “Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System”, ASTM International, West
Conshohocken, PA.
• IMT (2007). M-MMP-1-07 Límites de Consistencia. Suelos y Materiales para Terracerías. Métodos de Muestreo y Pruebas de Materiales. Secretaria de
Comunicaciones y Transportes. San Fandila, Querétaro.
• IMT (2003). M-MMP-1-04 Contenido de Agua. Suelos y Materiales para Terracerías. Métodos de Muestreo y Pruebas de Materiales. Secretaria de
Comunicaciones y Transportes. San Fadila, Querétaro.
• British Standard Institution (1990). Method of test for soils for civil engineering purposes (BS 1377 Part: 2), London.

24. FURTHER INFORMATION
You can scan the following QR code for a copy of this presentation and additional references about this project.
Rafael’s e-mail: rortiz10@alumnos.uaq.mx
LinkedIn: https://www.linkedin.com/in/raforther/