The piezocone penetration test (CPTu) is a in situ testing method used to determine the geotechnical engineering properties of soils and assessing subsurface stratigraphy, relative density, strength and equilibrium groundwater pressures.

1. CPTu Dissipation Tests
Theory to Practice – Webinar #11 of Dr. Peter Robertson.
Professor:
Nader Shariatmadari, Ph.D
Professor of Geotech. & Geoenv. Engng At IUST.
Presented by:
Abolfazl Najafi, B.Sc
M.Sc student in Geotech. Eng. At IUST.
School of Civil Engineering, Geotech. Engng Dept.
Iran University of Science and Technology | © Dec. 2019

2. Basic Cone Parameters
2A. NAJAFI, 2019
 Friction Ratio =
𝑓𝑓𝑠𝑠
𝑞𝑞𝑐𝑐
∗ 100%
 Sleeve Friction: 𝑓𝑓𝑠𝑠 =
𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿
2𝜋𝜋𝜋𝜋ℎ
 Pore Pressure: 𝑢𝑢2
 Tip Resistance: 𝑞𝑞𝑐𝑐 =
𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿
𝜋𝜋𝜋𝜋2

3. Example CPTu Test
3
• CPT: Univ. of BC Mc.D.
Farm, Vancouver, Canada
• Total Depth: 29.35 m
 Refrence
Campanella & Robertson,
1983

4. Example CPTu Test
4
 Test depth: 20 m
 Excess pore pressure is 3-4
times greater than
hydrostatic line
 Depth of piezometric
surface (GWT) is 1.04 m
 Dissipation test takes 20
mins.

5. Dissipation test
5A. NAJAFI, 2019
• Provides information on:
 Equilibrium pore pressure, u0 (at
that location and time)
• Piezometric profile (is it hydrostatic?)
• Piezometric surface (i.e. GWT)
 Rate of dissipation
• Controlled primarily by coefficient of
consolidation (ch) and permeability
(hydraulic conductivity, kh)
• Varies by orders of magnitude
(very fast to very slow)

6. 6
Equilibrium pore pressure (u0)
• CPTu dissipation tests provide a profile of equilibrium piezometric pressures
 Identify general flow regime
• No (vertical) flow (Hydrostatic conditions)
• Upward (vertical) flow (Artesian conditions)
• Downward (vertical) flow
 Effective even if filter is not 100% saturated
 Recommend at least one per CPT to confirm piezometric profile (GWL)
• Dissipation test in sand layers - faster

7. 7
Rate of dissipation - theory
 Main theories (many others):
• Tortensson (1977) – Cavity Expansion (CE)
• Baligh & Levadoux (1986) – Strain Path
• Houlsby & Teh* (1988, 1991) – Strain Path +FE
• Burns & Mayne* (1998) – CE & CSSM
*Teh/Houlsby and Burns/Mayne the most significant
Guy T. HoulsbyPaul W. Mayne

8. 8
Theory – key findings
• Dissipation controlled mostly by horizontal 𝐶𝐶ℎ
• Initial distribution of excess pore pressures has a major influence on process
• Consolidation predominantly in recompression mode especially for times less
than 50%
• Rigidity index (IR= G/su) is important
• Somewhat similar to 1-D consolidation in laboratory – horizontal (1-D) radial
consolidation

9. 9
Strain Path Method (Houlsby & Teh, 1988, 1991)
• Monotonic decay of ∆𝑢𝑢 with time
• Often applied with measured time to 50% completion, 𝑡𝑡50 (single point)
• Need estimate of undrained rigidity index, 𝐼𝐼𝑅𝑅 = �𝐺𝐺
𝑆𝑆𝑢𝑢
• Can handle different filter element locations, e.g. 𝑢𝑢1 and 𝑢𝑢2 location
• Easy to use chart based on 𝑡𝑡50

10. Strain Path Method
Degree of Consolidation:
𝑈𝑈 = 1 − �∆𝑢𝑢
∆𝑢𝑢𝑖𝑖
where ∆𝑢𝑢𝑖𝑖= 𝑢𝑢 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 − 𝑢𝑢0 during
penetration
∆𝑢𝑢 = remaining excess pore pressure.
T∗= modified theoretical time factor
10A. NAJAFI, 2019

11. 11A. NAJAFI, 2019
Monotonic CPTu dissipation
𝐶𝐶ℎ =
(𝑇𝑇50
∗
)𝑟𝑟2
𝐼𝐼𝑅𝑅
𝑡𝑡50
where 𝑇𝑇50
∗
= 0.245 for 𝑢𝑢2 (= 0.118 for 𝑢𝑢1)
r = probe radius
= 1.78 cm for 10 − 𝑐𝑐𝑐𝑐2cone
= 2.20 cm for 15 − 𝑐𝑐𝑐𝑐2
cone
𝐼𝐼𝑅𝑅 = ⁄𝐺𝐺 𝑆𝑆𝑢𝑢 : Rigidity Index

12. 12
Simple chart for monotonic dissipation
• Calculate your Rigidity Index
• Choose t50 on the horizontal axis
• Go up straight to reach the line
accorded to your rigidity index
• Go back-ward in horizontal direction
• Here is your Coef. Of consolidation

13. 13
Example monotonic dissipation
Tailings profile Dissipation tests at 19m
Fine-grained, low PI silt tailings

14. 14
Example monotonic dissipation
Logtime
Squareroottime

15. Unloading from push rods
Rapid change in pore pressure due to
unloading from push rods
Fix (clamp) push rods to avoid
unloading
Mostly 𝑢𝑢1 location
15

16. 16
Complex soil behaviour
 Most theories assume simple stress-strain response
• Elastic-perfectly plastic
• Simple non-linear stress-strain (e.g. Cam Clay)
 Real soil response more complex
• Highly non-linear
• Strain softening (sensitive)
• Rate and stress path dependent
 Complex loading around cone
• Unloading around shoulder of tip

17. 17
Many uncertainties
• Initial distribution of u (esp. for OCR > 4)
• Soil non-homogeneity (stratigraphy)
• Soil macrofabric and anisotropy
• Influence of non-horizontal dissipation
• Soil disturbance due to penetration
• Filter element clogging/smearing
• Filter not 100% saturated
Accurate to within an order of magnitude, unless soil is
soft and homogeneous
Dissipation test

18. Permeability from CPT
18A. NAJAFI, 2019
Based on theory via dissipation test, t50
𝑘𝑘ℎ = �(𝑐𝑐ℎ 𝛾𝛾𝑤𝑤)
𝑀𝑀
where:
M is the 1-D constrained modulus
𝜸𝜸𝒘𝒘 is the unit weight of water, in compatible units.
M can be estimated from

19. Generalized CPT Soil
Behavior Type
19
CPT Soil Behavior
A: Drained-dilative
B: Drained-contractive
C: Undrained-dilative
D: Undrained-contractive

20. 20
• Saturate pore pressure sensor
• Stop penetration & record pore pressure with time
• Frequent rate at first then more slowly
• Logarithmic rate of dissipation
• Fix rods (i.e. do not remove load during dissipation)
• avoid changing stress around cone by removing load
Dissipation test procedures
(James A. Schneider 2007)

21. Summary
21
• Dissipation tests very useful
• Equilibrium piezometric pressure, u0
• Rate of dissipation – ch & kh
• Accurate to within an order of magnitude, unless soil is soft and
homogeneous
• Useful to perform at least 1 dissipation test in each CPT
sounding (very fast in sand layers)
• Helpful to re-saturate sensor
• Helpful to evaluate time delay for pile load test
Photo Credit: Cone Penetration Test (CPT) Presentation – Course CE 483.

22. Refrences
• Peter K. Robertson (Nov. 2013), “CPTu dissipation tests - theory to practice, Webinar #11” - Gregg Drilling &
Testing, Inc.
• James A. Schneider, B. M. L., Fernando Schnaid (2007). "Velocity effects on piezocone measurements in
normally and over consolidated clays." International Journal of Physical Modelling in Geotechnics.
22A. NAJAFI, 2019

23. Thank you
abolfazl_najafi@civileng.iust.ac.ir
https://najafice.github.io
Photo Credit: Univ. of Tehran, Dept. of Civil Engineering website.