Ohio Department of Transportation

John R. Kasich, Governor Jerry Wray, Director

Development of CPT Spread Footing Direct Design Methodology for ODOT

October 26, 2016

Alexander B.C. Dettloff, P.E.Foundations Engineer

Division of EngineeringOffice of Geotechnical Engineering

2ODOT CPT Rig

3

ODOT CPT Rig

Manufactured by A.P. Van den Berg

Track-Mounted CPT unit Hyson 200 kN

VTS-D4-3550-700-TM30 Crawler

John Deere diesel engine, 93kW = 125 hp

40 1039mm CPT Rods = 41.56m = 136.36’

Maximum Push = 200 kN = 45 kips

15cm2 Type 2 Piezo-Cone w/ Seismic Module

Seismic Hammer Trigger Beam on jacks

ODOT CPT Rig

4ODOT CPT Rig

5CPT Exploration

6CPT Probe

7CPT Probe

8CPT Probe

9CPT Probe

10CPT Probe

11CPT Probe

12CPT Probe

13

CPT Probe

Ac = projected area of cone

(not surface area of cone)

As = area of friction sleeve

An = cross-sectional area of load cell

or shaft

a or an = Cone Net Area Ratio

qc/u2 from laboratory triaxial cell calibration

(this is not An/Ac)

CPT Probe

14

CPT Probe Readings

Qc = force acting on the cone

measured by load cell

Fs = force acting on the friction sleeve

measured by load cell

u2 = Pore Water Pressure

measured by transducer behind the cone

CPT Probe Readings

15

CPT Probe Readings

qc = Cone (Tip) Resistance

= Qc/Ac (typically in MPa)

fs = Sleeve Friction

= Fs/As (typically in MPa or kPa)

u2 = Pore Water Pressure

from transducer (typically in kPa)

CPT Probe Readings

16CPT Probe Readings

17CPT Cone (Tip) Net Area Ratio

18CPT Cone (Tip) Net Area Ratio

19Corrected Tip Resistance

qc is not necessarily directly useful

Need to correct for pore water pressure

qt = Corrected Tip Resistance

= qc + u2(1-a)

20

CPT Probe Data

Direct Readings

Qc, Fs, u2

Primary (Derived) Values

qc, fs

Secondary (Derived) Values

qt, Rf = Friction Ratio = (fs/qt) x 100%

Tertiary (or further) Correlations

Su, ϕ′, γ, Qt, Fr, Ic, Soil Behavior Type, etc.

CPT Probe Data

21

CPT Probe Data

Correlations exist for the following:

Su, undrained shear strength

ϕ′, effective internal friction angle

γ, soil unit weight

Soil Behavior Type (SBT)

Qt, Normalized Cone Resistance

Fr, Normalized Friction Ratio

Ic, Soil Behavior Type Index

N60, SPT blow count normalized at ER 60%

CPT Probe Data

22

CPT Probe Data

Correlations exist for the following:

St, soil sensitivity

OCR, overconsolidation ratio

Ko, in-situ stress ratio

Dr, Relative Density

ψ, state parameter

E, Young’s modulus

k, coefficient of permeability

consolidation characteristics (cv, ch, M)

CPT Probe Data

23CPT Direct Design

24CPT Direct Design

25CPT Direct Design

26CPT Direct Design

27CPT Direct Design

28CPT Direct Design

29CPT Direct Design

30CPT Direct Design

31CPT Direct Design

32CPT Direct Design

33CPT Direct Design

Ic (s/B)max

≤1.8 12%

2.0 11%

2.2 10%

2.3 9%

2.4 8%

2.4 7%

2.5 6%

2.5 5%

2.8 4%

≥3.0 3%

(s/B)max versus Ic

34CPT Direct Design

35CPT Direct Design

36CPT Direct Design

37

This is for Ultimate Capacity of the foundation, but we

don’t want an ultimate capacity foundation failure to

occur!

We have two options:

Use a resistance factor to limit the ultimate capacity to a

Strength I Limit State factored resistance.

Limit the deformation based on Service Limit

requirements to get to a Service I Limit State nominal

resistance.

We looked at both by comparing the results of CPT

direct design to the results of the Terzaghi bearing

equation, using correlated soil properties.

CPT Direct Design

38

For the first case (Strength Limit State), we have

qn = qult = qtnet avg. × hs × (s/B)max

For the second case (Service Limit State), we have

qn = qall = qtnet avg. × hs × (s/B)lim,

where (s/B)lim = 0.01 typically,

which equates to (s/B)0.5 = 0.1

or (s/B)lim = (slim/B)

where slim = 1.00 inch (or less, depending on

serviceability requirements of the structure).

For both cases, qtnet avg. = the average qtnet over 1.5B below

the foundation elevation.

CPT Direct Design

39CPT Direct Design

40

CPT Direct Design Inputs

Backfill Soil (bf):

γbf, φbf, δbf, βbf, αbf => ka, EH, EV

Foundation Soil (f):

γf, φf, δf, Suf, c′f, qtnet, Ic => hs, (s/B)max

Surcharge Soil (q):

γq

CPT Direct Design Inputs

41

qtnet, Net Total Cone Resistance

qtnet = qt – σvo

qt = Corrected Tip Resistance

qt = qc + u2(1-a)

γbf, γf, depth => σvo

CPT Direct Design Inputs

42

Ic, Soil Behavior Type Index

Ic = ((3.47 - log Qt)2 + (log Fr + 1.22)2)0.5

Qt = normalized cone penetration

resistance (dimensionless)

= (qt – σvo)/σ′vo

Fr = Normalized Friction Ratio (%)

= (fs/(qt – σvo)) x 100%

Ic is used in various other correlations

CPT Direct Design Inputs

43

Ic, Soil Behavior Type Index

Generally,

Ic < 0.5 = Gravel (A-1-a)

Ic = 0.5-1.5 = Sandy Gravel (A-1-b)

Ic = 1.5-2.0 = Sand to Silty Sand (A-3)

Ic = 2.0-2.5 = Sandy Silt to Silt (A-4)

Ic = 2.5-3.0 = Silt and Clay to Silty Clay (A-6)

Ic > 3.0 = Clay (A-7)

Ic = 2.0 is the approximate dividing line

between Granular and Cohesive soils.

CPT Direct Design Inputs

44

Cohesive Soils

Various correlations exist for Su,

“Peak Shear Strength”

However, we consider Peak Shear Strength to

be unconservative for design.

Instead, we use Remolded Shear Strength.

Su = fs = Sleeve Friction

This gives a close agreement with

conventional design using SPT correlations.

CPT Direct Design Inputs

45

Granular Soils

Various correlations exist for ϕ′,

“Peak Friction Angle”

However, we consider Peak Friction Angle to

be unconservative for design, giving values up

to 166% of what could be expected with SPT

correlations.

Instead, we are correlating ϕ′, effective internal

friction angle, to N60 and N160, based on

AASHTO LRFD Bridge Design Specifications

Article 10.4.6.2.4.

CPT Direct Design Inputs

46

SPT N60 Correlation

We use a correlation developed by

Robertson (2012):

(qt/pa)/N60 = 10(1.1268 – 0.2817Ic)

qt = Corrected Tip Resistance = qc + u2(1-a)

pa = Atmospheric Pressure

Ic = Soil Behavior Type Index

CPT Direct Design Inputs

47

γ, Total Soil Unit Weight

We use a correlation developed by

Robertson (2010):

γ/γw = 0.27 [log Rf] + 0.36 [log(qt/pa)] +1.236

Rf = Friction Ratio = (fs/qt) x 100%

γw = Unit Weight of Water

pa = Atmospheric Pressure

CPT Direct Design Inputs

48Indirect Design (Terzaghi Bearing Capacity)

49

Sand

Indirect Design (Terzaghi Bearing Capacity)

50Indirect Design (Terzaghi Bearing Capacity)

51Indirect Design (Terzaghi Bearing Capacity)

52Indirect Design (Terzaghi Bearing Capacity)

53Indirect Design (Terzaghi Bearing Capacity)

54Indirect Design (Terzaghi Bearing Capacity)

55Indirect Design (Terzaghi Bearing Capacity)

56Indirect Design (Terzaghi Bearing Capacity)

57

Cohesive Soil

Indirect Design (Terzaghi Bearing Capacity)

58Indirect Design (Terzaghi Bearing Capacity)

59Indirect Design (Terzaghi Bearing Capacity)

60Indirect Design (Terzaghi Bearing Capacity)

61Indirect Design (Terzaghi Bearing Capacity)

62

Conclusions

Terzaghi and Mayne essentially agree

for ultimate strength of denser granular

soils (Medium Dense to Very Dense).

Terzaghi bearing with FS = 3.0 or 2.5 straddles

the 1.0-inch displacement line per Mayne.

Deflection (Service) and Strength limit behavior

are in conformance for these soils (neither

really controls over the other).

Mayne versus Terzaghi

63

Conclusions

For looser granular soils (Very Loose to

Loose) Terzaghi General Shear Failure

is extremely unconservative (this is

acknowledged by Terzaghi).

Terzaghi Local Shear Failure is conservative

compared to Mayne (by a factor of around 2).

Deflection does not control in bearing capacity

design of footings by Local Shear Failure.

Mayne versus Terzaghi

64

Conclusions

For low-plasticity cohesive soils (A-4a

or A-6a) Terzaghi bearing capacity is

conservative compared to Mayne.

In the ultimate state, Terzaghi bearing capacity

is conservative by a factor of around 2).

In the service state, Terzaghi bearing capacity

is conservative by a factor of from 2 to 5).

Deflection does not control in footing design in

these soils by Terzaghi bearing capacity.

Mayne versus Terzaghi

65

Conclusions

For looser granular soils and low-

plasticity cohesive soils (A-4a or A-6a)

we could realize a substantial savings

in bearing capacity design by using a

displacement-based CPT Direct Design

methodology.

Mayne versus Terzaghi

66

Direct Design

How to fit a displacement-dependent

design method into an LRFD Limit State

framework?

For Service Limit State, no problem:

Compare Service Limit State Nominal

Load with Allowable Load at (s/B)lim

pn ≤ pall = qn = qall

Trickier at the Strength Limit State

Direct Design Methodology

67Direct Design Methodology

68Direct Design Methodology

69Direct Design Methodology

70Direct Design Methodology

71Direct Design Methodology

72Direct Design Methodology

73Direct Design Methodology

74

Strength Limit State

Service Load = pn

Strength Load = pfactored

pn × γp = pfactored (γp = pfactored / pn)

qult = qtnet avg. × hs × (s/B)max = qn

qall = qtnet avg. × hs × (s/B)lim

FSmin = qult / qall

Direct Design Methodology

75

Strength Limit State

ϕb = γp / FSmin

(ϕb is variable)

qR = ϕb × qn = ϕb × qult

pfactored ≤ qR

FS = qult / pn

FS ≥ FSmin

Direct Design Methodology

76

Example: Medium Dense Sand with Service Limit State load of 100 kips

pn = 100 kips

pfactored = 147 kips

γp = pfactored / pn = 1.47

qult = 313.15 kips = qn

qall = 130.48 kips (at 1.00 inch settlement) = pall

pn ≤ pall OK Service Limit State

FSmin = qult / qall = 2.40

ϕb = γp / FSmin = 0.61

qR = ϕb × qn = 0.61 x 313.15 kips = 191.80 kips

pfactored ≤ qR OK Strength Limit State

Direct Design Methodology

77

References

AASHTO. (2014). “LRFD Bridge Design Specifications.”

7th Edition, AASHTO, Washington, DC.

ASTM D5778-12. (2012). “Standard Test Method for

Electronic Friction Cone and Piezocone Penetration

Testing of Soils.” ASTM International, West

Conshohocken, PA, www.astm.org.

Hansen, J.B. (1970) “A Revised and Extended Formula

for Bearing Capacity”, Bulletin No. 28, Danish

Geotechnical Institute, Copenhagen.

References

78

References

Mayne, P. and Woeller, D. (2014) "Generalized Direct

CPT Method for Evaluating Footing Deformation

Response and Capacity on Sands, Silts, and Clays." Geo-

Congress 2014 Technical Papers: pp. 1983-1997.

Munfakh, G., A. Arman, J. G. Collin, J. C.-J. Hung, and R.

P. Brouillette. (2001) “Shallow Foundations Reference

Manual,” FHWA-NHI-01-023. Federal Highway

Administration, U.S. Department of Transportation,

Washington, DC.

NCHRP (2007) “Synthesis 386: Cone Penetration

Testing.” Transportation Research Board, Washington,

DC, www.TRB.org.

References

79

References

Robertson, P.K., Cabal, K.L. (2014). “Guide to Cone

Penetration Testing for Geotechnical Engineering.” 6th

Edition, Gregg Drilling & Testing, Inc., Signal Hill, CA.

Terzaghi, Karl (1943), Theoretical Soil Mechanics, J.

Wiley & Sons, New York.

Vesic, A.S. (1975), Chapter 3, Bearing Capacity of

Shallow Foundations, Foundation Engineering Handbook,

First Edition, H.F. Winterkorn and H.T. Fang, eds., Van

Nostrand Reinhold Company, New York, NY, pp. 121-147.

References

80

Thank You

Questions?

Comments?

End of Presentation