ESLAMI AND FELLENIUS CPTU

METHODCEE 445: Foundation Engineering, Lehigh University

Overview

CPTu developementToe ResistanceShaft Resistance Axial Load Capacity

Background: Cone Penetrometer

Developed to simulate a mini-pile load test

Measures cone resistance qc, cone side friction fs

Figure 1: CPT Cone Figure 2: CPT Truck (inside)

Piezocone CPTu

CPT cone with a piezometer to measure total pore water pressure

Figure 3: Piezocone Sketch Figure 4: Piezocone Photo

Pore Water Pressure

utotal = uhydrostatic + uadvancing

where utotal is piezocone pore water pressure measurement

uhydrostatic is hydrostatic pore water pressure

uadvancing is excess pore water pressure

induced by advancing cone

Note: uadvancing is small in sand and large in clay.

So, this method can be used with conventional CPT data in sand if groundwater table is known

and no artersian conditions are present.

Effective Cone Resistance

qE = qt – u2

where qE is effective cone resistance

qt is corrected cone resistance for pore water

pressure on the shoulder u2 is hydrostatic pore water

pressure

Soil Profiling Chart

Figure 5: Soil Profiling Chart

Toe Resistance

rt = Ct qEG

where rt is pile unit toe resistance, also qt’

Ct is toe correlation coefficient

qEG is geometric average of the effective cone resistance over the influence zone

qEG =[ (qE)1(qE)2 (qE)3… (qE)n]1/n

for piles installed through weak into dense soil: 8b above and 4b below pile toe

for piles installed through dense into weak soil:2b above and 4b below pile toe

where b is pile diameter

Ct toe correlation coefficient

Usually Ct=1 Since the larger the pile diameter, the

larger the movement required to mobilize the toe resistance, for b > 0.4 m,

Ct = 1/(3b) [meters]

Ct = 12/b [inches]

where b is pile diameter

Shaft Resistance

rs = Cs qE

where rs is pile unit shaft resistance; also fs

Cs is shaft correlation coefficient, which is a function of soil type from the Eslami- Fellenius soil profiling chart and Table 1

qE is effective cone resistance

Cs shaft correlation coefficientTable 1: Cs shaft correlation coefficient

Axial Load Capacity

Pa = rt At + ΣrsAs

(Coduto) FS

where Pa is allowable axial load

rs is pile unit shaft resistance; also fs

At is toe-bearing contact area

rt is pile unit toe resistance, also qt’

As is side friction contact area

FS is factor of safety

Limitations

More suitable for noncohesive soils because cohesive soils require pile installation depth greater than explored by CPTu

CPTu is a short term test that does not include the long term pile effect soil set-up (capacity gain over time)

Must be calibrated to site specific conditions

Image Sources

Figure 1: http://geology.about.com/b/2007/09/10/cone-penetrometer-testing.htm

Figure 2: http://geology.about.com/b/2007/09/10/cone-penetrometer-testing.htm

Figure 3: http://www.conepenetration.com/online-book/piezocone-cone-penetration-test-with-porewater-pressure-management

Figure 4: http://www.theinsitugroup.com/ Figure 5: Eslami, A., Fellenius, B.H. (1997). Table 1: Fellenius, B.H. (2006).

References

Eslami, A., Fellenius, B.H. (1997). “Pile Capacity by direct CPT and CPTu methods applied to 102 case histories.” Canadian Geotechnical Journal, 34(6), 886-904.

Fellenius, B.H. (2006). Basic Foundation Design, Electronic Ed.

Coduto, D. P. (2001. Foundation Design, 2nd Ed. Prentice Hall, Upper Saddle River, New Jersey.