pressuremeter testing an introduction .doc

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Pressuremeter TestingPaul J. Cosentino, Ph.D., P.E.

3/19/2016

I. In-situ testing:

Advantages

1.larger samples tested

2.less disturbance

3.much faster than lab tests

Disadvantages

1.can not control initial state of stress during testing (i.e.σo’)

2.many times stresses induced during testing are horizontal while building loads

are vertical

3.many times results are empirical

Types (most common)

1.pressuremeter (PMT)

2.cone penetrometer (CPT)

3.dilatometer (DMT)

4.vane shear (VST)

5.standard penetration test (SPT)

II.Pressuremeter (PMT) testing

Introduction

1.developed in 1954 by Ménard at University of Illinois

2.insert long cylindrical balloon type device into soil and during inflation with

water measureσ − εresponse of soil

Theory and Data Reduction

1.during inflation long cylinder expands radially producing plain strain conditions

2.injected water volumes are converted to volumetric strains to yield a stress-strain

plot that can be analyzed3.typically elastic moduli, lift-off (or at-rest) and limit pressures are determined

from plots

Uses

1.lateral loads on foundations especially piles and drilled shafts (or piers)

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2.empirical bearing capacity (i.e. ultimate soil capacity) predictions

3.empirical settlement predictions (have been shown to be more reliable than

Terzaghi’s One-dimensional consolidation predictions)

4.elastic moduli for finite element programs, pavement designs, immediatesettlement predictions

Advantages

1.fast testing: field testing can be completed in 10-20 minutes

2.fast analysis: computerized data reduction can be completed in 5 to 60 minutes

3.large sample tested (10 to 18-inches length depending upon model used)

4.test simulates lateral loads on piles and piers

5.simple procedures available to determine settlement, bearing capacity, etc.,

6.relatively simple testing procedure, especially with automation7.equipment relatively inexpensive ($8,000 to $12,000); therefore costs can be

recouped quickly

8.new procedures for pushing saves a SIGNIFICANT amount of time

9.new instrumentation software also save SIGNIFICANT time

Disadvantages

1.test hole MUST be carefully prepared, if pre-bored

2.membrane failure causes ½ day delay!

3.requires specialist to conduct test

Overview of test procedure

1.Prepare borehole and lower probe to desired test depth or

1.Hydraulically push cone or Pencel Pressuremeter to desired depth

2.Inject equal volume increments of water; wait for system to stabilize and record

corresponding pressures

3.Test is complete once either 90 cm3 or 1200 cm3 is injected depending upon the

PMT model used

4.Apply three calibrations to raw data;

one for inherent membrane resistance; a second for system or volumetric expansion (i.e. the tubing expands and

membrane contracts)

and a third for the test depth (i.e. hydrostatic pressure at test depth must be

added to pressure read off gage)

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III. Pressuremeter Models

There are several PMT models currently available. They vary based on the length to

diameter ratio and whether they are tri-cell or mono-cell probes.

Ménard first developed a tri-cell probe as shown below. There are two outer cells,

called guard cells that are expanded first to ensure plain strain conditions during

testing with a center cell that is expanded at predetermined pressure increments to

complete the test. The disadvantages of the Ménard probe are that 1) a stress

controlled test is conducted resulting in few data 2) the testing procedure is complex

and 3) that a gas supply is required to conduct the test.

3

Figure 1 Ménard pressuremeter

Guard CellsMeasuring Cell

Pressure Gauge

Gas Supply

Volume Measurement

Gas

Gas


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To simplify the testing procedure Briaud developed a mono-cell probe. This

simplification yields a strain-controlled test producing more data points as known as

about 20 equal volume increments of water are injected into the probe and the

corresponding pressures are recorded.

There are two mono-cell models currently available, the standard size PMT known

as the TEXAM and the cone penetrometer size version known as the PENCEL PMT.

A schematic of a typical mono-cell PMT is shown below. As the actuator is turned a

known volume of water is forced into the probe through nylon tubing and pressures

are recorded from the pressure gage. For the TEXAM; 60 cm3 volume increments up

to 1200 cm3 are injected while for the PENCEL; 5 cm3 increments are injected up to

90 cm3.

4

Schematic of a mono-cell pressuremeter

VolumeIndicator

Piston

Cylinder

Pressure Gauge

Tubing

Pressuremeter

Actuator

Control Unit

Figure 2 Schematic of Mono-Cell Pressuremeter


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The smaller PENCEL PMT is depicted below. The diameter of the probe is 1.35-inches

which is nearly the same the diameter of the Cone Penetrometer (CPT). It allows this probe

to be attached to cone rods and pushed into the soil. This feature allows a significant

number of tests to be conducted quickly. A photograph of the internal components of thecontrol unit is shown on the following page. It details the plumbing used to run the water

from the cylinder to the probe. It also includes the latest digital instrumentation that

enables operators to digitally acquire the reduced stress-strain data. The volume counter

runs from 0 to 135 cm3 and the pressure gage typically included reads pressures up to 2500

kPa (about 310 psi).

5

Figure 2: PENCEL Pressuremeter. 1. Probe, 2. Pressure Gauge, 3. VolCounter, 4. Actuator, . !ubing, ". Calibration !ube

Figure 3 PENCEL Pressuremeter. 1. Probe, 2. Analogue Pressure Gage, 3. AnalogueVolume Counter, 4. Actuator, 5. Tubing, 6 Calibration Tube for System Expansion


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Figure 4 Internal components of PENCEL Pressuremeter Control Unit

6

Analogue

Pressure Gage

Electronics’

Module

Digital Pressu

Transducer

Cylinder

Linear

Potentiometer

Volume Counter

and Crank

Handle Assembly


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Figure 5 Typical reduced pressuremeter data with definitions of key portions

A typical set of results is shown in Figure 5. This data indicates that after the soil reaches

the existing at rest pressure, it displays a relatively linear response up to about 400 kPa and

a nonlinear response typical of granular materials up to a limit pressure of about 650 kPa.

7

Sr

Si

po

pL

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60 70 80 90

Volume (cm3)

P r e s s u r e ( k P a )

Plastic

Phase

Elastic

Phase

At-Rest Soil

Pressure

ElasticReload

Phase

Limit

Pressure

Unloading


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IV Applicable Pressuremeter Theories

There are two key parameters that define any material, the stiffness and the

strength. The stiffness is based on the elastic response of a material and for thepressuremeter test the soil response which is nonlinear must be evaluated.

The basis for the pressuremeter theories is the assumption that the pressuremeter

probe causes the soil to expand according to plane strain conditions. Plane strain

typically occurs when one dimension is significantly very long compared to other

dimensions (Holtz and Kovacs 1981). The pressuremeter probe is thus

considered to be an infinitely long cylinder, expanding uniformly in the radial

direction. This assumption allows the soil moduli to be determined based on

linear elastic theory according to the equation:

E = 2 1+ ν ( )

∆ P ∆V

V m (1a)

where, E = Young’s modulus

∆P = change in stress∆V = change in volume related to∆P

V m = average volume over∆P ν = Poisson's Ratio (typically assumed to be 0.3 for unsaturatedconditions and 0.5 for saturated)

The relative radius increase in probe radius can be substituted into Equation 1a,

yielding the following equation used in analysis to determine moduli (Tucker

and Briaud 1986):

E = 1+ ν( ) 1+

∆R 1R

o

2

+ 1+

∆R 2R

o

2

∆P

1+ ∆R 1

R o

2

− 1+ ∆R 2

R o

2 (1b)

where, ∆R 1 = radius increase at point 1∆R 2 = radius increase at point 2

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V. Settlement with the Pressuremeter

Menard and Rousseau (1962) developed the basic settlement equation from PMT

data. It is composed of two parts a deviatoric component and a spherical

component. Several empirical factors are required to perform the calculations

but the basic equation is:

s =2

9 E d qBo λ d

B

Bo

α

+ α

E cqλ c B (2)

where: s = total footing settlement

Ed = pressuremeter modulus within the deviatoric zone of influence

Ec = pressuremeter modulus within the spherical zone of influence

q= net bearing pressure of the footing

λd = shape factor for deviatoric term from the figure below

λd= shape factor for spherical term from the figure below

α =rheological factor from the table below

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Figure 6 Variation in Menards shape factors versus footing dimensions

To perform the calculations, divide the soil layers beneath the footing into layers

B/2 thick. Us the PMT modulus within the first layer for Ec and an averagemodulus over a depth of 16 layers each B/2 thick for Ed. Briaud (1992)

recommends a harmonic mean calculation for this deviatoric modulus.

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Menard's Shape Factors for Settlement

1

1.25

1.5

1.75

2

2.25

2.5

0 1 2 ! 5 " 7 # 9 10

Length/Width

d

c


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Table 1Menard Rheological Factor

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Soil Type E/pL*

E/pL*

E/pL*

E/pL*

E/pL*

$%er&onsolidated ' 1" 1 ' 1! 2( ' 12 1(2 ' 10 1(

)ormally

Consolidated1 9*1" 2( #*1! 1(2 7*12 1( "*10 1(!

+eat,ered and(or

Remolded7*9 1(2 1(2 1( 1(!

Ro&- 1( 1(2 2(

Sand & GraelSilt

$t,er Slig,tly ra&tured or E/tremely

+eat,ered

!eat "lay Sand

ig,ly ra&tured


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VI. Applications for Laterally Loaded Piles

The Robertson et al, Pushed in PMT Method (1986)

Robertson et al. (1986) suggested a method that uses the results of pushed-in

PMT to evaluate p-y curves of a driven pile. According to the authors, the results

provide an excellent comparison with lateral loaded pile test measurements. The

pressure component of the PMT curve is multiplied by anα-factor to obtain the

corrected p-y curve. Using finite element analysis Byrne and Atukorala (1983)

confirmed this factor, which was initially suggested by Hughes et al. (1979),

Robertson et al. (1986) corrected the factorα near the surface assuming that the

PMT response is affected by the lower vertical stress.The factor increases linearly

up to a critical depth, which is assumed to be four pile diameter (Dc = 4) as

shown in Figure 6.

Fig#re $ "orrection Factor % ers#s (elatie )epth From (o+ertson et al, -.01

To obtain the p-y curve, the PMT curve is re-zeroed to the lift-off pressure that is

assumed to be equivalent to the initial lateral stress around the pile. The stress is

multiplied by the pile width and the strain component

∆ R

R

is multiplied by the

pile half width. For a small strain condition

∆ R

R is assumed equal to

∆

V

V

2

whereRandV are radius and volume of the PMT respectively.

12

Multiplying Factor a

1

!

"

#

0.5 1.0 1.5 2.00

Cohesionless Soils

(sand)

Cohesie Soils

(clay)

R e l a t i e ! e p t h " # B p i l e


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Since the installation of the pushed in PMT produces an initial pressure on the

probe, an unload/reload sequence is often used. The portion of the corrected

PMT curve from the beginning of reload through the maximum volume is

recommended for determining p-y curves of driven piles, while the initial slopefrom the PMT tests is recommended for constructing p-y curves for augured

piles. The following equations outline the process for driven piles:

a) Determine the initial radius of the probe:

π 2

Proeo n&eCir&umere3nitial0 = R (2)

b) Calculate the initial volume of the probe (Vo):

Memraneo 4engt,2

P0 RV π = (3)

c) DetermineP in units of force / length:

First a correction factor,α , is applied toP according to Figure 6, where the

relative depth is the depth from the ground surface to the center of the

membrane. Note that forzppmt

Bpile

≥ 4 α = 1.5for sands and2.0for clays and if

zppmt

Bpile


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P = F + Q (8)

where

F = friction resistance

Q = front resistance

Briaud suggested for the full displacement driven piles, that the reload portion

of the PMT curves be used. Graphically, the p-y curve is shown as the addition of

the F-y curve and the Q-y curve in Figure 7.

Figure 8 Front and side resistance components for P-y curve construction

Smith (1983) showed excellent correlations between the pressures obtained from

the PMT response and those acting on the pile. The front pressure contribution,Q, is found from the net limit pressure pL* determined as:

0 p p p $ $ −= (9)

where; pL is the limit pressure and p0is the horizontal stress at rest pressure

obtained from the PMT curve. The frontal resistance, Q is obtained by choosing

pressure points from the reduced PMT plot and using the equation:

78787878 % pile pmt &ront S B p% ××= (10)The side friction,F(side), of the pile is taken as a constant with depth and is given

by the equation:

78787878 F pile soil side S B F ××=τ (11)To obtain the associated lateral pile deflections, choose PMT deflections and

apply the following equation. The deflections must be less than those obtained

from the PMT test and would equate to the change in radii obtained during

expansion.

780

78

7878

pmt

pile

pmt pile R

R y y ×= (12)

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P = Q + F

Q

F

y

p

(d)

F Q

Friction Front

Resistance

Pressure Q

shear F

PPILE

(a) (b) (c)


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Where:Q(front) = soil resistance due to front reaction with unit of force /unit

length of pile

F(side) = soil resistance due to friction resistance with unit of force /unit

length of pile p(pmt) = pL* = net pressuremeter pressure

B(pile) = pile width or diameter

>(soil) = maximum soil shear stress-strain at the soil-pile interface

S(Q) = shape factor ( = 0.8 or π/4 for circular piles, 1.0 for square piles)

S(F) = shape factor ( = 1.0 for circular piles, 2.0 for square piles)

y(pile) = lateral deflection of the pile

y(pmt) = increase in radius of the soil cavity in the PMT test or radial

displacement.

R(pile) = pile half-width or radiusR0(pmt) =R0 = initial radius of the soil cavity in PMT test

This method does rely on an accurate estimate of the shear strength, which could

be found from other field-testing performed during the site investigation.

The displacement of soil around the laterally loaded pile is also influenced by

the ground surface. A reduction in the corrected PMT pressures is recommended

for values near the ground surface. A critical depth (Dc), to which pressures and

displacements are influenced, depends on the pile load, diameter and stiffness.

Briaud suggested using a relative rigidity factor,RR, given by:

!

78 5

1

$ pile p

E'

B RR = (13)

EI = pile flexural stiffness (E= pile modulus, I = pile moment of inertia)

78 pile B = pile diameter or width

pL* = net PMT limit pressure

Briaud et al. (1992) relationship results in relative rigidities slightly greater than

10 for most laterally loaded piles in soft clays and the resulting critical depth will be near 4, therefore Robertson’s value of 4 is recommended. The critical depths

for the PMT as recommended by (Baguelin et al., 1978) are 15 PMT diameters for

cohesive soils, and 30 PMT diameters for cohesionless soils.

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The Briaud et al. (1992) suggested reduction factorβ is shown in Figure 8 as a

function of relative depth (z/zc). The PMT curve is then corrected by using:

β

p pcorr = (14)

Fig#re . 2ria#d3s recommended !MT press#re red#ction factor for al#esnear the gro#nd s#rface

VII. References

1.Cosentino, Paul J., Edward Kalajian, Ryan Stansifer, ,J Brian Anderson,

Kishore Kattamuri, Graduate Research Assistant, Sunil Sundaram,

Graduate Research Assistant, Farid Messaoud, Thaddeus J. Misilo, Marcus

A Cottingham (2006) Final Report,Standardizing the Pressuremeter Test for

Determining p-y Curves for Laterally Loaded Piles, Florida Institute of

Technology, Civil Engineering Department ,Florida Department of

Transportation, Contract Number BC-819.

2.Briaud, J.L., 1997.“Simple Approach for Lateral Loads on Piles”. Journal

ofGeotechnical and Geoenvironmental Engineering, ASCE, Vol. 123, No. 10pp. 958-964.

3.Robertson, P. K., Campanella, R. G., Brown, P. T., Grof, I., and Hughes, J.

M., (1985). “Design of Axially and Laterally Loaded Piles Using In Situ

Tests : A Case History”, Canadian Geotechnical Journal, Vol. 22, No. 4,

pp.518-527.

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4.Robertson, P.K., Davis, M.P., Campanella, R.G.. (1989). “Design of

Laterally Loaded Driven Piles Using the Flat Dilatometer”, ASTM

Geotechnical Testing Journal, Vol.12, No. 1, pp30-38.

5.Robertson, P.K., Hughes, J.M.O., Campanella, R.G., and Sy, A., 1983.“Design of Laterally Loaded Displacement Piles Using a Driven

Pressuremeter”. ASTM STP 835, Design and Performance of Laterally

Loaded Piles and Piles Groups, Kansas City, Mo.

6.Robertson, P.K., Hughes, J.M.O., Campanella, R.G., Brown, P., and

McKeown, S., 1986. “Design of Laterally Loaded Displacement Piles Using

the Pressuremeter”. ASTM STP 950, pp. 443-457.

7.Robertson, P.K., and Hughes, J.M.O., 1985. “Determination of Properties

of Sand from Self-Boring Pressuremeter Tests”. The Pressuremeter and Its

Marine Applications, Second International Symposium.

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pressuremeter testing an introduction .doc

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